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Synthesis of CCOS and CRPAQS study findings
... The Fresno CSN site is located in California's Central Valley at an elevation of 96 m above mean sea level (MSL). Particulate air pollution has been studied extensively in Fresno and California's San Joaquin Valley (Reynolds et al. 2012). The Kaiser Wilderness IMPROVE site is in the Sierra Nevada mountain range at 2598 m above MSL and is 73 km northeast of the Fresno site. ...
The replacement of the Desert Research Institute (DRI) model 2001 with model 2015 thermal/optical analyzers (TOAs) results in continuity of the long-term organic carbon (OC) and elemental carbon (EC) database, and it adds optical information with no additional carbon analysis effort. The value of multiwavelength light attenuation is that light absorption due to black carbon (BC) can be separated from that of brown carbon (BrC), with subsequent attribution to known sources such as biomass burning and secondary organic aerosols. There is evidence of filter loading effects for the 25% of all samples with the highest EC concentrations based on the ratio of light attenuation to EC. Loading corrections similar to those used for the seven-wavelength aethalometer need to be investigated. On average, nonurban Interagency Monitoring of PROtected Visual Environments (IMPROVE) samples show higher BrC fractions of short-wavelength absorption than urban Chemical Speciation Network (CSN) samples, owing to greater influence from biomass burning and aged aerosols, as well as to higher primary BC contributions from engine exhaust at urban sites. Sequential samples taken during an Everglades National Park wildfire demonstrate the evolution from flaming to smoldering combustion, with the BrC fraction increasing as smoldering begins to dominate the fire event.
Implications:
The inclusion of seven wavelengths in thermal/optical carbon analysis of speciated PM2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm) samples allows contributions from biomass burning and secondary organic aerosols to be estimated. This separation is useful for evaluating control strategy effectiveness, identifying exceptional events, and determining natural visibility conditions.
... ). Concentrations increased from 12/27/2000 to 1/7/2001 during a prolonged high-pressure system between storms. PM 2.5 NO 3 − and OC concentrations during the haze period were elevated, confirming that NO 3 − is a large PM 2.5 component, as is well known for winter in central California(Reynolds et al., 2012). Large diurnal variations are also found with elevated NO 3 − and OC during the 16:00-24:00 local standard time. ...
A thermal/optical carbon analyzer (TOA), normally used for quantification of organic carbon (OC) and elemental carbon (EC) in PM2.5 speciation networks, was adapted to direct thermally-evolved gases to an electron impact quadrupole mass spectrometer (QMS), creating a TOA-QMS. This approach produces spectra similar to those obtained by the Aerodyne Aerosol Mass Spectrometer (AMS), but the ratios of the mass to charge (m/z) signals differ and must be re-measured using laboratory-generated standards. Linear relationships are found between TOA-QMS signals and ammonium (NH4⁺), nitrate (NO3⁻), and sulfate (SO4²⁻) standards. For ambient samples, however, positive deviations are found for SO4²⁻, compensated by negative deviations for NO3⁻, at higher concentrations. This indicates the utility of mixed-compound standards for calibration or separate calibration curves for low and high ion concentrations. The sum of the QMS signals across all m/z after removal of the NH4⁺, NO3⁻, and SO4²⁻ signals was highly correlated with the carbon content of oxalic acid (C₂H₂O₄) standards. For ambient samples, the OC derived from the TOA-QMS method was the same as the OC derived from the standard IMPROVE_A TOA method. This method has the potential to reduce complexity and costs for speciation networks, especially for highly polluted urban areas such as those in Asia and Africa.
Implications
Ammonium, nitrate, and sulfate can be quantified by the same thermal evolution analysis applied to organic and elemental carbon. This holds the potential to replace multiple parallel filter samples and separate laboratory analyses with a single filter and a single analysis to account for a large portion of the PM2.5 mass concentration.
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