Article

Loss of fine particle ammonium from denuded nylon filters

August 2006
Atmospheric Environment (1967) 40(25):4797-4807

August 2006
40(25):4797-4807

DOI:10.1016/j.atmosenv.2006.03.061

Authors:

Oak Ridge National Laboratory

Hankuk University of Foreign Studies

Sonia M Kreidenweis

Colorado State University

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Ammonium is an important constituent of fine particulate mass in the atmosphere, but can be difficult to quantify due to possible sampling artifacts. Losses of semivolatile species such as NH4NO3 can be particularly problematic. In order to evaluate ammonium losses from aerosol particles collected on filters, a series of field experiments was conducted using denuded nylon and Teflon filters at Bondville, IL (February 2003), San Gorgonio, CA (April 2003 and July 2004), Grand Canyon NP, AZ (May, 2003), Brigantine, NJ (November 2003), and Great Smoky Mountains National Park (NP), TN (July–August 2004). Samples were collected over 24 h periods. Losses from denuded nylon filters ranged from 10% (monthly average) in Bondville, IL to 28% in San Gorgonio, CA in summer. Losses on individual sample days ranged from 1% to 65%. Losses tended to increase with increasing diurnal temperature and relative humidity changes and with the fraction of ambient total N(−III) (particulate NH4++gaseous NH3) present as gaseous NH3. The amount of ammonium lost at most sites could be explained by the amount of NH4NO3 present in the sampled aerosol. Ammonium losses at Great Smoky Mountains NP, however, significantly exceeded the amount of NH4NO3 collected. Ammoniated organic salts are suggested as additional important contributors to observed ammonium loss at this location.

A Quantitative Method to Measure and Speciate Amines in Ambient Aerosol Samples

Article

Full-text available

Jul 2020

Ambient reactive nitrogen is a mix of nitrogen-containing organic and inorganic compounds. These various compounds are found in both aerosol- and gas-phases with oxidized and reduced forms of nitrogen. Aerosol-phase reduced nitrogen is predominately thought to include ammonium and amines. In ambient samples, the ammonium concentration is routinely determined, but the contribution of amines is not. We developed a method to discretely measure amines from ambient aerosol samples. It employs ion chromatography using a Thermo Scientific IonPac Dionex CS-19 column with conductivity detection and a three-step separation using a methanesulfonic acid eluent. This method allows for the quantification of 18 different amines, including the series of methylamines and the different isomers of butylamine. Almost all amines quantifiable by this technique were measured regularly when applying this method to ambient filter samples collected in Rocky Mountain National Park (RMNP) and Greeley, CO. The sum of the amines was ~0.02 µg m−3 at both sites. This increased to 0.04 and 0.09 µg m−3 at RMNP and Greeley, respectively, at the same time they were impacted by smoke. Analysis of separate, fresh biomass burning source samples, however, suggests that smoke is likely a minor emission source of amines in most environments.

Constraining Ammonia Emissions in Vehicle Plumes Utilizing Nitrogen Stable Isotopes

Preprint

Full-text available

Mar 2020

Abstract. Vehicle emissions have been identified as an important urban source of ammonia (NH<sub>3</sub>). However, there are large uncertainties regarding the contribution of vehicle emissions to urban NH<sub>3</sub> budgets, as well as its role in spatiotemporal fine particulate matter (PM<sub>2.5</sub>) formation and nitrogen (N) deposition patterns. The N stable isotopic composition (δ<sup>15</sup>N) may be a useful observational constraint to track NH<sub>3</sub> emission sources and chemical processing, but previously reported vehicle δ<sup>15</sup>N(NH<sub>3</sub>) emission signatures have reported a wide range of values, indicating the need for further refinement. Here we have characterized δ<sup>15</sup>N(NH<sub>3</sub>) spatiotemporal variabilities from vehicle plumes in stationary and on-road measurements in the US and China using a laboratory- and field-verified NH<sub>3</sub> collection technique shown to be accurate for characterizing δ<sup>15</sup>N(NH<sub>3</sub>) on the order of hourly time resolution. Significant spatial and temporal δ<sup>15</sup>N(NH<sub>3</sub>) variabilities were observed and suggested to be driven by vehicle fleet composition and influences from NH<sub>3</sub> dry deposition on tunnel surfaces. The reactive NH<sub>3</sub> sink associated with particulate ammonium (pNH<sub>4</sub><sup>+</sup>) formation was found to have a minimal impact on the vehicle plume δ<sup>15</sup>N(NH<sub>3</sub>) measurements due to the vast majority of NH<sub> x </sub> (= NH<sub>3</sub> + pNH<sub>4</sub><sup>+</sup>) residing as NH<sub>3</sub>. Overall, a consistent δ<sup>15</sup>N(NH<sub>3</sub>) signature of 6.6 ± 2.1 ‰ ( x ± 1σ; n = 80) was found in vehicle plumes with fleet compositions typical of urban regions. Overall, these measurements constrain the δ<sup>15</sup>N(NH<sub>3</sub>) urban traffic plume signature, which has important implications for tracking vehicle NH<sub>3</sub> in urban-affected areas.

Source apportionment of PM10 in Delhi, India using PCA/APCS, UNMIX and PMF

Article

Apr 2018
PARTICUOLOGY

In the present study, source apportionment of respirable particulate matter (PM10) was carried out in Delhi, India using two receptor models, principal component analysis with absolute principal component scores (PCA/APCS) and UNMIX. The results were also compared to previous estimate using positive matrix factorization (PMF) receptor model. The chemical compositions [organic carbon (OC), elemental carbon (EC), water soluble inorganic ions (WSIC) and trace elements] of PM10 was used for source apportionment, collected during January 2013 to June 2014. The average concentration of PM10 was recorded as 249.7 ± 103.9 µg m-3 (range: 61.4 to 584.8 µg m-3) during the study period. The study focuses on comparing the results from three different receptor models to form judgment about adequacy of the models for source apportionment. The UNMIX resolved five sources solution [soil dust (SD), vehicular emission (VE), secondary aerosols (SA), mixed type (biomass burning + sea salt), industrial emission (IE)] and PCA/APCS showed five sources out of which two were mixed type and they were identified as soil dust, vehicular emission, crustal/road dust + sea salt, secondary aerosols + biomass burning + sea salt and industrial emission whereas the PMF analysis well differentiated the sources by their corresponding tracers and identified seven sources (soil dust, vehicular emission, secondary aerosols, biomass burning, sea salt, industrial emission, fossil fuel combustion) of PM10. The three applied models were able to identify the main sources contributing to the PM10 and reconfirmed that vehicular emissions, secondary aerosols, biomass burning and soil/road dust were dominant contributors to PM10 in Delhi.

Characterizing the spatiotemporal nitrogen stable isotopic composition of ammonia in vehicle plumes

Article

Full-text available

Oct 2020
ATMOS CHEM PHYS

Vehicle emissions have been identified as an important urban source of ammonia (NH3). However, there are large uncertainties regarding the contribution of vehicle emissions to urban NH3 budgets, as well as the role of NH3 in spatiotemporal fine particulate matter (PM2.5) formation and nitrogen (N) deposition patterns. The N stable isotopic composition (δ15N) may be a useful observational constraint to track NH3 emission sources and chemical processing, but previously reported vehicle δ15N(NH3) emission signatures have reported a wide range of values, indicating the need for further refinement. Here we have characterized δ15N(NH3) spatiotemporal variabilities from vehicle plumes in stationary and on-road measurements in the USA and China using an active NH3 collection technique demonstrated to accurately characterize δ15N(NH3) on the order of hourly time resolution. Significant spatial and temporal δ15N(NH3) variabilities were observed and suggested to be driven by vehicle fleet composition and influences from NH3 dry deposition on tunnel surfaces. Overall, a consistent δ15N(NH3) signature of 6.6±2.1 ‰ (x‾±1σ; n=80) was found in fresh vehicle plumes with fleet compositions typical of urban regions. Our recommended vehicle δ15N(NH3) signature is significantly different from previous reports. This difference is due to a large and consistent δ15N(NH3) bias of approximately −15.5 ‰ between commonly employed passive NH3 collection techniques and the laboratory-tested active NH3 collection technique. This work constrains the δ15N(NH3) urban traffic plume signature, which has important implications for tracking vehicle NH3 in urban-affected areas and highlights the importance of utilizing verified collection methods for accurately characterizing δ15N(NH3) values.

Assessing the Challenges of Surface-Level Aerosol Mass Estimates From Remote Sensing During the SEAC 4 RS and SEARCH Campaigns: Baseline Surface Observations and Remote Sensing in the Southeastern United States

Article

Full-text available

Jun 2018

The Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC⁴RS) campaign conducted in the southeast United States (SEUS) during the summer of 2013 provided a singular opportunity to study local aerosol chemistry and investigate aerosol radiative properties and PM2.5 relationships, focusing on the complexities involved in simplifying the relationship into a linear regression. We utilize three Southeastern Aerosol Research and Characterization network sites and one Environmental Protection Agency Chemical Speciation Network station that afforded simultaneous Aerosol Robotic Network (AERONET) aerosol optical depth (AOD) and aerosol mass, chemistry, and light scattering monitoring. Prediction of AERONET AOD using linear regression of daily-mean PM2.5 during the SEAC⁴RS campaign yielded r² of 0.36-0.53 and highly variable slopes across four sites. There were further reductions in PM2.5 predictive skill using Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging SpetroRadiometer (MISR) AOD data, which have shorter correlation lengths and times relative to surface PM2.5. Long-term trends in aerosol chemistry and optical properties in the SEUS are also investigated and compared to SEAC⁴RS period data, establishing that the SEUS experienced significant reduction in aerosol mass, corresponding with changes in both aerosol chemistry and optical properties. These changes have substantial impact on the PM2.5-AOD linear regression relationship and reinforce the need for long-term aerosol observation stations in addition to concentrated field campaigns.

Inconsistency of ammonium-sulfate aerosol ratios with thermodynamic models in the eastern US: A possible role of organic aerosol

Article

Full-text available

Apr 2017
ATMOS CHEM PHYS

Thermodynamic models predict that sulfate aerosol (S(VI) ≡ H2SO4(aq) + HSO4⁻+ SO4²⁻) should take up available ammonia (NH3) quantitatively as ammonium (NH4⁺) until the ammonium sulfate stoichiometry (NH4)2SO4 is close to being reached. This uptake of ammonia has important implications for aerosol mass, hygroscopicity, and acidity. When ammonia is in excess, the ammonium–sulfate aerosol ratio R = [NH4⁺] ∕ [S(VI)] should approach 2, with excess ammonia remaining in the gas phase. When ammonia is in deficit, it should be fully taken up by the aerosol as ammonium and no significant ammonia should remain in the gas phase. Here we report that sulfate aerosol in the eastern US in summer has a low ammonium–sulfate ratio despite excess ammonia, and we show that this is at odds with thermodynamic models. The ammonium–sulfate ratio averages only 1.04 ± 0.21 mol mol⁻¹ in the Southeast, even though ammonia is in large excess, as shown by the ammonium–sulfate ratio in wet deposition and by the presence of gas-phase ammonia. It further appears that the ammonium–sulfate aerosol ratio is insensitive to the supply of ammonia, remaining low even as the wet deposition ratio exceeds 6 mol mol⁻¹. While the ammonium–sulfate ratio in wet deposition has increased by 5.8 % yr⁻¹ from 2003 to 2013 in the Southeast, consistent with SO2 emission controls, the ammonium–sulfate aerosol ratio decreased by 1.4–3.0 % yr⁻¹. Thus, the aerosol is becoming more acidic even as SO2 emissions decrease and ammonia emissions stay constant; this is incompatible with simple sulfate–ammonium thermodynamics. A tentative explanation is that sulfate particles are increasingly coated by organic material, retarding the uptake of ammonia. Indeed, the ratio of organic aerosol (OA) to sulfate in the Southeast increased from 1.1 to 2.4 g g⁻¹ over the 2003–2013 period as sulfate decreased. We implement a simple kinetic mass transfer limitation for ammonia uptake to sulfate aerosols in the GEOS-Chem chemical transport model and find that we can reproduce both the observed ammonium–sulfate aerosol ratios and the concurrent presence of gas-phase ammonia. If sulfate aerosol becomes more acidic as OA ∕ sulfate ratios increase, then controlling SO2 emissions to decrease sulfate aerosol will not have the co-benefit of suppressing acid-catalyzed secondary organic aerosol (SOA) formation.

Enhanced concentrations of reactive nitrogen species in wildfire smoke

Article

Oct 2016
ATMOS ENVIRON

During the summer of 2012 the Hewlett Gulch and High Park wildfires burned an area of 400 km² northwest of Fort Collins, Colorado. These fires both came within 20 km of the Department of Atmospheric Science at Colorado State University, allowing for extensive measurements of smoke-impacted air masses over the course of several weeks. In total, smoke plumes were observed at the measurement site for approximately 125 h. During this time, measurements were made of multiple reactive nitrogen compounds, including gas phase species NH3, NOx, and HNO3, and particle phase species NO3⁻ and NH4⁺, plus an additional, unspeciated reactive nitrogen component that is measured by high temperature conversion over a catalyst to NO. Concurrent measurements of CO, levoglucosan and PM2.5 served to confirm the presence of smoke at the monitoring site. Significant enhancements were observed for all of the reactive nitrogen species measured in the plumes, except for NH4⁺ which did not show enhancements, likely due to the fresh nature of the plume, the presence of sufficient regional ammonia to have already neutralized upwind sulfate, and the warm conditions of the summer measurement period which tend to limit ammonium nitrate formation. Excess mixing ratios for NH3 and NOx relative to excess mixing ratios of CO in the smoke plumes, ΔNH3/ΔCO (ppb/ppb) and ΔNOx/ΔCO (ppb/ppb), were determined to be 0.027 ± 002 and 0.0057 ± 0.0007, respectively. These ratios suggest that smoldering combustion was the dominant source of smoke during our plume interceptions. Observations from prior relevant laboratory and field measurements of reactive nitrogen species are also briefly summarized to help create a more comprehensive picture of reactive nitrogen and fire.

Evaluation of the sequential spot sampler (S3) for time-resolved measurement of PM2.5 sulfate and nitrate through lab and field measurements

Article

Full-text available

Feb 2016

The Sequential Spot Sampler (S3), a newly developed instrument to collect aerosols for time-resolved chemical composition measurements, was evaluated in the laboratory and field for the measurement of particulate sulfate and nitrate. The S3 uses a multi-temperature condensation growth tube to grow individual aerosols to droplets which are then deposited as a ∼ 1 mm diameter dry spot at the end of the growth tube in a 100 µL well of a multi-well plate. The well plate advances automatically to provide a sequence of time-resolved samples. The collected aerosols are subsequently analyzed in the laboratory. The sample is concentrated during the collection process, and the laboratory extraction and analysis steps can be automated. The well plate, as received from the field, is placed onto a needle-based autosampler that adds liquid for sample extraction and injects sample extract from each well onto an ion chromatograph for analysis. Laboratory evaluation for sulfate and nitrate ions showed that poly ether ether ketone (PEEK) used as well plate material does not contribute any artifacts; a 60 min extraction procedure leads to the recovery of sulfate and nitrate from the dry spots at above 95 % extraction efficiency; and samples stored frozen and analyzed up to 23 months later show less than a 10 % change in sulfate and nitrate concentrations. The limit of detection was 0.5 µg m−3 for sulfate and 0.2 µg m−3 for nitrate for a 1 h sampling period. In a month-long field study conducted in southern California, two S3s were deployed alongside a URG denuder–filter-pack and a Particle-Into-Liquid Sampler combined with an Ion Chromatograph (PILS-IC). Collocated S3 sampler concentrations compared by linear regression show good agreement, with r2 = 0.99 and slope = 0.99 (±0.004) µg m−3 for sulfate and r2 = 0.99 and slope = 1.0 (±0.006) µg m−3 for nitrate. When compared to the URG denuder–filter-pack and the PILS-IC, the S3 sulfate and nitrate concentrations yielded correlations above 0.84 for the square of the correlation coefficient and regression slopes close to 1.

