Indications of heterogeneous chlorine activation on moderately cold aerosol based on chlorine observations in the Arctic stratosphere

During two balloon flights in the Arctic winter in February 1994 two distinctly different vertical profiles of ClO were measured. On 1 February enhanced values of ClO were observed between about 420 and 620 K inside the polar vortex in an air mass that had experienced moderately cold temperatures about 10 h prior to our measurements. In contrast, on 9 February an unstructured vertical profile was observed outside of the vortex with lower mixing ratios of ClO. Based on calculations with a photochemical box trajectory model we show that the observed enhancement in ClO can be explained by heterogeneous processing of chlorine reservoir species on supercooled liquid aerosols. When NAT formation is forced in the model or when lee wave processing is assumed, the mixing ratios of ClO inside the vortex are overpredicted. Under conditions where air masses are only cooled to low temperatures for a short period of time this may be a very important chlorine activation mechanism.