Properties of Mount Pinatubo aerosols as derived from Nimbus 7 total ozone mapping spectrometer measurements

Abstract

The perturbations to the radiances measured by the Nimbus 7 total ozone mapping spectrometer (TOMS) during the 18 months after the eruption of Mount Pinatubo are used to derive weekly zonal mean values for the stratospheric aerosol optical thickness at 312.5 nm and zonal‐mean values for the area‐weighted or effective radius. The method uses the TOMS observations, on both sides of the orbital track, of the detailed structure in the backscattering region of the aerosol‐scattering phase function. Spatial and temporal evolution of the aerosol optical thickness and effective radius is obtained for the tropical region (25°N to 25°S) during most of the period mid‐July 1991 to December 1992. The largest derived value of optical thickness was 0.22 (+36%/−10%), obtained for the latitude zone from 5° to 15°S at the end of July 1991. By the end of 1992, tropical optical depths varied from 0.02 to 0.06 over the 25°N to 25°S geographical area. The main source of uncertainty in the derived optical depth is the altitude of the aerosol layer. The inferred time evolution of the effective radius clearly shows an increase in particle size. At the end of July 1991, effective radius values of about 0.5 μm were derived, while in the fall of 1992, these values were between 0.7 and 1.4 μm. Corrections and error estimates are obtained for the measured ozone amounts. The zonal average retrieved ozone amounts corrected for the presence of aerosols are within 1% of the unconnected zonal averages. Individual scan angles can have ozone amount corrections of ±3%, with a nadir view correction of 2%.