Since the El Chichón eruption cloud was first detected over Mauna Loa, Hawaii in April 1982, the aerosols measured by lidar there have extended downward from the stratosphere into the upper troposphere, without a sharply-defined base. Aerosol concentrations in the upper troposphere decreased graduually with decreasing altitude. November lidar data showed a return to typical pre-El Chichón profiles in the upper troposphere, with few aerosols and cleanest air at the top of the troposphere, near the tropopause.

The Mauna Loa lidar cannot reliably measure aerosol concentrations above about 30 km altitude, but the presence of a distinct break in slope in the recorded profile at roughly 39 km in past months has suggested enhanced aerosol concentrations to that altitude. However, lidar measurements on 15 and 27 November showed no structure between 30 and 40 km altitude, suggesting that no aerosols were present. The 27 November data also showed a substantial decrease in the integrated aerosol backscattering, suggesting a decline in the stratospheric aerosol load. The decrease in integrated backscattering appeared real and the instrument signal looked typical, but the presence of a heavy cirrus layer at the altitude where the instrument is usually normalized may have distorted the data. No similar decreases in aerosol backscattering were observed at Fukuoka, Japan or Hampton, Virginia.

Lidar data at Firenze, Italy showed no firm evidence of aerosols from the eruption of Mayon (Philippines) in September, but integrated backscattering increased from the end of October through the end of November. Aerosol loading seemed quite continuous from about 14 km to 22-23 km altitude. Measurements at the end of November showed more evidence of inhomogeneity of aerosol distribution with height.

William Fuller reports that a ground truth measurement experiment took place over Laramie, Wyoming 29-30 November during the overflight of the newly-launched SAGE II satellite. Sun photometer and airborne lidar measurements were conducted on board the Ames Research Center CV 990 aircraft. Aerosol, water vapor, ozone, and NO₂ measurements were made using balloon-borne samplers. Excellent data sets were obtained on each day.

Information Contacts: T. DeFoor, MLO; W. Fuller, NASA; M. Fujiwara and M. Hirono, Kyushu Univ., Japan; L. Stefanutti, Isto. di Ricera Sulle Onde Electromagnetiche, Italy.