This paper presents one of the few quantitative estimates of surface aerosol forcing made directly from surface irradiance observations. The method described within yields estimates of the forcing accurate to 20%. The study was conducted from February to March 1998 at the Kaashidhoo Climate Observatory (KCO) during the First Field Phase of the Indian Ocean Experiment (INDOEX-FFP). For the 400-700 nm region studied here, the forcing is -7.6 +/- 1.5 W m(-2). The data are obtained from two photodiode radiometers measuring global and diffuse irradiance in five channels in the visible and ultraviolet. The instruments were chosen, calibrated, and deployed specifically for a precise measurement of aerosol forcing. The angular, spectral, and absolute response characteristics of the instruments are determined in the laboratory and used to calibrate the data, as described here. The accuracy in the calibrated data is 2.4% for the global irradiance and 1.8% for the diffuse irradiance, Direct aerosol forcing is obtained from the measured aerosol forcing efficiency, which is determined by two methods: hybrid and differential, The hybrid method uses a radiative transfer model to subtract out the contribution from the aerosol-free atmosphere. The differential method assumes that changes in 400-700 nm solar flux are forced by changes in aerosol optical depth. By using flux changes, the differential method is not sensitive to the small calibration uncertainties, and is independent of model assumptions about the single-scatter properties of the aerosol. For this soot-laden marine region south of India, a 0.1 change in aerosol optical depth produces a -4.0 +/- 0.8 W m(-2) change in the 400-700 nm surface flux; 55% of this forcing is observed in the 400-540 nm region. The global and diffuse data agree to within 5 W m(-2) of results calculated by a Monte Carlo radiative transfer model. The model assumes an aerosol consistent with the spectral optical depth, lidar vertical profiles, and surface optical properties measured simultaneously at KCO.
