Tropospheric aerosols are thought to cause a significant direct and indirect climate forcing, but the magnitude of this forcing remains highly uncertain because of poor knowledge of global aerosol characteristics and their temporal changes. The standard long-term global NOAA product, the one-channel AVHRR aerosol optical thickness over the ocean, relies on a single predefined aerosol model and can be inaccurate in many cases. TOMS data can be used to detect absorbing aerosols over land, but are insensitive to aerosols located below 1 km. It is thus clear that innovative approaches must be employed in order to extract a more quantitative and accurate aerosol climatology from available satellite and other measurements, thereby enabling more reliable estimates of the direct and indirect aerosol forcings.

This project was a response to the recommendations of the 1997 Aerosol Workshop held at the NASA Goddard Institute for Space Studies (GISS) in June of 1997 and the resulting call for a focused effort aimed at extracting an improved multi-decadal aerosol record from existing satellite measurements as outlined in the Aerosol Radiative Forcing NRA. Specifically, the main objective has been to develop advanced global aerosol climatologies for the full period of satellite data, supplement them by improved modeling results, and to make these aerosol datasets broadly available and suitable for use in studies of the direct and indirect effects of aerosols on climate.

In the framework of GACP, multichannel aerosol retrieval algorithms, as suggested or reviewed by the GACP Science Team, have been systematically applied to the full period of satellite measurements. The algorithms have been tested and refined using ground-based/in situ data and contemporaneous EOS results. The full period of available satellite data has been reprocessed as improved algorithms or data calibration were achieved. The satellite retrievals over oceans and limited surface measurements have been used to calibrate 3-D tracer aerosol models for individual aerosol types and sources. These calibrated tracer models have thus provided an alternative estimate of the aerosol distribution over continents. Still further refinements of the aerosol climatologies may be possible using TOMS and in situ/ground-based data.

The results of this project have been used in outreach programs, thus providing a mechanism to test the use of the aerosol research as a tool for teaching science.