Inferring the Specific Absorption and Concentration of Black Carbon from AERONET Aerosol Retrievals

Abstract

Black carbon is ubiquitous in the atmosphere and the main anthropogenic absorbing particulate. Absorption by black carbon is thought to be comparable to the cooling associated with sulfate aerosols, although present day satellites are incapable of obtaining this measurement and model estimates are highly uncertain. More measurements of black carbon concentration are necessary for improving and validating transport and general circulation models. The aerosol robotics network (AERONET) of 180 worldwide radiometers offers an opportunity to obtain these measurements. We use the Maxwell Garnett effective medium approximation to infer the column-averaged black carbon concentration and specific absorption of AERONET retrievals at 46 locations. The yearly-averaged black carbon column concentrations exhibit the expected regional dependence, with remote island sites having values about an order of magnitude lower than the biomass burning locations. The yearly-averaged specific absorption cross section is consistent with other measured values, 9.9 m2 g−1 for 19591 retrievals, but varies from 7.7 to 12.5 m2 g−1. We also observe this variability in calculations associated with nine aerosol climatologies where specific absorption varies by a factor of 2 or more. We attribute this variability to the details of the size distributions and the fraction of black carbon contained in the aerosol mixture. Transport models generally use aerosol size distributions with externally-mixed black carbon and are unable to account for this variability of specific absorption associated with internal mixing. We also used the Maxwell Garnett equations to parameterize the imaginary refractive index with respect to the black carbon volume fraction, enabling simple but accurate absorption estimates for aerosol mixtures. The black carbon concentrations that we derive from AERONET measurements are validated with surface measurements and represent an alternative to absorption optical thickness in the link between models and AERONET measurements.