Sensitivity study of air trajectory long-range transport modelling

A forward trajectory model for long-term regional scale simulation of transport and deposition of sulphur from a single elevated point source is used for sensitivity tests of the deposition pattern and suspension ratio, subject to a spatial and temporal variable mixing depth, contra constant mixing depth different oxidation rates of SO₂, wet deposition rates and different source heights. The deposition is studied up to 1800 km from the source using meteorological data measured every 6 h covering Europe. For dry conditions, the inclusion of a diurnally variable mixing depth causes longer residence times of the airborne S and larger deposition far from the source. An effective wet deposition rate, however, has a tendency to suppress the change in the dry deposited mass induced by variable mixing height. A change from an oxidation rate of about 1 to 3%h⁻¹ increases both the suspension ratio and the deposition significantly at large distances from the source, while a change to a low value, 0.5 % h⁻¹ has the opposite effect. Halving the wet removal rates results in about 30% larger deposition at remote locations and 30% lower deposition occurs when the removal rates are doubled. Similar changes occur in the suspension ratio. A low source causes about 60% higher local depositions and consequently reduced suspension ratios and distant depositions of about 30%. These results support the assumption that the uncertainty in predictions of total deposition and residence times is within about ±50% when the uncertainty in the parameters considered is taken into account.