A widely-distributed and voluminous cloud of aerosols remained in the upper troposphere in early March. Aircraft observations indicated that aerosols had been disseminated over broad areas of middle and lower northern latitudes by mid-February. Although the cloud was clearly of volcanic origin, no eruption has been unequivocally identified as its source.

Solar irradiance measurements. On 11 January a pyrheliometer (which measures direct solar irradiance over a broad spectrum at the earth's surface) at Mauna Loa Observatory, Hawaii detected a substantial decrease in solar radiation, nearly as large as that measured after the major eruption of Agung in 1963. A small decrease measured by this instrument during its preceding reading 4 January was within the noise level. Lidar data collected at Mauna Loa indicated that material in the upper troposphere, between 10 and 13 km altitude, was responsible for the initial decrease. Solar irradiance has remained low at Mauna Loa, as it did for about 3 years after the Agung eruption. A pyrheliometer at Aspendale Observatory, Australia (about 38°S, 145°E) had shown no major increase in atmospheric turbidity as of early March. No other atmospheric data are presently available from the Southern Hemisphere.

Lidar and balloon measurements. Stratospheric aerosols were first detected by lidar on 23 January at Kyushu University, Fukuoka, Japan as a thin layer centered at about 17 km altitude. Scattering ratios were at background levels during their next observation 26 January but each measurement since then, beginning 30 January, has recorded backscatter from the cloud, with peak concentrations on 2 February. Their most recent data, on 3 March, indicated the presence of an aerosol layer between 9 and 17 km altitude.

No stratospheric layer was evident on the Mauna Loa lidar until 28 January (9 days after the preceding measurement). It continued to detect stratospheric material through late February and on the 26th the aerosol layer was about 3 km thick, centered at 18 km altitude. Lidar data collected during poor conditions 5 March showed an apparently weak layer centered at 17.3 km. No aerosol layer was detected during a balloon flight from Laramie, Wyoming 5 February, but instruments aboard balloons launched 17 and 27 February measured aerosol concentrations between 13.5 and 18 km altitude that were similar to those observed following the 18 May, 1980 eruption of St. Helens. William Fuller reports that lidar data obtained 26 January from Hampton, Virginia indicated that the stratosphere at this latitude was reasonably clear, with peak backscattering ratios at about background levels. Measurements made with the NASA Langley Research Center airborne lidar 10 February from the ground at Wallops Island, Virginia showed a considerable increase in stratospheric material. The base of the layer was at 12 km and it extended to 18 km with a peak backscattering ratio 4-5 times greater than normal. Stratospheric lidar measurements will continue on a regular basis from Hampton with a ground-based 48-inch lidar and possibly with the airborne lidar. During the night of 8-9 March, the ground-based lidar at Hampton detected the aerosol layer between 12.9 and 16.5 km.

Airborne lidar measurements - Virginia to Costa Rica. By mid-February, stratospheric aerosols were distributed over a broad area of the Northern Hemisphere, as described in the following report from William Fuller.

"A flight was conducted 13 February from Wallops Island to San José, Costa Rica (9.9°N, 84.1°W) with the lidar on board the NASA Wallops Electra aircraft. Lidar measurements indicated that the very intense stratospheric layer was present along the entire flight path. As the aircraft proceeded S, the layer became very intense until the maximum scattering ratio occurred at 21°N, 87.1°W, just E of the Yucat n Peninsula. The stratospheric layer remained strong to 10°N, 82-83°W where the southernmost measurement was made. On the return flight 21 February, lidar measurements were made from New Orleans (30.0°N, 90.05°W) to Wallops Island and the stratospheric layer continued to be present."

Aerial sampling. On 6 March at about 1900 GMT, impact samples of the aerosol cloud were collected at about 20°N, 96.2°W (SW Gulf of Mexico) on a grid carried by a NASA U-2 aircraft. The samples were similar to those collected from the St. Helens aerosol cloud 1-2 weeks after the 18 May, 1980 eruption. More than 70 particles/cm³ greater than 0.6 µm in diameter, all liquid droplets, were recovered at 16.75 km altitude. Preliminary analysis by the Atmospheric Experiments Branch of the NASA Ames Research Center showed that substantial quantities of H₂SO4 were present in the cloud, indicating that its source was volcanic. Virtually no mineral grains were recovered. Hand-held geiger counters in the aircraft recorded radioactivity at only one-half of normal background. Additional analyses of the 6 March samples are planned, as are a series of additional flights.

Unusual sunsets - Arizona. Aden and Marjorie Meinel saw sunset glows from Tucson, Arizona beginning 23 January, when late twilight coloration indicated possible enhancement in the region between 18 and 20 km altitude. Glow was stronger the next night and on 15 January glowset was 43 minutes after sunset. The glow was weaker 26 and 27 January, and after cloudy weather 28-29 January, no enhancement was visible 30 January. The Meinels observed a faint glow 19 February, similar to that of 24 January, but after two nights of cloudiness saw none 22 February. Glow was next visible from Tucson 2 March and was present for the next 2 nights.

Information Contacts: W. Fuller, P. McCormick, and M. Fujiwara, NASA, VA; B. Ragent, G. Ferry, V. Overbeck, K. Snetsinger, and D. Hayes, NASA Ames Research Center, CA; K. Coulson, Mauna Loa Observatory (MLO), HI; B. Mendonça, NOAA, CO; A. and M. Meinel, Univ. of Arizona; J. Rosen, Univ. of Wyoming; M. Hirono, Kyushu Univ., Japan; J. Gras, CSIRO, Australia.