A test of the partitioning between ClO and ClONO₂ using simultaneous UARS measurements of ClO, NO₂, and ClONO₂

The photochemical theory of the partitioning hetween the chlorine species ClONO 2 and ClO in the lower and mid stratosphere is tested for the first time using simultaneous measurements of ClO, NO 2 , and ClONO 2 . The theory suggests that over most of the day the production of ClONO 2 through the three‐body reaction of ClO and NO 2 is balanced by the loss of ClONO 2 through photolysis. Our analysis tests this theory by utilizing simultaneous measurements of these species from instruments aboard the Upper Atmosphere Research Satellite (UARS), as well as calculations of the photolysis rate of ClONO 2 and laboratory measurements of the rate constant for the reaction between ClO and NO 2 . Between 32 and 10 hPa (∼24 and 32 km), averaged instantaneous production and loss rates of ClONO 2 agree within ∼10%. At 46 hPa (∼21 km), the agreement between averaged instantaneous production and loss of ClONO 2 is poorer than at higher altitudes, but still within uncertainties. This analysis provides no evidence for any problems in our understanding of the partitioning between the chlorine species ClONO 2 and ClO over the range of pressures (46 to 10 hPa) and latitudes (60°S to 60°N) considered here.