Changes in Aerosols and Cloud Reflectivity (1979-2008) From 30 Years of Radiance Data using Multiple Satellites: N7-TOMS, EP-TOMS, SBUV-2 Series, SeaWiFS, and OMI

The amount of solar radiation reflected back to space or reaching the Earth's surface is primarily governed by the amount of cloud cover, and, to a much lesser extent, by aerosols and various absorbing gases (e.g., O3, NO2, H2O) in their relative bands of absorption. A useful measure of the effect of cloud plus aerosol cover is given by the amount that the UV (331nm to 400 nm) Lambert Equivalent Reflectivity (LER) of a scene exceeds the surface reflectivity for snow/ice-free scenes. A 30-year reflectivity time series is presented by combining data from several satellites: N7 (Nimbus-7 TOMS; 331 nm) from 1979 to 1992, SBUV-2 series (Solar Backscatter UltraViolet-NOAA N9, N11, N16, N17, N18; 331 nm) 1985 to 2007, EP (Earth-Probe TOMS; 331 nm) 1997 to 2006, SW (SeaWiFS; 412 nm) 1998 to 2007, and OMI (Ozone Measuring Instrument; 331 nm) 2004 - 2007. Only N7 and SW have a sufficiently long data record and are adequately calibrated for long-term reflectivity trend estimation by themselves. Values derived from these instruments and the SBUV-2 series obtained during the overlapping years are compared. Key issues in determining long- term reflectivity changes that have occurred during the N7 and SW operating periods are discussed, as are the problems in re-calibrating all of the TOMS and SBUV satellite data. The preliminary combined 30-year reflectivity data set shows a small global increase in radiation reflected back to space suggestive of global dimming at the surface.