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Airborne whole air samples collected over the western Pacific in spring, 2001 showed depletion of tetrachloroethene (C2Cl4) in every upper tropospheric (UT) air parcel that had interacted with large areas of cirrus less than three days upwind. The amount of C2Cl4 depletion showed a negative correlation with time since the interaction, consistent with the C2Cl4-depleted air parcels mixing with the ambient air as they moved downwind of the cirrus. Ethane and C2Cl4 both react relatively quickly with atomic chlorine (Cl) but ethane was not significantly depleted in these same air parcels, indicating that the C2Cl4 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 × 104 atom Cl cm-3 is indirectly estimated for heterogeneous chlorine activation by cirrus clouds in the UT in tropical- and mid-latitudes during spring. © 2003 by the American Geophysical Union.