Models overestimate diffuse clear-sky surface irradiance: A case for excess atmospheric absorption

Radiative transfer models consistently overestimate surface diffuse downward irradiance in cloud-free atmospheres by 9 to 40% at two low altitude sites while correctly calculating direct-normal Solar irradiance. For known systematic and random measurement errors and for realistic aerosol optical properties, the discrepancy can be resolved by a reduction in the vertical aerosol optical thickness (AOT) inferred from sunphotometric measurements by an average 0.02 +/- 0.01 for 32 cases examined, together with a compensating increase in a continuum-like atmospheric absorptance over the solar spectrum of similar to 5.0% +/- 3.0%. This phenomenon is absent at two high altitude sites, where models and measurements agree to within their mutual uncertainties. Examination of apparent AOT at several locations around the globe also indicates presence of such excess atmospheric absorption. The proposed absorption and corresponding reduction in AOT would have important consequences for climate prediction and remote sensing.