Airborne measurements of cirrus-activated C sub(2)Cl sub(4) depletion in the upper troposphere with evidence against Cl reactions

Airborne whole air samples collected over the western Pacific in spring, 2001 showed depletion of tetrachloroethene (C sub(2)Cl sub(4)) in every upper tropospheric (UT) air parcel that had interacted with large areas of cirrus less than three days upwind. The amount of C sub(2)Cl sub(4) depletion showed a negative correlation with time since the interaction, consistent with the C sub(2)Cl sub(4)- depleted air parcels mixing with the ambient air as they moved downwind of the cirrus. Ethane and C sub(2)Cl sub(4) both react relatively quickly with atomic chlorine (Cl) but ethane was not significantly depleted in these same air parcels, indicating that the C sub(2)Cl sub(4) depletion cannot be attributed to Cl chemistry alone. Based on the minimum ethane depletion that can be detected by our measurements, a daytime upper limit of roughly 3 x 10 super(4) atom Cl cm super(-3) is indirectly estimated for heterogeneous chlorine activation by cirrus clouds in the UT in tropical- and mid-latitudes during spring.