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 toanalyse the contribution of the different terms in the continuity equationto the budget of ozone, NOx and PAN. Both models cover largeparts of Europe and describe the processes relevant for troposphericchemistry and dynamics. One of the models is designed to simulate episodesin the order of 1–2 weeks (EURAD), the other is focussing on theseasonal scale (LOTOS). Based on EURAD simulations it is found that theatmospheric boundary layer (ABL) in Central Europe during a summer-smogepisode in 1990 acts as a source of ozone, which is partly exported from theproduction region in Central Europe. About 40% of the ozone producedchemically in the ABL is lost from Central Europe due to net transport(large-scale and turbulent), 40% are deposited within the domain. Vertical mass exchange of ozone is dominated by the prevailing subsidenceand averaged vertical mass fluxes are directed downward. Averaged massfluxes of PAN, which has no stratospheric source, are upward in the upperpart of the ABL. The results from LOTOS are discussed for the same episodeand for a two month period (July/August 1990). The budget calculation showlarger chemical production for the LOTOS model compared to EURAD. Therelative importance of deposition and net transport, however, is in the sameorder. Differences between the two-month calculation and the one weekepisode are only important for Western Europe where the chemical production is enhanced by 30% during the summer-smog episode. The dependence ofthe results on initial and boundary values is discussed for ozone on thebasis of a simple sensitivity study with EURAD where ozone in the FT is setto 10 ppb initially. This leads to a reversal in the direction of averagedozone mass fluxes in the upper part of the ABL.