Effect of the eruption of El Chichon on stratospheric aerosol size and composition

Abstract

Dominant effects of the El Chichon eruption on stratospheric aerosols at 19.8 to 20.7 km are: (1) vapor depositional growth of the small‐aerosol (background) mode; (2) development of a large‐particle mode by sedimentation from the highest altitudes in the cloud; (3) a change in the large‐particle mode from sulfate‐coated silicates to sulfate aerosols, some with silicate cores; (4) a 100‐fold increase in sulfate mass in the large particle mode. Terminal velocities of large silicate particles, maximum r = 2.3 µm, sampled 1 month after eruption, and calibrated with the aid of lidar data, indicate initial injection to 26 to 27 km. Smaller velocities of sulfate aerosols, median r = 0.5 µm, are compatible with major growth in 2 to 3 months at 27 to 28 km. Aerosol settling accounts for the descent of the main lidar return to 26.5 km in August and to 20 to 21 km in December. Additional growth by condensation and/or coalescence during descent is required to explain the largest aerosols, r = 0.5 to 0.9 µm. Although sedimentation is the dominant cause of bimodality, mixing of air from low and high latitudes can produce bimodal or trimodal distributions.