Surface aerosol radiative forcing at Gosan during the ACE-Asia campaign

[1] Surface radiation measurements were made at Gosan, Jeju, Republic of Korea, during the ACE-Asia field campaign aimed at assessing the impact of aerosols, specifically the "Asian yellow dust,'' throughout the region. Downwelling total, direct, and diffuse radiative fluxes were measured in the total solar spectrum as well as near-infrared and visible portions of the spectrum. Aerosol optical depth measurements at 500 nm were also made using a scanning shadow band radiometer. Surface radiative forcing values were determined during clear-sky conditions made from 25 March to 4 May 2001. The diurnal forcing efficiency, determined by taking the slope of the best fit line through the flux versus optical depth plot, was found to be - 73.0 +/- 9.6, - 35.8 +/- 5.5, and - 42.2 +/- 4.8 W m(-2)/tau(500) for the total solar, near-infrared, and visible spectral regions. We also introduce a new radiative forcing parameter, the fractional forcing efficiency, defined to express the radiative forcing relative to the total energy incident at the top of the atmosphere. The fractional diurnal forcing efficiency at Gosan during ACE-Asia was - 18.0 +/- 2.3, - 16.2 +/- 2.4, and - 26.7 +/- 3.3%/tau(500) for the same band passes, indicating that a larger percentage of the flux at visible wavelengths is radiatively forced compared to the total and near-infrared portions of the solar spectrum.