Do we understand trends in atmospheric sulfur species?

Anomalies appear to exist in our understanding of atmospheric sulfur compou nds, specifically as evidenced in the time trends of the different chemical forms of these compounds. Trends determined at a number of locations by se veral different groups seem to indicate that, responding to emission reduct ions across North America, the concentration of SO2 in the atmosphere is de clining more rapidly than that of aerosol SO42-. A number of possible reaso ns for this discrepancy are examined, but it is not possible to provide a d efinitive answer at this stage. The intent is to stimulate debate, because shortcomings in our understanding of the processes involved could have prof ound implications for the credibility of abatement strategies and policies for both acid deposition and fine particulate matter (PM).