Impact of heterogeneous chemistry on model predictions of ozone changes

A two-dimensional chemical/transport model of the middle atmosphere is used to assess the importance of chemical heterogeneous processes both in the polar regions (on polar stratospheric clouds (PSCs)) and at other latitudes (on sulfate aerosols). When conversion on type I and type II PSCs of N₂O₅ into HNO₃ and of ClONO₂ into reactive forms of chlorine is taken into account, enhanced ClO concentrations lead to the formation of a springtime "ozone hole" over the Antarctic continent. No such major reduction in the ozone column is found in the Arctic region. When conversion of nitrogen and chlorine compounds is assumed to occur on sulfate particles present in the lower stratosphere at all latitudes, significant perturbations in the chemistry are also found. For background aerosol conditions, the concentration of nitric acid is enhanced and agrees with observed values, while that of nitrogen oxides is reduced and agrees less than if heterogeneous processes are ignored in the model calculations. The concentration of the OH radical is significantly increased. Ozone number density appears to become larger between 16 and 30 km but smaller below 16 km, especially at high latitudes. The ozone column is only slightly modified, except at high latitudes where it is substantially reduced if the ClONO₂ conversion into reactive chlorine is taken into account. After a large volcanic eruption such as that of Mount Pinatubo in June 1991, these changes are further exacerbated. The ozone budget in the lower stratosphere becomes less affected by nitrogen oxides but is largely controlled by the ClO_x and HO_x chemistries. A substantial decrease in the ozone column is predicted as a result of the Pinatubo eruption, mostly in winter at midlatitudes and high latitudes. The predicted values depend on the assumption made for the evolution of the aerosol surface area density but is expected to be of the order of 10% at mid-latitudes in February and March 1992. An enhanced mixing ratio of NO₂ of the order of 100 parts per trillion by volume and a reduced mixing ratio of NO₂ below 25 km should be detected outside the polar vortex, especially in air masses with high levels of volcanic aerosols.