2 PARAMETRIZATIONS OF THE DRY DEPOSITION EXCHANGE FOR SO2 AND NH3 IN A NUMERICAL-MODEL

The sensitivity of total sulphur + sulphate and ammonia + ammonium dep osition to the descriptions of dry deposition exchange was investigate d by comparing the results from two different dry deposition exchange approaches in a numerical model. The first parametrization depended on the meteorological and biological variables available, and included t he concept of bidirectional NH3-fluxes, by using a compensation point approach. The other parametrization applied a less detailed empirical dry deposition velocity (the base case). Averaged over a year the chan ges in total deposition were a 0-20% reduction and a 0-25% increase in total sulphur and ammonia, respectively, over central Europe when the more detailed description was used compared to the base case. Further away from the emission sources there was a 0-15% increase in total su lphur + sulphate deposition and a 10-50% decrease in total ammonia + a mmonium deposition. Emission of NH3 from stomata was found to be in th e order of 0.1% of the total NH3-emission over Europe, but may give la rge emission contributions on certain days over crop land and where th e average dry deposition velocities can be influenced.