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

摘要

The Mass Absorption Cross section (MAC) and Absorption ?ngstr?mExponent (AAE) have been commonly estimated for ambient aerosols but rarelyfor black carbon (BC) or organic aerosol (OA) alone in the ambientconditions. Here, we provide estimates of BC (and OA) MAC and AAE in EastAsian outflow, by analyzing field data collected at the Gosan ABC supersite. At this site, EC (and OC) carbon mass, the aerosol absorptioncoefficient at 7 wavelengths and PM mass density were continuously measuredfrom October 2009 to June 2010.We remove the absorption data with significant dust influence using the massratio of PM₁₀ to PM_2.5. The remaining data shows an AAE of about1.27, which we suggest represent the average carbonaceous aerosol (CA) AAE atGosan.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 associatedwith weak absorption signal. After the filtering, absorption coefficient isregressed 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 nmrespectively. From the estimated BC and OA MAC, we find that OA contributesabout 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 isfound to be 1.6–1.8. Compared with a previous estimate of OA MAC andAAE near biomass burning, our estimates at Gosan strongly suggest that thestrongly-absorbing so-called brown carbon spheres are either unrelated tobiomass burning or absent near the emission source.