Airborne measurements of cirrus‐activated C₂Cl₄ 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 2 Cl 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 2 Cl 4 depletion showed a negative correlation with time since the interaction, consistent with the C 2 Cl 4 ‐depleted air parcels mixing with the ambient air as they moved downwind of the cirrus. Ethane and C 2 Cl 4 both react relatively quickly with atomic chlorine (Cl) but ethane was not significantly depleted in these same air parcels, indicating that the C 2 Cl 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 × 10 4 atom Cl cm −3 is indirectly estimated for heterogeneous chlorine activation by cirrus clouds in the UT in tropical‐ and mid‐latitudes during spring.