Incomplete sulfate aerosol neutralization despite excess ammonia in the eastern US: a possible role of organic aerosol

Article

Full-text available

Jun 2016
ACP

Acid-base neutralization of sulfate aerosol (S(VI) ≡ H2SO4(aq) + HSO4− + SO42−) by ammonia (NH3) has important implications for aerosol mass, hygroscopicity, and acidity. Surface network and aircraft observations across the eastern US show that sulfate aerosol is not fully neutralized even in the presence of excess ammonia, at odds with thermodynamic equilibrium models. The sulfate aerosol neutralization ratio (f = [NH4+]/2[S(VI)]) averages only 0.51 ± 0.11 mol mol−1 at sites in the Southeast and 0.78 ± 0.13 mol mol−1 in the Northeast in summer 2013, even though ammonia is in large excess as shown by the corresponding [NH4+]/2[S(VI)] ratio in wet deposition fluxes. There is in fact no site-to-site correlation between the two quantities; the aerosol neutralization ratio in the Southeast remains in a range 0.3–0.6 mol mol−1 even as the wet deposition neutralization ratio exceeds 3 mol mol−1. While the wet deposition neutralization ratio has increased by 4.6 % a−1 from 2003 to 2013 in the Southeast US, consistent with SO2 emission controls, the aerosol neutralization ratio has decreased by 1.0–3.2 % a−1. Thus the aerosol is becoming more acidic even as SO2 emissions decrease. One possible explanation is that sulfate particles are increasingly coated by organic material, retarding the uptake of ammonia. The ratio of organic aerosol (OA) to sulfate increases over the 2003–2013 period as sulfate decreases. We implement a kinetic mass transfer limitation for ammonia uptake to sulfate aerosols in the GEOS-Chem chemical transport model and find improved agreement with surface and aircraft observations of the aerosol neutralization ratio. If sulfate aerosol becomes more acidic as OA/sulfate ratios increase, then controlling SO2 emissions to decrease sulfate aerosol will not have the co-benefit of suppressing acid-catalyzed secondary organic aerosol (SOA) formation.

Optimization of sampling conditions to minimize sampling errors of both PM 2.5 mass and its semi-volatile inorganic ion concentrations

Article

Oct 2023
AEROSOL SCI TECH

Considerations for realistic atmospheric environments: An application to corrosion testing

Article

Full-text available

May 2023
SCI TOTAL ENVIRON

Measured salt compositions in dust collected over roughly the last decade from surfaces of in-service stainless-steel alloys at four locations around the United States are presented, along with the predicted brine compositions that would result from deliquescence of these salts. The salt compositions vary greatly from ASTM seawater and from laboratory salts (i.e., NaCl or MgCl2) commonly used on corrosion testing. The salts contained relatively high amounts of sulfates and nitrates, evolved to basic pH values, and exhibited deliquescence relative humidity values (RH) higher than seawater. Additionally, inert dust components were quantified and considerations for laboratory testing with inert dust are presented. The observed environments are discussed in terms of the potential corrosion behavior and are compared to commonly used accelerated testing protocols. Finally, ambient weather conditions and their influence on diurnal fluctuations in temperature (T) and RH on heated metal surfaces are evaluated and a relevant diurnal cycle for laboratory testing a heated surface has been developed. Suggestions for future accelerated tests are proposed that include exploration of the effects of inert dust particles on atmospheric corrosion, chemistry considerations, and realistic diurnal fluctuations in T and RH. Understanding mechanisms in both realistic and accelerated environments will allow development of a corrosion factor (i.e., scaling factor) for the extrapolation of laboratory-scale test results to real world applications.

Bayesian Inference Approach to Quantify Primary and Secondary Organic Carbon in Fine Particulate Matter Using Major Species Measurements

Article

Mar 2023
ENVIRON SCI TECHNOL

The determination of primary organic carbon (POC) and secondary organic carbon (SOC) in fine particulate matter using ambient measurements is essential in atmospheric chemistry. A novel Bayesian inference (BI) approach is proposed to achieve such quantification using only major component measurement data and tested in two case studies. One case study composes of filter-based daily compositional data made in the Pearl River Delta region, China, during 2012, while the other uses online measurement data recorded at the Dianshan Lake monitoring site in Shanghai in wintertime 2019. Source-specific organic trace measurement data are available in both the cases so that positive matrix factorization (PMF) analysis is performed, where PMF-resolved POC and SOC are used as the best available reference values for model evaluation. Meanwhile, traditional techniques, i.e., minimum ratio value, minimum R squared, and multiple linear regression, are also employed and evaluated. For both the cases, the BI models have shown significant advantages in accurately estimating POC and SOC amounts over conventional methods. Further analysis suggests that using sulfate as the SOC tracer in BI model gives the best model performance. This methodological advance provides an improved and practical tool to derive POC and SOC levels for addressing PM-related environmental impacts.

An ensemble-variational inversion system for the estimation of ammonia emissions using CrIS satellite ammonia retrievals

Article

Full-text available

May 2022
ATMOS CHEM PHYS

An ensemble-variational inversion system is developed for the estimation of ammonia emissions using ammonia retrievals from the Cross-track Infrared Sounder (CrIS) for use in the Global Environmental Multiscale – Modelling Air quality and Chemistry (GEM-MACH) chemical weather model. A novel hybrid method to compare logarithmic retrieval parameters to model profiles is presented. Inversions for the monthly mean ammonia emissions over North America were performed for May to August 2016. Inversions using the hybrid comparison method increased ammonia emissions at most locations within the model domain, with total monthly mean emissions increasing by 11 %–41 %. The use of these revised emissions in GEM-MACH reduced biases with surface ammonia observations by as much as 25 %. The revised ammonia emissions also improved the forecasts of total (fine + coarse) ammonium and nitrate, as well as ammonium wet deposition, with biases decreasing by as much as 13 %, but they did not improve the forecasts of just the fine components of ammonium and nitrate. A comparison of biases resulting from inversions using different comparison methods shows favourable results for the hybrid comparison method.

Quantifying the importance of vehicle ammonia emissions in an urban area of northeastern USA utilizing nitrogen isotopes

Article

Full-text available

Oct 2022
ATMOS CHEM PHYS

Atmospheric ammonia (NH3) is a critical component of our atmosphere that contributes to air quality degradation and reactive nitrogen deposition; however, our knowledge of NH3 in urban environments remains limited. Year-long ambient NH3 and related species were measured for concentrations and the nitrogen isotopic compositions (δ15N) of NH3 and particulate ammonium (pNH4+) were measured to understand the temporal sources and chemistry of NH3 in a northeastern US urban environment. We found that urban NH3 and pNH4+ concentrations were elevated compared to regional rural background monitoring stations, with seasonally significant variations. Local and transported sources of NHx (NH3+ pNH4+) were identified using polar bivariate and statistical back trajectory analysis, which suggested the importance of vehicles, volatilization, industry, and stationary fuel combustion emissions. Utilizing a uniquely positive δ15N(NH3) emission source signature from vehicles, a Bayesian stable isotope mixing model (SIMMR) indicates that vehicles contribute 46.8±3.5 % (mean ±1σ) to the annual background level of urban NHx, with a strong seasonal pattern with higher relative contribution during winter (56.4±7.6 %) compared to summer (34.1±5.5 %). The decrease in the relative importance of vehicle emissions during the summer was suggested to be driven by temperature-dependent NH3 emissions from volatilization sources, seasonal fuel-combustion emissions related to energy generation, and change in seasonal transport patterns based on wind direction, back trajectory, and NH3 emission inventory analysis. This work highlights that reducing vehicle NH3 emissions should be considered to improve wintertime air quality in this region.

Insights into the performance of the two contrasting dynamic adsorption platforms in the removal of gaseous benzene on microporous carbon materials

Article

Jun 2022
J CLEAN PROD

High-speed purification system, made of packed-bed adsorber, is often subject to some apparent demerits (e.g., pressure drop and/or high energy consumption) when treating waste gas. As a potential alternative to such system, a denuder sorbent tube has been constructed by coating powder activated carbon (PAC) on the inner surface of the sorbent tube with the help of polyvinyl alcohol (PVA) as adhesive. The designed denuder sorbent tube was then used to treat a gaseous stream containing 1 Pa benzene (as a model volatile organic compound (VOC)) under various volumetric flow rates (100–500 mL min⁻¹) at near-ambient conditions. Accordingly, the packed bed outperformed the denuder system by a factor of at least two (in terms of the breakthrough volume (BTV, L·g⁻¹) and adsorption capacity (Q, mg·g⁻¹) at 10% BT) when tested at higher flow rates (200–500 mL min⁻¹). In contrast, the pattern was reversed at the lowest flow rate of 100 mL min⁻¹ so that the denuder (BTV 10% = 615 L g⁻¹) outperforms the packed bed (330 L g⁻¹). According to the adsorption kinetic analysis, the surface-interaction and film diffusion have a rate-determining adsorption step in the denuder, while the boundary/pore diffusion mechanism should play a crucial role in the packed-bed system. Overall, the experimental data suggest the potential of an AC-based denuder system as an efficient alternative to replace traditional packed-bed columns for removing VOCs in the air.

Significant impactor sampling artifacts of ammonium, nitrate, and organic acids

Article

Feb 2022
ATMOS ENVIRON

The evaporation losses of semi-volatile compounds such as ammonium nitrate and organic acids on denuded and undenuded impactors can significant underestimate their mass loadings. In this study, sampling artifacts of formate, oxalate, and other semi-volatile inorganic components were measured using a combination of an upstream-denuded and a downstream-denuded MOUDI system in Shanghai. Compared to regular sampling, the concentrations of semi-volatile ammonium, nitrate, formate, and oxalate collected on the upstream-denuded MOUDI sampler decreased by 11%–43%, suggesting removing acidic and alkaline gaseous compounds from the sampled air had significant negative sampling artifacts for semi-volatile inorganic salts and low-molecular-weight organic acids. Evaporation losses from regular MOUDI sampling were confirmed by the higher concentrations of acidic and alkaline gaseous compounds in the downstream denuder. The concentration of aerosol formate was seriously underestimated, with average sampling efficiencies of 36.7 ± 10% and 55.2 ± 8.5% on denuded and undenuded MOUDIs, respectively. The mass ratio of total organic acids to sulfate increased to approximately 0.5 for the particle population smaller than 0.1 μm, indicating that organic acids might influence particle acidity and chemical behaviors in this size range.

Improving Estimates of Sulfur, Nitrogen, and Ozone Total Deposition through Multi-Model and Measurement-Model Fusion Approaches

Article

Jan 2022
ENVIRON SCI TECHNOL

Earth system and environmental impact studies need high quality and up-to-date estimates of atmospheric deposition. This study demonstrates the methodological benefits of multimodel ensemble and measurement-model fusion mapping approaches for atmospheric deposition focusing on 2010, a year for which several studies were conducted. Global model-only deposition assessment can be further improved by integrating new model-measurement techniques, including expanded capabilities of satellite observations of atmospheric composition. We identify research and implementation priorities for timely estimates of deposition globally as implemented by the World Meteorological Organization.

An Ensemble-Variational Inversion System for the Estimation of Ammonia Emissions using CrIS Satellite Ammonia Retrievals

Preprint

Full-text available

Sep 2021

An ensemble-variational inversion system is developed for the estimation of ammonia emissions using ammonia retrievals from the Cross-track Infrared Sounder (CrIS) for use in the Global Environmental Multiscale – Modelling Air quality and Chemistry (GEM-MACH) chemical weather model. A novel hybrid method to compare logarithmic retrieval parameters to model profiles is presented. Inversions for the monthly mean ammonia emissions over North America were performed for May to August 2016. Inversions using the hybrid comparison method increased ammonia emissions at most locations within the model domain, with total monthly mean emissions increasing by 11–41 %. The use of these revised emissions in GEM-MACH reduced biases with surface ammonia observations by as much as 25 %. The revised ammonia emissions also improved the forecasts of total (fine+coarse) ammonium and nitrate and ammonium wet deposition, with biases decreasing by as much as 13 %, but did not improve the forecasts of just the fine components of ammonium and nitrate. An additional area of 1.3 × 105 km2 of upland forests in Canada were estimated to exceed the ecosystem's critical load due to the changes in ammonia deposition from the inversion. A comparison of biases resulting from inversions using different comparison methods shows favourable results for the hybrid comparison method.

Current challenges of improving visibility due to increasing nitrate fraction within PM2.5 during the haze days in Beijing, China

Article

Aug 2021
ENVIRON POLLUT

The annual mean PM2.5 mass concentration has decreased because of the stringent emission controls implemented in Beijing, China in recent years, whereas the nitrate (NO3–) mass fraction within PM2.5 increases gradually. Low-visibility events occur frequently even though PM2.5 pollution has been mitigated significantly, with the daily mean PM2.5 mass concentration mostly less than 75 μg/m³. In this study, the non-linear relationship was analyzed between atmospheric visibility and PM2.5 based on chemical composition from a two-year field observation. Our results showed that NO3– became the main constituent of PM2.5, especially during the haze pollution episodes. A localized parameterization scheme was proposed between the atmospheric extinction coefficient (σext) and major chemical constituents of PM2.5 by multiple linear regression. The contribution of NO3– to σext increased with increasing air pollution, and NO3– became the most important contributor for PM2.5 above 75 μg/m³. The visibility decreased with increasing NO3– mass fraction for the same PM2.5 mass concentration when PM2.5 was above 20 μg/m³. The hygroscopicity of PM2.5 increased with increasing mass fraction of hygroscopic NO3–. These results stressed the importance of reducing particulate NO3– and its precursors (for instance, NH3) through effective emission control measures as well as the tightening of PM2.5 standards to further improve air quality and visibility in Beijing.

Collocated speciation of PM2.5 using tandem quartz filters in northern nanjing, China: Sampling artifacts and measurement uncertainty

Article

Nov 2020
ATMOS ENVIRON

In this study, collocated samples of particulate matter with aerodynamic diameter less than 2.5 μm (PM2.5) were collected every sixth day on quartz filters (Qf) at a suburban site in northern Nanjing, China for one year. A backup quartz filter (Qb) was installed behind Qf to estimate positive artifacts. The analysis of gravimetric mass, water-soluble inorganic ions (WSIIs), total organic (OC) and elemental carbon (EC), water-soluble OC (WSOC), total nitrogen (WSTN) and organic nitrogen (WSON) were performed on both Qf and Qb. Due to the high mass loadings on Qf (17.9 ± 7.82 mg filter⁻¹, 4.71–38.5 mg filter⁻¹), the collocated precision of gravimetric mass and dominant species (NH4⁺, NO3⁻, SO4²⁻, OC, and EC) is better than that from previous work. Except K⁺, EC, and WSON, all other target components were detected on Qb with average Qb/Qf ratios ranging from 3.02 ± 3.48–21.7 ± 22.4%. The final concentrations and uncertainties of PM2.5 components were determined based on duplicate Qf–Qb data. The results suggest that using an error fraction of 10% will underestimate the uncertainty of less concentrated species (e.g., Ca²⁺ and Mg²⁺) in PM2.5. Due to the evaporation loss of semi-volatile materials from the Qf, the Qf–Qb calculation would lead to an estimate of the lower limit for particulate NH4⁺, NO3⁻, and OC. Synchronized hourly data of PM2.5 mass and components were obtained at downtown Nanjing. The comparisons of gravimetric versus reconstructed PM2.5 and filter-based versus continuous measurements of PM2.5 components indicate that a substantial fraction of the unexplained gravimetric PM2.5 can be attributed to aerosol water.

