MULTIYEAR STRATOSPHERIC AEROSOL AND GAS EXPERIMENT-II MEASUREMENTS OFUPPER-TROPOSPHERIC AEROSOL CHARACTERISTICS

Measurements of aerosol extinction at wavelengths of 0.525 and 1.02 mu m, made by the Stratospheric Aerosol and Gas Experiment (SAGE) II sol ar occultation satellite experiment, have been used to Study the globa l-scale characteristics of the upper tropospheric aerosol. Extinction measurements, in which only aerosols occurred along the optical path, have been separated from those that included high-altitude cloud by ex amining the wavelength variation of the extinction. Data for the time period October 1984 to May 1991 show that the two main influences on t he upper tropospheric aerosol were seasonal lifting of material from b elow and downward transfer of volcanic aerosol from the stratosphere. Maximum lifting of surface material occurs in local spring in both hem ispheres and is observed at all latitudes between 20 degrees N and 80 degrees N and 20 degrees S and 60 degrees S; the data also show a stro ng hemispheric asymmetry with more aerosol in the northern hemisphere. Downward transfer of volcanic aerosol is particularly observed polewa rd of 40 degrees latitude; where a substantial enhancement of material occurs down to altitudes 2-3 km below the tropopause. By comparing tr opospheric aerosol concentrations at different times during the period of observation, it has been possible to differentiate the effects of volcanic aerosols from those of the background, or baseline, aerosols. A simple model, based on the ratio of the extinctions at the two meas urement wavelengths, has bean used to calculate the aerosol mass densi ty and effective radius. It was found that in 1984-1985, approximately 15% of the Volcanic aerosol still present: from the eruption of El Ch ichon in 1982, resided in the upper troposphere. Particle sizes for th e volcanic aerosol in the lower stratosphere and upper troposphere wer e of the order of 0.5 mu m, while those for the baseline aerosol were about 0.15 mu m. Slightly larger aerosol sizes, of the order of 0.25 m u m, were observed at altitudes 6-8 km during the springtime enhanceme nts. The low-latitude aerosol enhancements in both hemispheres appear to have the characteristics of material derived from arid surface regi ons, while the higher-latitude aerosol in the northern hemisphere appe ars more likely to be derived from anthropogenic sources.