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 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 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.