Marginal PM2.5: Nonlinear aerosol mass response to sulfate reductions in the Eastern United States

Reductions in airborne sulfate concentration may cause inorganic fine parti culate matter (PM2.5) to respond nonlinearly, as nitric acid gas may transf er to the aerosol phase. Where this occurs, reductions in sulfur dioxide (S O2) emissions will be much less effective than expected at reducing PM2.5. As a measure of the efficacy of reductions in sulfate concentration on PM2. 5 we define marginal PM2.5 as the local change in PM2.5 resulting from a sm all change in sulfate concentration. Using seasonal-average conditions and assuming thermodynamic equilibrium, we find that the conditions for PM2.5 t o respond nonlinearly to sulfate reductions are common in the eastern Unite d States in winter, occurring at half of the sites considered, and uncommon in summer, due primarily to the influence of temperature. Accounting for d iurnal and intraseasonal variability, we find that seasonal-average conditi ons provide a reasonable indicator of the time-averaged PM2.5 response. The se results indicate that reductions in sulfate concentration may be up to 5 0% less effective at reducing the annual-average PM2.5 than if the role of nitric acid is neglected. Further, large reductions in sulfate will also ca use an increase in aerosol nitrate in many regions that are the most acidic .