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Measurement of trace gas fluxes using tunable diode laser spectroscopy
Abstract
A fast-response, high precision, tunable diode laser spectrometer has been developed for field measurements of methane and nitrous oxide fluxes using the eddy correlation method. The instrument uses a novel multiple-pass absorption cell with astigmatic mirrors to provide a long absorption path length (36 m) in a small volume (0.31). The combination permits rapid response with sufficient sensitivity for eddy correlation flux measurements over a wide range of meteorological conditions. Extensions of the method to measurements of surface-reactive gases such as nitric acid and ammonia are possible using rapid sampling techniques to minimize surface interactions. -from Authors
... A more widely acceptance can be expected if TDLAS can be better used in process control and monitoring. [3,4] The first part of the paper makes commercially successful identification of applications easier since TDLAS is a versatile and reliable but also relatively expensive tool. ...
... In our experience it turns out that (beside the low detection limits and selectivity) the high measurement speed [4], the chance to run remote monitoring and the universal feature of the Lambert-Beer law are key advantages triggering broader need. The importance of the universal aspect may surprise at the first moment. ...
... QC lasers in this application can be used to reduce the size, complexity, and operator attention compared to our earlier instruments using lead salt TDLs. 4 The data acquisition and signal processing software product, TDLWINTEL, which uses direct absorption with rapid sweep integration to retrieve absolute trace gas concentrations, has been extensively modified to accommodate pulsed QC lasers. The program can control up to four lasers and report simultaneous concentrations for eight gases using spectroscopic data from the HITRAN data base. ...
By means of Tunable Diode Laser Absorption Spectroscopy (TDLAS) superior detection limits can be achieved in analytics of many gases. [1,2]. However, application is still limited to laboratory applications or very small series production and the measurement technique is not yet broadly established (in comparison to methods like FTIR). A more widely acceptance can be expected if TDLAS can be better used in process control and monitoring. [3,4] The first part of the paper makes commercially successful identification of applications easier since TDLAS is a versatile and reliable but also relatively expensive tool. In our experience it turns out that (beside the low detection limits and selectivity) the high measurement speed [4], the chance to run remote monitoring and the universal feature of the Lambert-Beer law are key advantages triggering broader need. The importance of the universal aspect may surprise at the first moment. However, oxygen is a good example here: At low target temperatures paramagnetic sensors are widely used, while at elevated temperatures ZrO sensors are most common. From an end-user point of view a standardized sensor for all temperature would be appreciated and the optical sensor based on TDLAS can do the job.[5] Furthermore, the paper shortly reviews the product range.of LASER COMPONENTS GmbH useful for TDLAS: We do offer single mode lasers in spectroscopic quality under the name "SPECDILAS". The IR series is based on IV-VI technology and covers the range from 3-10 µm. The range from 10-20 µm can also be covered by lasers based on iV-VI technology, but in multimode quality only. The V series (single mode VCSEL) is available at 760 nm (for oxygen detection), 850 nm and most recently also between 1.5 and 2 µm. The D-series (DFB) is available from 1.25 to 1.75 µm. Recently, the QUANTA diode laser assembled with peltier cooling having emission at 5.5 µm using pulsed conditions and the referring GENPULSE controller are launched into the market.
... Laser absorption spectroscopy (LAS) with tunable semiconductor lasers is a commonly used technique for spectroscopic measurements of trace gas concentrations. It offers great flexibility, relatively simple and robust setup, and the ability to achieve high measurement precisions [1][2][3][4][5][6][7]. However, a common drawback observed in field deployable sensors is the lack of long-term stability, which is the most critical parameter to achieve high accuracy guaranteed over extended periods of time (e.g., [8][9][10]). ...
... Methods to mitigate fringe-induced drifts include: careful optical design, precise temperature control of the entire sensing instrument, or use of costly materials with low coefficients of thermal expansion (e.g., [5,[14][15][16][17]). Unfortunately, not only are these methods expensive or consume much power, but the improvement they provide is usually still not sufficient to obtain absolute long-term stability of a sensing system. Therefore, in order to ensure accuracy of the concentration measurement over long time, periodic calibration of the sensor with certified gas mixtures is usually performed [2,12]. ...
... It is a common practice in direct LAS sensing to use spectral fitting algorithms to retrieve an accurate concentration of the target analyte [2,5,19]. Such algorithms require precise calibration of the optical frequency axis (I(t)→I(ν) for frequency or I(t)→I(λ) for wavelength) in order to perform a line-by-line spectral transmission simulation based on molecular transition parameters obtained from a spectral database. ...
A real-time drift correction and calibration method using spectral correlation based on a revolving in-line gas cell for laser-based spectroscopic trace-gas measurements has been developed and evaluated experimentally. This technique is relatively simple to implement in laser spectroscopy systems and assures long-term stability of trace-gas measurements by minimizing the effects of external sources of drift in real-time. Spectroscopic sensitivity sufficient for environmental monitoring and effective drift suppression has been achieved for long-term measurements of CO2 with a quantum cascade laser based spectrometer.
... 2.4) from the HCl-TILDAS sampling line were calculated as an objective measure of the sampling method performance. Such timescales for sticky gases (including HCl) have been previously determined by fitting data to a bi-exponential model (Roscioli et al., 2016;Zahniser et al., 1995;Ellis et al., 2010;Pollack et al., 2019): ...
... The τ 90 achieved utilizing active passivation in this study is the shortest reported instrument response time for changes in HCl mixing ratios to date ( Table 1) and demonstrates that the use of PFBS is effective for reducing HCl-surface interactions. Previous studies have suggested that a bi-exponential model (Eq. 1) may better physically represent sticky gas flow through an instrument Zahniser et al., 1995;Ellis et al., 2010;Pollack et al., 2019); in this approach, τ 1 may represent the air residence time within the instrument, while τ 2 will represent the factor(s) that cause the analyte to lag through the instrument (e.g., surface interactions). Our results were not inconsistent with this postulation since the unpassivated cases were well represented by the bi-exponential model (i.e., significant τ 1 and τ 2 equation terms within Eq. 1), while passivated cases were better represented by the single-exponential model (i.e., dominant τ 1 but negligible τ 2 ). ...
The largest inorganic, gas-phase reservoir of chlorine atoms in the atmosphere is hydrogen chloride (HCl), but challenges in quantitative sampling of this compound cause difficulties for obtaining high-quality, high-frequency measurements. In this work, tunable infrared laser direct absorption spectroscopy (TILDAS) was demonstrated to be a superior optical method for sensitive, in situ detection of HCl at the 2925.89645 cm−1 absorption line using a 3 µm inter-band cascade laser. The instrument has an effective path length of 204 m, 1 Hz precision of 7–8 pptv, and 3σ limit of detection ranging from 21 to 24 pptv. For longer averaging times, the highest precision obtained was 0.5 pptv with a 3σ limit of detection of 1.6 pptv at 2.4 min. HCl-TILDAS was also shown to have high accuracy when compared with a certified gas cylinder, yielding a linear slope within the expected 5 % tolerance of the reported cylinder concentration (slope = 0.964 ± 0.008). The use of heated inlet lines and active chemical passivation greatly improve the instrument response times to changes in HCl mixing ratios, with minimum 90 % response times ranging from 1.2 to 4.4 s depending on inlet flow rate. However, these response times lengthened at relative humidities >50 %, conditions under which HCl concentration standards were found to elicit a significantly lower response (−5.8 %). The addition of high concentrations of gas-phase nitric acid (>3.0 ppbv) were found to increase HCl signal (<10 %), likely due to acid displacement with HCl or particulate chloride adsorbed to inlet surfaces. The equilibrium model ISORROPIA suggested a potential of particulate chloride partitioning into HCl gas within the heated inlet system if allowed to thermally equilibrate, but field results did not demonstrate a clear relationship between particulate chloride and HCl signal obtained with a denuder installed on the inlet.
... The application of IR-spectroscopy for various field techniques implies serious problems in drift and low sensitivity (Zahniser et al., 1995). Closed-path analysers employ advanced technologies such as wavelength scanned cavity ring down spectroscopy (WS-CRDS) or integrated cavity output spectroscopy ...
... Since the 1980s, infrared absorption spectrometry has become popular for measurements of trace gases in the lab. The development of various field techniques has revealed serious problems with drift and low sensitivity (Zahniser et al., 1995). Today, the commonly implemented usage for CO 2 continuous measurements is the nondispersal infrared spectroscopy (LICOR CO 2 detector; Schneider et al., 1992, 2006), which was a major breakthrough for unmaintained detection of CO 2 partial pressures from surface waters. ...
... Sample flow rate is measured using a 30 SLPM 20 (Standard Litres Per Minute) mass flow meter (M10MB01334CS3BV, MKS Instruments UK Ltd, UK), placed directly upstream of a 0.5 µm sintered particle filter (SS-4F-05, Swagelok, USA). The choice of sample cell pressure is a balance between two effects: higher pressures increase the absorption, thus improving the signal-to-noise ratio of the measurement, whilst pressure broadening of the spectral lines increases spectral overlap and line mixing (as discussed by Zahniser et al., 1995). Airborne operation is also compli- cated by the large variation in inlet pressures typically encountered over the course of a flight (down to ∼ 250 hPa at 10 km altitude). ...
