DIMINISHED EFFECTS OF EL-CHICHON ON STRATOSPHERIC AEROSOLS, EARLY 1984 TO LATE 1986

Stratospheric aerosol collections by wire impactors, taken mainly over the western United States from early 1984 to late 1986, show the diminishing effects of El Chichon's 1982 eruption, and provide a set of data for judging subsequent volcanic effects (e.g. Pinatubo). Decrease in sulfate burden during the study time is due to preferential gravitational settling-out of large (>0.5 mum diameter) sulfate aerosol droplets. As a result of settling from higher levels, lower altitude (12.2-15.2 km) air early in 1984 tends to contain more sulfate than higher level (19.8-21.3 km) air. As of late 1986, however, high- and low-altitude sulfate contents have decreased and are similar, suggesting large-particle settling has been completed. The later sulfate collections also have size distributions that in shape resemble unimodal background spectra, whereas earlier ones are bimodal. Average sulfate load for all altitudes decreases during the period of study from 0.4 to 0.08 mug m-3. The latter value is somewhat higher than a volcanically unenriched pre-El Chichon level (0.05 mug m-3) suggesting that even as of 1987, stratospheric background had not been obtained.