INCREASED STRATOSPHERIC OZONE DEPLETION DUE TO MOUNTAIN-INDUCED ATMOSPHERIC WAVES

Chemical reactions on polar stratospheric cloud (PSC) particles are re sponsible for the production of reactive chlorine species (chlorine 'a ctivation') which cause ozone destruction(1). Gas-phase deactivation o f these chlorine species can take several weeks in the Arctic winter s tratosphere, so that ozone destruction can be sustained even in air pa rcels that encounter PSCs only intermittently(2,3). Chlorine activatio n during a PSC encounter proceeds much faster at low temperatures when cloud particle surface area and heterogeneous reaction rates are high er(4). Although mountain-induced atmospheric gravity waves are known t o cause local reductions in stratospheric temperature of as much as 10 -15 K (refs 5-9), and are often associated with mesoscale PSCs10-12, t heir effect on chlorine activation and ozone depletion has not been co nsidered. Here we describe aircraft observations of mountain-wave-indu ced mesoscale PSCs in which temperatures were 12 K lower than expected synoptically, Model calculations show that despite their localized na ture, these PSCs can cause almost complete conversion of inactive chlo rine species to ozone-destroying forms in air flowing through the clou ds. Using a global mountain-wave model(8), we identify regions where m ountain waves can develop, and show that they can cause frequent chlor ine activation of air in the Arctic stratosphere. Such mesoscale proce sses offer a possible explanation for the underprediction of reactive chlorine concentrations and ozone depletion rates calculated by three- dimensional models of the Arctic stratosphere(13-17).