Do we understand trends in atmospheric sulfur species?

Anomalies appear to exist in our understanding of atmospheric sulfur compounds, specifically as evidenced in the time trends of the different chemical forms of these compounds. Trends determined at a number of locations by several different groups seem to indicate that, responding to emission reductions across North America, the concentration of SO2 in the atmosphere is declining more rapidly than that of aerosol SO4(2-). A number of possible reasons for this discrepancy are examined, but it is not possible to provide a definitive answer at this stage. The intent is to stimulate debate, because shortcomings in our understanding of the processes involved could have profound implications for the credibility of abatement strategies and policies for both acid deposition and fine particulate matter (PM).

The predicted effect of SO2 emission controls on the water quality of eastern Canadian lakes.

Changes in SO inf4 (sup2-) deposition predicted to occur in response to implementation of announced SO2 emission control programs in Canada and the U.S.A. have been used as input to water chemistry models thereby giving an estimate of the changes in lake acid neutralizing capacity (ANC) and pH that can be expected from these programs. Eastern Canada has been divided into 22 subregions for the purpose of this analysis. Relative to the current level (1982-86) of SO inf4 (sup2-) deposition (Scenario 1), the effect of the Canadian SO2 emission control program alone (Scenario 2) is compared to that obtained when controls are implemented throughout North America (Scenarios 3 and 4). SO2 emission reduction will effect a shrinkage of the high wet SO inf4 (sup2-) deposition field in NE North America such that under Scenario 4 conditions, almost no area will remain in Canada that receives >20 kg ha(-1) yr(-1). The greatest decrease in deposition and resulting change in lake chemistry occurs in southern Ontario and southwestern Quebec. ANC distributions shift to higher concentrations and the percentage of lakes having pH<6 decreases in these areas. The Atlantic Provinces will obtain only a minor benefit from the control programs, i.e. experiencing only a small decrease in deposition and improvement in water quality. High sensitivity of the terrain in many parts of Atlantic Canada means that large numbers of lakes will remain acidic (i.e. ANC<0) and/or have pH<6 (an important biological threshold) even after full implementation of the current plans for SO2 control in Canada and the U.S.