Filter samples from LASL high-altitude aircraft and data from NASA's SAGE satellite provided information about the height and dispersal of the eruption cloud ejected by Ulawun in October 1980, and the LASL aircraft also collected tephra probably produced by the May 1981 eruption of Pagan.

Ulawun's brief but powerful eruption took place 6-7 October 1980, producing a cloud estimated by ground observers to have reached 7-10 km in height. On 24 October, the aircraft sampled the lower stratosphere at about 19 km altitude, between the equator and 5°N at about 80°W (just S of Panama). Data from these samples indicated atmospheric concentrations of as much as 6 parts per billion (ppb) of sulfate by mass, of which only 25-50% could be attributed to the 18 May eruption of St. Helens. The SAGE satellite detected tephra from St. Helens N of 40°N in October 1980, but also detected a large cloud of new material from 26°N to 10°S (data collection was truncated at 10°S) between 125°W and the International Date Line. SAGE next collected data from the equatorial region in mid-November, when a zone of significant particle enhancement (roughly 5 times background), extending upward from the tropopause (about 16.5 km altitude at the equator) to about 22 km, circled the globe in an irregular band 10-20° wide between 20°S and 10°N.

Pagan's eruption began 15 May. Japanese weather radar recorded the top of the eruption column at 18-20 km altitude and weather satellite images showed that the high-altitude cloud traveled SSE. Ten weeks later, filter samples collected by the LASL aircraft just S of Panama (from the equator to 5°N) on 24 July showed lower stratospheric sulfate concentrations of about 5 ppb by mass at altitudes of 18.2, 19, and 19.2 km. Between 5°N and 35°N at 16.8 km altitude (above the tropopause), sulfate concentrations were as high as 4 ppb by mass; only about 1 ppb could be attributed to material remaining from the St. Helens eruption 14 months earlier. High concentrations of sulfate aerosols and silicate particles collected at mid to high northern latitudes in early July are probably from the late April-early May eruption of Alaid. From these data, the average lower stratospheric sulfate concentration over the entire Northern Hemisphere in July 1981 was calculated to be about 2.5 ppb by mass, primarily contributed by the eruptions of Pagan and Alaid but including a little material from the St. Helens eruption. The same calculations made from July 1980 data yielded a slightly lower concentration, about 2.3 ppb by mass, with St. Helens as the dominant source. In the last decade, only two relatively brief periods can be identified as showing "background" sulfate concentrations, without a substantial volcanic component: mid-1973 through mid-1974 (about 0.34 ppb sulfate by mass), and late 1978 through late 1979 (about 0.47 ppb sulfate by mass).

Information Contacts: W. Sedlacek, LASL; P. McCormick, NASA.