Chlorine activation near the midlatitude tropopause

[1] We report on substantial chlorine activation near the midlatitude tropopause over the central United States (39-42 degrees N 102-105 degrees W) during the WB-57F Aerosol Mission (WAM) in April 1998. The extent of activation was determined by simultaneous in situ measurements of ClO and ozone and calculations of total inorganic chlorine (Cl-y) based on recently reported relationships between HCl, ozone, and halocarbons for similar latitudes. Levels of [ClO]/[Cly] approaching 50% occurred in a region of strong gravity wave activity, high particulate surface areas (5-20 mu m 2 cm(-3)), low ozone, and relatively high humidities (20-25 ppm of H2O, yet undersaturated with respect to ice), consistent with a heterogeneous mechanism on particles formed in recently lofted tropospheric air as it mixed into the lowermost stratosphere. These observations are similar to a previous one of chlorine activation on volcanic aerosol, suggesting a common heterogeneous chemical mechanism involving HCl, ClNO3, and HOCl that even in volcanically quiescent years can impact ozone photochemistry in regions of the lowermost stratosphere influenced by mixing from the tropopause region. Models not incorporating this chlorine activation process may be underestimating the impact on ozone in the midlatitude lowermost stratosphere by decomposition of very short lived halocarbon compounds, including substitutes for ozone-depleting compounds.