Quantifying organic matter and functional groups in particulate matter filter samples from the southeastern United States – Part 1: Methods

Article

Full-text available

Oct 2019
AMT

Comprehensive techniques to describe the organic composition of atmospheric aerosol are needed to elucidate pollution sources, gain insights into atmospheric chemistry, and evaluate changes in air quality. Fourier transform infrared absorption (FT-IR) spectrometry can be used to characterize atmospheric organic matter (OM) and its composition via functional groups of aerosol filter samples in air monitoring networks and research campaigns. We have built FT-IR spectrometry functional group calibration models that improve upon previous work, as demonstrated by the comparison of current model results with those of previous models and other OM analysis methods. Laboratory standards that simulated the breadth of the absorbing functional groups in atmospheric OM were made: particles of relevant chemicals were first generated, collected, and analyzed. Challenges of collecting atmospherically relevant particles and spectra were addressed by including interferences of particle water and other inorganic aerosol constituents and exploring the spectral effects of intermolecular interactions. Calibration models of functional groups were then constructed using partial least-squares (PLS) regression and the collected laboratory standard data. These models were used to quantify concentrations of five organic functional groups and OM in 8 years of ambient aerosol samples from the southeastern aerosol research and characterization (SEARCH) network. The results agreed with values estimated using other methods, including thermal optical reflectance (TOR) organic carbon (OC; R2=0.74) and OM calculated as a difference between total aerosol mass and inorganic species concentrations (R2=0.82). Comparisons with previous calibration models of the same type demonstrate that this new, more complete suite of chemicals has improved our ability to estimate oxygenated functional group and overall OM concentrations. Calculated characteristic and elemental ratios including OM∕OC, O∕C, and H∕C agree with those from previous work in the southeastern US, substantiating the aerosol composition described by FT-IR calibration. The median OM∕OC ratio over all sites and years was 2.1±0.2. Further results discussing temporal and spatial trends of functional group composition within the SEARCH network will be published in a forthcoming article.

The sensitivity of PM 2.5 acidity to meteorological parameters and chemical composition changes: 10-year records from six Canadian monitoring sites

Article

Full-text available

Jul 2019

Aerosol pH is difficult to measure directly but can be calculated if the chemical composition is known with sufficient accuracy and precision to calculate the aerosol water content and the H+ concentration through the equilibrium among acids and their conjugate bases. In practical terms, simultaneous measurements of at least one semi-volatile constituent, e.g. NH3 or HNO3, are required to provide a constraint on the calculation of pH. Long-term records of aerosol pH are scarce due to the limited monitoring of NH3 in conjunction with PM2.5. In this study, 10-year (2007–2016) records of pH of PM2.5 at six eastern Canadian sites were calculated using the E-AIM II model with the input of gaseous NH3, gaseous HNO3 and major water-soluble inorganic ions in PM2.5 provided by Canada's National Air Pollution Surveillance (NAPS) Program. Clear seasonal cycles of aerosol pH were found with lower pH (∼2) in summer and higher pH (∼3) in winter consistently across all six sites, while the day-to-day variations of aerosol pH were higher in winter compared to summer. Tests of the sensitivity of aerosol pH to meteorological parameters demonstrate that the changes in ambient temperature largely drive the seasonal cycle of aerosol pH. The sensitivity of pH to chemical composition shows that pH has different responses to the changes in chemical composition in different seasons. During summertime, aerosol pH was mainly determined by temperature with limited impact from changes in NHx or sulfate concentrations. However, in wintertime, both meteorological parameters and chemical composition contribute to the variations in aerosol pH, resulting in the larger variation during wintertime. This study reveals that the sensitivity of aerosol pH to chemical composition is distinctly different under different meteorological conditions and needs to be carefully examined for any particular region.

Investigating daytime and night-time differences with the seasonal trend and sources of inorganic fine aerosols in Indo-Gangetic plain

Article

Mar 2019

This study investigates the seasonal distribution of inorganic aerosols in Central New Delhi and identifies their potential source regions using concentration weighted trajectories (CWTs). Secondary inorganic aerosols (\(\hbox {NO}_{3}^{-}\), \(\hbox {SO}_{4}^{2-}\), \(\hbox {NH}_{4}^{+})\) are the largest contributors to fine particulate matter in New Delhi, India. The concentrations of secondary inorganic aerosols showed very distinct seasonal patterns with higher concentrations in winter and post-monsoon seasons. Inorganic ions \(\hbox {Ca}^{2+}\), \(\hbox {Mg}^{2+}\) and \(\hbox {K}^{+}\) were also examined to understand their temporal trends. The primary aerosols were found to have smaller diurnal differences than secondary aerosols. The higher coefficient of divergence for secondary aerosols indicated a significant difference in their chemistry and/or meteorology during daytime and night-time, respectively. The backward trajectory analysis revealed the advection of ionic species from distant sources responsible for their significant seasonality. The highest concentrations of \(\hbox {K}^{+}\) during the post-monsoon season were mainly influenced by air masses arriving from Punjab and Haryana regions resulting from the prominent agricultural crop residue burning in these areas. CWT also identified the Thar Desert and Punjab as potential regions for enhanced levels of \(\hbox {Ca}^{2+}\) and \(\hbox {K}^{+}\), respectively. Also, the brick kilns located in western UP were observed as the major potential sources for \(\hbox {NO}_{3}^{-}\) and \(\hbox {SO}_{4}^{2-}\).

Evidence for the Importance of Semivolatile Organic Ammonium Salts in Ambient Particulate Matter

Article

Dec 2018

The gas/particle phase partitioning behavior of NH3/NH4+ and other semi-volatile constituents was measured by a custom-designed Denuder-MOUDI-Denuder integrated sampling system in Toronto, Canada. In this setup, upstream denuders were used to capture alkaline and acidic gaseous components, and particle phase components were captured by the filters on MOUDI stages. Downstream denuders captured any alkaline and acidic gases that exited the MOUDI apparatus, likely representing semi-volatile constituents. In the ambient gas phase HCOOH was the most abundant acidic gas, with an average mixing ratio ~2-3 times higher than that of SO2 and HNO3. It was found that the majority (49-96%) of filter-collected NH4+ volatilized during collection. NO3- volatilization could only explain 0.9-15% of NH4+ loss from the filters. Instead, a strong correlation and nearly 1:1 molar ratio between downstream HCOO- and NH4+ indicated that most of the semi-volatile NH4+ was originally balanced by organic acids in the ambient particle phase. The thermodynamic properties of HCOOH/HCOO- suggest that it should not have been present at high levels in the ambient particle phase, and we interpret its detection in the downstream denuder as evidence for larger organic acids that reacted to generate HCOOH prior to our offline measurement.

Field performance of a semi-continuous monitor for ambient PM2.5 water-soluble inorganic ions and gases at a suburban site

Article

Nov 2016
ATMOS ENVIRON

To reduce sampling artifacts and to improve time-resolved measurements of inorganic aerosol system, a recently commercialized semi-continuous In-situ Gas and Aerosol Composition (IGAC) monitoring system was evaluated against a reference annular denuder system (ADS; denuder/two-stage filter pack) at a suburban site over a year, during which the average PM2.5 was 37.0 ± 24.8 μg/m³. A suite of eight ions SO4²⁻, NO3⁻, Cl⁻, NH4⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺ and two gases SO2 and NH3 were the target species. In comparison to the reference ADS method, the IGAC performed well in measuring the major ions SO4²⁻, NO3⁻ and NH4⁺, and the SO2. For those species, the linear slopes, intercepts and R² values between the two methods all passed the performance evaluation criteria outlined by earlier similar studies. The performance of IGAC on Cl⁻, Na⁺, K⁺ and NH3 was marginally acceptable, whereas Ca²⁺ and Mg²⁺ could not be properly evaluated due to the low concentrations (<0.2 μg/m³) and hence inadequate amount of sample size. The ionic balance of the hourly IGAC samples averaged very close to unity, as did the daily ADS samples, though the former was considerably more variable than the latter. The overall performance of the IGAC has been shown to be comparable to other similar monitors and its improvements are discussed.

Estimation of columnar concentrations of absorbing and scattering fine mode aerosol components using AERONET data: Aerosol Concentrations From AERONET Data

Article

Oct 2016

Columnar concentrations of absorbing and scattering components of fine mode aerosols were estimated using AERONET data for a site downwind of Seoul. The study period was between March 2012 and April 2013 including the period of the Distributed Regional Aerosol Gridded Observation Networks (DRAGON)-Asia campaign in March to May 2012. The Maxwell Garnett mixing rule was assumed for insoluble components embedded in a host solution while the volume average mixing rule was assumed for the aqueous solution of soluble components. During the DRAGON-Asia campaign the surface concentrations of major components of fine particles were measured. The columnar mass fractions of black carbon (BC), organic carbon (OC), mineral dust (MD), and ammonium sulfate (AS) were 1.5, 5.9, 6.6, and 52%, respectively, which were comparable to the mass fractions measured at the surface for BC, OC, and secondary inorganic aerosols at 2.3, 18, and 55%. The vertical distributions of BC and AS were investigated by employing the concept of a column height. While the column height for BC was similar to the planetary boundary layer (PBL) height, that for AS was 4.4 times higher than the PBL height and increased with air temperature from March to May. The monthly variations of the columnar mass concentrations during the study period were generally well explained in term of meteorology and emission characteristics. However, certain variations of MD were different from those typically observed primarily because only fine mode aerosols were considered.

Concurrent measurements of size-segregated particulate sulfate, nitrate and ammonium using quartz fiber filters, glass fiber filters and cellulose membranes

Article

Sep 2016
ATMOS ENVIRON

Chemical Composition of PM2.5 and its Impact on Visibility in Guangzhou, Southern China

Article

Full-text available

Oct 2016

A one-year field experiment was conducted in 2013 at an urban and suburban site in Guangzhou, Southern China to study the chemical compositions of PM2.5 and reconstruct the IMPROVE Algorithm to investigate the impact of aerosol components on visibility. Annual average PM2.5 mass concentration was 61.3 ± 27.6 and 54.2 ± 29.7 µg m–3 at the urban and suburban site, with organic matter (OM), sulfate (SO42–), and nitrate (NO3–) among the dominant components, accounting for 40.3%, 16.3%, and 8.0% of the PM2.5 mass concentration respectively. Based on the modified IMPROVE Algorithm, the localized Mass Scattering Efficiencies (MSE) for sulfate and nitrate was obtained, with values of 2.16 ± 0.34 m² g–1 and 2.63 ± 0.66 m² g–1 at urban site and 2.22 ± 0.31 m² g–1 and 2.76 ± 0.84 m² g–1 at suburban site. Hygroscopic growth factors (Gf) for OM and EC were also taken into consideration with average values of 1.28 ± 0.13 m² g–1 and 1.15 ± 0.11 m² g–1 at the urban site and 1.18 ± 0.09 m² g–1 and 1.10 ± 0.09 m² g–1 at the suburban site. The estimated total light extinction coefficient was 294.7 ± 106.9 and 255.8 ± 119.0 Mm–1 at the urban and suburban site, with OM, SO42– and NO3– contributing 41.7%, 16.5% and 11.5% at the urban site, and 42.1%, 21.3% and 9.1% at the suburban site. Secondary water-soluble ions and OM as a whole was becoming increasingly vital under polluted conditions, with nitrate in particular being more important under heavily polluted conditions in Guangzhou.

The European aerosol budget in 2006

Article

Full-text available

Feb 2011

This paper presents the aerosol budget over Europe in 2006 calculated with the global transport model TM5 coupled to the size-resolved aerosol module M7. Comparison with ground observations indicates that the model reproduces the observed concentrations quite well with an expected slight underestimation of PM<sub>10</sub> due to missing emissions (e.g. resuspension). We model that a little less than half of the anthropogenic aerosols emitted in Europe are exported and the rest is removed by deposition. The anthropogenic aerosols are removed mostly by rain (95%) and only 5% is removed by dry deposition. For the larger natural aerosols, especially sea salt, a larger fraction is removed by dry processes (sea salt: 70%, mineral dust: 35%). We model transport of aerosols in the jet stream in the higher atmosphere and an import of Sahara dust from the south at high altitudes. Comparison with optical measurements shows that the model reproduces the Ångström parameter very well, which indicates a correct simulation of the aerosol size distribution. However, we underestimate the aerosol optical depth. Because the surface concentrations are close to the observations, the shortage of aerosol in the model is probably at higher altitudes. We show that the discrepancies are mainly caused by an overestimation of wet-removal rates. To match the observations, the wet-removal rates have to be scaled down by a factor of about 5. In that case the modelled ground-level concentrations of sulphate and sea salt increase by 50% (which deteriorates the match), while other components stay roughly the same. Finally, it is shown that in particular events, improved fire emission estimates may significantly improve the ability of the model to simulate the aerosol optical depth. We stress that discrepancies in aerosol models can be adequately analysed if all models would provide (regional) aerosol budgets, as presented in the current study.

Qualitative and quantitative determination of water in airborne particulate matter

Article

Full-text available

Oct 2012
ACP

This paper describes the optimization and validation of a new simple method for the quantitative determination of water in atmospheric particulate matter (PM). The analyses are performed by using a coulometric Karl-Fisher system equipped with a controlled heating device; different water contributions are separated by the application of an optimized thermal ramp (three heating steps: 50–120 °C, 120–180 °C, 180–250 °C). The analytical performance of the method was verified by using standard materials containing 5.55% and 1% by weight of water. The recovery was greater than 95%; the detection limit was about 20 μg. The method was then applied to NIST reference materials (NIST1649a, urban particulate matter) and to real PM<sub>10</sub> samples collected in different geographical areas. In all cases the repeatability was satisfactory (10–15%). When analyzing the reference material, the separation of four different types of water was obtained. In real PM<sub>10</sub> samples the amount of water and its thermal profile differed as a function of the chemical composition of the dust. Mass percentages of 3–4% of water were obtained in most samples, but values up to about 15% were reached in areas where the chemical composition of PM is dominated by secondary inorganic ions and organic matter. High percentages of water were also observed in areas where PM is characterized by the presence of desert dust. A possible identification of the quality of water released from the samples was tried by applying the method to some hygroscopic compounds that are likely contained in PM (pure SiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>, ammonium salts, carbohydrates and dicarboxylic acids) and by comparing the results with those obtained from field samples.

Modelling the partitioning of ammonium nitrate in the convective boundary layer

Article

Full-text available

Mar 2012

An explanatory model study is presented on semi-volatile secondary inorganic aerosols on three clear days in May 2008 during the IMPACT campaign at the Cabauw tower in the Netherlands. A single column model in combination with the equilibrium aerosol model ISORROPIA is used. This model uses surface observations from IMPACT and calculates the gas-aerosol partitioning of ammonium nitrate. The calculated gas-aerosol equilibrium overestimates the gas phase fraction during daytime, and overestimates the aerosol phase fraction during night-time. This discrepancy can partly be solved when the approach of the gas-aerosol equilibrium is forced to proceed with a delay timescale of up to two hours. Although it is shown that the delay itself has a small effect, the most important effect is caused by the mixing of air from higher altitudes at which the equilibrium is shifted to the aerosol phase. Thus, vertical mixing is shown to have a significant influence on the calculated partitioning at the surface. On some occasions, the correspondence to the observed partitioning improves dramatically. Even though gas-aerosol partitioning of ammonium nitrate is not instantaneous, observations show that a different equilibrium in the upper boundary layer causes aerosol ammonium nitrate concentrations to increase with altitude. Our model calculates similar vertical gradients depending on the assumed speed of gas-aerosol equilibrium. The calculated optical properties of the aerosol show a similar behaviour. The aerosol optical properties depend on the aerosol size distribution both directly, because light scattering depends on particle size, and indirectly, because the equilibration timescale depends on the aerosol sizes. Future studies should therefore focus on a fully size-resolved treatment of the gas-aerosol partitioning. Finally, coarser-resolution models may treat the gas-aerosol equilibrium of ammonium nitrate by calculating the equilibrium with a temperature and humidity sampled at a different altitude. We found that the equilibrium at an altitude of 200 m (night) up to 600 m (day) is representative for the partitioning of ammonium nitrate at the surface in the beginning of May 2008.

