Stratospheric aerosol following Pinatubo, comparison of the north and south mid latitudes using in situ measurements

Abstract

In situ measurements of stratospheric aerosol concentrations have been conducted approximately monthly at Laramie, Wyoming (41°N), and approximately yearly at Lauder, New Zealand (45°S), from shortly after Pinatubo through November 1996. Vertical aerosol profiles showed quite similar characteristics at the two conjugate sites by November 1992. There were differences in the altitude profiles within the first 8 months following the eruption and in the November 1994 and 1995 measurements. Size distributions fitted to the data were used to calculate aerosol surface area concentrations. Distributions of aerosol surface area versus radius at the altitude of the peak in aerosol surface area also showed similar properties at the two sites. The distributions contained two modes, one centered near 0.1 μm and one near 0.5 μm. At both sites the median radii in the small and large modes decreased from 0.18 and 0.6 μm in November 1992 to 0.08 and 0.4 μm in November 1995 and 1996. In the same time the maximum surface area in these modes decreased by factors of 5 and 10 for the small and large modes. The descent of the altitude of the peak aerosol surface area in the southern hemisphere is similar to the fall speed of a 0.5 μm particle, 3–4 km yr −1. The decay of column integrals of aerosol effective radius, surface area, and volume shows very similar patterns at both sites with the exception of the initial 8 months following the eruption and in November 1994 and 1995 when the Lauder column integrals of surface area and volume are a factor of 2–3 above the Laramie measurements.