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38. This extraordinary variation is caused by: (1) different source types, e.g. windblown dust, biomass burning, marine aerosols, industrial haze etc.; (2) each aerosol type has a distinct chemical composition, size distribution as well as optical and radiative properties; (3) tropospheric aerosols also have a short residence time (order of a week) which restricts their influence to well defined regions, with numerous factors influencing their atmospheric life history.
39. The most commonly measured aerosol variables are light scattering coefficient, vertical optical depth, mass concentration and chemical composition in several size classes.
40. The measured data supplied by data providers seldom match the needs of data users. For example, research on climate effects requires vertically integrated backscattering and absorption while the measured variable may be aerosol mass concentration at the earth surface. Biogeochemical and acid deposition studies, on the other hand, need deposition rates that are difficult to estimate from remotely sensed optical data. The translation from the available to the needed variables is a major source of uncertainty and it is a an impediment to the effective diffusion and use of data.
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< a name=42>42. L. L. Stowe, R. Hitzenberger, A. Deepak, Report on Experts Meeting on Space Observations of Tropospheric Aerosols and Complementary Measurements, WCRP-48, WMO/TD-No. 389 (World Meteorological Organization, 1990).
43. R. B. Husar, L. L. Stowe, A. Deepak, Report of the Meeting of Experts on Global Aerosol Data System (GADS), Global Atmosphere Watch, No. 73, WMO/TD-No.438, (World Meteorological Organization, 1992). GADS is being implemented through the Global Aerosol Information Network (GAIN). As an illustration of GAIN, the electronic version of this paper, the data sets, figures, animations and software tools are accessible via Internet through FTP, gain@capita.wustl.edu or WWW Mosaic httl://capita.wustl.edu/gain/oceanaer/oceanaer.http
44. We thank Craig Long for providing the aerosol data files and Attila P. Husar for preparing contour maps. This research was partially supported by the NOAA grant #NA 16RC0517.
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