Lidar observations from Hampton, VA continued to show a second aerosol layer above 23 km altitude through late February, but it was thinner by the 25th, and appeared to be absent the first week in March. Aerosols over Mauna Loa, Hawaii were weaker in February than in January, with most of the decline in the lower stratosphere (figure 38).

Figure 37. Lidar data from Hawaii and Virginia, showing altitudes of aerosol layers. Note that some layers have multiple peaks. Backscattering ratios 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. Altitudes of maximum backscattering ratios and coefficients are shown for each layer at Mauna Loa. Note that the backscattering ratio for the 10 February measurement at Hampton has been revised from the previously reported value. An unusually low tropopause (8 km) over Hampton on 12 February resulted in an increased integrated value that night.

Figure 38. Average monthly lidar profiles from Mauna Loa, Hawaii. September 1986-February 1987. The dotted line superimposed on each profile represents the average 5-22 November data, before the arrival of the Ruiz aerosols.

January 1985-December 1986 integrated backscatter data from Mauna Loa shows decay of the El Chichón aerosol through May 1985, when a sudden increase was detected from an unknown source, then a decline until the appearance of Ruiz material in November 1985. Data have been irregular since then, but 1986 has shown a slight increase in mean integrated backscatter, despite the absence of any known injection of volcanic aerosols into the stratosphere.

Information Contacts: Thomas DeFoor, Mauna Loa Observatory, P.O. Box 275, Hilo, HI 96720 USA; William Fuller, NASA Langley Research Center, Hampton, VA 23665 USA.