Influence of anthropogenic aerosols on the total and spectral irradiance at the sea surface during the Indian Ocean Experiment (INDOEX) 1999

Abstract

Unique measurements of the spectral signature of the aerosol forcing at the surface between 350 and 1050 nm wavelength during the 1999 haze event over the tropical Indian Ocean are presented and discussed. The aerosol visible optical depths reached values as high as 0.7 in the northern Indian Ocean and decreased to about 0.1 south of the Intertropical Convergence Zone (ITCZ) in the Southern Hemisphere (SH) tropical oceans. Measurements of aerosol optical depth and global (direct plus diffuse) irradiance have been taken with a grating spectroradiometer with a resolution of 3 nm onboard R/V Ronald H. Brown. The radiometer was calibrated onboard in real time. We inverted the direct solar spectra to obtain aerosol optical depth and to compare these with three independent sunphotometer optical depth at selected spectral intervals to assess the validity of the retrieved optical depth. We took the difference between aerosol optical depth between polluted and pristine air mass spectra to obtain the spectral signature. The difference in optical depth between the polluted regime north of the ITCZ and the pristine air masses south of the ITCZ were between 0.5 at 350 nm wavelength, 0.35 at 500 nm wavelength, and 0.1 at 1050 nm wavelength. Next, we obtained the aerosol forcing by correlating optical depth variations (for the duration of the cruise) with corresponding global irradiance variations. We found the Southern Asian aerosol reduces the solar irradiance (for a unit increase in optical depth at 500 nm wavelength) by about 25% at 350 nm decreasing to about 10% in the near‐infrared. The reduction in the direct solar was a factor of 2 to 3 larger. The spectral data shown here will provide a critical test for aerosol‐radiation models used for aerosol‐forcing estimates.