... This instrument uses a mid-IR, thermoelectrically cooled, continuouswave, distributed feedback QCL (Alpes Laser, Switzerland) as a light source. The laser beam is directed through an astigmatic Herriott multipass absorption cell, providing an effective optical pathlength through the sample of 76 m(McManus et al., 1995), and Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ...
Spectroscopic measurements of atmospheric N2O and CH4 mole fractions were made on board the FAAM (Facility for Airborne Atmospheric Measurements) large Atmospheric Research Aircraft. We present details of the mid-IR Aerodyne Research Inc. Quantum Cascade Laser Absorption Spectrometer (QCLAS) employed, including its configuration for airborne sampling, and evaluate its performance over 17 flights conducted during summer 2014. Two different methods of correcting for the influence of water vapour on the spectroscopic retrievals are compared and evaluated. A new in-flight calibration procedure to account for the observed sensitivity of the instrument to ambient pressure changes is described, and its impact on instrument performance is assessed. Test flight data linking this sensitivity to changes in cabin pressure is presented. Total 1σ uncertainties of 1.81 ppb for CH4 and 0.35 ppb for N2O are derived. We report a mean difference in 1 Hz CH4 mole fraction of 2.05 ppb (1σ = 5.85 ppb) between in-flight measurements made using the QCLAS and simultaneous measurements using a previously characterised Los Gatos Research Fast Greenhouse Gas Analyser (FGGA). Finally, a potential case study for the estimation of a regional N2O flux using a mass balance technique is identified, and the method for calculating such an estimate is outlined.
... Therefore, they cover the IR fundamental bands with strong absorption for the most atmospheric trace gases [1]. They are used almost exclusively for spectroscopic applications [10][11][12][13][14]. Since IV-VI lasers and their associated detectors operate at cryogenic temperatures, they are more expensive Fig. 1. ...
... A fast laser-optical sensor is the key issue for trace gas flux measurements based on the so-called eddy correlation technique [12][13][14]. As high time resolution and high chemical resolution were the prerequisites for the success of the measurements, highfrequency modulation (FM) spectroscopy has been selected. ...
Laser-optical sensors are applied whenever sensitive, selective and fast in-situ gas analysis is required. This contribution illustrates the performance of selected near- and mid-infrared spectrometers based on tunable diode-lasers. Applications of lead-salt diode-lasers for NO2 and CH4 sensing, of antimonide lasers for CH4 and HCHO sensing in the 3-4 μm range and a near-infrared gas sensor for CO2 based on a room temperature 2 μm Indium-Phosphide laser will be presented and spectrometer performance will be discussed.
... The improvements in measurement precision and ease of use must be matched with a corresponding increase in calibration efforts to achieve comparable gains in compatibility. Many sensors rely on the accuracy of spectroscopic parameters (e.g., line strengths and their pressure and temperature dependencies) to derive in situ " spectroscopically calibrated " mixing ratios from raw spectra (Rothman et al., 2009; Zahniser et al., 1995). The use of these raw data is often appropriate, particularly if (1) a sensor is linear with respect to the range of observed concentrations, and (2) the quantity of interest is the relative enhancement of one chemical tracer versus another or versus background values measured on the same sensor. ...
... The CO 2 spectrum appears inverted because this particular air sample has less CO 2 than the calibration air flowing through the reference cell. Optical-based measurements are particularly sensitive to fluctuations in temperature and pressure (Zahniser et al., 1995) and careful controls must be implemented, particularly during flight where large dynamic ranges in both variables are observed (Fried et al., 2009 ). In-flight calibrations at regular intervals from gas cylinders are used to track sensor drift. ...
We present an evaluation of aircraft observations
of the carbon and greenhouse gases CO2, CH4, N2O, and
CO using a direct-absorption pulsed quantum cascade laser
spectrometer (QCLS) operated during the HIPPO and Cal-
Nex airborne experiments. The QCLS made continuous 1 Hz
measurements with 1� Allan precisions of 20, 0.5, 0.09, and
0.15 ppb for CO2, CH4, N2O, and CO, respectively, over
> 500 flight hours on 79 research flights. The QCLS measurements
are compared to two vacuum ultraviolet (VUV) CO
instruments (CalNex and HIPPO), a cavity ring-down spectrometer
(CRDS) measuring CO2 and CH4 (CalNex), two
broadband non-dispersive infrared (NDIR) spectrometers
measuring CO2 (HIPPO), two onboard gas chromatographs
measuring a variety of chemical species including CH4, N2O,
and CO (HIPPO), and various flask-based measurements of
all four species. QCLS measurements are tied to NOAA and
WMO standards using an in-flight calibration system, and
mean differences when compared to NOAA CCG flask data
over the 59 HIPPO research flights were 100, 1, 1, and 2 ppb
for CO2, CH4, N2O, and CO, respectively. The details of
the end-to-end calibration procedures and the data quality assurance
and quality control (QA/QC) are presented. Specifically,
we discuss our practices for the traceability of standards
given uncertainties in calibration cylinders, isotopic
and surface effects for the long-lived greenhouse gas tracers,
interpolation techniques for in-flight calibrations, and the effects
of instrument linearity on retrieved mole fractions.
... Pierwsze analizatory metanu mogące pracować z odpowiednio wysoką częstotliwością pomiarów (np. 10 Hz) wyprodukowała firma Aerodyne Research [224]. Wymagały one jednak chłodzenia ciekłym azotem, wobec czego były niezwykle mało wygodne w zastosowaniach środowiskowych [494,495]. Podobnie pierwsze dostępne analizatory firmy Campbell Scientific nie mogły pracować bez chłodzenia detektora ciekłym azotem, ze względu na jego wysoki szum termiczny. Do pierwszej dekady XXI wieku firma LI-COR Biosciences (USA) była jedynym komercyjnym dostawcą niskokosztowego urządzenia pomiarowego (model LI-7700) o wysokiej precyzji pomiaru, służącego do określania strumieni metanu [224,496]. ...
... Detecting and quantifying this gas to measure microbial activity is well established (Johnson et al., 2014;Li et al., 2020;Metcalfe, Alahmari, Smith, & Hippler, 2019). TDLAS uses frequency modulation spectroscopy which is capable of detecting in situ specific gas concentration reliably (Philippe & Hanson, 1993;Zahniser, Nelson, McManus, & Kebabian, 1995). TDLAS has been applied to detect changes in headspace gas content in response to microbial growth of media-filled vials Brueckner, Roesti, et al., 2017;Danilović et al., 2018). ...
The sensitivity and reliability of Tunable Diode Laser Absorption Spectroscopy (TDLAS) for monitoring the growth of microorganisms in real food matrices was explored. The rapid and non-destructive TDLAS equipment uses laser light to monitor carbon dioxide changes due to microbial growth in the container head space which has the additional advantage of detecting contamination in products and packaging where visual inspection is not possible. TDLAS growth detection of Bacillus fengqiuensis, Candida albicans, Lactococcus lactis, Microbacterium luteolum, Paenibacillus chitinolyticus and Staphylococcus pasteuri was studied in various Ready To Feed (RTF) infant formula products. Detection of growth was correlated with cell numbers and the reliability of detection was tested using multiple inoculum levels. Overall, detection varies based on the matrices and the characteristics of the test organisms. TDLAS was capable of detecting growth of L. lactis within 20 h and S. pasteuri in 55 h when foods were contaminated with as low as ∼100 CFU/ml. However, the spore former B. fengqiuensis was not detected after 72 h in three matrices when inoculated at low levels. The lowest cell density detected at 4.47 CFU/ml was for the yeast (C. albicans) after 28.99 ± 1.82 h and the highest at 8.53 CFU/mL was for the actinomycete (M. luteolum) at 37.02 ± 1.84 h in RTF infant formula matrices. A strong linear relationship (R² value ≥ 0.827) between initial inoculum and Time To Detection (TTD) for multiple inoculum levels was observed and growth rates (μ) calculated from TTD data was comparable with viable plate count methods for P. chitinolyticus but showed higher variability for L. lactis. Therefore, the TDLAS equipment was shown to be reliable with some specific limitations in identifying microbiological contamination by typical spoilage microbes in commercially sterile dairy beverage products.
... CH 4 is an important GHG and a crucial part of the carbon balance of many (especially wetland) ecosystems. While the earliest landscape-scale CH 4 flux measurements date back to the 1990s (Verma et al., 1992;Zahniser et al., 1995;Suyker et al., 1996), considerable advances in laser absorption spectroscopy (LAS) within the last 10 years have led to a wide application of fast LAS-based sensors as part of eddy covariance (EC) setups. Intercomparisons of the available sensors are given by Tuzson et al. (2010), Detto et al. (2011) and Peltola et al. (2013, 2014. ...
