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

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Author(s): C. E. Chung | S.-W. Kim | M. Lee | S.-C. Yoon | S. Lee

Journal: Atmospheric Chemistry and Physics Discussions
ISSN 1680-7367

Volume: 12;
Issue: 2;
Start page: 4507;
Date: 2012;
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ABSTRACT
Carbon mass of aerosols and its division between EC and OC sources were continuously measured at hourly intervals from October 2009 to June 2010. During this 9-month period, we also measured the aerosol absorption coefficient at 7 wavelengths and obtained PM mass density data at 1-h resolution. The measurement was made at the Gosan ABC super site, which is an ideal location for monitoring long-range transported aerosols from China. We remove the absorption data with significant dust influence using the mass ratio of PM10 to PM2.5. The remaining data shows an Absorption Ångström Exponent (AAE) of about 1.27, which we suggest represent the average carbonaceous aerosol (CA) AAE at Gosan. CA AAE is highest in winter, in which the monthly value is near 1.4. 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. Black carbon (BC) and organic aerosol (OA) absorption cross sections per carbon mass are found to be 5.1 (4.2–6.0) and 1.4 (1.1–1.8) m2 g−1 at 520 nm respectively. From the estimated BC & OA MAC, we find that OA contributes about 45% to CA absorption at 520 nm. OA AAE is found to be 1.7 (1.4–2.1). Compared with a previous estimate of OA MAC and AAE, 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.

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