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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.

The Global Aerosol Climatology Project (GACP) was established in 1998 in response to the recommendations of the 1997 Aerosol Workshop held at the NASA Goddard Institute for Space Studies (GISS) 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.

The project was part of the NASA Radiation Sciences Program and the Global Energy and Water Cycle Experiment (GEWEX). Its main objectives were to:

  1. Analyze satellite radiance measurements and field observations in order to infer the global distribution of aerosols, their properties, and their seasonal and interannual variations; and
  2. Perform advanced global and regional modeling studies of the aerosol formation, processing, and transport.

To accomplish these objectives, the Earth Science Enterprise (formerly called Mission to Planet Earth) of NASA Headquarters established a processing center at the Goddard Institute for Space Studies and issued a research announcement (NRA-97-MTPE-16). The principal investigators of the successful proposals formed the GACP Science Team. Besides pursuing their individual research objectives, the members of the Science Team have participated in selecting candidate algorithms to be applied to the full period of satellite measurements in order to estimate parameters of significance to the aerosol climatology. The processing center has been employing the algorithms recommended by the Science Team to produce the satellite element of the combined GACP aerosol product.

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.

A major outcome of this collective research effort is a 23-year global aerosol climatology compiled from channel-1 and -2 AVHRR data and supplemented by data from other satellites, field observations, and chemical-transport modeling. The resulting datasets and analysis products have been documents in numerous peer-reviewed publications, including a special issue of the Journal of Atmospheric Sciences. They have been used to improve the understanding and modeling of the climate forcing due to changing aerosols, including both the direct radiative forcing by the aerosols and the indirect radiative forcing caused by the effects of changing aerosols on cloud properties.

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


Hal Maring
Program Manager
Radiation Sciences Program
Sun-Earth System Division
NASA Headquarters
300 E ST SW
Washington DC 20546-0001
Phone: (202) 358-1679
Fax: (202) 358-2770

Paul D. Try
International GEWEX Project Office
1010 Wayne Ave.
Suite 450
Silver Spring, MD 20910 USA
Phone: (301) 565-8345
Fax: (301) 565-8279

Joyce E. Penner
Science Team Leader
Dept. of Atmospheric, Oceanic and Space Sciences
University of Michigan
2455 Hayward Street
Ann Arbor, MI 48109-2143
Phone: (734) 936-0519
Fax: (734) 764-5137

Michael Mishchenko
Project Manager
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025
Phone: (212) 678-5590
Fax: (212) 678-5552