With respect to their role in the global carbon cycle, natural peatlands are characterized by their ability to sequester atmospheric carbon. This trait is strongly connected to the water regime of these ecosystems. Large parts of the soil profile in natural peatlands are water saturated, leading to anoxic conditions and to a diminished decomposition of plant litter. In functioning peatlands, the rate of carbon fixation by plant photosynthesis is larger than the decomposition rate of dead organic material. Over time, the amount of carbon that remains in the soil and is not converted back to carbon dioxide grows. Land use of peatlands often goes along with water level manipulations and thereby with alterations of carbon flux dynamics. In this study, carbon dioxide (CO2) and methane (CH4) flux measurements from a bog site in northwestern Germany that has been heavily degraded by peat mining are presented. Two contrasting types of management have been implemented at the site: (1) drainage during ongoing peat harvesting on one half of the central bog area and (2) rewetting on the other half that had been taken out of use shortly before measurements commenced. The presented 2-year data set was collected with an eddy covariance (EC) system set up on a central railroad dam that divides the two halves of the (former) peat harvesting area. We used footprint analysis to split the obtained CO2 and CH4 flux time series into data characterizing the gas exchange dynamics of both contrasting land use types individually. The time series gaps resulting from data division were filled using the response of artificial neural networks (ANNs) to environmental variables, footprint variability, and fuzzy transformations of seasonal and diurnal cyclicity. We used the gap-filled gas flux time series from 2 consecutive years to evaluate the impact of rewetting on the annual vertical carbon balances of the cutover bog. Rewetting had a considerable effect on the annual carbon fluxes and led to increased CH4 and decreased CO2 release. The larger relative difference between cumulative CO2 fluxes from the rewetted (13±6 mol m−2 a−1) and drained (22±7 mol m−2 a−1) section occurred in the second observed year when rewetting apparently reduced CO2 emissions by 40 %. The absolute difference in annual CH4 flux sums was more similar between both years, while the relative difference of CH4 release between the rewetted (0.83±0.15 mol m−2 a−1) and drained (0.45±0.11 mol m−2 a−1) section was larger in the first observed year, indicating a maximum increase in annual CH4 release of 84 % caused by rewetting at this particular site during the study period.
... Cavity ring-down spectroscopy (CRDS [18]) and off-axis integrated-cavity output spectroscopy (OA-ICOS [19]) were developed and commercialized to measure CO 2 /H 2 O fluxes with closed-path. Zahniser et al. firstly introduced a field deployment for eddy covariance employing a multi-pass absorption cell based on tunable diode laser spectroscopy (TDLAS) [20]. The available gas analyzer technology limits the measurements to areas of relatively complicated and ever-changing air-sea fluxes in marine boundary layers [21]. ...
We report here the development of a compact, open-path CO2 and H2O sensor based on the newly introduced scanned-wavelength modulation spectroscopy with the first harmonic phase angle (scanned-WMS-θ1f) method for high-sensitivity, high temporal resolution, ground-based measurements. The considerable advantage of the sensor, compared with existing commercial ones, lies in its fast response of 500 Hz that makes this instrument ideal for resolving details of high-frequency turbulent motion in exceptionally dynamic coastal regions. The good agreement with a commercial nondispersive infrared analyzer supports the utility and accuracy of the sensor. Allan variance analysis shows that the concentration measurement sensitivities can reach 62 ppb CO2 in 0.06 s and 0.89 ppm H2O vapor in 0.26 s averaging time. Autonomous field operation for 15-day continuous measurements of greenhouse gases (CO2/H2O) was performed on a shore-based monitoring tower in Daya Bay, demonstrating the sensor’s long-term performance. The capability for high-quality fast turbulent atmospheric gas observations allow the potential for better characterization of oceanographic processes.
... 20 CH 4 is an important GHG and a crucial part of the carbon balance of many (especially wetland) ecosystems. While the earliest landscape scale CH 4 flux measurements date back to the 1990s (Verma et al., 1992;Zahniser et al., 1995;Suyker et al., 1996), considerable advances in laser absorption spectroscopy (LAS) within the last ten years have led to a wide application of fast LAS-based sensors as part of eddy covariance (EC) setups. Intercomparisons of the available sensors are given by Tuzson et al. (2010), Detto et al. (2011), Peltola et al. (2013 and Peltola et al. (2014). ...
Abstract. With respect to their role in the global carbon cycle, natural peatlands are characterized by their ability to sequester atmospheric carbon. This trait is strongly connected to the water regime of these ecosystems. Large parts of the soil profile in natural peatlands are water-saturated, leading to anoxic conditions and to a diminished decomposition of plant litter. In functioning peatlands, the rate of carbon fixation by plant photosynthesis is larger than the decomposition rate of dead organic material. Over time, the amount of carbon that remains in the soil and is not converted back to carbon dioxide grows. Land use of peatlands often goes along with water level manipulations and thereby with alterations of carbon flux dynamics. In this study, carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) flux measurements from a bog site in NW Germany that has been heavily degraded by peat mining are presented. Two contrasting types of management have been implemented at the site: (1) drainage during ongoing peat-harvesting on one half of the central bog area and (2) rewetting on the other half that had been taken out of use shortly before measurements commenced. The presented two-year data set was collected with an eddy covariance (EC) system set up on a central railroad dam that divides the two halves of the (former) peat harvesting area. We used footprint analysis to split the obtained CO<sub>2</sub> and CH<sub>4</sub> flux time series into data characterizing the gas exchange dynamics of both contrasting land use types individually. The time series gaps resulting from data division were filled using the response of artificial neural networks (ANNs) to environmental variables, footprint variability and fuzzy transformations of seasonal and diurnal cyclicity. We used the gap-filled gas flux time series from two consecutive years to evaluate the impact of rewetting on the annual vertical carbon balances of the cutover bog. Rewetting had a considerable effect on the annual carbon fluxes and led to increased CH<sub>4</sub> and decreased CO<sub>2</sub> release.
The larger relative difference between cumulative CO<sub>2</sub> fluxes from the rewetted (22 ± 7 mol m<sup>−2</sup> a<sup>−1</sup>) and drained (13 ± 6 mol m<sup>−2</sup> a<sup>−1</sup>) section occurred in the second observed year when rewetting apparently reduced CO<sub>2</sub> emissions by 40 %. The absolute difference in annual CH<sub>4</sub> flux sums was more similar between both years while the relative difference of CH<sub>4</sub> release between the rewetted (0.83 ± 0.15 mol m<sup>−2</sup> a<sup>−1</sup>) and drained (0.45 ± 0.11 mol m<sup>−2</sup> a<sup>−1</sup>) section was larger in the first observed year indicating a maximum increase of annual CH<sub>4</sub> release of 84 % caused by rewetting; at this particular site during the study period.
... Kaimal and Gaynor, 1991), and by chemical analysers that are able to sense an increasing variety of scalar concentrations at fast rates, such as Tunable Diode Laser absorption spectrometers (TDL) in the case of N 2 O (see e.g. Zahniser et al., 1995;Fowler et al., 1995). A fast response ultrasonic anemometer (model USA-1, METEK GmbH, Elmshorn, Germany) was used to measure the three components of the wind at a frequency of 10 Hz. ...
Managed grasslands are known to be an important source of N<sub>2</sub>O with estimated global losses of 2.5 Tg N<sub>2</sub>O-N yr<sup>−1</sup>. Chambers are to date the most widely used method to measure N<sub>2</sub>O fluxes, but also micrometeorological methods are successfully applied. In this paper we present a comparison of N<sub>2</sub>O fluxes measured by non-steady state chambers and eddy covariance (EC) (using an ultra-sonic anemometer coupled with a tunable diode laser) from an intensively grazed and fertilised grassland site in South East Scotland. The measurements were taken after fertilisation events in 2003, 2007 and 2008. In four out of six comparison periods, a short-lived increase of N<sub>2</sub>O emissions was observed after mineral N application, returning to background level within 2–6 days. Highest fluxes were measured by both methods in July 2007 with maximum values of 1438 ng N<sub>2</sub>O-N m<sup>−2</sup> s<sup>−1</sup> (EC) and 651 ng N<sub>2</sub>O-N m<sup>−2</sup> s<sup>−1</sup> (chamber method). Negative fluxes above the detection limit were observed in all comparison periods by EC, while with chambers, the recorded negative fluxes were always below detection limit. Median and average fluxes over each period were always positive. Over all 6 comparison periods, 69% of N<sub>2</sub>O fluxes measured by EC at the time of chamber closure were within the range of the chamber measurements. N<sub>2</sub>O fluxes measured by EC during the time of chamber closure were not consistently smaller, neither larger, compared to those measured by chambers: this reflects the fact that the different techniques integrate fluxes over different spatial and temporal scales. Large fluxes measured by chambers may be representing local hotspots providing a small contribution to the flux measured by the EC method which integrates over a larger area. The spatial variability from chamber measurements was high, as shown by a coefficient of variation of up to 139%. No diurnal pattern of N<sub>2</sub>O fluxes was observed, possibly due to the small diurnal variations of soil temperature. The calculation of cumulative fluxes using different integration methods showed EC data provide generally lower estimates of N<sub>2</sub>O emissions than chambers.
