A study of stratospheric chlorine partitioning based on new satellite measurements and modeling

Two recent satellite instruments -- the Microwave Limb Sounder (MLS) on Aura and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on SCISAT-1 -- provide an unparalleled opportunity to investigate stratospheric chlorine partitioning in detail. We use a suite of measurements from MLS and ACE-FTS, with a focus on ClO, HCl, and ClONO2, to study the evolution of reactive and reservoir chlorine throughout the lower stratosphere (400-750 K) during two Arctic and two Antarctic winters. The measurements are compared to results from the recently-updated SLIMCAT three-dimensional chemical transport model. In both hemispheres at middle latitudes and at high latitudes at the beginning of winter, HCl greatly exceeds ClONO2. Measured and modeled values typically agree well at these locations/times. In contrast, in all winters studied the model slightly overestimates the magnitude, spatial extent, and duration of chlorine activation inside the polar vortex, most likely because of its simplified parameterization of polar stratospheric clouds. The measurements show that in the Antarctic, although differences in timing are seen, chlorine deactivation proceeds in a similar manner in both winters, with a rapid rise in HCl accompanying the decrease in ClO. A different picture of chlorine recovery is found for the two Arctic winters: In 2004/2005, deactivation occurs through initial reformation of ClONO2 followed by slow repartitioning between ClONO2 and HCl, in agreement with the canonical view, whereas in 2005/2006, HCl and ClONO2 rise at comparable rates in some regions.