Global component aerosol direct radiative effect at the top of atmosphere

The two-step approach of combining Clouds and the Earth's Radiant Energy System (CERES)/Moderate Resolution Imaging Spectroradiometer (MODIS) shortwave (SW) flux and aerosol optical thickness (AOT) at 0.55m with the component AOT fractions from the Goddard Space Flight Centre (GSFC)/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model to derive top of atmosphere (TOA) component aerosol direct radiative effect (ADRE) over the global cloud-free oceans proposed by the first author in a previous publication has been extended to cloud-free land areas for nearly global coverage. Validation has also been performed by comparing the ADRE computation with calculations from the Fu-Liou radiative transfer model at globally distributed AErosol RObotic NETwork (AERONET) sites by using the aerosol optical properties observed from AERONET and surface reflectance obtained from MODIS observations as the model inputs. The promising validation results provide support for extending the two-step approach from global clear-sky oceans to global clear-sky land areas. The global annual mean values of ADRE for clear-sky condition are +0.3 +/- 0.2W m-2 for black carbon, -1.0 +/- 0.6W m-2 for organic carbon; -2.3 +/- 0.7W m-2 for sulphate; -1.6 +/- 0.5W m-2 for dust; -2.2 +/- 0.6W m-2 for sea salt; -2.4 +/- 0.8W m-2 for anthropogenic aerosol; -4.5 +/- 1.2W m-2 for natural aerosol; and -6.8 +/- 1.7W m-2 for total aerosols. For global average cloudy skies, the all-sky values of component ADRE are about 42% of their clear-sky counterparts. The major sources of uncertainty in the estimates are also discussed.