... The choice of sample cell pressure is a balance between two effects: higher pressures increase the absorption, thus improving the signal-to-noise ratio of the measurement, whilst pressure broadening of the spectral lines increases spectral overlap and line mixing (as discussed by Zahniser et al., 1995). Airborne operation is also complicated by the large variation in inlet pressures typically encountered over the course of a flight (down to ∼ 250 hPa at 10 km altitude). ...
Spectroscopic measurements of atmospheric N2O and CH4 mole fractions were made on board the FAAM (Facility for Airborne Atmospheric Measurements) large atmospheric research aircraft. We present details of the mid-infrared quantum cascade laser absorption spectrometer (QCLAS, Aerodyne Research Inc., USA) employed, including its configuration for airborne sampling, and evaluate its performance over 17 flights conducted during summer 2014. Two different methods of correcting for the influence of water vapour on the spectroscopic retrievals are compared and evaluated. A new in-flight calibration procedure to account for the observed sensitivity of the instrument to ambient pressure changes is described, and its impact on instrument performance is assessed. Test flight data linking this sensitivity to changes in cabin pressure are presented. Total 1σ uncertainties of 2.47 ppb for CH4 and 0.54 ppb for N2O are derived. We report a mean difference in 1 Hz CH4 mole fraction of 2.05 ppb (1σ = 5.85 ppb) between in-flight measurements made using the QCLAS and simultaneous measurements using a previously characterised Fast Greenhouse Gas Analyser (FGGA, Los Gatos Research, USA). Finally, a potential case study for the estimation of a regional N2O flux using a mass balance technique is identified, and the method for calculating such an estimate is outlined.
... Taking into account the achieved signal-to-noise ratio in the spectra and all other sources of errors due to the retrieval process, mixing ratios are determined within 5%-10% pre-cision error. Indeed, the achieved signal-to-noise ratio in the absorption spectra makes it possible to increase the temporal resolution from 1 s to 10 ms and with a fairly constant accuracy in the concentration retrieval; it could be of interest to address new topics in dynamics of the atmosphere (turbulence or measurement of fluxes; see Zahniser et al. 1995). ...
The Spectromètre à Diodes Laser Accordables (SDLA), a balloonborne near-infrared diode laser spectrometer, was developed to provide simultaneous in situ measurements of methane and water vapor in the troposphere and the lower stratosphere. The instrument was flown several times from stratospheric balloons operated by the Centre National d'Etudes Spatiales within the framework of the Third European Stratospheric Experiment on Ozone in 1998-2000. The SDLA is based on a multipass optical cell open to the atmosphere. Two near-infrared telecommunication-type laser diodes are connected with optical fibers to the cell to take in situ absorption spectra of methane (in the 6047 cm-1 spectra region) and water vapor (in the 7181 cm-1 spectra region) at 1-s intervals. Mixing ratios are obtained, with a precision error of within 5%-10%, from a nonlinear fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements. The SDLA is described, and achieved atmospheric methane and water vapor vertical concentration profiles are reported.
... Prospective solutions for mobile robot applications are laser spectrometers. As an example, a fast-response, high precision, tunable diode-laser spectrometer developed for field measurements of methane and nitrous oxide fluxes can be mentioned [64]. The instrument uses a multiple-pass absorption cell to provide a long absorption path length (36 m). ...
This paper presents the concept, structural design and implementation of components of a multifunctional sensory network, consisting of a Mobile Robotic Platform (MRP) and stationary multifunctional sensors, which are wirelessly communicating with the MRP. Each section provides the review of the principles of operation and the network components' practical implementation. The analysis is focused on the structure of the robotic platform, sensory network and electronics and on the methods of the environment monitoring and data processing algorithms that provide maximal reliability, flexibility and stable operability of the system. The main aim of this project is the development of the Robotic Nurse (RN)-a 24/7 robotic helper for the hospital nurse personnel. To support long-lasting autonomic operation of the platform, all mechanical, electronic and photonic components were designed to provide minimal weight, size and power consumption, while still providing high operational efficiency, accuracy of measurements and adequateness of the sensor response. The stationary sensors serve as the remote "eyes, ears and noses" of the main MRP. After data acquisition, processing and analysing, the robot activates the mobile platform or specific sensors and cameras. The cross-use of data received from sensors of different types provides high reliability of the system. The key RN capabilities are simultaneous monitoring of physical conditions of a large number of patients and alarming in case of an emergency. The robotic platform Nav-2 exploits innovative principles of any-direction motion with omni-wheels, navigation and environment analysis. It includes an innovative mini-laser, the absorption spectrum analyser and a portable, extremely high signal-to-noise ratio spectrometer with two-dimensional detector array.
... Apart from concentration, it is also possible to determine the temperature, pressure, velocity, and mass flux of the gas under observation. 7,8 Just as its name implies, this technique uses tunable diode laser sources, which offer small size, lower cost, lower power requirements, and easier tuning, as well as fast response, thus making them useful for spectroscopic applications, such as remote sensing of environmental gases and pollutants in the atmosphere, homeland security, 9 industrial exhaust emissions for process control, 10 and medical diagnostics such as breath analyzers, 11 as well as plasma chemistry. 12 Compared with classic chemical and chromatographic measurement techniques, TDLAS shows high sensitivity and selectivity, rapid responsibility, nondestructive, environmentally friendly analysis (i.e., without use of chemicals and without releasing harmful by-products into the environment), multivariate, and remote sensing. ...
This paper presents a novel methodology-based discrete wavelet transform (DWT) and the choice of the optimal wavelet pairs to adaptively process tunable diode laser absorption spectroscopy (TDLAS) spectra for quantitative analysis, such as molecular spectroscopy and trace gas detection.
The proposed methodology aims to construct an optimal calibration model for a TDLAS spectrum, regardless of its background structural characteristics, thus facilitating the application of TDLAS as a powerful tool for analytical chemistry. The performance of the proposed method is verified
using analysis of both synthetic and observed signals, characterized with different noise levels and baseline drift. In terms of fitting precision and signal-to-noise ratio, both have been improved significantly using the proposed method.
... Since the 1980s, infrared absorption spectrometry has become popular for measurements of trace gases in the lab. The development of various field techniques has revealed serious problems with drift and low sensitivity (Zahniser et al. 1995). Today, the commonly implemented usage for CO 2 continuous measurements is the nondispersal infrared spectroscopy (LICOR CO 2 detector; Schneider et al. 1992Schneider et al. , 2006, which was a major breakthrough for unmaintained detection of CO 2 partial pressures from surface waters. ...
A new system is presented that allows the continuous measurement of methane and carbon dioxide concentrations in surface waters autonomously using ships of opportunity. The analytical setup consists of a methane carbon dioxide-Analyzer (MCA, Los Gatos Research) joined to an established equilibrator setup. The analyzer uses off-axis integrated cavity output spectroscopy (ICOS) and combines a highly specific infrared band laser with a set of strongly reflective mirrors to obtain an effective laser path length of several kilometers. This allows detecting methane and carbon dioxide in the equilibrated gas phase with high precision (less than 0.1%) and frequency. The system was installed on the cargo ship Finnmaid (Finnpartner) in November 2009, which commutes regularly in the Baltic Sea between Travemunde (Germany), Gdynia (Poland), and Helsinki (Finland). Methane concentrations of the equilibrated gas phase measured by gas chromatography and by the MCA during lab tests are in excellent agreement. The comparison of carbon dioxide data measured by the MCA system to CO(2) values gathered from the same type of equilibrator in combination with a LICOR CO(2) detector (Schneider et al. 1992) during onboard operation show concordant results. The time constant for the system in freshwater at room temperature was determined to be 676 s for CH(4) and 226 s for CO(2). Additional performance tests are presented. First field results show large regional differences with remarkable features, especially in shallow regions, demonstrating the need for the high spatiotemporal data coverage provided by the instrument.
... Wider usage of stable isotopes in ecosystem exchange studies has been impeded by the complication that existing instrumentation using isotope ratio mass spectrometry (IRMS)the standard method for determining trace gas stable isotope ratios for many applications other than measuring eddy covariance fluxes -requires samples to be collected in flasks and returned to the laboratory for analysis, a tedious method for measurements that we would like to make continuously. Although recent advances in automated sample collection systems (Flanagan, et al. 1997;Bowling, et al. 1999;Bowling, et al. 2001;Mortazavi and Chanton 2002;Lai, et al. 2003;Torn, et al. 2003) make IRMS-based applications less cumbersome, tunable infrared laser differential absorption (TILDAS), a broad class of instrumentation which includes tunable diode laser absorption spectroscopy, TDLAS, which has been applied to eddy covariance flux measurements of such trace gases as CH 4 , N 2 O, and NO 2 (Zahniser et al., 1995;Fowler et al., 1995;Horii et al., 1999;Werle et al., 2001;Kormann et al., 2001), promises simple in situ isotope measurements at high-frequency that IRMS will never match for eddy covariance applications. ...