Atmospheric ammonia and particulate inorganic nitrogen over the United States

Article

Full-text available

Aug 2012
ACP

We use in situ observations from the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network, the Midwest Ammonia Monitoring Project, 11 surface site campaigns as well as Infrared Atmospheric Sounding Interferometer (IASI) satellite measurements with the GEOS-Chem model to investigate inorganic aerosol loading and atmospheric ammonia concentrations over the United States. IASI observations suggest that current ammonia emissions are underestimated in California and in the springtime in the Midwest. In California this underestimate likely drives the underestimate in nitrate formation in the GEOS-Chem model. However in the remaining continental United States we find that the nitrate simulation is biased high by a factor of 1.5 or more year-round. None of the uncertainties in precursor emissions, the uptake efficiency of N<sub>2</sub>O<sub>5</sub> on aerosols, OH concentrations, the reaction rate for the formation of nitric acid, or the dry deposition velocity of nitric acid are able to explain this. We find that reducing nitric acid concentrations to 2/3 of their simulated values corrects the bias in nitrate (as well as ammonium) in the US. However the mechanism for this potential reduction is unclear and may be a combination of errors in chemistry, deposition and sub-grid near-surface gradients. This "updated" simulation reproduces PM and ammonia loading and captures the strong seasonal and spatial gradients in gas-particle partitioning across the United States. We estimate that nitrogen makes up 15–35% of inorganic fine PM mass over the US, and that this fraction is likely to increase in the coming decade, both with decreases in sulfur emissions and increases in ammonia emissions.

Variation of Urban Atmospheric Ammonia Pollution and its Relation with PM2.5 Chemical Property in Winter of Beijing, China

Article

Full-text available

Jun 2016

To understand the air pollution problem in megacities such as Beijing, field measurement investigating the variation of NH3 and its association with PM2.5 chemical property was conducted from 25 November to 24 December 2013. The results indicated that the daily concentration of wintertime NH3 tended to be high on the days with relatively high temperatures and low wind speeds. Affected by the synoptic condition, NH3 concentration showed a bimodal diurnal variation pattern, which tended to peak at around 09:00 and 22:00 of the day. As the sole precursor for NH4+, NH3 exerted a significant impact on the ion chemistry of PM2.5 through enhancing the nighttime NH4Cl formation and promoting both homogeneous and heterogeneous formation of NO3-. During heavy pollution episodes with PM2.5 concentrations over 200 mu g m(-3), the NH3 levels and NH4+/NH3 ratios grew simultaneously with the increase of PM2.5 levels, indicating that NH3 is one of the key reasons for heavy pollution events. Revealed by the features of measured ionic species in PM2.5, in conjunction with the acidity analysis using thermodynamic model, our results suggested that NH3 was frequently sufficient in wintertime atmosphere of urban Beijing and the fine particulates were neutralized nearly fully by NH3.

Evaluation of a Sequential Spot Sampler (S3) for time-resolved measurement of PM2.5 sulfate and nitrate through lab and field measurements

Article

Full-text available

Oct 2015

The Sequential Spot Sampler (S3), a newly developed instrument to collect aerosols for time resolved chemical composition measurements, was evaluated in the laboratory and field for the measurement of particulate sulfate and nitrate. The S3 uses a multi-temperature condensation growth tube to grow individual aerosols to droplets which are then deposited as a ~ 1 mm diameter dry spot at the end of the growth tube on a 100 μL well of a multi-well plate. The well plate advances automatically to provide a sequence of time-resolved samples. The collected aerosols are subsequently analyzed in the laboratory. The sample is concentrated during the collection process and the laboratory extraction and analysis steps can be automated. The well plate, as received from the field, is placed onto a needle-based autosampler that adds liquid for sample extraction and injects sample extract from each well onto an ion chromatograph for analysis. Laboratory evaluation for sulfate and nitrate ions showed that PEEK used as well plate material does not contribute any artifacts; a 60 min extraction procedure leads to the recovery of sulfate and nitrate from the dry spots at above 95 % extraction efficiency; and samples stored frozen and analyzed up to 23 months later show less than a 10 % change in sulfate and nitrate concentrations. In a month long field study conducted in Southern California, two S3s were deployed alongside a URG denuder/filter-pack and a Particle-Into-Liquid Sampler combined with an Ion Chromatograph (PILS-IC). Collocated S3 sampler concentrations compared by linear regression show good agreement with r2 = 0.99 and slope = 0.99 (±0.004) μg m−3 for sulfate and r2 = 0.99 and slope =1.0 (±0.006) μg m−3 for nitrate. When compared to the URG denuder/filter-pack and the PILS-IC, the S3 sulfate and nitrate concentrations yielded correlations above 0.84 for the square of the correlation coefficient and regression slopes close to one.

Atmospheric reduced nitrogen: Sources, transformations, effects, and management

Article

May 2024
J Air Waste Manag Assoc

Comparison of the Chemical Characteristics and Source Apportionment of PM1.0 and PM2.5 Using Real-time Air Quality Monitoring Network Data of Air Pollutants

Article

Aug 2023

Active vs. passive isotopic analysis: Insights from urban Beijing field measurements and ammonia source signatures

Article

Sep 2023

Need for Improved Monitoring of Spatial and Temporal Trends of Reduced Nitrogen

Article

Jul 2019

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10032039/

Seasonal Aerosol Acidity, Liquid Water Content and Their Impact on Fine Urban Aerosol in SE Canada

Article

Full-text available

Jun 2022

This study explores the drivers of aerosol pH and their impact on the inorganic fraction and mass of aerosol in the S.E. Canadian urban environments of Hamilton and Toronto, Ontario. We find that inter-seasonal pH variability is mostly driven by temperature changes, which cause variations of up to one pH unit. Wintertime acidity is reduced, compared to summertime values. Because of this, the response of aerosol to precursors fundamentally changes between seasons, with a strong sensitivity of aerosol mass to levels of HNO3 in the wintertime. Liquid water content (LWC) fundamentally influences the aerosol sensitivity to NH3 and HNO3 levels. In the summertime, organic aerosol is mostly responsible for the LWC at Toronto, and ammonium sulfate for Hamilton; in the winter, LWC was mostly associated with ammonium nitrate at both sites. The combination of pH and LWC in the two sites also affects N dry deposition flux; NO3− fluxes were comparable between the two sites, but NH3 deposition flux at Toronto is almost twice what was seen in Hamilton; from November to March N deposition flux slows down leading to an accumulation of N as NO3− in the particle phase and an increase in PM2.5 levels. Given the higher aerosol pH in Toronto, aerosol masses at this site are more sensitive to the emission of HNO3 precursors compared to Hamilton. For both sites, NOx emissions should be better regulated to improve air quality during winter; this is specifically important for the Toronto site as it is thermodynamically more sensitive to the emissions of HNO3 precursors.

Wintertime haze and ozone at Dinosaur National Monument

Article

Mar 2022

Dinosaur National Monument (DINO) is located near the northeastern edge of the Uinta Basin and often experiences elevated levels of wintertime ground-level ozone. Previous studies have shown that high ozone mixing ratios in the Uinta Basin are driven by elevated levels of volatile organic compounds (VOCs) and nitrogen oxides (NOx) from regional oil and gas development coupled with temperature inversions and enhanced photochemistry from persistent snow cover. Here we show that persistent snow cover and temperature inversions, along with abundant ammonia, also lead to wintertime haze in this region. A study was conducted at DINO from November 2018 through May 2020 where ozone, speciated fine and coarse aerosols, inorganic gases, and VOCs were measured. Three National Ambient Air Quality Standards (NAAQS) ozone exceedances were observed in the first winter, and no exceedances were observed in the second winter. In contrast, elevated levels of particulate matter were observed both winters, with 24-h averaged particle light extinction exceeding 100 Mm-1. These haze events were dominated by ammonium nitrate, and particulate organics were highly correlated with ammonium nitrate. Ammonium nitrate formation was limited by nitric acid in winter. As such, reductions in regional NOx emissions should reduce haze levels and improve visibility at DINO in winter. Long-term measurements of particulate matter from nearby Vernal, Utah, suggest that visibility impairment is a persistent issue in the Uinta Basin in winter. From April through October 2019, relatively clean conditions occurred, with average particle extinction of ~10 Mm-1. During this period, ammonium nitrate concentrations were lower by more than an order of magnitude, and contributions from coarse mass and soil to haze levels increased. VOC markers indicated that the high levels of observed pollutants in winter were likely from local sources related to oil and gas extraction activities.

Effect of aerosol sampling conditions on PM2.5 sampling accuracy

Article

Feb 2022
J AEROSOL SCI

Filter-based devices were found to underestimate PM2.5 mass concentrations due to the evaporation loss of semi-volatile inorganic materials (SVIM). To reduce the evaporation-induced PM2.5 loss, the chilled Teflon filter sampler (CTF) was developed in which the sampling air was chilled to low temperatures (T) of 4–7 °C after dehumidification. The CTF with the aerosol flow dehumidified to low relative humidity (RH) of 25.50 ± 4.88% by using a Nafion dehumidifier and chilled at 4 °C showed an accurate measurement for the total ion concentration with the mean normalized bias (MNB) of +4.17 ± 8.96% as compared to the actual value measured by the porous denuder sampler (PDS). In comparison, the normal single Teflon filter sampler (STF) sampled PM2.5 ion concentration at ambient T and RH showed a negative MNB of −14.26 ± 13.66%. It indicates that the 4 °C CTF can suppress the evaporation loss of SVIM and measure actual ion concentrations accurately. However, 4 °C CTF over-measured PM2.5 concentrations with the MNB of +15.01 ± 7.99% as compared to −10.40 ± 5.92% of the STF, due to normal and capillary condensations of water vapor although the condensed water on particles prevented SVIM evaporation loss. After correcting for the remaining water concentration determined by using surrogate TiO2 nanoparticles, the accuracy of PM2.5 concentrations was improved significantly with the MNB of only about +2.93 ± 8.56%. To avoid excessive remaining water, the CTF was chilled to 7 °C and found to be able to reduce the evaporation loss of SVIM while measuring PM2.5 concentrations accurately with the MNB of +4.92 ± 6.52% without remaining water correction in most sampling days.

A comparative study of two-way and offline coupled WRF v3.4 and CMAQ v5.0.2 over the contiguous US: performance evaluation and impacts of chemistry–meteorology feedbacks on air quality

Article

Full-text available

Nov 2021
GMD

The two-way coupled Weather Research and Forecasting and Community Multiscale Air Quality (WRF-CMAQ) model has been developed to more realistically represent the atmosphere by accounting for complex chemistry–meteorology feedbacks. In this study, we present a comparative analysis of two-way (with consideration of both aerosol direct and indirect effects) and offline coupled WRF v3.4 and CMAQ v5.0.2 over the contiguous US. Long-term (5 years from 2008 to 2012) simulations using WRF-CMAQ with both offline and two-way coupling modes are carried out with anthropogenic emissions based on multiple years of the U.S. National Emission Inventory and chemical initial and boundary conditions derived from an advanced Earth system model (i.e., a modified version of the Community Earth System Model/Community Atmospheric Model). The comprehensive model evaluations show that both two-way WRF-CMAQ and WRF-only simulations perform well for major meteorological variables such as temperature at 2 m, relative humidity at 2 m, wind speed at 10 m, precipitation (except for against the National Climatic Data Center data), and shortwave and longwave radiation. Both two-way and offline CMAQ also show good performance for ozone (O3) and fine particulate matter (PM2.5). Due to the consideration of aerosol direct and indirect effects, two-way WRF-CMAQ shows improved performance over offline coupled WRF and CMAQ in terms of spatiotemporal distributions and statistics, especially for radiation, cloud forcing, O3, sulfate, nitrate, ammonium, elemental carbon, tropospheric O3 residual, and column nitrogen dioxide (NO2). For example, the mean biases have been reduced by more than 10 W m−2 for shortwave radiation and cloud radiative forcing and by more than 2 ppb for max 8 h O3. However, relatively large biases still exist for cloud predictions, some PM2.5 species, and PM10 that warrant follow-up studies to better understand those issues. The impacts of chemistry–meteorological feedbacks are found to play important roles in affecting regional air quality in the US by reducing domain-average concentrations of carbon monoxide (CO), O3, nitrogen oxide (NOx), volatile organic compounds (VOCs), and PM2.5 by 3.1 % (up to 27.8 %), 4.2 % (up to 16.2 %), 6.6 % (up to 50.9 %), 5.8 % (up to 46.6 %), and 8.6 % (up to 49.1 %), respectively, mainly due to reduced radiation, temperature, and wind speed. The overall performance of the two-way coupled WRF-CMAQ model achieved in this work is generally good or satisfactory and the improved performance for two-way coupled WRF-CMAQ should be considered along with other factors in developing future model applications to inform policy making.

15N natural abundance of vehicular exhaust ammonia, quantified by active sampling techniques

Article

Jun 2021
ATMOS ENVIRON

Vehicular exhaust has been identified as an important ammonia (NH3) emission source in urban areas using the stable isotope tracer technique (δ¹⁵N); however, its δ¹⁵N signature remains poorly constrained due to difficulties in concentrating and speciating NHx (NH3 + particulate NH4⁺). In this study, we deployed two active sampling techniques, MCIs (Multi-nozzle Cascade Impactor sampler; “particle-gas” order) and CCSCs (ChemComb Speciation Cartridge sampler; “gas-particle” order), to collect vehicular exhaust NHx in a long urban tunnel in Shenyang, China, and quantified the ¹⁵N from the collected NH3 and NH4⁺. While no significant difference in tunnel NH3 concentrations between the two samplers was observed (n = 21), the δ¹⁵N-NH3 was slightly higher using the MCIs (6.3 ± 1.6‰) compared to the CCSCs (4.8 ± 2.3‰) during the tunnel operation period. This minor absolute difference of 1.5 ± 2.8‰ may derive from denuder-saturation used within the CCSCs that lead to NHx speciation ambiguity, which also caused significant δ¹⁵N-pNH4⁺ differences between the two sampling approaches. Our results suggest that both active samplers can well characterize the δ¹⁵N-NHx in the ambient environment, but the “particle - gas” sampling approach performs better in elevated NH3 concentration environments. We recommend that the δ¹⁵N value of 6.3‰ is used as the isotope end member for vehicle NH3 when performing NH3 source apportionment.

Photochemical Oxidation of Water-Soluble Organic Carbon (WSOC) on Mineral Dust and Enhanced Organic Ammonium Formation

Article

Nov 2020

Water-soluble organic carbon (WSOC), which is closely related to biogenic emissions, is of great importance in the atmosphere for its ubiquitous existence and rich abundance. Levoglucosan, a typical WSOC, is usually considered to be stable and thus used as a tracer of biomass burning. However, we found that levoglucosan can be photo-oxidized on mineral dust, with formic acid, oxalic acid, glyoxylic acid, 2,3-dioxopropanoic acid, dicarbonic acid, performic acid, mesoxalaldehyde, 2-hydroxymalonaldehyde, carbonic formic anhydride, and 1,3-dioxolane-2,4-dione detected as main products. Further, we observed the heterogeneous uptake of NH3 promoted by the carboxylic acids stemming from the photocatalytic oxidation (PCO) of levoglucosan. The mineral-dust-initiated PCO of levoglucosan and enhanced heterogeneous uptake of NH3, which are highly influenced by irradiation and moisture conditions, were for the first time revealed. The reaction mechanisms and pathways were studied in detail by diffuse reflection infrared Fourier transform spectroscopy (DRIFTS), high-pressure photon ionization time-of-flight mass spectrometry (HPPI-ToF-MS) and flow reactor systems. Diverse WSOC constituents were studied as well, and the reactivity toward NH3 is related to the number of hydroxyl groups of the WSOC molecules. This work reveals a new precursor of secondary organic aerosols and provides experimental evidence of the existence of organic ammonium salts in atmospheric particles.

