Stratospheric Ozone Depletion: Analysis of Heterogeneous Chemistry in the Antarctic

In spite of the success of the Montreal protocol and its amendments and adjustments in reducing the stratospheric chlorine loading, due to the long atmospheric lifetime of chlorine source gases, the Antarctic ozone hole will continue to occur for decades (WMO, 2014). The ozone hole arises from ozone destruction driven by elevated levels of ozone destroying (“active”) chlorine in Antarctic spring. The established picture of the development of the ozone hole is that high levels of active chlorine are maintained in Antarctic spring by a competition between chlorine deactivation through gas-phase formation of ClONO$_{2}$ and HCl and activation of ClONO$_{2}$ and HCl by heterogeneous reactions (Solomon et al., 2014, 2015). In the core of vortex for ozone hole conditions in the lower stratosphere (i.e. 16-18 km or 100-70 hPa) formation of ClONO$_{2}$ is of minor importance and the formation of HCl by reaction of Cl with CH$_{4}$ and CH$_{2}$O cannot lead to deactivation because it is balanced by immediate reactivation in effective reaction cycles involving the heterogeneous reaction HCl+HOCl. For the (observed) complete activation of stratospheric chlorine the production of HOCl via HO$_{2}$ + ClO,with the HO$_{2}$ resulting from CH$_{2}$O photolysis, is essential. These results are key for assessing the impact of changes of the future stratospheric composition on the recovery of Antarctic ozone. Our simulations indicate that future increased methane concentrations will not lead to enhanced chlorine deactivation and that extreme ozone destruction to levels below $\approx$ 0.1 ppm will occur until mid-century. Besides the reactions involved in the activation and rapid deactivation of chlorine are analysed here thoroughly