Effects of El Chichón on stratospheric aerosols late 1982 to early 1984

Stratospheric aerosols collected over the western United States from late 1982 to early 1984 show the strong effects of El Chichón's eruption. Although mineral particles disappeared during this period, large (> 0.9‐μm diameter) acid droplets were still common. Because these have never been seen in prevolcanic, background‐level collections, they apparently result from increased droplet growth made possible by the unusual abundance of sulfate. Aerosol size distributions show a wide variety of multimodal curves due to mixing of air masses containing aerosols of various ages or histories. Toward the end of the study time there are fewer large aerosols because of gravitational settling and poleward transport. The result is a steady reduction in sulfate, as most aerosol mass is concentrated in a small number of large droplets. Even the later sulfate levels (0.3 μg m −3 ) are, however, 5 times typical prevolcanic background contents (about 0.06 μg m −3 ). Thus the influence of El Chichón on high‐altitude (15–21 km) air was still considerable 22 months after eruption.