STRATOSPHERIC OZONE DEPLETION BY CIONO2 PHOTOLYSIS

SPRINGTIME ozone depletion over Antarctica is thought1,2 to be due to catalytic cycles involving chlorine monoxide, which is formed as a result of reactions on the surface of polar stratospheric clouds (PSCs). When the PSCs evaporate, ClO in the polar air can react with NO2 to form the reservoir species ClONO2. High concentrations of ClONO2 can also be found at lower latitudes because of direct transport of polar air or mixing of ClO and NO2 at the edges of the polar vortex. ClONO2 can take part in an ozone-depleting catalytic cycle18, but the significance of this cycle has not been clear. Here we present model simulations of ozone concentrations from March to May both within the Arctic vortex and at a mid-latitude Northern Hemisphere site. We find increasing ozone loss from March to May. The ClONO2 cycle seems to be responsible for a significant proportion of the simulated ozone loss. An important aspect of this cycle is that it is not as limited as the other chlorine cycles to the timing and location of PSCs; it may therefore play an important role in ozone depletion at warm middle latitudes.