This study describes the application of atmospheric modelling to assess transboundary transport from the USA and the influence of natural emissions sources for the 12 most populated sub regions across Ontario, Canada. The impact of Ontario emissions was determined by the difference between model runs including all emissions and model runs with Ontario’s anthropogenic NOx, SO2, VOC and primary particulate matter emissions shut off. The modelling has been done for the entire May through September 1998 period. Since there were many days with high ozone and/or high PM2.5 that summer, the model runs provide estimates of the variations in Ontario’s impact from episode to episode and from the spring to the summer.

A comprehensive mercury model system using the Eulerian reference frame of the Acid Deposition and Oxidant Model (ADOM) has been developed under the Canada–Germany Science & Technology Co-operation Agreement and applied within the European Union MArine Science and Technology–BAltic Sea SYstem Study (MAST-III-BASYS) and the Environment & Climate project Mercury Species over Europe (MOE), to study the regional transport and deposition fluxes of atmospheric mercury species. The model is able to simulate long-range transport of mercury over the entire depth of the troposphere with a basic time step of 1h and incorporates current knowledge of physico-chemical forms and transformation reactions of atmospheric mercury species. Model predicted concentration and deposition pattern of mercury species over Europe are presented and concentrations of total gaseous mercury in ambient air and total mercury in precipitation calculated by the model are compared with observed values from a BASYS monitoring network study in February/March 1998. Concentrations in air agree within a factor of about 2 with observed values, thus indicating that the model is capable of reproducing observations satisfactorily even on an hourly basis. Observed monthly average concentrations in precipitation at four monitoring stations at the Baltic Sea coast are reproduced by the model within a factor of 1.3 suggesting that the chemical scheme in the model is based on an adequate parameterisation of aqueous phase chemistry.

The importance of the three major aqueous reactions thought to be responsible for the in-cloud conversion of SO2 to SO42- was studied using the acidic deposition and oxidants model by supressing each reaction individually and all reactions simultaneously. The reactions are the oxidation of SO2 by H2O2, or O3 and catalytic oxidation by O2 in the presence of Fe and Mn. The model simulations were 19–24 April 1981. It was found that SO42- precipitation concentrations were generally more sensitive to H2O2 oxidation than to O3 oxidation. The contribution of catalytic oxidation of SO2 in the presence of Fe and Mn is insignificant everywhere and at all times. The contributions of H2O2 oxidation to SO42- in precipitation is strongest in light precipitation areas while O3 oxidation can be greater than H2O2 oxidation in heavy precipitation areas. The effect of supressing one reaction is mitigated by compensation through another mechanism. This is seem from the significant difference observed in the effects when individual suppressions were added together and when all reactions were suppressed simultaneously. From this, it is estimated that the contribution of aqueous oxidation of SO2 to SO42- in precipitation is approximately 50–80 per cent. Further simulations show that the relationship between SO2 emissions and SO42- production in the aqueous-phase through the oxidation reaction with O3 is always non-linear in view of the pH dependence of the reaction rates.

An Eulerian atmospheric model with complex chemistry (Acidic Deposition and Oxidant Model) and a Lagrangian model with linear chemistry (Ontario Ministry of the Environment Trajectory Model) were used to simulate the wet SO42− deposition pattern over eastern North America for 16 days during April 1981.