EVALUATION OF THE EFFECTS OF MOUNT-PINATUBO AEROSOL ON DIFFERENTIAL ABSORPTION LIDAR MEASUREMENTS OF STRATOSPHERIC OZONE

Substantially increased aerosol backscattering and extinction after a major volcanic eruption can lead to errors in differential absorption lidar (DIAL) measurements of stratospheric ozone. Mie calculations, pe rformed for the wavelengths 308 and 353 nm and based on size distribut ions measured over Laramie, Wyoming (41 degrees), were used to assess size and temporal evolution of these errors. In many situations, negle cting the different aerosol backscattering at the absorption and refer ence wavelengths can lead to relative errors in the ozone concentratio n larger than 100% for the 308-, 353-nm pair. The error due to neglect ing the differential aerosol extinction, however, will rarely exceed 2 %. A correction for this differential extinction should only be attemp ted when high concentrations (>100 cm(-3)) of small aerosol particles with radii below 0.1 mu m are present, e.g., shortly after an eruption . A correction for the differential backscatter can be made by using a dditional lidar measurements at a second reference wavelength or by ha ving general size distribution information on the aerosol. Possible co rrections were tested and will usually reduce the error in the ozone c oncentration considerably. For the 308-, 353-nm pair, both Mie calcula tions and a comparison with ozone profiles from electrochemical cell s ondes show, however, that even after the correction the uncertainty in the ozone concentration within some regions of the strongly enhanced Mt. Pinatubo aerosol layer can still be substantial, of the order of 1 0-50%. Wavelength separation smaller than 40 nm or use of wavelengths shorter than 300 nm will reduce the error. The best solution seems to be the addition of Raman channels. It avoids the large error due to th e differential backscatter term.