Assessing PM2.5 Model Performance for the Conterminous U.S. with Comparison to Model Performance Statistics from 2007-2015

Article

Jul 2019
ATMOS ENVIRON

Previous studies have proposed that model performance statistics from earlier photochemical grid model (PGM) applications can be used to benchmark performance in new PGM applications. A challenge in implementing this approach is that limited information is available on consistently calculated model performance statistics that vary spatially and temporally over the U.S. Here, a consistent set of model performance statistics are calculated by year, season, region, and monitoring network for PM2.5 and its major components using simulations from versions 4.7.1-5.2.1 of the Community Multiscale Air Quality (CMAQ) model for years 2007-2015. The multi-year set of statistics is then used to provide quantitative context for model performance results from the 2015 simulation. Model performance for PM2.5 organic carbon in the 2015 simulation ranked high (i.e., favorable performance) in the multi-year dataset, due to factors including recent improvements in biogenic secondary organic aerosol and atmospheric mixing parameterizations in CMAQ. Model performance statistics for the Northwest region in 2015 ranked low (i.e., unfavorable performance) for many species in comparison to the 2007-2015 dataset. This finding motivated additional investigation that suggests a need for improved speciation of wildfire PM2.5emissions and modeling of boundary layer dynamics near water bodies. Several limitations were identified in the approach of benchmarking new model performance results with previous results. Since performance statistics vary widely by region and season, a simple set of national performance benchmarks (e.g., one or two targets per species and statistic) as proposed previously are inadequate to assess model performance throughout the U.S. Also, trends in model performance statistics for sulfate over the 2007 to 2015 period suggest that model performance for earlier years may not be a useful reference for assessing model performance for recent years in some cases. Comparisons of results from the 2015 base case with results from five sensitivity simulations demonstrated the importance of parameterizations of NH3 surface exchange, organic aerosol volatility and production, and emissions of crustal cations for predicting PM2.5 species concentrations.

Selective Collection of Particulate Ammonium for Nitrogen Isotopic Characterization Using a Denuder-Filter Pack Sampling Device

Article

May 2019
ANAL CHEM

Nitrogen stable isotope analysis (δ15N) of particulate ammonium (NH4+) may provide additional constraints on this critical component of fine particulate matter; however, no previous collection method has been verified for its ability to accurately and precisely characterize δ15N(NH4+). This is a critical point due to the difficulty of quantitative NH4+ collection and possible sampling artifacts. Here, we report on δ15N(NH4+) precision using an established denuder-filter pack combination with two filter configurations including (1) a Nylon filter plus an acid impregnated cellulose filter and (2) an acid impregnated glass fiber filter for NH4+ collection in both laboratory-controlled environments and ambient air samples. Laboratory NH4+ were generated from the nebulization of ammonium salt solutions and collected using a filter pack sampling train for off-line concentration and isotopic measurement. Quantitative collection of NH4+ was achieved using both filter configurations in both laboratory and field collections. Laboratory experiments indicate a δ15N(NH4+) precision of ±0.9‰ (1σ; n=24) and ±0.6‰ (n=9) for the nylon plus citric acid impregnated cellulose filter and for the citric acid impregnated glass fiber filter, respectively. Field sample reproducibility was assessed from 24-hour collected side-by-side samples and indicated δ15N(NH4+) to be reproducible within 1.1‰, consistent with the laboratory findings. This work represents the first established method for speciated NH4+ collection for isotopic analysis with important implications for furthering our understanding of its atmospheric dynamics.

The sensitivity of PM<sub>2.5</sub> acidity to meteorological parameters and chemical composition changes: 10-year records from six Canadian monitoring sites

Article

Full-text available

Mar 2019
ACP

Aerosol pH is difficult to measure directly but can be calculated if the chemical composition is known with sufficient accuracy and precision to calculate the aerosol water content and the H⁺ concentration through ion balance. In practical terms, simultaneous measurements of at least one semi-volatile constitute, e.g. NH3 or HNO3, are required to provide a constraint on the calculation of pH. Long-term records of aerosol pH are scarce due to the limited monitoring of NH3 in conjunction with PM2.5. In this study, 10-year (2007–2016) records of pH of PM2.5 at six eastern Canadian sites were calculated using the E-AIM II model with the input of gaseous NH3, gaseous HNO3 and major water-soluble inorganic ions in PM2.5 provided by Canada's National Air Pollution Surveillance (NAPS) Program. Clear seasonal cycles of aerosol pH were found with lower pH (~2) in summer and higher pH (~3) in winter consistently across all six sites, while the day-to-day variations of aerosol pH were higher in winter compared to summer. Tests of the sensitivity of aerosol pH to meteorological parameters demonstrate that the changes in ambient temperature largely drive the seasonal cycle of aerosol pH. The sensitivity of pH to chemical composition shows that pH has different responses to the changes in chemical composition in different seasons. During summertime, aerosol pH was mainly determined by temperature with limited impact from changes in NHx or sulfate concentrations. However, in wintertime, both meteorological parameters and chemical composition contribute to the variations in aerosol pH, resulting in the larger variation during wintertime. This study reveals that the sensitivity of aerosol pH to chemical composition is distinctly different under different meteorological conditions and needs to be carefully examined for any particular region.

Observation of Secondary Organic Aerosol and New Particle Formation at a Remote Site in Baengnyeong Island, Korea

Article

Dec 2017

To improve the understanding of secondary organic aerosol (SOA) formation from the photo-oxidation of anthropogenic and biogenic precursors at the regional background station on Baengnyeong Island, Korea, gas phase and aerosol chemistries were investigated using the Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-ToF-MS) and the Aerodyne High Resolution Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS), respectively. HR-ToF-AMS measured fine particles (PM1; diameter of particle matter less than 1 μm) at a 6-minute time resolution from February to November 2012, while PTR-ToF-MS was deployed during an intensive period from September 21 to 29, 2012. The one-minute time-resolution and high mass resolution (up to 4000 m Δm⁻¹) data from the PTR-ToF-MS provided the basis for calculations of the concentrations of anthropogenic and biogenic volatile organic compounds (BVOCs) including oxygenated VOCs (OVOCs). The dominant BVOCs from the site are isoprene (0.23 ppb), dimethyl sulphide (DMS, 0.20 ppb), and monoterpenes (0.38 ppb). Toluene (0.45 ppb) and benzene (0.32 ppb) accounted for the majority of anthropogenic VOCs (AVOCs). OVOCs including acetone (3.98 ppb), acetaldehyde (2.67 ppb), acetic acid (1.68 ppb), and formic acid (2.24 ppb) were measured. The OVOCs comprise approximately 75% of total measured VOCs, suggesting the occurrence of strong oxidation processes and/or long-range transported at the site. A strong photochemical aging and oxidation of the atmospheric pollutants were also observed in aerosol measured by HR-ToF-AMS, whereby a high f44: f43 value is shown for organic aerosols (OAs); however, relatively low f44: f43 values were observed when high concentrations of BVOCs and AVOCs were available, providing evidence of the formation of SOA from VOC precursors at the site. Overall, the results of this study revealed several different SOA formation mechanisms, and new particle formation and particle growth events were identified using the powerful tools scanning mobility particle sizer (SMPS), PTR-ToF-MS, and HR-ToF-AMS.

Composition and sources of winter haze in the Bakken oil and gas extraction region

Article

Full-text available

Feb 2017
ATMOS ENVIRON

In the past decade increased use of hydraulic fracturing and horizontal drilling has dramatically expanded oil and gas production in the Bakken formation region. Long term monitoring sites have indicated an increase in wintertime aerosol nitrate and sulfate in this region from particulate matter (PM2.5) measurements collected between 2000 and 2010. No previous intensive air quality field campaign has been conducted in this region to assess impacts from oil and gas development on regional fine particle concentrations. The research presented here investigates wintertime PM2.5 concentrations and composition as part of the Bakken Air Quality Study (BAQS). Measurements from BAQS took place over two wintertime sampling periods at multiple sites in the United States portion of the Bakken formation and show regionally elevated episodes of PM2.5 during both study periods. Ammonium nitrate was a major contributor to haze episodes. Periods of air stagnation or recirculation were associated with rapid increases in PM2.5 concentrations. Volatile organic compound (VOC) signatures suggest that air masses during these episodes were dominated by emissions from the Bakken region itself. Formation rates of alkyl nitrates from alkanes revealed an air mass aging timescale of typically less than a day for periods with elevated PM2.5. A thermodynamic inorganic aerosol model (ISORROPIA) was used to investigate gas-particle partitioning and to examine the sensitivity of PM2.5 concentrations to aerosol precursor concentrations. Formation of ammonium nitrate, the dominant component, was most sensitive to ammonia concentrations during winter and to nitric acid concentrations during early spring when ammonia availability increases. The availability of excess ammonia suggests capacity for further ammonium nitrate formation if nitrogen oxide emissions increase in the future and lead to additional secondary formation of nitric acid.

Quantification of Sampling Artifacts in PM2.5 Inorganic Ion Species using Teflon Filter

Article

Feb 2007

Artifacts due to inter-particle and gas-particle interactions during PM2.5 sampling were quantified by comparing the measurement results between the annular denuder-filterpack system and the filterpack system without denuder. Measurements were carried in Seoul for 10 days in each season; Nov. 2004, Jan. 2005, Mar. 2005, and Jul. 2005, respectively. In each day, two 12-h samples were obtained. The concentrations of nitrate and chloride showed seasonal variations mainly due to the availability of ammonium to neutralize nitrate or chloride. Nitrates and chloride losses were prominent in summer. Since most of ammonia was used to neutralize sulfuric acid and formed ammonium sulfate in summer, nitrate and chloride could not exist in particles and ammonium loss was smaller than other seasons.

Denuder tubes for sampling of gaseous species. A review

Article

Full-text available

Jul 1989
ANALYST

A review is presented, with 59 references, of the development and applications, in particular for sampling of environmental materials, of the cited denuder tubes.

Water soluble dicarboxylic acids and related compounds in Antarctic aerosols

Article

Full-text available

Aug 1996

Antarctic aerosols collected at Syowa Station were studied for water soluble organic compounds by employing a water extraction and dibutyl ester derivatization and using a capillary gas chromatography (GC) and GC/mass spectrometry (GC/MS). Total carbon and nitrogen were also determined. A homologous series of alpha,omega-dicarboxylic acids (C2-C11), omega-oxocarboxylic acids (C2-C9), and alpha-dicarbonyls (C2-C3) were detected, as well as pyruvic acid and aromatic (phthalic) diacid. Succinic (C4) or oxalic (C2) acid was found to be the dominant diacid species, followed by azelaic (C9), adipic (C6), or malonic (C3) acid. Concentration range of the total diacids was 5.9-88 ngm-3, with an average of 29 ngm-3. Highest concentrations were observed in the summer sample with a predominance of succinic acid (61.5 ngm-3), which comprised approximately 70% of the total diacids and accounted for 3.5% of total aerosol carbon (1020 ngm-3). The succinic acid (C4) is likely produced by photooxidation of 4-oxocarboxylic acids, which are present in the atmosphere as intermediates of the photooxidation of unsaturated fatty acids. These results indicate that the Antarctic organic aerosols originate from marine-derived lipids and are transformed largely by photochemical oxidations. omega-Oxocarboxylic acids (C2-C9, 0.36-3.0 ngm-3) also showed the highest concentration in the summer sample, again suggesting a secondary production in the atmosphere of the Antarctic and in the Southern Ocean.

Ambient air sampling

Chapter

Full-text available

Jan 2001

Analysis of Aerosols

Article

Full-text available

Oct 1997
CHIMIA

Urs Baltensperger

Atmospheric aerosols have recently attracted considerable attention due to their effects on climate, atmospheric chemistry, and health. This interest has triggered a variety of activities including the development of new analytical methods for a better characterization of atmospheric aerosol particles, some of which are presented here. Emphasis is laid on the description of methods for on-line and in situ analysis, since the analysis of many processes requires a good time resolution, and since in situ analysis methods are less prone to sampling artefacts.

Intercomparison and closure calculations using measurements of aerosol species and optical properties during the Yosemite Aerosol Characterization Study

Article

Full-text available

Jul 2005
J GEOPHYS RES

1] Physical and optical properties of inorganic aerosols have been extensively studied, but less is known about carbonaceous aerosols, especially as they relate to the non-urban settings such as our nation's national parks and wilderness areas. Therefore an aerosol characterization study was conceived and implemented at one national park that is highly impacted by carbonaceous aerosols, Yosemite. The primary objective of the study was to characterize the physical, chemical, and optical properties of a carbon-dominated aerosol, including the ratio of total organic matter weight to organic carbon, organic mass scattering efficiencies, and the hygroscopic characteristics of a carbon-laden ambient aerosol, while a secondary objective was to evaluate a variety of semi-continuous monitoring systems. Inorganic ions were characterized using 24-hour samples that were collected using the URG and Interagency Monitoring of Protected Visual Environments (IMPROVE) monitoring systems, the micro-orifice uniform deposit impactor (MOUDI) cascade impactor, as well as the semi-continuous particle-into-liquid sampler (PILS) technology. Likewise, carbonaceous material was collected over 24-hour periods using IMPROVE technology along with the thermal optical reflectance (TOR) analysis, while semi-continuous total carbon concentrations were measured using the Rupprecht and Patashnick (R&P) instrument. Dry aerosol number size distributions were measured using a differential mobility analyzer (DMA) and optical particle counter, scattering coefficients at near-ambient conditions were measured with nephelometers fitted with PM 10 and PM 2.5 inlets, and ''dry'' PM 2.5 scattering was measured after passing ambient air through Perma Pure Nafion 1 dryers. In general, the 24-hour ''bulk'' measurements of various aerosol species compared more favorably with each other than with the semi-continuous data. Semi-continuous sulfate measurements correlated well with the 24-hour measurements, but were biased low by about 0.15 mg/m 3 . Semi-continuous carbon concentrations did not compare favorably with 24-hour measurements. Fine mass closure calculations suggested that the factor for estimating organic mass from measurements of carbon was approximately 1.8. Furthermore, fine scattering closure calculations showed that the use of 4.0 m 2 /g for the fine organic mass scattering coefficient was an underestimate by at least 30% for periods with high organic mass concentrations.

Method Comparisons for Particulate Nitrate, Elemental Carbon, and PM2.5 Mass in Seven U.S. Cities

Article

Full-text available

Aug 2000

Methods that measure PM2.5 mass, total particulate NO3-, and elemental carbon (EC) were evaluated in seven U.S. cities from 1997 to 1999. Sampling was performed in Bakersfield, CA; Boston, MA; Chicago, IL; Dallas, TX; Philadelphia, PA; Phoenix, AZ; and Riverside, CA. Evaluating and validating methods that measure the components of fine mass are important to the effort of establishing a speciation-monitoring network. The Harvard Impactor (HI), which measures fine particle mass, showed excellent agreement (r2 = 0.99) with the PM2.5 Federal Reference Method (FRM) for 81 24-hr samples in Riverside and Bakersfield. The HI also showed good precision (4.8%) for 243 24-hr collocated samples over eight studies. The Aethalometer was employed in six of the sampling locations to measure black carbon (BC). These values were compared to EC as measured from a quartz filter using thermal analysis. For the six cities combined, the two methods were highly correlated (r2 = 0.94; 187 24-hr samples); however, the BC values were approximately 24% less than the EC measurements consistently across all six cites. This compares well to results observed for EC/BC measurements observed in other semi-urban areas. Particulate NO3- was measured using the Harvard-EPA Annular Denuder System (HEADS). This was compared to the NO3- measured from the HI Teflon (DuPont) filter to assess NO3- artifacts. Significant NO3- losses (approximately 50% of total NO3-) were found in Riverside, Philadelphia, and Boston, while minimal artifacts were observed in the other sites. Two types of HEADS configurations were employed in five cities. One system used a Na2CO3-coated glass fiber filter, and the other type used a nylon filter to collect volatilized NO3- from the Teflon filter. The HEADS with the Na2CO3-coated filter consistently underestimated the total particulate NO3- by approximately 20% compared to the nylon HEADS.

