BUDGET CALCULATIONS FOR OZONE AND ITS PRECURSORS - SEASONAL AND EPISODIC FEATURES BASED ON MODEL SIMULATIONS

Results from two air quality models (LOTOS, EURAD) have been used to a nalyse the contribution of the different terms in the continuity equat ion to the budget of ozone, NOx and PAN. Both models cover large parts of Europe and describe the processes relevant for tropospheric chemis try and dynamics. One of the models is designed to simulate episodes i n the order of 1-2 weeks (EURAD), the other is focussing on the season al scale (LOTOS). Based on EURAD simulations it is found that the atmo spheric boundary layer (ABL) in Central Europe during a summer-smog ep isode in 1990 acts as a source of ozone, which is partly exported from the production region in Central Europe. About 40% of the ozone produ ced chemically in the ABL is lost from Central Europe due to net trans port (large-scale and turbulent), 40% are deposited within the domain. Vertical mass exchange of ozone is dominated by the prevailing subsid ence and averaged vertical mass fluxes are directed downward. Averaged mass fluxes of PAN, which has no stratospheric source, are upward in the upper part of the ABL. The results from LOTOS are discussed for th e same episode and for a two month period (July/August 1990). The budg et calculation show larger chemical production for the LOTOS model com pared to EURAD. The relative importance of deposition and net transpor t, however, is in the same order. Differences between the two-month ca lculation and the one week episode are only important for Western Euro pe where the chemical production is enhanced by 30% during the summer- smog episode. The dependence of the results on initial and boundary va lues is discussed for ozone on the basis of a simple sensitivity study with EURAD where ozone in the FT is set to 10 ppb initially. This lea ds to a reversal in the direction of averaged ozone mass fluxes in the upper part of the ABL.