Measurement of the sulfuric acid weight percent in the stratospheric aerosol from the El Chichón eruption.

Abstract

During a slow balloon descent from 30 km over southern Texas on October 23, 1982, the intake tube to a particle counter capable of measuring the concentrations of r ≥0.15 µm and r ≥0.25 µm aerosol was heated to 150°C and allowed to cool periodically to determine aerosol volatility. At the time of the measurement, the El Chichón injected aerosol was characterized by two main layers centered at about 17 and 24 km. The upper layer contained the larger particles (main mode radius of ~0.3 µm as compared to ~0.1 µm in the lower layer). The aerosol, when heated, indicated a  concentration of ~ 1 % of the ambient values suggesting that most of the particles were highly volatile or had a highly volatile sheath with possibly nonvolatile cores of radii < 0.15 µm. The vertical distribution of the remaining nonvolatile component could be resolved. By observing the temperature at which the bulk of the aerosol was removed (vaporization point) at several altitudes (pressures), a vapor pressure curve was constructed. The results indicate that the volatile material in the upper layer consisted of ~80% H₂SO₄ - 20% H₂O (by weight) while the lower layer consisted of a 60 - 65% acid aerosol. This difference is due mainly to the higher temperatures in the upper layer. The measured sulfuric acid weight percentages are in good agreement with theoretical values as calculated for observed temperatures and typical water vapor concentrations.