Particulate Nitrate Measurement Using Nylon Filters

Article

Full-text available

Sep 2005

Nylon filters are a popular medium to collect atmospheric fine particles in different aerosol monitoring networks, including those operated by the U.S. Environmental Protection Agency and the Interagency Monitoring of Protected Visual Environments (IMPROVE) program. Extraction of the filters by deionized water or by a basic aqueous solution (typically a mixture of sodium carbonate and sodium bicarbonate) is often performed to permit measurement of the inorganic ion content of the collected particles. Whereas previous studies have demonstrated the importance of using a basic solution to efficiently extract gaseous nitric acid collected using nylon filters, there has been a recent movement to the use of deionized water for extraction of particles collected on nylon filters to eliminate interference from sodium ion (Na+) during ion chromatographic analysis of inorganic aerosol cations. Results are reported here from a study designed to investigate the efficiency of deionized water extraction of aerosol nitrate (NO3-) and sulfate from nylon filters. Data were obtained through the conduct of five field experiments at selected IMPROVE sites. Results indicate that the nylon filters provide superior retention of collected fine particle NO3-, relative to Teflon filters, and that deionized water extraction (with ultrasonication) of collected NO3- and sulfate is as efficient, for the situations studied, as extraction using a basic solution of 1.7 mM sodium bicarbonate and 1.8 mM sodium carbonate.

Property Rights and the Agency Problem in China's Enterprise Reform.

Article

Full-text available

Feb 1993
CAMB J ECON

Keun Lee

This paper analyzes problems in China's enterprise reform from property rights and agency problem perspectives. Reform efforts by the mid 1980s are interpreted as dealing with only the agency problem by relocating the decision process among agents, but not solving the hierarchical collusion problem. The shareholding system experiment is the first effort to deal with property rights relations. Although it seems to have contributed to some improvement in enterprise performance, there still remains the problem of expropriation of state assets by agents seeking irregular private gains. Copyright 1993 by Oxford University Press.

Sampling of Selected Labile Atmospheric Pollutants

Chapter

Jun 1993

B. R. Appel

Nitric acid, particulate nitrate, and particulate organic carbon may be termed "labile" atmospheric pollutants; this name reflects the ease with which they undergo physical or chemical changes while an integrated sample is being collected. Manual sampling methods of varying accuracy are described for these species, together with sources of error. For nitric acid and particulate nitrate, the filter pack method, usually with Teflon and nylon filters in tandem, is the least accurate but simplest sampling procedure. More accurate techniques use diffusion denuders to separate gaseous HN03 from fine particulate nitrate. Most methods remain subject to interference from nitrous acid. Sampling techniques for particulate organic C remain relatively primitive. Tandem filter sampling permits correction for the error due to sorption of gaseous C on the filter medium. Denuder-based techniques, such as those described here for polyaromatic hydrocarbons, may hold the key to future development of improved samplers for particulate organic C as well.

Aerosol Ion Characteristics during the Big Bend Regional Aerosol and Visibility Observational Study

Article

May 2004

The ionic compositions of particulate matter with aerodynamic diameter ≤2.5 μm (PM2.5) and size-resolved aerosol particles were measured in Big Bend National Park, Texas, during the 1999 Big Bend Regional Aerosol and Visibility Observational study. The ionic composition of PM 2.5 aerosol was dominated by sulfate (SO42-) and ammonium (NH4+). Daily average SO42- and NH4+ concentrations were strongly correlated (R2 = 0.94). The molar ratio of NH4+ to SO42- averaged 1.54, consistent with concurrent measurements of aerosol acidity. The aerosol was observed to be comprised of a submicron fine mode consisting primarily of ammoniated SO 42- and a coarse particle mode containing nitrate (NO 3-). The NO3- appears to be primarily associated with sea salt particles where chloride has been replaced by NO3-, although formation of calcium nitrate (Ca(NO 3)2) is important, too, on several days. Size-resolved aerosol composition results reveal that a size cut in particulate matter with aerodynamic diameter ≤1 μm would have provided a much better separation of fine and coarse aerosol modes than the standard PM2.5 size cut utilized for the study. Although considerable nitric acid exists in the gas phase at Big Bend, the aerosol is sufficiently acidic and temperatures sufficiently high that even significant future reductions in PM2.5 SO42- are unlikely to be offset by formation of particulate ammonium nitrate in summer or fall.

The effect of atmospheric SO2 photochemistry upon observed nitrate concentrations in aerosols

Article

Dec 1977
Atmos Environ

In this study, X-ray photoelectron spectroscopy was used to observe the relationship between the nitrate and sulfate content of photochemically formed aerosol particles which were produced by exposing initially particle-free ambient air in a 14 m3 transparent Teflon chamber to sunlight with various amounts of NO2, SO2, and C3H6 added to the reaction mixture. It was observed that the photochemical oxidation of SO2 to SO42− decreased the amount of nitrates present in the aerosol significantly below that observed in the absence of SO2. It was also found that nitrate could be removed from an aerosol sample on a glass fiber filter by passing air with sulfate-containing aerosols through the filter. These observations indicate that H2SO4 formed from the photochemical oxidation of SO2 in the reaction chamber released NO3− from the surface of the aerosol particles, probably in the form of HNO3. An examination of analyses of ambient aerosols from the Los Angeles area showed a number of cases where a similar inverse relationship between particulate sulfate and nitrate content existed.

Determination of Microgram Quantities of Inorganic Sulfate in Atmospheric Particulates

Article

Jun 1980
J Air Pollut Contr Assoc

Jack Wolfson

This paper describes a simple, manual procedure for the determination of microgram quantities of inorganic sulfate in aqueous extracts of airborne particulates collected on membrane-type filters. Its main feature is the use of a cyclone prefilter to remove particulates greater than 3.5 microns. The prefilter requires a restricted flow so that a 24 hour sample is only 2.6 m3, with a 37 mm membrane filter disc. In this paper the modified turbidimetric method of Tabatabai is tested, adapted, and shown to be adequate for the determination of water soluble inorganic sulfate in our ambient air particulate samples. The validity of this method was verified by comparison with the automated methylthymol blue method.

Determination of inorganic and organic volatile acids, NH 3 , particulate SO 4 2− , NO 3 2− and Cl − in ambient air with an annular diffusion denuder system

Article

Nov 1988

An annular denuder system for simultaneous determination of gaseous and particulate pollutants in ambient air was developed. Inorganic acids (HCl, HNO3) were collected in a NaF-coated denuder, while organic acids (HCOOH, CH3COOH) were trapped in one coated with KOH. NH3 was sampled with a H3PO4-coated denuder tube. Particulate H2SO4 was evaporated at elevated temperature (145°–155°C) and deposited on a NaF-coating together with HCl and HNO3 originating from thermal decomposition of NH4Cl and NH4NO3. NH3 resulting from deammoniation of (NH4)2SO4 as well as NH3 remaining from NH4Cl and NH4NO3 were collected in a H3PO4-coated denuder. The practical collection capacity of the tubes ranged from 0.35 to 1.0 µmol calculated from an experimentally determined sorption efficiency of at least 90%. The precision, expressed as relative standard deviation, of sampling and analytical procedures was determined from duplicate measurements in ambient air. The reproducibility varied from 9% to 14% for the gaseous components, while that of the particulate compounds ranged from 12% to 23%. Aqueous extracts of the denuder coatings were analysed for ionic components by ion chromatography using conductivity detection. The minimum detectable concentration in air was found to be 1.5 to 14 nmol/m3 for the different compounds calculated for 1.6 m3 sample volume, based on 3-h measurements at a flow of 9 l/min.

Spatial and Seasonal Trends in Particle Concentration and Optical Extinction in the United-States

Article

Jan 1994

In the spring of 1988 an interagency consortium of Federal Land Managers and the Environmental Protection Agency initiated a national visibility and aerosol monitoring network to track spatial and temporal trends of visibility and visibility-reducing particles. The monitoring network consists of 36 stations located mostly in the western United States. The major visibility-reducing aerosol species, sulfates, nitrates, organics, light-absorbing carbon, and wind-blown dust are monitored as well as light scattering and extinction. Sulfates and organics are responsible for most of the extinction at most locations throughout the United States, while at sites in southern California nitrates are dominant. In the eastern United States, sulfates contribute to about two thirds of the extinction. In almost all cases, extinction and the major aerosol types are highest in the summer and lowest during the winter months.

Identification of C3-C10 aliphatic dicarboxylic acids in airborne particulate matter

Article

Mar 1978

Airborne particulate samples collected during photochemical smog episodes were subjected to solvent extraction and fractionation, and the derivatized acid fractions were analyzed by combined gas chromatography-mass spectrometry using capillary columns and methane chemical ionization. Fifteen aliphatic dicarboxylic acids ranging from malonic acid (C3) to sebacic acid (C10) were identified, nine of which (including all seven branched-chain acids) are reported for the first time. Possible hydrocarbon precursors of these acids in urban photochemical smog are discussed.

Evaporative loss of fine particulate nitrates during sampling

Article

Dec 1992
Atmos Environ Gen Top

Laboratory and field data for losses of nitrates during sampling with filters and impactors are compared with theoretical predictions for evaporative losses. Predicted sampling efficiencies are in qualitative agreement with measured values. Both theoretical analysis and experimental data indicate that significant nitrate losses from filters and impactors can occur, although losses from impactors are far less than from filters. For both types of sampler, evaporative losses increase as the ratio of the gas-phase concentration to the particle concentration increases. For filters, losses become significant when this ratio exceeds a value in the range of 0.1–1.0; a value in the range 1.0–10.0 is required for losses to be significant for impactors.

The nitrogen species methods comparison study: An overview

Article

Dec 1988
Atmos Environ

Douglas R. Lawson

Theoretical Analysis of Evaporative Losses from Impactor and Filter Deposits

Article

Dec 1987
Atmos Environ

The pressure drop in aerosol samplers can lead to evaporation of the collected particles. A theoretical analysis for the evaporative losses from both impactor and filter deposits is developed. Quantitative expressions for the losses are expressed in terms of three dimensionless parameters: dimensionless pressure drop, ratio of the equilibrium vapor density of the aerosol species to its mass concentration, and dimensionless sampling time. In typical atmospheric aerosol sampling, the first two parameters are the dominant factors affecting evaporative loss rates. The results that are presented account for evaporative losses of pure, single component aerosols that are saturated at the inlet of the samplers.The analysis shows that evaporative losses will typically be substantial (10% or more) when the vapor-to-particle mass ratios of the aerosol are larger than about 1.0. It follows that for species present at mass concentrations of 10 μg m−3, evaporative losses will be substantial when vapor pressures exceed about 10−9 atm. The analysis also suggests that evaporative losses from single-stage impactor samplers may be less than from typical filter media. It follows that impactors may offer some advantages over filters for sampling volatile aerosol species.

Thermodynamic prediction of the water activity, NH4NO3 dissociation constant, density and refractive index for the NH4NO3-(NH4)2SO4-H2O system at 25°C

Article

Dec 1982
Atmos Environ

A.W. Stelson

The thermodynamic properties, water activity, density and refractive index of NH4NO3-(NH4)2SO4-H2O aerosols are estimated from binary solution data and existing mixing rules. Particle growth is shown to be predictable from the particle composition, the NH4NO3-(NH4)2SO4-H2O phase diagram and the water activity calculation technique of C.L. Kusik and H.P. Meissner (1978, A.I.Ch. E. Symp. 173, 14–20). Good agreement between the theoretical predictions and the experimental measurements of I.N. Tang et al. (1981, Atmospheric Environment15, 2463–2471), J. Thudium (1978,Pageoph. 116, 130–148) and H.H. Emons and W. Hahn (1970, Wiss Z.12, 129–132) is shown. Also, the effect of (NH4)2SO4 on the relative humidity dependence of the NH4NO3 dissociation constant is evaluated.

Organic acids in continental background aerosols

Article

Jun 1999
ATMOS ENVIRON

With a newly developed method aerosol samples from three distinctly different continental sites were analyzed: an urban site (Vienna), a savanna site in South Africa (Nylsvley Nature Reserve, NNR) and a free tropospheric continental background site (Sonnblick Observatory, SBO). In all samples a range of monocarboxylic acids (MCAs) and dicarboxylic acids (DCAs) has been identified and quantified. The three most abundant MCAs in Vienna were the C18, C16 and C14 acids with concentrations of 66, 45 and 36ngm-3, respectively. At the mid tropospheric background site (SBO) the three most abundant MCAs were the C18, C16 and C12 acid. For the DCAs at all three sites oxalic, malonic and succinic acid were the dominant compounds. For some individual compounds an information about the sources could be obtained. For example the determined unsaturated MCAs in South Africa appear to result from biogenic sources whereas in Vienna those acids are considered to be derived from combustion processes. Oxalic and glyoxalic acid appear to have a free tropospheric air chemical source. The relative high amounts at SBO in comparison to Vienna can only be explained by secondary formation of oxalic acid in the atmosphere.

Atmospheric concentrations of carboxylic acids and related compounds at a semiurban site

Article

Jan 1995
ATMOS ENVIRON

Haider Khwaja

Atmospheric gas- and particle-phase carboxylic acids and related compounds were measured during October 1991 in a semiurban site in northeastern United States. Formic and acetic acid were present in the atmosphere mostly in the gaseous form with less than 10% in the particle phase. Concentrations of formic acid and acetic acid were in the 0.80–2.5 and 0.60–3.4 ppbv ranges, respectively. Formic acid was correlated with acetic acid (r=0.94). Diurnal variations of formic acid were similar to those of ozone. This pattern is consistent with photochemical production as an important source. Formic-to-acetic acid ratios less than one were recorded, likely due to an increase in acetic acid contribution from direct emissions associated with vehicular traffic. Formaldehyde levels began to decrease after the mid-afternoon maxima; concentrations varied from 0.63 to 3.7 ppbv. Seven carboxylic acids (formic, acetic, pyruvic, glyoxalic, oxalic, succinic, and malonic) in the particle phase were identified. Aerosol carboxylic acid concentrations varied from 26 to 360 ng m−3. About 80% of the carboxylic acid aerosol mass was in the size fraction below 1.0 μm diameter. Of the dicarboxylic acids, oxalic acid was the most abundant species, followed by succinic acid and malonic acid. Particulate total organic carbon exhibited a concentration range of 12.6 × 103−49.9 × 103 ng Cm−3. The observed amounts of carboxylic acids in the particle phase accounted for a small fraction of the organic carbon. Results indicated that photochemical processes and anthropogenic emissions such as automobile exhaust are major sources of atmospheric carboxylic acids.

Evaporation loss of ammonium nitrate particles during filter sampling

Article

Dec 1997
J AEROSOL SCI

This study examines the evaporation loss of submicron, monodisperse ammonium nitrate particles during filter sampling when the upstream saturation ratio is zero. The proposed model has considered the upstream particle concentration, the porosity of the particle bed, particle diameter, upstream temperature, pressure drop. According to the present results, upstream particle concentration heavily influences the evaporation loss of ammonium nitrate particles. Theoretical results agree reasonably well with the experimental data obtained in the laboratory under well-controlled conditions. Results presented herein confirm that the simplified theory by Zhang and McMurry (1992) provides a reasonable yet somewhat lower collection efficiency of volatile species during filter sampling owing to the assumptions in the downstream saturation ratio and pressure drop.

Determination of organic acids by ion chromatography in rain water in the State of Zulia, Venezuela

Article

Jun 1994
J CHROMATOGR A

Ion-exchange chromatography with a Dionex Model QIC system was used to identify and determine formic and acetic acid in event rains collected at the La Esperanza site (Zulia, Venezuela). Pyruvic acid was observed infrequently and always at very low concentrations. The method can be adapted for the routine determination of these acids, giving results in less than 12 min (only the chromatogram without column clean-up). A set of calibration graphs for mixed standards with different concentration ranges (0.5–80 μM) with a good linear regression (R2 = 0.9992–1.0000) were used. The estimated limit of quantification was <0.2 μM for both acids. Replicate analyses of four different fractions of a certain sample taken and preserved with chloroform showed relatively good reproducibility (R.S.D. ca. 7%) for both acids, and the results were well within acceptable data quality limits. Both the absolute organic acid concentrations and the ratio of organic acid concentrations to inorganic acid concentrations were significantly lower in La Esperanza than those reported at other rural Venezuelan sites. Organic and inorganic anion balances revealed a low potential contribution (ca. 7%) of formic and acetic acid to the acidity of the rain (volume-weighted average pH = 4.1). In other rural Venezuelan sites these acids contributed over 60% to the free acidity.

