Global optical climatology of the free tropospheric aerosol from 1.0‐μm satellite occultation measurements

Measurements of the aerosol/molecular extinction ratio at l‐μm wavelength, obtained from the SAGE I, SAGE II, and SAM II solar occultation satellite experiments between 1978 and 1986, have been used to study the global‐scale behavior of the upper tropospheric aerosol. The distribution of extinction ratio values shows a pronounced mode between about 0.5 and 5 in all data subsets, regardless of latitude and season. Within a given latitude band and season the mode value is nearly constant over the altitude range from about 5 km above Earth's surface to 3 km below the tropopause. The mode shows a distinct seasonal variation, with maxima in local spring and summer, and is significantly enhanced following volcanic injection of material into the stratosphere. South of latitude 20°N, mode values in the absence of volcanic contamination are normally between 0.5 and 1.0. North of 20°N, values up to about 5 are observed, probably associated with aerosol derived from surface dust or anthropogenic sources. A secondary mode, with extinction ratios of 30 or greater and little or no variation of extinction with wavelength, is apparent just below the tropopause. This mode is believed to be associated with thin cloud along the ray path from the Sun to the satellite.