Long-range transport to Europe: Seasonal variations and implications for the European ozone budget

We use a chemical transport model (GEOS-CHEM) to quantify the contribution of long-range transported pollution to the European ozone (O-3) budget for the year 1997. The model reproduces the main features observed over Europe for O-3, carbon monoxide and nitrogen dioxides, as well as two events of enhanced O-3 of North American origin over the eastern North Atlantic and over Europe. North American O-3 fluxes into Europe experience a maximum in spring and summer, reflecting the seasonal variation in photochemical activity and in export pathways. In summer, North American O-3 enters Europe at higher altitudes and lower latitudes because of deep convection, and because the flow over the North Atlantic is mostly zonal in that season. The low-level inflow is only important in spring, when loss rates in the boundary layer over the North Atlantic are weaker. Asian O-3 arrives mainly via the westerlies, and usually at higher altitudes than North American O-3 because of stronger deep convection over Asia. In addition, Asian O-3 fluxes are at a maximum in summer during the monsoon period because of enhanced convection over Asia, increased nitrogen oxides sources from lightning and direct transport towards Europe via the monsoon easterlies. Over Europe, total background accounts for 30 ppbv at the surface. North American and Asian O-3 contribute substantially to the annual O-3 budget over Europe, accounting for 10.9% and 7.7%, respectively, while the European contribution only accounts for 9.4%. We find that in summer, at the surface, O-3 decreases over Europe from 1980 to 1997, reflecting the reduction of European O-3 precursor emissions. In the free troposphere, this decrease is compensated by the increase in O-3 due to increasing Asian emissions. This may explain the lack of trends observed over most of the European region, especially at mountain sites.