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) ODs – 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 ODc, 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 ODs showing the least variability 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 ODs. We find the ratio between in situ particulate absorption optical depth and ODs to be inversely proportional to aerosol SSA. Our findings underscore that ODs is a better optical depth measure than ODc for applying appropriate correction factors when estimating particle-phase light absorption from filter-based techniques.