Influence of mountain venting in the Alps on the ozone chemistry of the lower free troposphere and the European pollution export

The influence of mountain venting over the Alps on ozone (O₃) mixing ratios in the lower free troposphere (FT) and on export of European emissions was investigated with a Lagrangian chemical box-model system. During summertime fair-weather conditions mountain venting is an efficient tropospheric lifting mechanism that carries atmospheric boundary layer (ABL) pollutants to the FT. Little is known about the effect of mountain venting on net O₃ production in the FT. Chemistry in a FT and an ABL air parcel was initialized with typical European summertime background FT and Alpine ABL mixing ratios, respectively, which were derived from measurements. A parameterization was developed based on previous quantification of mountain venting in the Swiss Alps, describing the injection of air from the ABL into the FT. For simulations at constant altitude (3500 m above mean sea level (MSL)), net O₃ production was generally positive for day 0 of the simulation, when mountain venting occurred. Whether O₃ was produced or destroyed from day 1 onward critically depended on initial nitrogen oxides (NO_x) plus peroxy acetyl nitrates (PANs) mixing ratios in the ABL, and to a smaller extent on initial H₂O and O₃ mixing ratios, photolysis rates, and PANs chemistry. For a simulation period of 8 days O₃ mixing ratios remained higher in the simulations with venting than in simulations for the FT background without venting. Simulations on cluster average forward trajectories initialized over the Alps showed that European emissions vented in the Alps have a strong influence on the Mediterranean region. O₃ production on these trajectories was enhanced by the release of NO_x from PANs that acted as a reservoir species while the air parcel was descending over the Mediterranean. The O₃ production efficiency with regard to NO_x was about 20 molecules O₃ produced per molecule NO_x consumed at 3500 m MSL on day 0 of the venting simulation. On the southward moving trajectories O₃ production efficiency was in the range of 6-11 until day 3 of the simulation when all injected NO_y was consumed.