Field comparison of methods for the measurement of gaseous and particulate contributors to acidic dry deposition

Article

Dec 1990
Atmos Environ Gen Top

A field study was conducted to compare methods for sampling and analysis of atmospheric constituents that are important contributors to acidic dry deposition. Three multicomponent samplers were used: the Canadian filter pack (FP), the annular denuder system (ADS), and the transition flow reactor (TFR). A tunable diode laser absorption spectrometer (TDLAS) provided continuous reference measurements of NO2 and HNO3. Nitrogen dioxide was also monitored with continuous luminol-based chemiluminescence monitors and with passive sampling devices (PSDs). The study was designed to provide a database for statistical comparison of the various methods with emphasis on the multicomponent samplers under consideration for use in a national dry deposition network. The study was conducted at the EPA dry deposition station in Research Triangle Park, NC between 29 September and 12 October, 1986. Daily averaging and/or sampling times were employed for the 13-day study; weekly samples were also collected, but results from these samples are not compared in the paper. Different measurements of ambient concentrations of the following constituents are compared: total particulate and gaseous NO3(-), HNO3, NO2, total particulate NH4(-), NH3, total particulate SO4(-), and SO2.

Evaporation of ammonium nitrate particles

Article

Dec 1987
Atmos Environ

Microscopic anhydrous ammonium nitrate particles are levitated in a vacuum in an electric quadrupole trap at 25°C. Size and mass changes of the evaporating particles are monitored continuously in situ using light scattering and gravity balancing. Both supersaturated liquid droplets at the zero-solvent limit and crystalline solids are studied. The measured vapor pressure of the liquid is p = (3.18 ± 0.45) × 10−6 torr. Fresh solid particles evaporate at a rate where r is an equivalent radius. Beyond 4 h the rate is a constant −0.06 Å s−1. These rates are consistent with that predicted from thermodynamics if the mass accommodation coefficient is α = 0.02 initially and 0.004 after 4 h. Aerosol sampling by several groups indicates the presence of solid ammonium nitrate particles in the atmosphere. Long lifetime and long-range transport of these particles are predicted regardless of ammonia and nitric acid vapor concentrations.

Atmospheric particulate nitrate sampling errors due to reactions with particulate and gaseous strong acids

Article

Dec 1981
Atmos Environ

The significance of particulate nitrate loss from inert filters due to reactions with particulate and gaseous strong acids was assessed. Laboratory as well as atmospheric results from California's South Coast Air Basin support such loss by reaction of ammonium nitrate with paniculate H2SO4. Nitrate loss by reaction with gaseous HCl, not previously reported, was found to be significant in laboratory trials.

Low molecular weight carboxylic acids in an urban atmosphere: Winter measurements in SÃ£o Paulo City, Brazil

Article

Jul 1999
ATMOS ENVIRON

Atmospheric gas and particle-phase carboxylic acids were measured during July 1996, Winter, in an urban area of São Paulo, a highly polluted Latin American city. Ion chromatography and capillary electrophoresis techniques were used to determine the species. As oxalic (36.2±21.4%), pyruvic (15.0±7.9%), β-hydroxy-butyric (9.15±9.00%) and glycolic (3.55±2.26%) acids were determined in aerosol particles, formic and acetic acids were determined both in the gaseous (4.36±2.70 and 3.66±2.63 ppbv, respectively) and particulate phases (17.8±12.4 and 18.2±9.8%, respectively). Approximately 98% of the total acetic and formic acids were in the gas-phase and the gas–aerosol equilibrium was influenced by high levels of relative humidity. Gaseous formic-to-acetic ratios fell in the 0.94–1.85 range. Photochemical production appeared to be a very likely source of the gaseous acetic and formic acid levels found in this investigation. Direct emissions, mainly motor exhaust of vehicles also contributed to their presence in air.

A Summary of Airborne Concentrations of Sulfur and Nitrogen-Containing Pollutants in the Northeastern United States

Article

Aug 1999

Joseph E. Sickles II

Airborne concentrations of SO2, SO4 , HNO3, NO3 , NH4+, and O3 were monitored over the six-year period from September 1, 1989, through August 31, 1995, at 10 largely rural Clean Air Status and Trends Network (CASTNet) sites in the northeastern United States. Each of the sulfur- and nitrogen-containing air pollutants monitored by CASTNet displays regular, seasonal cyclical behavior and also exhibits a relatively strong high-to-low spatial concentration gradient from southwest to northeast. On average, more than 70% of the measured airborne sulfur is present as SO2, except during the summer, when the figure drops to about 50%. During the summer, the SO2 concentration is the lowest, SO4 is the highest, and the fraction of airborne sulfur present as SO4 varies considerably with location, ranging from an average of 42% at five sites in Pennsylvania to 70% at two sites in New England. Studywide, more than 70% of the measured, oxidized, airborne nitrogen (N) is present as HNO3, except during the winter, when the figure drops to about 60%. The concentrations of gaseous SO2 and HNO3 are usually comparable but not always larger than the corresponding concentrations of measured sulfur and nitrogen aerosols. Nevertheless, the relatively faster deposition velocities for gases are sufficient to ensure that SO2 and HNO3 are usually the dominant contributors to dry sulfur and nitrogen deposition. Observed changes of 1990–1995 annual average airborne sulfur and N concentrations at 10 CASTNet sites in the Northeast are generally consistent with changes in emissions estimated to have occurred in the Northeast over the same period.

Photochemical Atmospheric Pollutants Derived from Nitrogen Oxides

Article

Dec 1977
ATMOS ENVIRON

Chester Spicer

Concern is now being focused on the health effects of nitrogen oxides and nitrogen oxides reaction products in the atmosphere. This paper reviews 3 years of investigation into the formation, distribution and fate of the nitrogen oxides and their reaction products. Smog-chamber and field-monitoring studies of the nature and concentration of gaseous and particulate nitrogen compounds are described, along with pertinent problems in analytical methodology.

Measurement of Atmospheric Nitric Acid in Gas Phase and Nitrate in Particulate Matter by means of Annular Denuders

Article

Jan 1994

A denuder technique for the determination of nitric acid and particulate nitrate is described. The technique relies on the use of four annular denuders set in series followed by a downstream filter-pack, and allows the reliable and accurate measurement of gaseous nitric acid, coarse nitrate and fine nitrate, even in the presence of high concentration levels of other nitrogen gaseous species and in case of high particulate to gaseous phase ratio. Field data collected by using the described system in extreme conditions, ranging from very polluted areas to extremely remote sites, are discussed. The data are analyzed in terms of mass distribution of the analyte on the different stages of the set-up; time resolution, minimum detectable concentration and precision of the measurements are also discussed.

Measurement of Atmospheric Pollutants Relevant to Dry Acid Deposition

Article

Jul 1989

The transfer of acid gases and particles from the atmosphere to the earth's surface, where they exert adverse environmental effects, is called acid deposition. Relevant removal processes occur during dry weather conditions, during the formation of clouds or by scavenging of falling rain or other hydrometeors. The first removal process is called acid dry deposition while the last two, known as rainout and washout, respectively, are lumped together into the phenomenon of acid wet deposition.

Relative humidity and temperature dependence of the ammonium nitrate dissociation constant

Article

Dec 1982
ATMOS ENVIRON

Expressions for predicting the temperature and relative humidity dependence of the NH4NO3 dissociation constant are derived from fundamental thermodynamic principles. The general trends predicted by the theory agree with the atmospheric data of Appel et al. (1979,1980), Pitts (1978,1979) and Tuazon et al. (1980).

Secondary Formation and the Smoky Mountain Organic Aerosol: An Examination of Aerosol Polarity and Functional Group Composition During SEAVS

Article

Jan 1998

Size-resolved particle samples were collected in the Smoky Mountains at Look Rock, TN, during the Southeastern Aerosol and Visibility Study (SEAVS) July−August 1995 and analyzed directly by Fourier transform infrared (FTIR) spectroscopy for functional group and chemical bond information. Twenty-eight samples were also gently rinsed in hexane, acetone, and water and reanalyzed after each rinse. Direct FTIR analyses of substrates rinsed with solvents enabled separation by polarity and identification of sulfur-containing organics even though samples were too small for traditional extraction and analysis (approximately 10−15 μg). The submicron organic aerosol was predominantly polar. Most of the nonpolar material, including aliphatic carbon and various minerals, was concentrated in particles greater than 1.0 μm and is most likely from primary biogenic and geogenic emissions, such as plant waxes and windblown soil dust. Unlike Los Angeles, carbonyl size distributions were unimodal and usually peaked in the 0.5−1.0 μm diameter size range. The predominance of sulfate, carbonyl, and organosulfur absorbances, the polarity of the aerosol, and the carbonyl size distributions indicate that secondary formation processes have a large influence on the concentrations, composition, and size distributions of the Smoky Mountain aerosol.

Strong and weak acidity of aerosols collected over the Northeastern United States

Article

Jun 1983

Total sulfate, acidity, and principal cations were measured in aerosol samples collected from an aircraft over the northeastern United States and at ground level on filters protected by a denuder tube for NH/sub 3/. Aerosol acidity was determined by aqueous extraction of the filters and titration with NaOH. Strong and weak acid components were resolved by using Gran plots, and for 30 fall and summer samples the weak acidity comprised x +/- s = 26 +/- 12% of the total acidity. The strong acidity averaged 25 +/- 18% of the total sulfate equivalents. Hydrogen ion plus ammonium ion balanced closely the sulfate ion concentration with the ratio of equivalents of ((H/sup +/) + (NH/sub 4//sup +/))/(SO/sub 4//sup 2 -/) = 1.03 +/- 0.14 for 36 samples. A phase diagram for the H/sub 2/O-H/sub 2/SO/sub 4/-(HN/sub 4/)/sub 2/SO/sub 4/ system was used to determine that the aerosol particles were liquid at the ambient relative humidity in 24 of 47 cases. For the 24 liquid samples the calculated pH average 0.11 +/- 0.59.

Collection efficiencies of a tandem sampling system for atmospheric aerosol particles and gaseous ammonia and sulfur dioxide

Article

Jun 1989

The collection efficiencies of aerosol particles and gaseous NH3 and SO2 were tested for a tandem sampling system consisting of a cyclone separator followed by a 1.0 μ pore size 47-mm Millipore Teflon particle filter and four 47-mm Whatman 41 filters coated with oxalic acid and either K2CO3 or LiOH. The collection efficiency of the cyclone was compared with an 8.0 μm pore size Nuclepore filter using NaCl particles. Both the cyclone and the filter had a 50% collection efficiency at 0.9 μm (50 standard L/min, 55 cm s-1 filter face velocity). Known amounts of NH3 and SO2 were generated and collected on the coated filters. The collection efficiency of the system for NH3 was found to be 103 ± 30%. The SO2 collection efficiency on K2CO3- and LiOH-coated filters was 100 ± 21 and 88 ± 9%, respectively, and was not affected by the presence of reduced sulfur gases or ozone in the sampled air stream.

Theoretical analysis of evaporative losses of adsorbed or absorbed species during atmospheric aerosol sampling

Article

Mar 1991

Evaporative losses of adsorbed or absorbed species from particle deposits during sampling is discussed. The theory that is developed focuses on evaporative losses that occur as a result of the pressure drop within the sampling device. The theory assumes that temperature and gas and particle concentrations remain constant during sampling. When the atmospheric aerosol is in equilibrium, the evaporative losses from filter samples are predicted to be small. The losses from impactor samples may be large for impactor stages with large pressure drop, especially for chemical species that are predominantly in the gas phase. When vapor denuders are used upstream, predicted sampling efficiencies for filters are always poorer than for impactors. In all cases, sampling efficiencies decrease with increasing values of the equilibrium ratio of gas to particle concentrations of the evaporating species.

A Comparison of High Volume and Diffusion Denuder Samplers for Measuring Semivolatile Organic Compounds in the Atmosphere

Article

Nov 2000

A detailed comparison of two inherently different ambient air samplers has been conducted to fully characterize and compare their performance in sampling and measuring the partitioning of a suite of 29 semivolatile organic compounds (SVOCs). A high volume air sampler (hi-vol) utilizing polyurethane foam (PUF) adsorbent for vapor-phase trapping and an annular diffusion denuder sampler were operated concurrently. Sampling artifacts were observed and can be related to the physicochemical properties of the analytes and the designs of the samplers. The results suggest that high volume air samplers equipped with PUF are unsuitable for measuring those organochlorine compounds and 2- and 3-ring PAHs which have subcooled liquid vapor pressures (p°L) greater than ca. 0.2 Pa (log[p°L] = −0.7), owing to their breakthrough on PUF sampling media at relatively low sample volumes (170 m3) and ambient temperatures typical of temperate regions (mean = 11 °C, max = 18 °C). Theoretical calculations of breakthrough volumes for SVOCs on PUF are presented and in most cases these successfully predict observed behavior. The denuder sampler is more efficient at measuring the relatively volatile SVOCs. For total SVOC measurements the hi-vol and denuder were in good agreement for those compounds which were efficiently sampled, and the denuder yielded total SVOC concentrations which differed by a mean factor of 1.2 relative to those obtained with the hi-vol sampler. The hi-vol sampler provides good agreement with the Junge-Pankow model for partitioning of the relatively less volatile PAHs (log[p°L] < −3), though the fraction of the PCBs in the particle-phase is underestimated. The results from the diffusion denuder indicate that for the more volatile SVOCs (log[p°L] > −3), particulate loadings are overestimated with respect to the Junge-Pankow model, and for less volatile SVOCs, particulate loadings tend to be underestimated. An important observation is that the results from the denuder indicate that PCBs may be adsorbed on atmospheric particulate matter to a similar degree as PAHs.

Simultaneous nitric acid, particulate nitrate and acidity measurements in ambient air

Article

Dec 1980
ATMOS ENVIRON

A laboratory and field study was conducted to evaluate measurement methods for atmospheric nitric acid and to elucidate the formation of artifact particulate nitrate on filter media. Nitric acid collection with nylon and NaCl-impregnated filters was compared employing Teflon pre-filters to remove atmospheric particulate matter. The two collection procedures provided > 97% collection of nitric acid at the levels expected under ambient conditions. At higher levels, the efficiency of the nylon filters decreased. Nylon filters supplied by Millipore and Ghia Corporations were shown to be equally effective. Clean Teflon pre-filters permitted passage of > 98% of the HNO3 to the collection filters. However, atmospheric particulate matter on the pre-filters retained HNO3. Ammonium nitrate on otherwise clean pre-filters caused positive error in HNO3 measurement (and a corresponding loss of particulate nitrate) when clean air was passed through the sample. Dissociation to NH3 and HNO3 is a likely source of these errors. A lower limit of 22 (ppb)2 is obtained for the NH4NO3 dissociation constant at 21°C.Atmospheric sampling was done in Pittsburg, California. On average, results with the two HNO3 collection procedures agreed within 3% for levels in the range 0.3–1.5 ppb. Levels of HNO3 and NH3 were below those needed for saturation with respect to NH4NO3 formation. A glass fiber filter was shown to serve as a total inorganic nitrate sampler, collecting both particulate nitrate and HNO3.