... The instruments use a multiple-pass absorption cell with astigmatic mirrors to provide a long absorption path length (36 m). As an example, the precision for methane and nitrous oxide detection is 0.1% of ambient levels with a 1 s averaging time [52]. However, spectrometers are too large for mobile sensing, and are very susceptible to mechanical vibrations. ...
... Modern atmospheric research on gas exchange between the biosphere and the atmosphere requires sensitive, reliable and fast-response chemical sensors. Therefore, techniques for fast and simultaneously sensitive trace gas measurements based on tunable diode-laser absorption spectroscopy have been successful applied to micrometeorological trace gas flux measurement techniques as the eddy covariance technique (Zahniser et al. 1995). The availability of such sensors allows for example a validation of closed chamber measurements and also can provide information about CH 4 emissions on a larger scale, which is the basis for any up-scaling effort from a regional to a global scale. ...
Optical sensors based on semiconductor lasers are at the threshold of routine applications in gas analysis and increasingly these sensors are used for industrial and environmental monitoring applications whenever sensitive, selective and fast in-situ analysis in the near- and mid-infrared spectral region is required. With the increasing complexity of processes, online gas analysis is becoming an issue in automated control of various industrial applications such as combustion and plasma diagnostics, investigations of engines and automobile exhaust measurements. Other challenges are online analysis of high purity process gases, medical diagnostics and monitoring of agricultural and industrial emissions (VDI 2002). The need to meet increasingly stringent environmental and legislative requirements has led to the development of analyzers to measure concentrations of a variety of gases based on near- and mid-infrared absorption spectroscopy.
... Although several other open path techniques exist [7][8][9][10][11][12][13][14], LAnTeRN is a different implementation which simultaneously transmits and receives both the on and the offline wavelengths. This key difference makes many of the noise sources including scintillation common to both wavelengths and since this is a ratiometric measurement they cancel in the ratio. ...
In 2012, we developed a proof-of-concept system for a new open-path laser absorption spectrometer concept for measuring atmospheric CO2. The measurement approach utilizes high-reliability all-fiber-based, continuous-wave laser technology, along with a unique all-digital lock-in amplifier method that, together, enables simultaneous transmission and reception of multiple fixed wavelengths of light. This new technique, which utilizes very little transmitted energy relative to conventional lidar systems, provides high signal-to-noise (SNR) measurements, even in the presence of a large background signal. This proof-of-concept system, tested in both a laboratory environment and a limited number of field experiments over path lengths of 680 m and 1,600 m, demonstrated SNR values >1,000 for received signals of similar to 18 picoWatts averaged over 60 s. A SNR of 1,000 is equivalent to a measurement precision of +/- 0.001 or similar to 0.4 ppmv. The measurement method is expected to provide new capability for automated monitoring of greenhouse gas at fixed sites, such as carbon sequestration facilities, volcanoes, the short-and long-term assessment of urban plumes, and other similar applications. In addition, this concept enables active measurements of column amounts from a geosynchronous orbit for a network of ground-based receivers/stations that would complement other current and planned space-based measurement capabilities.
... The TDLAS method, which determines absolute concentrations based on tabulated spectroscopic parameters, is preferable for long term monitoring at the Harvard Forest site. TDLAS instruments are also routinely employed for measurement of eddy covariance fluxes 17 . ...
A dual tunable diode laser absorption spectrometer (TDLAS) for continuous field measurement of nitric acid and nitrogen dioxide eddy covariance fluxes is described and preliminary field results are presented. The dual TDLAS simultaneously measures nitric acid (HNO3) and nitrogen dioxide (NO2) by direct absorption spectroscopy over a long path enclosed in an astigmatic Herriott multipass cell. The technique provides sufficient precision and time response (200 ppt RMS in 1 second) needed to record ambient variations and deposition rates by the eddy-covariance method. Real-time fitting of the integrated spectra over multiple absorption features makes the system appropriate for continuous field measurements while retaining the highly selective quality of direct absorption measurements and minimizing potential interferences. This method also produces an absolute, spectroscopic determination of concentration within the multipass cell, eliminating the need for calibrated gas mixtures in the field. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints
... This is particularly true for airborne measurements in the upper troposphere/lower stratosphere. High resolution IR spectroscopy systems based on Pb-salt tunable diode lasers [17][18][19][20], LN 2 -cooled CW QC lasers [6,9,21], and TE-cooled pulsed QC lasers [8][9][10][11] have been developed during the last years for rapid and sensitive detection of CH 4 , N 2 O and CO with sufficient time response for eddy covariance flux measurements [22]. ...
We present an overview of the dual QC laser spectrometer developed at Aerodyne Research and various examples of its application for atmospheric trace gas detection. The instrument incorporates two pulsed QC lasers, a compact 76-m (or 56-m) multipass absorption cell, a dual HgCdTe detector, and a sophisticated signal generation, data acquisition and processing system. Recent findings and hardware innovations are highlighted. Our results show that the precision and minimal detectable absorbance obtainable with pulsed QC lasers are comparable to those achieved with cryogenically cooled CW Pb-salt lasers in spite of the broader laser linewidths inherent to pulsed operation. This is demonstrated through in situ measurements of several trace gases, including methane, nitrous oxide, carbon monoxide, formaldehyde, formic acid, nitrous acid and ethylene. Recent measurements of HCHO and HCOOH on board a NOAA aircraft are presented. The precision, stability and intrinsic accuracy of the instrument were assessed through inter-comparisons measuring CH4 and CO. These measurements were made either comparing two QC lasers sweeping over different transitions or comparing the dual QCL spectrometer and a standard instrument (NDIR CO). The absorbance precision achieved is typically 2x10-5 Hz-1/2. For long-lived species, such as CH4 and N2O, this implies 1-Hz fractional precisions of 0.1% or better, which fulfill the requirement for meaningful measurements from aircraft platforms. Spectroscopically derived mixing ratios are accurate within 5% or better. The spectrometer is equipped to perform automatic, periodic calibrations with zero and span gases whenever higher accuracy is required.
... Since the 1980s, infrared absorption spectrometry has become popular for measurements of trace gases in the lab. The development of various field techniques has revealed serious problems with drift and low sensitivity (Zahniser et al. 1995). Today, the commonly implemented usage for CO 2 continuous measurements is the nondispersal infrared spectroscopy (LICOR CO 2 detector; Schneider et al. 1992Schneider et al. , 2006, which was a major breakthrough for unmaintained detection of CO 2 partial pressures from surface waters. ...
Though systems to assess the sea surface concentrations of climate-relevant trace gases were first designed three decades ago, only for carbon dioxide the technology has advanced far enough to allow quasi non-maintained data acquisition based on ships of opportunity. One of the reasons for this is the fact that until now, only for carbon dioxide the concentrations in surface waters are high enough to allow the use of non gas-consuming, IR-spectroscopy-based detection of the gas, usually provided by a LICOR gas detector. This causes problems to estimate the marine fluxes of other important trace gases such as methane or nitrous oxide, which are usually strongly bound to coastal and estuarine zones, and thus would require long-term, spatio-temporal data acquisition for a robust marine source assessment. Here, we present a new system which allows to measure methane and carbon dioxide in surface waters autonomously and continuously using a non-gas consuming optical detection system. The analytical setup consists of a CH4/CO2- Analyzer (MCA; Los Gatos Research) joint with a bubble-type equilibration system. The analyzer uses off-axis integrated cavity output spectroscopy (ICOS) which combines two highly specific band lasers with a set of strongly reflective mirrors to obtain an effective laser path length of several kilometers. While a first system was installed in November 2009 on the cargo ship Finnmaid (Finnpartner) that commutes regularly between Travemünde (Germany) and Helsinki (Finnland) in the Baltic Sea, a 2nd system was build to be used on board of research vessels and successfully monitored the gas concentrations along the ship track during a 3,5 week long research cruise of RV Maria S. Merian (MSM16/1) in the Baltic Sea in August, 2010. Very low post-bloom surface pCO2 values and distinct patterns of surface methane concentrations pointing to local sources were amongst the results of the surface survey. During the expedition, the system was also linked to the outlet of a pump-CTD system which was lowered into the euxinic, methane-enriched part of the water column, revealing a distinct effect of H2S concentrations on the water vapor band, which needs to be assessed before the system is used for this kind of investigations.
