Balloonborne particle counter observations of the El Chichón aerosol layers in the 0.01 - 1.8 µm radius range

Abstract

Through the use of balloonborne optical particle counters, the following conclusions concerning the stratospheric aerosol effects of the eruption of El Chichón have been drawn. Initially, two aerosol layers formed with a major layer ofvery large droplets (main mode radius ~0.3μm at 25 km and a !ayer of nearly the same concentration but of lesser size (main mode radius ~0.15μm ) at about 18 km. Both layers were ≥98°1o volatile at 130°c indicating a predominant sulfuric acid composition. Toe particle size distribution at 25 km was initially highly bimodal with modes near 0.02μm and O. 7 μm suggesting the nucleation of new droplets from the gas phase and extensive growth of the pre-eruption distribution. New aerosol nucleation appeared to cease after about 3 months but droplet growth was still present after 9 months. By December, the aerosol layers had largely spread to the latitude of Laramie. Aerosol growth characteristics suggest a sulfuric acid vapor concentration of ~10 7 molecules cm3 about 40 days after the eruption, and a vapor lifetime of 22-45 days. The total sulfuric acid aerosol mass is estimated to have been about 10 Tg sorne 9 months after the eruption andas muchas 20 Tg early in the event. In January 1983, a new, extensive cloud ofvery small (r ~0.02μm) sulfuric acid droplets was discovered over Laramie in the 30-35 km altitude region. These new droplets appear to have been thermally nucleated in polar regions and were probably derived from El Chichón vapors and/or vapors emanating through the evaporation of El Chichón sulfuric acid aerosol during polar stratospheric warming episodes.