Samples from the stratosphere obtained by U-2 aircraft after the first three major eruptions of Mount St. Helens contained large globules of liquid acid and ash. Because of their large size, these globules had disappeared from the lower stratosphere by late June 1980, leaving behind only smaller acid droplets. Particle-size distributions and mineralogy of the stratospheric ash grains demonstrate in-homogeneity in the eruption clouds.

Trace ammonia in laboratory air reacts easily with sulfuric acid aerosol samples to form crystalline ammonium sulfate. Using argon atmospheres, however, we have succeeded in protecting sampling surfaces from ammonia contamination. We find that atmospheric aerosols treated in this way contain only sulfuric acid. After an hour exposed to laboratory air, these same samples convert to ammonium sulfate. We are continuing to collect aerosol particles, using argon control, to determine if the absence of crystalline sulfate is common. But so far there is no evidence that aerosols are being neutralized by ammonia in the stratosphere.

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.

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.

We have obtained stratospheric aerosols from tropical to northern latitudes using special collectors on U-2 aircraft during 1976 and 1977. Aerosols characterized by large numbers of small particles are found in the tropical zone suggesting this is a region of particle growth; whereas aerosols containing mostly larger particles are distributed throughout the Northern Hemisphere indicating a well-mixed, mature population. We find the aerosol layer extends from higher altitudes near the equator to lower ones toward the pole. Although this gradient suggests mature aerosols may leave the stratosphere at high latitudes, the data are, as yet, inconclusive. Comparisons of our data with those of other investigators using different instruments are generally encouraging, suggesting that if similar populations were sampled, the results would be similar. When our calculated sulfate mass mixing ratios are compared with those measured directly by others, we find better agreement is achieved if we assume more dilute sulfate and water mixtures than previously proposed.