Two parametrizations of the dry deposition exchange for SO2 and NH3 in a numerical model

The sensitivity of total sulphur + sulphate and ammonia + ammonium deposition to the descriptions of dry deposition exchange was investigated 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 the 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 changes 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 sulphur + sulphate deposition and a 10-50% decrease in total ammonia + ammonium deposition. Emission of NH3 from stomata was found to be in the order of 0.1% of the total NH3-emission over Europe, but may give large emission contributions on certain days over crop land and where the average dry deposition velocities can be influenced.