High-altitude aerial sampling 9, 10, and 13 July revealed at extensive zone of sulfate aerosols and silicate fragments just below the tropopause at high northern latitudes.

On 9 July, instruments aboard a Los Alamos Scientific Laboratories (LASL) B-57 research aircraft, flying at an altitude of about 13.5 km from Seattle, Washington (47.5°N, 122.5°W) about 2,300 km to Anchorage, Alaska (62°N, 149°W), sampled a very constant sulfate concentration of 0.7-0.8 mg/m³, well above the normal mid-latitude background of about 0.1 mg/m³. The next day, flying directly N from Anchorage at a constant 12 km altitude from 62°N to 75°N (a distance of more than 1,500 km) along the 145° meridian, sampling instruments measured sulfate concentrations of about 1.5 mg/m³. A few silicate particles, larger than 1 mm and probably coated with acid, were also recovered. At 75°N, the aircraft climbed through the tropopause, just above the 12 km altitude of the northbound flight path, to about 16.5 km, then flew S at that altitude back to 62°N. The zone of high sulfate and silicate concentration terminated sharply at the tropopause and no unusual concentrations were recorded in the stratosphere during the return flight.

A U-2 aircraft operating from the NASA Ames Research Center near San Francisco (37.33°N, 121.92°W) flew at gradually increasing altitude to about 50°N, 155°W on 13 July. As the aircraft climbed toward the tropopause, marked by the polar jet stream, concentrations of sulfate increased gradually from about 2 mg/m³ to about 4.5 mg/m³. Some silicate particles were also collected. Sulfate concentrations of 2 mg/m³ or higher were measured for about 2 hours of flight time, representing a lateral distance of about 1,800 km. As in the 9-10 July flights, the zone of high sulfate and silicate concentration was truncated abruptly at the tropopause. While descending on the return leg of the flight, a sulfate concentration of 5 mg/m³ was recorded at the top of the polar jet stream.

While the sulfates and silicates sampled from the aircraft were almost certainly of volcanic origin, it is not yet possible to pinpoint their source, nor their time of eruption [but see 6:10]. Airmass movement in the days prior to 9 July will be analyzed to help locate a probable source area and the presence or absence of significant variation in silicate chemistry should help determine whether all are from a single eruption or whether multiple sources are likely.

Information Contacts: R. Chuan, Brunswick Corp., CA; W. Rose, Jr., Michigan Tech. Univ.