... It does not require any specific calibration and is insensitive to pressure and temperature fluctuations in the air sampling stream; however, it requires accurate knowledge of the reference gas concentration (Pattey et al., 2006a). Other custom-made TDL based trace gas analyzers are available (e.g., TDL-36, Aerodyne Research, Billerica, MA, USA), and have been used by several research teams in Europe and USA (Weinhold et al., 1994;Zahniser et al., 1995;Laville et al., 1999). The TGA100 has been used extensively to measure trace gas concentration gradients in order to determine the flux using the flux-gradient technique (Edwards et al., 1994(Edwards et al., , 2001(Edwards et al., , 2003Wagner-Riddle et al., 1996a,b;Wagner-Riddle and Thurtell, 1998;Simpson et al., 1995Simpson et al., , 1997Pattey, 1999, 2003). ...
... The lower sensitivity limit of the system has been checked until now by determining the minimum detectable absorption of the ammonia spectral lines used for the wavelength calibration. Recent results shows that sweep integration can provide in most cases sensitivity levels comparable with those achieved with wavelength modulation techniques (Webster 1988;Brassington 1994;Zahniser et al. 1995;Werle 1996), with the further advantage of a simpler electronic equipment and the direct evaluation of the absorption level. In our case, the acquisition electronics introduces an overall noise of about 1 ϫ 10 Ϫ7 with respect to the overall signal amplitude, when averaging over 64 000 spectra in the conditions described above. ...
A new instrument realized for measuring the HNO 3 concentration in air is described. The device is a midinfrared absorption spectrometer based on a tunable diode laser and a multipass absorption cell. The instrument is specifically designed for airborne operation on board the M-55 Geophysica, in the frame of the Airborne Polar Experiment project, taking into account all the related environmental and operational constraints. The device is part of a complete measurement package for the measurement of the chemical content of the polar stratospheric clouds and other atmospheric aerosols. Furthermore, it can be used as a stand-alone detector of molecular trace gases. The primary purpose of the instrument is to perform in situ diagnostic measurements in the upper troposphere–lower stratosphere. Design criteria include a new optical setup, one much less sensitive to the vibration and thermal stresses with respect to the conventional diode laser spectrometers. Furthermore, the authors developed a novel detection scheme for quicker acquisition and better signal-to-noise ratio. This paper reports calibration and testing measurements, including a detection lower limit both for the HNO 3 and ammonia. This last gas is used as a wavelength and absorption reference.
... A flow restrictor at the outlet of the Tedlar bag guaranteed the inflation of the bag. The N20 concentration was measured off-line by tunable diode laser spectroscopy (Aerodyne, TDLAS 004) [Zahniser e! al., 1995] ...
Concentration profiles of N2O in a grassland soil and dynamic response curves to disturbance of the soil concentration (relaxation curves) were measured with a new membrane tube technique. Diffusive properties of the soil were derived from 222Rn measurements. The mathematical analysis of the relaxation curves yielded N2O uptake rates U, soil diffusivities Ds, scale lengths z*, and production rates P at different levels under the surface. The following ranges were found during 2 days of measurements: Ds=(0.4-5)×10-7m2s-1, U=(1-20)×10-4s-1, z*=0.7-2.8cm, and P=0.02-4.4ppbs-1. These values were used to reproduce the measured N2O concentration profiles with a one-dimensional diffusive transport model of N2O in the soil air-filled pore space and to deduce flux profiles. Bidirectional fluxes occurred with small deposition fluxes up to a few ppt ms-1 during intensive growing phases of the grass. Uptake rates were high enough that N2O produced at greater depth did not reach the atmosphere.
... Fan et al. (1992) used a Zeeman split He}Ne laser as light source for the methane detection in a closed multipass cell arrangement. However, most authors have used closed single or multipass optical arrangements for mid-infrared lead salt diode laser light absorption, e.g., Verma et al. (1992), Shurpali et al. (1993), Edwards et al. (1994), Suyker et al. (1996), Zahniser et al. (1995 or Hargreaves and Fowler (1998). The latter two use a direct absorption detection scheme, whereas the other instruments use a so-called wavelength-modulation (lock-in technique at the kilohertz-range) detection scheme to improve the instruments detection limit. ...
Eddy covariance measurements of methane were carried out over the fen “Murnauer Moos” in the south of Germany in order to evaluate the performance of a newly developed eddy covariance measurement system, based on a frequency-modulated tunable diode laser spectrometer as a fast chemical sensor. During a six-day period, an average daytime methane emission of (5.4±1.8)mgCH4m−2h−1 was measured. We find this value moderate, considering the favorable meteorological and soil conditions for methane emission. Diurnal cycles of the fluxes of methane and carbon dioxide as well as of sensible and latent heat are presented. Results are discussed in terms of relevant micrometeorological quantities, and quality control procedures based on Allan variance and spectral analysis are discussed.
Ammonia (NH3) from animal feeding operations (AFOs) is an important source of reactive nitrogen in the US, but despite its ramifications for air quality and ecosystem health, its near‐source evolution remains understudied. To this end, Phase I of the Transport and Transformation of Ammonia (TRANS²Am) field campaign was conducted in the northeastern Colorado Front Range in summer 2021 and characterized atmospheric composition downwind of AFOs during 10 research flights. Airborne measurements of NH3, nitric acid (HNO3), and a suite of water‐soluble aerosol species collected onboard the University of Wyoming King Air research aircraft present an opportunity to investigate the sensitivity of particulate matter (PM) formation to AFO emissions. We couple the observations with thermodynamic modeling to predict the seasonality of ammonium nitrate (NH4NO3) formation. We find that during TRANS²Am northeastern Colorado is consistently in the NH3‐rich and HNO3‐limited NH4NO3 formation regime. Further investigation using the Extended Aerosol Inorganics Model reveals that summertime temperatures (mean: 23°C) of northeastern Colorado, especially near the surface, inhibit NH4NO3 formation despite high NH3 concentrations (max: ≤114 ppbv). Finally, we model spring/autumn and winter conditions to explore the seasonality of NH4NO3 formation and find that cooler temperatures could support substantially more NH4NO3 formation. Whereas NH4NO3 only exceeds 1 μg m⁻³ ∼10% of the time in summer, modeled NH4NO3 would exceed 1 μg m⁻³ 61% (88%) of the time in spring/autumn (winter), with a 10°C (20°C) temperature decrease relative to the campaign.
Phase One of the Transportation and Transformation of Ammonia (TRANS²Am) field campaign took place in northeastern Colorado during the summer of 2021. One of the goals of TRANS²Am was to measure ammonia (NH3) emissions from cattle feedlots and dairies. Most of these animal husbandry facilities are co‐located within oil and gas development, an important source of methane (CH4) and ethane (C2H6) in the region. Phase One of TRANS²Am included 12 near‐source research flights. We present estimates of NH3 emissions ratios with respect to CH4 (NH3 EmR), with and without correction of CH4 from oil and gas, for 29 feedlots and dairies in the region. The data shows larger emissions ratios than previously reported in the literature with a large range of values (i.e., 0.1–2.6 ppbv ppbv⁻¹). Facilities housing cattle and dairy had a mean (std) of 1.20 (0.63) and 0.29 (0.08) ppbv ppbv⁻¹, respectively. We also found that only 15% of the total ammonia (NHx) is in the particle phase (i.e., NH4+ ${\text{NH}}_{4}^{+}$) near major sources during the warm summertime months. We examined the evolution of NH3 in one plume that was sampled at different distances and altitudes up to 25 km downwind and estimated the NH3 lifetime against deposition and partitioning to the particle phase to be 87–120 min. Finally, we calculated estimates of NH3 emission rates from four optimally sampled facilities. These ranged from 4 to 29 g NH3 · h⁻¹ · hd⁻¹.
The largest inorganic, gas phase reservoir of chlorine atoms in the atmosphere is hydrogen chloride (HCl), but the challenges in quantitative sampling of this compound cause difficulties for obtaining high-quality, high-frequency measurements. In this work, tunable infrared laser direct absorption spectroscopy (TILDAS) was demonstrated to be a superior optical method for sensitive, in situ detection of HCl at the 2925.89645 cm-1 absorption line using a 3 𝜇m interband cascade laser. The instrument has an effective path length of 204 m, 1 Hz precision of 7–8 pptv, and 3𝜎 limit of detection ranging from 21–24 pptv. For longer averaging times, the highest precision obtained was 0.5 pptv and 3𝜎 limit of detection of 1.6 pptv at 2.4 minutes. HCl TILDAS was also shown to have high accuracy when compared with a certified gas cylinder, yielding a linear slope within the expected 5 % tolerance of the reported cylinder concentration (slope = 0.964 ± 0.008). The use of heated inlet lines and active chemical passivation greatly improve the instrument response times to changes in HCl mixing ratios, with minimum 90 % response times ranging from 1.2–4.4 s, depending on inlet flow rate. However, these response times lengthened at relative humidities > 50 %, conditions under which HCl concentration standards were found to elicit a significantly lower response (-5.8 %). The addition of high concentrations of gas phase nitric acid (> 4.0 ppbv) were found to increase HCl signal (< 10 %), likely due to acid displacement with HCl or particulate chloride adsorbed to inlet surfaces. The equilibrium model ISORROPIA suggested a potential of particulate chloride partitioning into HCl gas within the heated inlet system if allowed to thermally equilibrate, but field results did not demonstrate a clear relationship between particulate chloride and HCl signal obtained with a denuder installed on the inlet.
