Characterization of the Nonlinear Change in Annual Sulfur Deposition for a Change in Emissions

Abstract

A central question for atmospheric processes research posed by the National Acid Precipitation Assessment Program (NAPAP) of the United States has been: What degree of nonproportionality in the response in sulfur deposition is expected to be associated with a specified change in precursor emissions? Nonlinearity (or nonproportionality) exists where the fractional changes in emissions of a primary pollutant are not matched by a commensurate (proportional) change in the deposition of its primary or secondary product. Nonlinearity is only expected to occur in wet deposition processes. When sulfur dioxide in the aqueous phase is converted to sulfate, the most reactive oxidant, hydrogen peroxide, can become depleted in the cloud (oxidant limited). During the in-cloud-water conversion if there is more sulfur dioxide than is needed to react fully with available hydrogen peroxide, the hydrogen peroxide can be depleted and unreacted sulfur dioxide left over (an excess). Subsequently, if emissions of sulfur dioxide are reduced, the “excess” sulfur dioxide is reduced, leading to less than proportional (nonlinear) reductions in sulfate deposition. One concern is that if emission controls are implemented, deposition may not reduce in proportion to emissions reductions.