ON THE ORIGIN OF MIDLATITUDE OZONE CHANGES - DATA-ANALYSIS AND SIMULATIONS FOR 1979-1993

Satellite data show large declines in global (4.5%) and midlatitude (1 0%) ozone in the mid-1980s and during 1992 and 1993. Analyses of ozone , temperature, and aerosol records and two-dimensional chemical transp ort simulations have been carried out to develop an understanding of t he causes of these changes. Simulations include contemporary homogeneo us and heterogeneous chemistry. Also included are the effects of trace gas increases, dilution and denitrification associated with the Antar ctic ozone destruction, solar cycle effects including relativistic ele ctron precipitation (REP),variable diabatic transport fields and tempe rature, and variable sulfate aerosol surface area density and acidity. Simulated global and midlatitude ozone agree very well with observati ons for the entire period. Mid-1980s near-global ozone declines calcul ated by the model were found to be due to solar cycle (including REP) effects, -1.9%; volcanic effects, -1.5%; dilution effects, -1.1%; tran sport and temperature effects, -1%; and trace gas effects, -0.2%. The maximum effects of these different processes occur at different times. The observed 10% reductions in midlatitude ozone are reproduced in th e simulations and are primarily due to 1 to 2-year transport and tempe rature variations.