ON THE OCCURRENCE OF CLO IN CIRRUS CLOUDS AND VOLCANIC AEROSOL IN THETROPOPAUSE REGION

Airborne observations during descents of the ER-2 through layers of vo lcanic aerosol (Mount Pinatubo eruption of 1991) and a cirrus cloud ar e utilized to study the the abundance of ClO and its relation to aeros ol surface area in the midlatitude tropopause region. During a cirrus event near the tropopause ClO mixing ratios up to 2.7 pptv were detect ed and near the tropopause ClO levels from a few to 70 pptv were found . These measurements are associated with large experimental uncertaint ies but demonstrate the possible presence of ClO inside cirrus clouds and near the tropopause. Model calculations show that possible enhance ments of ClO by heterogeneous chemistry on cirrus cloud particles coul d considerably increase the ozone loss rates and could have implicatio ns for the ozone budget in this region. It is not possible to decide f rom the limited data base whether the observed ClO levels are due to t ransport or heterogeneous chemistry.