Carbonaceous aerosol AAE inferred from in-situ aerosol measurements at the Gosan ABC super site, and the implications for brown carbon aerosol

Abstract

Abstract. The Mass Absorption Cross section (MAC) and Absorption Ångström Exponent (AAE) have been commonly estimated for ambient aerosols but rarely for black carbon (BC) or organic aerosol (OA) alone in the ambient conditions. Here, we provide estimates of BC (and OA) MAC and AAE in East Asian outflow, by analyzing field data collected at the Gosan ABC super site. At this site, EC (and OC) carbon mass, the aerosol absorption coefficient at 7 wavelengths and PM mass density were continuously measured from October 2009 to June 2010. We remove the absorption data with significant dust influence using the mass ratio of PM10 to PM2.5. The remaining data shows an AAE of about 1.27, which we suggest represent the average carbonaceous aerosol (CA) AAE at Gosan. We find a positive correlation between the mass ratio of OC to EC and CA AAE, and successfully increase the correlation by filtering out data associated with weak absorption signal. After the filtering, absorption coefficient is regressed on OC and EC mass densities. BC and OA MACs are found to be 5.1 (3.8–6.1) and 1.4 (0.8–2.0) m2 g−1 at 520 nm respectively. From the estimated BC and OA MAC, we find that OA contributes about 45% to CA absorption at 520 nm. BC AAE is found to be 0.7–1.0, and is probably even lower considering the instrument bias. OA AAE is found to be 1.6–1.8. Compared with a previous estimate of OA MAC and AAE near biomass burning, our estimates at Gosan strongly suggest that the strongly-absorbing so-called brown carbon spheres are either unrelated to biomass burning or absent near the emission source.