Artifact particulate sulfate and nitrate formation on filter media

Article

Dec 1984
ATMOS ENVIRON

Laboratory and atmospheric sampling studies were performed to evaluate glass fiber, quartz and Teflon filters for their abilities to form artifact particulate sulfate and nitrate with SO2and HNO3, respectively. The glass fiber filters were the types employed by the U.S. EPA as well as many State and local agencies for hi-vol particle sampling in the period 1977–1982.In 24-h laboratory trials, the sulfate artifact with the glass fiber filters ranged from 32 to 59μgcm−2 (8–15μg m−3 at simulated hi-vol sampling rates). With the exception of Whatman QMA, the ‘quartz’ and Teflon filters collected very low levels of SO2. At nitric acid dosages representative of those in atmospheric sampling, the glass fiber filters retained > 94% of the HNO3; the ‘quartz’ filters, 33 to > 99%; and Teflon filters, <2 % of the HNO3,. Because of competing acidic species in ambient air, these represent upper limits to the values to be expected in atmospheric sampling. Over the dosage range evaluated, artifact particulate sulfate and nitrate formation on ‘quartz’ and Teflon filters were highly correlated to each other from one filter to another, with a 4:1 molar ratio of nitrate: sulfate.The sulfate artifact in atmospheric sampling with Pallflex ‘quartz’ filters was consistent with laboratory findings while the nitrate artifact was much lower than predicted.

Atmospheric Equilibrium Model of Sulfate and Nitrate Aerosols

Article

Dec 1983
ATMOS ENVIRON

Given local rates of production of gas-phase sulfate and nitrate, ammonia concentration, relative humidity and temperature, a model is presented that enables calculation of the quantity of sulfate/nitrate/ammonium/water aerosol, its precise chemical composition and physical state. The model is based on a complete thermodynamic chemical and phase equilibrium calculation for the sulfate/nitrate/ammonium system. Detailed simulations of sulfate/nitrate/ammonium aerosol evolution are presented, and recent ambient data from Tanner are interpreted. Some new results on temperature variation of activity coefficients are presented in the Appendix.

Sampling of nitrates in ambient air

Article

Dec 1981
Atmos Environ

Methods for the measurement of nitric acid, particulate nitrate and total inorganic nitrate (i.e. HNO3 plus particulate nitrate) are compared using atmospheric samples from the Los Angeles Basin. Nitric acid was measured by (1) the nitrate collected on nylon or NaCl-impregnated cellulose filters after removal of particulate matter with Teflon prefilters, (2) long-path Fourier transform infrared spectroscopy (FTIR) performed by a collaborating investigator, and (3) the difference between total inorganic nitrate (TIN) and particulate nitrate (PN). TIN was measured by the sum of the nitrate collected with a Teflon prefilter and nylon or NaCl-impregnated after-filter. PN was measured by the nitrate able to penetrate a diffusion dénuder coated to remove acidic gases (e.g. HNO3). Losses of nitrate from Teflon prefilters were determined by comparing the nitrate retained by these filters to the nitrate penetrating the acid gas denuder. TIN and the nitrate collected with glass fiber filters were compared to assess the origin of the artifact particulate nitrate on the latter.

A Comparison of Three Methods for Measurement of Atmospheric Nitric Acid and Aerosol Nitrate and Ammonium

Article

Dec 1985
Atmos Environ

Three methods for measuring gaseous ambient nitric acid in the low concentration range 0–15 μ m−3 were compared under field conditions in Southwestern Ontario during 1–14 June 1982. The methods employed were (1) tunable diode laser absorption, (2) a tungstic acid denuder tube and (3) a filter pack containing a Teflon-nylon-W41 filter combination operated over 3-h sampling periods. In general, the three methods measured nitric acid with adequate sensitivity (sub ppb) and they correlated reasonably over the low ambient concentrations considered here. However, there were some notable differences (up to a factor of 2 at night) by the tungstic acid and filter pack methods. During the field comparison, particulate nitrate and ammonium were also measured (0–6 μg m−3 range) by the tungstic acid and filter pack methods. These correlated well but some differences (less than 20%) were observed in the particulate nitrate measurements. In view of the low concentrations measured and the resultant larger experimental error, specific loss or interfering mechanisms could not be unambiguously identified.

Determination of total inorganic nitrate utilizing collection of nitric acid on NaCl—impregnated filters

Article

Jan 1980
ATMOS ENVIRON

Cellulose filters impregnated with NaCl will collect nitric acid vapor from ambient atmospheres at efficiencies of ~95% when placed downstream from pretreated quartz particle filters in a high-volume sampler. No adsorption of NO2 on the NaCl filters with retention as nitrate was observed. Conversion of NO2 to nitrate on quartz (either retained as nitrate or released as artifact nitric acid) was negligibly low with the removal of the less than 1% of the incident NO2. At high humidities adsorption of HNO3 on the quartz prefilter can become significant Quartz filters which were preloaded with ambient particulate matter lost nitrate when exposed to aerosol H2SO4. Sulfur dioxide may be collected on potassium-carbonate-impregnated cellulose filters placed downstream from quartz and NaCl papers without any loss on the preceding filters. Ammonium nitrate on quartz filters is partially volatilized during ambient air sampling.

Interference in Sampling Atmospheric Particulate Nitrate

Article

Dec 1977
Atmos Environ

The vast majority of atmospheric data on particulate nitrate concentrations is based on filter collections employing glass fiber filters. Recent laboratory studies indicate that these data may be in error due to collection of gaseous nitrates and nitrate precursors on the filters. This communication documents the severity of the interference under ambient sampling conditions. Two types of glass fiber filters in widespread use were found to collect 10–20 times as much artifact nitrate as actual particulate nitrate.

Semivolatile behavior of dicarboxylic acids and other polar organic species at a rural background site (Nylsvley, RSA)

Article

Apr 2001
ATMOS ENVIRON

In this study aerosol samples from the South African savanna were analyzed for their polar organic constituents. Samples were collected with a front/back-up filter tandem system of quartz fiber filters (dual filter strategy). In all samples (n=15) dicarboxylic acids and a variety of phthalates, aldehydes and monocarboxylic acids were observed. Oxalic acid was the dominating compound with an average amount of 79.2 ng m−3 on the front filter and 11.3 ng m−3 on the back-up filter. The presence of significant concentrations of dicarboxylic acids on the back-up filter was rather unexpected. There are two possible sources to explain the presence of individual compounds on the back-up filter – particle penetration through the front filter or adsorption of compound parts from the gas phase. Interpretation of the data indicates that the dicarboxylic acid concentrations on the back-up filters appear to be caused by the adsorption of gaseous organic species. Dicarboxylic acids semivolatilic behavior is evident with this results. This conclusion refutes the commonly held view that dicarboxylic acids in the atmosphere were associated with the aerosol phase only. Additionally, it was found that the distribution of dicarboxylic acids between the gas and particle phase in the atmosphere is not only dependent on their vapor pressures. The actual gas phase concentration appears to be more determined by the chemical properties of the particles than by pure physical influences. Surprisingly, malonic acid exhibits an anomaly, as it does not show a semivolatile tendency.

Determination of Aerosol Strong Acidity Losses Due to Interactions of Collected Particles: Results from Laboratory and Field Studies

Article

Apr 1992
ATMOS ENVIRON

Existing methods of measuring atmospheric aerosol strong acidity adequately prevent neutralization of fine-particle acidity by removing course alkaline particles and gaseous ammonia from air samples. However, these techniques do not consider particle interactions on the collection medium; therefore, they may still underestimate the actual aerosol acidity. Assessment of acid neutralization due to such interactions is made possible using annular denuder technology in conjuction with a newly designed filter pack. The amount of sulfate-related acidity neutralized by the collected ammonium nitrate (and possibly ammonium chloride and organic acid ammonium salts) is determined. Laboratory data suggest that large fractions of sulfate-related aerosol acidity are neutralized by ammonium nitrate particles during collection on filter media. Field data from the Harvard Acid Aerosol Health Effects Study also suggest that ammonium nitrate and possibly other ammonium salts, such as ammonium chloride, neutralize collected acid aerosols. For low-acid aerosol concentrations, the correction factor is significant; whereas, for high-acid concentrations, correction is negligible.

A Fabric Denuder for Sampling Semi-Volatile Species

Article

Jul 2000

A new style of diffusion denuder has been evaluated specifically for sampling HNO3. A coated fabric is used as the denuder substrate, which can be loaded directly into a standard filter holder. This approach allows direct denuder sampling with no additional capital costs over filter sampling and simplifies the coating and extraction process. Potential denuder materials and coatings were evaluated in the laboratory to test the removal efficiency. NaCl coatings were used to assess more than 20 materials for HNO3 collection efficiency. Particle retention, which would cause a denuder to have a positive bias for gas concentration measurements, was evaluated by ambient air sampling using particulate sulfate as the reference aerosol. Particle retention varied from 0 to 15%, depending on the denuder material tested. The best performing material showed an average particle retention of less than 3%. Denuder efficiency of four fabric materials was tested under ambient conditions to determine removal efficiency. The fabric denuder method was compared with a long path-length Fourier transform infrared (FTIR) spectrometer, a tunable diode laser absorption spectrometer (TDLAS), and a denuder difference sampler to independently measure HNO3. HNO3 collection efficiency was typically 90% for the denuders, whether coated with NaCl or not. For 10-L/min sampling rates with the fabric denuder, the square of the correlation coefficient with the FTIR spectrometer was 0.73, compared to 0.24 with the TDLAS.

The Aerosol Research and Inhalation Epidemiology Study (ARIES): PM 25 Mass and Aerosol Component Concentrations and Sampler Intercomparisons

Article

Sep 2000

The Aerosol Research and Inhalation Epidemiology Study (ARIES) was designed to provide high-quality measurements of PM2.5, its components, and co-varying pollutants for an air pollution epidemiology study in Atlanta, GA. Air pollution epidemiology studies have typically relied on available data on particle mass often collected using filter-based methods. Filter-based PM2.5 sampling is susceptible to both positive and negative errors in the measurement of aerosol mass and particle-phase component concentrations in the undisturbed atmosphere. These biases are introduced by collection of gas-phase aerosol components on the filter media or by volatilization of particle phase components from collected particles. As part of the ARIES, we collected daily 24-hr PM2.5 mass and speciation samples and continuous PM2.5 data at a mixed residential-light industrial site in Atlanta. These data facilitate analysis of the effects of a wide variety of factors on sampler performance. We assess the relative importance of PM2.5 components and consider associations and potential mechanistic linkages of PM2.5 mass concentrations with several PM2.5 components. For the 12 months of validated data collected to date (August 1, 1998-July 31, 1999), the monthly average Federal Reference Method (FRM) PM2.5 mass always exceeded the proposed annual average standard (12-month average = 20.3 +/- 9.5 micrograms/m3). The particulate SO4(2-) fraction (as (NH4)2SO4) was largest in the summer and exceeded 50% of the FRM mass. The contribution of (NH4)2SO4 to FRM PM2.5 mass dropped to less than 30% in winter. Particulate NO3- collected on a denuded nylon filter averaged 1.1 +/- 0.9 micrograms/m3. Particle-phase organic compounds (as organic carbon x 1.4) measured on a denuded quartz filter sampler averaged 6.4 +/- 3.1 micrograms/m3 (32% of FRM PM2.5 mass) with less seasonal variability than SO4(2-).

Use of Two Different Acidic Aerosol Samplers To Measure Acidic Aerosols in Hsinchu, Taiwan

Article

Jan 2001

Acidic aerosol concentrations measured by an annular denuder system (ADS) and a honeycomb denuder system (HDS) in Hsinchu, Taiwan, were compared. Aerosols were also sampled by a MOUDI (micro-orifice uniform deposit impactor) and analyzed by an ion chromatograph to determine the size distributions of different species. Using the measured aerosol size distribution, theoretical analysis showed that positive HNO3 artifact due to volatilization of NH4NO3 is generally negligible for both samplers. Comparing two different denuder samplers, the average concentration of HNO3 measured by the ADS was found to be lower than that measured by the HDS, while the difference between the two samplers for the average concentration of other species was found to be within +/- 15%. A possible cause of the difference in HNO3 concentrations is due to a greater loss of HNO3 in the cyclone used by the ADS than in the impactor used by the HDS. The study also showed incomplete absorption of the evaporated HCl and HNO3 from the particles on the Teflon filter by the first nylon filter in the filter pack of the ADS. Collection efficiency and capacity of HCl and HNO3 by the nylon filters need further investigation.

Characterization of Organic Compounds Collected during Southeastern Aerosol and Visibility Study: Water-Soluble Organic Species

Article

Feb 2005

As part of the Southeastern Aerosol and Visibility Study (SEAVS), water-soluble organic species (WSOS) in fine aerosols collected from July 15 to August 25, 1995, at the Great Smoky Mountain National Park, Tennessee (USA), were chemically classified into seven groups, with concentrations ranging from around 1 to >200 ng/m3. Dicarboxylic acids represented the dominant identified compound class, and succinic acid was the most abundant dicarboxylic acid. The trends in data suggest that most WSOS collected in the SEAVS samples were mainly generated from secondary photochemical reactions, especially during the first (cleaner) half of the sampling campaign. High relative humidity at the sampling site resulted in substantial water uptake by the aerosols, which may have enhanced the levels of succinic acid by reducing its rate of photooxidation. Concurrent trends in malic and malonic acid concentrations suggest these were generated from the oxidation of succinic acid. Consistent with the conversion of 3-hydroxypropanoic acid to malonic acid, it appears that 4-hydroxybutanoic acid served as a major precursor contributing to high levels of succinic acid in the daytime. Nocturnal WSOS generally followed the trend of diurnal WSOS, but they exhibited different chemical compositions and lower concentrations, unlike what has been reported for an urban site. A nocturnal-to-diurnal ratio of succinic acid larger than 0.25 may indicate an atmosphere dominated by photochemical reactions, rather than by primary emissions.

Article

Effect of ambient NH3 levels on PM2.5 composition in the Great Smoky Mountains National Park

August 2005 · Atmospheric Environment

Continuous measurements were made of gaseous NH3, gaseous HNO3, PM2.5 sulfate, aerosol mass, air temperature, and relative humidity (RH), and 24-h filter samples of PM2.5 aerosols were analyzed to determine the effects of ambient NH3 levels on PM2.5 aerosol composition. These measurements were conducted from 2 to 15 August 2002, at Look Rock, TN, at an air monitoring station located on a mountain ... [Show full abstract] ridge (elevation ∼800 m MSL) on the southwestern edge of the Great Smoky Mountains National Park. A strong diurnal pattern was observed for gaseous NH3 levels at this site with higher daytime and much lower NH3 during the night hours. The diurnal variability of both gaseous NH3 concentrations and RH suggests that conditions controlling aerosol nitrate formation and acidic aerosol neutralization are complex. The observations during this 2-week long sampling period showed that measurable NH4NO3 aerosol was not present under the ambient conditions encountered. However, modest perturbations from observed ambient conditions could have enabled NH4NO3 aerosol production. The extent of neutralization of sulfate aerosol collected on filters indicates that there was insufficient regional NH3 to neutralize the acidic sulfate aerosols to the degree predicted from local NH3 and RH conditions. Incomplete neutralization of acidic sulfate could also result if neutralization proceeds more slowly under field conditions compared to laboratory conditions or model predictions. Continuous measurements of aerosol ammonium and/or acidity are needed to determine if ammonia availability or kinetic limitations (or both) restrict neutralization of acidic sulfate aerosols, a key factor for modeling and exposure studies.

Article

A Laboratory Study of the Interaction of NH3 and NO2 with Sea Salt Particles

April 1998 · Water Air and Soil Pollution

It is known from the literature that the reaction 2 NO2(aq) + Cl-(aq) → NO3-(aq) + ClNO(g) takes place in the presence of NO2 and sea salt aerosol. This work indicates that the reaction is first order in Cl- and second order in NO2(aq) and the rate coefficient was determined to be 8.9 ± 2.1) × 1010 M-2 s-1 at 293 K. The study shows that the formation rate of nitrate in liquid sea salt aerosol is ... [Show full abstract] not influenced by pH in the range between 5.1 and 7.5. The uptake of ammonia in liquid sea salt particles in the present experimental system is governed by the rate of the reaction CO2(aq) + H2O → H2CO3. The addition of NH3 to NO2 and dry sea salt particles does not change the extent of nitrate formation in the particles. The dry aerosol shows an unexpected content of ammonium which increases with decreasing relative humidity. The nitrate formation in frozen sea salt particles at 266 K exposed to only NO2, is about 5 times lower than for the corresponding droplets at 293 K.