Multi-parcel lagrangian model for quantification of influence of alpine air mass exchange on photo-oxidant production

The Swiss plateau, embedded between the Jura and the Alps, has high population density with strong anthropogenic emissions. The main processes leading to exchange of air masses between the Swiss plateau and the Alps are the thermally produced valley winds transporting pollution into the Alpine valleys during the day and their respective backflush during the night. During a cloudless summer day, about 40% of the whole air mass under the mixing height over the Swiss plateau is exchanged with Alpine air masses. In the present case study for 29 July 1993, the dynamics including this exchange process is calculated on three different scales by means of wind trajectories. The valley wind systems are described on a grid with a resolution of 500 m using scale-related parameterizations for each relevant physical process. The regional wind field and the long-range trajectories are obtained from different models, taking into account the general weather pattern. The chemistry is calculated in the framework of a Lagrangian model using the CBM IV mechanism. The anthropogenic emissions over the Swiss plateau as well as the biogenic hydrocarbons emitted by the forests are considered. Both emission inventories are given on a grid with a 5 km resolution. While moving over the ground, emissions and depositions of the species are simulated, taking into account mixing and exchange with air parcels of the upper layer. The results show a clear overall NOx limited ozone production situation in the Swiss plateau and a marginal influence of biogenic VOC emisions. Only south of the Alps and near the northeastern border the VOC decrease could lead to a substantial reduction of the ozone levels under weather conditions represented in this study.