Reactive nitrogen (Nr) within smoke plumes plays important roles in the production of ozone, the formation of secondary aerosols, and deposition of fixed N to ecosystems. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) field campaign sampled smoke from 23 wildfires throughout the western U.S. during summer 2018 using the NSF/NCAR C‐130 research aircraft. We empirically estimate Nr normalized excess mixing ratios and emission factors from fires sampled within 80 min of estimated emission and explore variability in the dominant forms of Nr between these fires. We find that reduced N compounds comprise a majority (39%–80%; median = 66%) of total measured reactive nitrogen (ΣNr) emissions. The smoke plumes sampled during WE‐CAN feature rapid chemical transformations after emission. As a result, within minutes after emission total measured oxidized nitrogen (ΣNOy) and measured total ΣNHx (NH3 + pNH4) are more robustly correlated with modified combustion efficiency (MCE) than NOx and NH3 by themselves. The ratio of ΣNHx/ΣNOy displays a negative relationship with MCE, consistent with previous studies. A positive relationship with total measured ΣNr suggests that both burn conditions and fuel N content/volatilization differences contribute to the observed variability in the distribution of reduced and oxidized Nr. Additionally, we compare our in situ field estimates of Nr EFs to previous lab and field studies. For similar fuel types, we find ΣNHx EFs are of the same magnitude or larger than lab‐based NH3 EF estimates, and ΣNOy EFs are smaller than lab NOx EFs.
This chapter reviews the methodology employed in soil nitrogen (N) research, following the review by J. M. Bremner and R. D. Hauck. Advances in analytical instrumentation and techniques have played a key role in the proliferation of research since 1980 concerning the nature and transformations of soil N, particularly as regards agricultural productivity and environmental quality. The application of enzymatic assay techniques in soil N research has focused primarily on the role of nitrogenase in biological N2 fixation and the hydrolysis of urea by soil urease. Environmental applications of molecular genetics have focused largely on xenobiotic degradation, but N-cycle processes also have been a subject of investigation, particularly N2 fixation, nitrification, and denitrification. Limmer et al. have utilized a different form of isotope dilution to detect emission of N2 generated during denitrification under field conditions. © 2008 by American Society of Agronomy, Inc. Crop Science Society of America, Inc. Soil Science Society of America, Inc.
This chapter discusses state-of-the-art, mid-infrared spectroscopic techniques used for the detection and monitoring of various specific molecular species, such as NH3, NO and NO2, based on laser absorption spectroscopy (LAS), cavity ring-down spectroscopy (CES), conventional and quartz-enhanced photoacoustic spectroscopy (CPAS and QEPAS) as well as Faraday rotation spectroscopy (FRS). A critical component for each technique is to use an optimum laser source and a detector matched to the detection technique with the option to apply wavelength, frequency and amplitude modulation to the laser source. A significant improvement to trace gas detection systems using spectroscopic techniques has been achieved with the development of high performance mid-infrared semiconductor lasers, in particular quantum cascade lasers (QCLs), since 1994. This has led to increased spectral resolution and high detection sensitivity of molecular trace gas species in the mid-infrared range. Spectroscopic sensors based on QCLs are capable of real time, ultra-sensitive detection of trace gas molecular species that vary in concentration from the per cent level down to parts per trillion (ppt). Such sensors can be used in a wide range of applications that include environmental monitoring, medical and biomedical diagnostics, public health issues, industrial process control and analysis as well as in national defense and security.
Infrared laser absorption spectroscopy (LAS) is a promising modern technique for sensing trace gases with high sensitivity, selectivity, and high time resolution. Mid-infrared quantum cascade lasers, operating in a pulsed or continuous wave mode, have potential as spectroscopic sources
because of their narrow linewidths, single mode operation, tunability, high output power, reliability, low power consumption, and compactness. This paper reviews some important developments in modern laser absorption spectroscopy based on the use of quantum cascade laser (QCL) sources. Among
the various laser spectroscopic methods, this review is focused on selected absorption spectroscopy applications of QCLs, with particular emphasis on molecular spectroscopy, industrial process control, combustion diagnostics, and medical breath analysis.
Methane is an important greenhouse gas and tropospheric ozone precursor. Simultaneous observation of ethane with methane can help identify specific methane source types. Aerodyne Ethane-Mini spectrometers, employing recently available mid-infrared distributed feedback tunable diode laser (DFB-TDL) lasers, provide 1 second ethane measurements with sub-ppb precision. In this work, an Ethane-Mini spectrometer has been integrated into two mobile sampling platforms, a ground vehicle and a small airplane, and used to measure ethane/methane enhancement ratios downwind of methane sources. Methane emissions with precisely known sources are shown to have ethane/methane enhancement ratios that differ greatly depending on the source type. Large differences between biogenic and thermogenic sources are observed. Variation within thermogenic sources are detected and tabulated. Methane emitters are classified by their expected ethane content. Categories include: biogenic (<0.2%), dry gas (1 - 6 %), wet gas (> 6%), pipeline grade natural gas (< 15 %) and processed natural gas liquids (> 30 %). Regional scale observations in the Dallas/Fort-Worth area of Texas show two distinct ethane/methane enhancement ratios bridged by a transitional region. These results demonstrate the usefulness of continuous and fast ethane measurements in experimental studies of methane emissions, particularly in the oil and natural gas sector.
The micrometeorological technique of conditional sampling has been used to measure the surface–atmosphere exchange of a number of radiatively active trace gases. This paper reviews the design and operational details of a system developed as part of the NERC TIGER programme. The system has been used to measure the exchange of carbon dioxide, methane, nitrous oxide, total hydrocarbons and nonmethane hydrocarbons. The necessary modifications to the basic system are described which permit such a wide range of compounds to be measured and typical flux estimates are given. The paper also includes some discussion of the relative sensitivity of the technique and suggests areas for future research to improve the stability and practicality of systems employing the conditional sampling approach.
Tunable diode laser absorption spectroscopy (TDLAS), as a noninvasive spectroscopic method, permits high-resolution, high-sensitivity, fast, in situ absorption measurements of atomic and molecular species and narrow spectral features in gaseous, solid, and liquid phases. Advances in new diode laser sources and laser spectroscopic techniques generally have triggered an increasing application of TDLAS in various disciplines (for example, atmospheric environmental monitoring, chemical analysis, industrial process control, medical diagnostics and combustion monitoring, etc.) over the last four decades. This article reviews some important developments in TDLAS, from its basic principles as a spectroscopic tool to the demonstration of gas absorption measurements, emphasizing signal enhancement and noise reduction techniques developed for improving current TDLAS performance.
We describe here a sensitive tunable diode laser absorption spectrometer (TDLAS) which was employed for ambient measurements of formaldehyde (HCHO) during the 1993 Idaho Hill/Fritz Peak Photochemistry Experiment. This system incorporated many new features and approaches including a novel astigmatic Herriott sampling cell which achieves a 100-m pathlength in a 3-L volume. We also describe procedures and tests carried out to ensure high accuracy, including the verification of HCHO standards by means of four techniques. During the field campaign, ambient HCHO measurements were acquired with an average 1σ measurement precision of 0.17 ppbv employing 1–5 min integration times. When combined with a maximum systematic uncertainty of 10%, ambient HCHO concentrations around 1.5 ppbv were measured with an average total (random plus systematic) 1σ uncertainty of 15% during the field campaign. In the intervening 2 years since the field experiment, additional features have been implemented for continuous unattended operation and improved performance. Rapid background subtraction now routinely allows HCHO measurements to be acquired with replicate precisions of 0.040 to 0.056 ppbv employing a 5-min integration period. This corresponds to routine minimum detectable absorbances of 1.2 to 1.6×10−6 in an actual mobile laboratory field environment.
An application of a dual tunable diode laser absorption spectrometer
(TDLAS) to ambient measurements of formaldehyde (HCHO), nitrogen dioxide
(NO2) and sulfur dioxide (SO2) is described.
During the PM2.5 Technology Assessment and Characterization Study-New
York (PMTACS-NY) 2002 intensive field campaign at Whiteface Mountain,
New York, a dual TDLAS was deployed during a field instrument comparison
study along with five other state-of-the-art trace gas measurement
techniques. Prior to the field study, a thorough laboratory
characterization of the instrument was undertaken to establish the
optimum operational conditions including the selection of absorption
features and the implementation of continuous laser frequency locking
and rapid background subtraction. The Allan variance method was used to
determine the optimal background subtraction cycle and to evaluate
instrument performance. During the field campaign, measurements of HCHO,
NO2 and SO2 were made by the TDLAS with the
measurement precisions (1-min time interval, 1σ) of 80 ppt, 30 ppt
and 40 ppt, respectively. The HCHO time series indicated a high
variability of HCHO concentrations ranging from below the detection
limit to 5 ppbv. The diurnal cycle of HCHO measurements showed higher
concentrations during the late afternoon and early morning hours. The
measured NO2 concentrations varied from less than the
detection limit to the maximum of 25 ppbv. Broad NO2 plumes
from long-distance transport of air masses and as well as high spikes
from local pollution sources were observed during the campaign. The
measured SO2 results also show high variability with the
average concentration of 0.75 ± 0.95 ppbv (1σ).
