Aerosol light absorption from optical measurements of PTFE membrane filter samples: sensitivity analysis of optical depth measures

Abstract

Mass absorption cross section (MAC) measurements of atmospherically relevant aerosols are required to quantify their effect on Earth's radiative budget. Estimating aerosol light absorption from transmittance and/or reflectance measurements through filter deposits is an attractive option because of their ease of deployment in field settings, low cost, and the ability to revisit previously analyzed samples. These measurements suffer from artifacts that depend on a given filter measurement system and aerosol optics. Empirical correction algorithms are available for commercial instruments equipped with optically thick fiber filters, but optically thin filter media have not been characterized in detail. Here, we present empirical relationships between particle light absorption optical depth -measured using multiwavelength integrated photoacoustic spectrometers -and filter optical depth measurements for polytetrafluoroethylene (PTFE) membrane filter samples of carbonaceous aerosols generated from combustion of diverse biomass fuels and kerosene (surrogate for fossil fuel combustion). Through radiative transfer modeling, we assessed the suitability of three measures of filter-based optical depth for robustly describing particulate-phase light absorption over a range of single scattering albedo (SSA) values: (1) OD s -a measure of transmission of the fraction of incident radiation that is not backscattered by the filter system -which utilizes transmittance and reflectance of the sample side of the filter; (2) the commonly used OD c , which uses transmittance and reflectance of the clean side of the filter; and (3) ATN or the Beer-Lambert attenuation. Modeling results were also validated experimentally, with OD s showing the least vari-ability around the mean in this multidimensional parameter space. We establish a simple, wavelength-independent formulation for calculating aerosol MAC and absorption coefficients from measurements of OD s . We find the ratio between in situ particulate absorption optical depth and OD s to be inversely proportional to aerosol SSA. Our findings underscore that OD s is a better optical depth measure than OD c for applying appropriate correction factors when estimating particle-phase light absorption from filter-based techniques.

Light Absorption by Organic Aerosol Emissions Rivals That of Black Carbon from Residential Biomass Fuels in South Asia

“…MAC PM values are approximately log-normally distributed with average (and 95% confidence intervals) of 0.54 (0.10–1.46) m 2 g –1 , consistent with values (0.4–1.5 m 2 g –1 ) derived from in situ (photoacoustic) absorption measurements and corresponding mass emission factors of particulate matter from biomass cookstoves in Nepal. MAC PM values in this study are slightly smaller than those from our earlier analysis on a smaller set of samples from Indian cookstove emissions, largely due to improved treatment of filter artifacts . Our estimates are significantly smaller than MAC PM (range of 0.5–6.6 m 2 g –1 ) calculated using other filter-based measurements of absorption from cookstove emissions in laboratory or field , settings.…”

“…MAC PM values in this study are slightly smaller than those from our earlier analysis 19 on a smaller set of samples from Indian cookstove emissions, largely due to improved treatment of filter artifacts. 36 Our estimates are significantly smaller than MAC PM (range of 0.5−6.6 m 2 g −1 ) calculated using other filter-based measurements of absorption from cookstove emissions in laboratory 41 or field 13,47 settings.…”

“…In a recent study, we showed that OD s is a reliable estimator of aerosol light absorption from filter optical measurements for a wide range of biomass combustion aerosols . The empirical correction scheme derived in that work was used to calculate particle-phase light absorption optical depth (ABS PM )­ …”

Light Absorption and Chemical Composition of Brown Carbon Organic Aerosol Produced from Burning of Selected Biofuels

“…Biomass burning experiments were carried out using 10 g of selected biofuels for each burn in a specialized experimental burn chamber at Washington University in St. Louis. 23,24,33,34 The sealed 21 m 3 stainless steel chamber is equipped with a computer-controlled ignition system and a recirculation fan. The ignition system is a 1 kW ring heater (McMaster-Carr 2927094A) coupled to a 1/ a K-type thermocouple to close the control loop.…”

“…Model estimates suggest that increased inhalation exposure to outdoor BBOA is responsible for a considerable number of premature deaths globally each year . The chemical composition and optical properties of BBOA are strongly influenced by regional vegetation types, − underscoring the need to understand biofuel-specific contributions to overall atmospheric BBOA and their impacts. This understanding is pivotal for accurate source apportionment and modeling of the individual and collective impacts of emissions from wild and prescribed fires.…”