Reductions in airborne sulfate concentration may cause inorganic fine particulate matter (PM 2.5 ) to respond nonlinearly, as nitric acid gas may transfer to the aerosol phase. Where this occurs, reductions in sulfur dioxide (SO 2 ) emissions will be much less effective than expected at reducing PM 2.5 . As a measure of the efficacy of reductions in sulfate concentration on PM 2.5 , we define marginal PM 2.5 as the local change in PM 2.5 resulting from a small change in sulfate concentration. Using seasonalaverage conditions and assuming thermodynamic equilibrium, we find that the conditions for PM 2.5 to respond nonlinearly to sulfate reductions are common in the eastern United States in winter, occurring at half of the sites considered, and uncommon in summer, due primarily to the influence of temperature. Accounting for diurnal and intraseasonal variability, we find that seasonal-average conditions provide a reasonable indicator of the time-averaged PM 2.5 response. These results indicate that reductions in IMPLICATIONS Reductions in SO 2 emissions are likely to be less effective than expected at reducing annual average PM 2.5 at many locations in the eastern United States, due to the consequent increase in aerosol nitrate. Where SO 2 emissions reductions are ineffective, a combination of controls on SO 2 with oxides of nitrogen (NO x ) or ammonia, or controls on organics, may be necessary to reduce PM 2.5 . Controls on NO x proposed to address ozone standards may therefore be a necessary component of a strategy to reduce PM 2.5 . This work also highlights the need for gasphase measurements of nitric acid and ammonia in order to estimate the PM 2.5 response.