A 3-DIMENSIONAL MODELING STUDY OF TRACE SPECIES IN THE ARCTIC LOWER STRATOSPHERE DURING WINTER 1989-1990

A three-dimensional (3D) radiative-dynamical-chemical model has been d eveloped and used to study the evolution of trace gases in the Arctic lower stratosphere during winter 1989-1990. A series of 10-day model i ntegrations were performed throughout this period. The model includes a comprehensive scheme of gas phase chemical reactions as well as a pa rameterization of heterogeneous reactions occurring on polar stratosph eric cloud (PSC) surfaces. An important element of a 3D chemical model is the transport scheme. In this study the transport of chemical spec ies is achieved by a non diffusive method well suited to the preservat ion of sharp gradients. During the winter studied temperatures were co ld enough for the formation of both type I and type II polar stratosph eric clouds from early December to early February. Model simulations i n late December show that inside the polar vortex air is rapidly proce ssed by polar stratospheric clouds converting HCl and ClONO2 to active chlorine. The possibility of ozone destruction depends strongly on th e amount of sunlight. In early February an average ozone loss of 15 pp bv (parts per billion by volume) /day is predicted in PSC-processed ai r at 50 hPa. giving a column loss of just under 1 DU/day. This loss in creases to 25 ppbv/day if PSCs persist until March with a column loss of around 1.5 DU/day. The relatively small magnitude of the ozone loss predicted in the model, compared to the variability of ozone induced by dynamics, highlights the problems in identifying the signature of c hemical ozone loss in the Arctic. In future years significant ozone de pletion could occur if PSCs persist until late March. rhe efficiency o f the catalytic cycles responsible for the ozone loss has been analyze d as a function of latitude, altitude and time. In general, the cycle involving ClO + ClO is the dominant loss mechanism in the polar lower stratosphere. Cycles involving BrO can make a relatively large contrib ution early in the season and when the levels of CIO are low. The cycl e initiated by ClO + O destroys ozone at altitudes above 30 hPa but th e loss is compensated, to some extent, by in situ ozone production. Th e results for trace species are validated, where possible, by comparis on with the available measurements, although the sparse nature of the observations does not effectively constrain the model.