POSSIBLE NITRIC-ACID COATING FORMATION OVER PINATUBO AEROSOLS INFERRED WITH A MICROPHYSICAL CODE - A CASE-STUDY DURING EASOE

Lidar observations at Sodankyla (Finland) on January 9 and 11 1992, ma de as part of the European Arctic Stratospheric Ozone Experiment (EASO E), showed very different profiles of backscattering ratio. A prominen t layer was seen between potential temperatures of 420 and 445K on Jan uary 9, which was nearly absent in the backscattering profile between these isentropic surfaces on the 11th. Examination of the trajectories of the airmasses reaching Sodankyla on these days lead us to suggest that the larger returns on January 9th could arise from the formation of a nitric acid coating on volcanic aerosols. Simulations using a mic rophysical model produce results consistent with this suggestion. We c onclude that nitric acid condensation on January 9 can explain the mea surements, but that independent information would be needed to quantif y the characteristics of the aerosol substrate.