Optical extinction properties of volcanic stratospheric aerosol derived from ground-based lidar and sun photometer measurements

Measurements of lidar-integrated backscatter and sun photometer optical thickness made at Mauna Loa, Hawaii, were used to compute the bulk columnar value of the extinction-to-backscatter ratio S(c) of stratospheric aerosol during a period of major stratospheric aerosol contamination following the eruption of El Chichon (Mexico, March-April 1982). Values of S(c) were also calculated using observations of integrated backscatter and aerosol optical thickness collected from various sources in the northern midlatitude region. The calculated values of S(c) were found to range between 40 and 70 sr at a wavelength of 0.694 micron during periods when high and moderate stratospheric aerosol concentrations were observed. Variations of S(c) with time of approximately 20-30 percent are noted. Similar S(c) results obtained by other investigators are compared with the results of this investigation. Overall, the agreement among the various results was found to be consistent within their uncertainties. Values of S(c) were also calculated for the stratospheric aerosol from the eruption of Mount Pinatubo (Philippines, June 1991). The Mount Pinatubo results were about 15 percent lower than the El Chichon results during their early stages. Differences between the El Chichon and the Pinatubo results are within their error bounds. (Author)