We have developed a mid-infrared continuous-wave quantum cascade laser
direct-absorption spectrometer (QCLS) capable of high frequency (≥1
Hz) measurements of 12CH4 and
13CH4 isotopologues of methane (CH4)
with in situ 1-s RMS ? precision of 1.5 ‰ and Allan-minimum
precision of 0.2 ‰. We deployed this QCLS in a well-studied New
Hampshire fen to compare measurements of CH4 isoflux by eddy
covariance (EC) to Keeling regressions of data from automated flux
chamber sampling. Mean CH4 fluxes of 6.5 ± 0.7 mg
CH4 m-2 hr-1 over two days of EC
sampling in July, 2009 were indistinguishable from mean autochamber
CH4 fluxes (6.6 ± 0.8 mgCH4 m-2
hr-1) over the same period. Mean ? composition of emitted
CH4 calculated using EC isoflux methods was -71 ± 8
‰ (95% C.I.) while Keeling regressions of 332 chamber closing
events over 8 days yielded a corresponding value of -64.5 ± 0.8
‰. Ebullitive fluxes, representing ˜10% of total
CH4 fluxes at this site, were on average 1.2 ‰
enriched in 13C compared to diffusive fluxes. CH4
isoflux time series have the potential to improve process-based
understanding of methanogenesis, fully characterize source isotopic
distributions, and serve as additional constraints for both regional and
global CH4 modeling analysis.
The greenhouse gas emissions from agricultural systems contribute significantly to the national budgets for most countries in Europe. Measurement techniques that can identify and quantify emissions are essential in order to improve the selection process of emission reduction options and to enable quantification of the effect of such options. Fast box emission measurements and mobile plume measurements were used to evaluate greenhouse gas emissions from farm sites. The box measurement technique was used to evaluate emissions from farmyard manure and several other potential source areas within the farm. Significant (up to 250 g CH4 m−2 day−1and 0.4 g N2O m−2 day−1) emissions from ditches close to stables on the farm site were found.
The commonly measured value of in the relaxed eddy accumulationmethod of about 0.56is shown to arise from the non-Gaussiannature of turbulence. Fourth-orderGram-Charlier functions forthe two-dimensional probability distributionsof variation in the horizontal component of wind velocityand concentrations of water vapour, carbondioxide and methane with respect to thevertical component of wind velocity are used to examinethe value of .An analytical solution for ispresented in terms of fourth-order moments.Under mean conditions, this solution givesa value for of0.557. Variation of is shown to be controlledprimarily by the ratio of the mean ofc'w3 (where c'is relevant to the entity of interest andw' is vertical component of windvelocity) to the correlationcoefficient between the entity concentrationand vertical component of wind velocity.
Most studies of soil/atmosphere greenhouse gas exchange in Arctic and Sub-Arctic wetlands have been conducted by the use of small scale chamber techniques during the growing season. To improve the knowledge about the processes in the transition period from winter to growth season, an experiment is presented here showing measurements of CH4, CO2 and H2O using both chambers (only CH4) and eddy correlation technique from the thaw period in early spring and during mid summer. The emphasis is on a comparison between eddy correlation and chamber measurements of methane fluxes during spring thawing in a subarctic mire near Abisko, northern Sweden. Methane exchange as measured by the two techniques is compared and evaluated in relation to temperature variations and atmospheric conductance. During the thaw period, integrated daily net fluxes of CH4 flux showed emission rates increasing from 2.6 mg m−2 d−1 to 22.5 mg m−2 d−1 within four days; the later rate corresponding to approximately 25% of the mid-summer flux. A profound diurnal cycle was observed in the release of methane, emphasising the importance of continuous measurements when calculating integrated fluxes.
We report the development of a field-deployable infrared laser spectrometer using new quantum cascade laser technology at
4.5 μm. The instrument is designed to measure in situ N2O concentrations at ground level even in bad weather conditions. We provide details of the instrument design and data processing.
The long-term stability of the instrument was evaluated using the Allan variance technique. A preliminary evaluation of the
instrument performance was realized by in situ measurements of N2O concentration outside the laboratory.
Eddy correlation measurements of methane exchange were conducted during a period of 43 days covering the summer season in high-arctic, NE Greenland. Measurements were carried out over a fen area and showed fluxes ranging from no exchange in the early part of the campaign to 120 mg m-2 d-1 during midsummer. The emission showed a clear variation related to soil temperatures and water table level in the late part of the summer, whereas the thickness of the active soil layer was a main controlling parameter in the thaw period, in the early part of the season. A model to assess methane emission dependency on physical parameters is found to give a realistic estimate for the seasonal variations in flux. The proportion of C returned to the atmosphere as CH4 relative to the total C cycling was around 2%, which is similar to ratios often reported in literature. On the basis of these findings, an estimate is given for the total summer season emission of CH4, in which a simple model including both physical parameters and net primary production (NPP) is adapted to reproduce CH4 exchange rates for periods when no measurements were undertaken. In this way the total emission of CH4 during the period June 1 to September 1 1997, is found to equal 3.7+/-0.57gm-2, which is a relatively high rate given the harsh climate in which the measurements were made.
We report laboratory studies of the detection scheme employed for in situ measurement of NO. in the atmosphere. In this technique, an air stream is passed over a hot metal (usually 24 karat (k) Au) catalyst in the presence of a reducing agent (usually CO), which converts the NO compounds to NO Using the NO species NO, NO 2, HNO 3, and isopropyl y ß y nitrate and the potential interferences HCN, CH3CN, NH 3, and N20, we investigated: (1) conversion efficiencies as a function of pressure and catalyst temperature; (2) conversion efficiencies as a function of reducing-agent concentration with both H 2 and CO; (3) the effect of humidity and 0 3 on conversion efficiency; (4) loss of NO in the catalyst; and (5) the efficacy and suitability as catalytic converters (or inlets) of several metals (24 k Au, 18 k Au, Au with 1% Co, Ag, Pt, stainless steel) and quartz. The most significant results are the discovery of a gas-phase process that contributes to the conversion of HNO 3 to NO and the identification of conditions under which HCN, CH3CN, and NH 3 are converted to NO with high efficiency. We discuss the implications of these results for in situ measurement of atmospheric NOy.
Measurements of N2O emission fluxes from a 3 ha field of winter wheat were measured using eddy covariance and relaxed eddy accumulation continuously over 10 days during April 1994. The measurements averaged fluxes over approximately 105 m2 of the field, which was fertilised with NH4NO3 at a rate of 43 kg N ha-1 at the beginning of the measurements. The emission fluxes became detectable after the first heavy rainfall, which occured 4 days after fertiliser application. Emissions of N2O increased rapidly during the day following the rain to a maximum of 280 ng N m-2s-1 and declined over the following week. During the period of significant emission fluxes, a clear diurnal cycle in N2O emission was observed, with the daytime maximum coinciding with the soil temperature maximum at 12 cm depth. The temperature dependence of the N2O emission was equivalent to an activation energy for N2O production of 108 kJ mol-1. The N2O fluxes measured using relaxed eddy accumulation, averaged over 30 to 270 min, were in agreement with those of the eddy covariance system within 60%. The total emission of N2O over the period of continuous measurement (10 days) was equivalent to about 10 kg N2O-N, or 0.77% of the N fertiliser applied.
Impacts of sea-salt-aerosol pH on oxidation processes, sulfur cycling, and surface-ocean fertilization are uncertain; estimates vary from pH9 and the pH-dependence of some transformations is poorly characterized. We modeled these processes under clean and polluted conditions. At pH 8, S(IV)+O3 in sea salt is the principal S-oxidation pathway. At pH 5.5, S(IV) oxidation by HOCl dominates. Decreased SO2 solubility at pH 3 slows S(VI) production. The relative contribution of H2SO4(g) scavenging to S(VI) in sea salt increases with decreasing pH. Significant sea-salt dehalogenation is limited to acidified aerosol. Volatilization rates of BrCl and Br2 do not vary significantly between pH 5.5 and 3, whereas HCl production via acid displacement increases by a factor of 20. At pH 5.5 and 8, virtually all HNO3 is scavenged by sea salt. Modeled HNO3 increases at pH 3 but remains substantially lower than particulate NO-3. Discrepancies between measurements and modeled results are assessed based on measurement artifacts, uncertainties in rate and equilibrium constants, organic reactants and surface films, and dynamics.
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