Multiyear Stratospheric Aerosol and Gas Experiment II measurements of upper tropospheric aerosol characteristics

Abstract

Measurements of aerosol extinction at wavelengths of 0.525 and 1.02 μm, made by the Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation satellite experiment, have been used to study the global‐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 examining the wavelength variation of the extinction. Data for the time period October 1984 to May 1991 show that the two main influences on the upper tropospheric aerosol were seasonal lifting of material from below and downward transfer of volcanic aerosol from the stratosphere. Maximum lifting of surface material occurs in local spring in both hemispheres and is observed at all latitudes between 20°N and 80°N and 20°S and 60°S; the data also show a strong hemispheric asymmetry with more aerosol in the northern hemisphere. Downward transfer of volcanic aerosol is particularly observed poleward of 40° latitude, where a substantial enhancement of material occurs down to altitudes 2–3 km below the tropopause. By comparing tropospheric 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 measurement wavelengths, has been used to calculate the aerosol mass density and effective radius. It was found that in 1984–1985, approximately 15% of the volcanic aerosol still present from the eruption of El Chichón in 1982 resided in the upper troposphere. Particle sizes for the volcanic aerosol in the lower stratosphere and upper troposphere were of the order of 0.5 μm, while those for the baseline aerosol were about 0.15 μm. Slightly larger aerosol sizes, of the order of 0.25 μm, were observed at altitudes 6–8 km during the springtime enhancements. The low‐latitude aerosol enhancements in both hemispheres appear to have the characteristics of material derived from arid surface regions, while the higher‐latitude aerosol in the northern hemisphere appears more likely to be derived from anthropogenic sources.