Chemical apportionment of shortwave direct aerosol radiative forcing at the Gosan super-site, Korea during ACE-Asia

Shortwave direct aerosol radiative forcing (DARF) at the surface as well as aerosol optical depth (AOD) were estimated and chemically apportioned on the basis of ground-based aerosol and radiation measurements at the Gosan super-site in Korea during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) in April 2001. An aerosol optical model and a radiative transfer model (RTM) were employed to calculate the aerosol extinction coefficient and radiative flux at the surface, respectively. The calculated scattering and absorption coefficients for Dp<10 μm aerosols agreed well with measured scattering and absorption coefficients with root mean square errors (RMSEs) of 23.6 and 3.0 Mm−1, respectively. The modeled direct and diffuse irradiances at the surface were also in good agreement with the measured direct and diffuse irradiances. In this study we found that the 17-day mean aerosol radiative forcing of −38.3 W m−2 at the surface is attributable to mineral dust (45.7%), water-soluble components (sum of sulfate, nitrate, ammonium, and water-soluble organic carbon (WSOC)) (26.8%), and elemental carbon (EC) (26.4%). However, sea salt does not play a major role. For the cases of Asian dust and smoke episodic events on 26 April 2001, a diurnal averaged forcing of −36.2 W m−2 was contributed by mineral dust (−18.8 W m−2), EC (−6.7 W m−2), and water-soluble components (−10.7 W m−2). The results of this study suggest that water-soluble and EC components as well as a mineral dust component are responsible for a large portion of the aerosol radiative forcing at the surface in the continental outflow region of East Asia.