**CANCELLED** PAOC Colloquium: Susan Solomon, MIT
Building 54, Cambridge, MA
Title: Monsoon circulations and tropical heterogeneous chlorine chemistry in the stratosphere
Abstract: This seminar presents novel findings that expand the range of latitudes over which heterogeneous chemistry operates in the stratosphere. It has long been thought that heterogeneous chlorine chemistry would only be important in the Antarctic and Arctic. Our new work shows that tropical heterogeneous chemistry is also important. Transport processes associated with the summer monsoons bring increased abundances of hydrochloric acid into contact with liquid sulfate aerosols in the cold tropical lowermost stratosphere, leading to heterogeneous chemical activation of chlorine species. The calculations indicate that the spatial and seasonal distributions of chlorine monoxide and chlorine nitrate near the monsoon regions of the northern hemisphere tropical and subtropical lowermost stratosphere could provide indicators of heterogeneous chlorine processing. In the model, these processes impact the local ozone budget and decrease ozoneabundances, implying a chemical contribution to longer-term northern tropical ozone profile changes at 16-19 km.