Abstract

The 1980 eruptions of Mount St. Helens ejected large amounts of volcanic material into the upper troposphere and lower stratosphere. When the fresh volcanic cloud was first observed at our location (37°N.-76.3°W.), the peak depolarization ratios in the volcanic layers were 0.15 to 0.18. From the first observations in June 1980, the depolarization decreased as the volcanic material diffused, sedimented, and changed chemical composition. In 9 months, atmospheric depolarization ratios returned to values measured prior to the Mount St. Helens eruptions.