An apparently new aerosol layer was detected at the base of the stratosphere on lidar and balloon data. The source of the aerosols was uncertain. No large explosive eruptions were documented in the weeks before the layer was first observed in early June. However, large forest fires that burned from about 6 to 22 May in N China produced extensive clouds of smoke that were tracked for thousands of kilometers on NOAA weather satellite imagery. One major smoke plume moved over Kamchatka, then covered most of the Gulf of Alaska before advancing over Canada's Yukon Territory. Another extended into the Arctic Basin in streamers several hundred kilometers wide and thousands of kilometers long. A third major zone of smoke entered a low pressure area and could be traced as far as Ellesmere Island, just W of northern Greenland.

Instruments on a balloon launched near Laramie, WY on 29 May detected strong particle enhancement from ground level through the tropopause into the lower stratosphere. Highest values were measured between 10 km and the top of the layer at about 13 km, with concentrations of particles larger than 0.15 µm exceeding 10/19cm³, compared to recent lower stratospheric values of about 0.5/cm³. Concentrations remained above 5/cm³ throughout the troposphere, increasing to more than 10/19cm³ near ground level. More than a month later (8 July), the next balloon flight from Laramie detected no unusual tropospheric material, but an aerosol layer extended from the tropopause at 11 km to about 14 km. Counts of particles larger than 0.15 µm reached 3/cm³. The layer was relatively smooth and the distribution of particle sizes suggested that it was probably several weeks old (figure 42). The ratios of the number of particles of 0.15 µm diameter to those of 0.25 µm size was somewhat less than 10 on 29 May, decreasing to about 5 on 8 July. Smaller particles are generally expected from biomass burning, typically yielding ratios of the 0.15/0.25 µm size fractions greater than 10, compared to values of 2-5 for eruption-generated aerosols.

Figure 42. Plots of concentrations of particles with radii >0.15 µm counted from balloons launched from Laramie, WY on 29 May and 8 July 1987. Note that the x axis is a logarithmic scale. The 8 July tropopause was at about 11 km altitude.

Lidar at Hampton, VA detected a distinct layer between the tropopause (13 km) and 15 km altitude on 10 June (figure 43). Eight days later, remnants of the layer were still present, but it was not evident on 25 or 29 June.

Figure 43. Lidar data from various locations, showing altitudes of aerosol layers. Note that some layers have multiple peaks. Backscattering ratios from Fukouka, Japan, are for the Nd-YAG wavelength of 1.06 µm; all others are for the ruby wavelength of 0.69 µm. Integrated values show total backscatter, expressed in steradians-1, integrated over 300-m intervals from 16-33 km at Mauna Loa and from the tropopause to 30 km at Hampton, Virginia. Altitudes of maximum backscattering ratios and coefficients are shown for each layer at Mauna Loa.

A strong but thin layer at the tropopause (15.6 km) was observed 19 May by lidar at Mauna Loa, Hawaii, and the upper troposphere was very turbid during the next measurement, on 26 May. Each of the five June lidar measurements at Mauna Loa showed a small layer at the base of the stratosphere, most strongly on 16 June. The upper troposphere remained turbid through June. On 11 June a very strong scattering layer that had not been present 2 days earlier appeared just above the tropopause at Fukuoka, Japan. A similar but weaker layer was detected again on 25 June, but was not evident on 16, 22, or 26 June. Late on 10 July, lidar at Garmisch-Partenkirchen, West Germany measured minor peaks at 12.5 and 14 km, probably near the tropopause, with scattering ratios estimated at 1.2-1.3. The next day, peaks of the same intensity were observed at 14 and 16 km, and on 12 July there was a peak of the same scattering ratio at 15 km.

Integrated backscatter data from Mauna Loa, January 1985-June 1987, show a rapid decline in the aerosol produced by the 1982 El Chichón eruption, a sudden increase in June 1985 from an unknown source, then a decline until the appearance of the Ruiz aerosol in December 1985. A gradual increase was observed through much of 1986, then values have diminished for most of the first half of 1987.

Information Contacts: David Hofmann and James Rosen, Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071 USA; William Fuller and Mary Osborn, NASA Langley Research Center, Hampton, VA 23665 USA; Thomas DeFoor, Mauna Loa Observatory, P. O. Box 275, Hilo, HI 96720 USA; Motowo Fujiwara, Physics Department, Kyushu University, Fukuoka 812, Japan; Horst Jäger, Fraunhofer-Institut für Atmosphärische Umweltforschung, Kreuzeckbahnstrasse 19, D-8100 Garmisch-Partenkirchen, West Germany; Will Gould, NOAA/NESDIS, World Weather Building, Camp Springs, MD 20746, USA.