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.
Highlights
A number of field studies have been conducted to monitor the optical properties of ambient aerosols (Kim et al, 2004; Huebert et al, 2003; Jung et al, 2009; Nakajima et al, 2007)
We find a positive correlation between the mass ratio of OC to EC and carbonaceous aerosol (CA) Absorption Angstrom Exponent (AAE), and successfully increase the correlation by filtering out data associated with weak absorption signal
From the estimated black carbon (BC) and organic aerosol (OA) Mass Absorption Cross section (MAC), we find that OA contributes about 45 % to CA absorption at 520 nm
Summary
A number of field studies have been conducted to monitor the optical properties of ambient aerosols (Kim et al, 2004; Huebert et al, 2003; Jung et al, 2009; Nakajima et al, 2007). Soot quickly evolves was found to be sulfate, ammonium, organics, nitrate and water (Moff into aggregates of fine particles (Katrinak et al, 1993) These fine particles are called spherules or monomers, and are commonly in the range of 20–60 nm in diameter in the ambient atmosphere (Alexander et al, 2008). A numb2e3r of studies (Schnaiter et al, 2005; Moffet and Prather, 2009; Chung et al, 2012) established the enhancement of BC absorption by a scattering coating shell. Alexander et al (2008) found the strongly-absorbing BrC aerosol to always exist as stand-alone particles. It appears that BrC is either seldom or never part of the BC coating shell. How much does OA AAE change from region to region? In the present study, we will discuss this issue
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