An inorganic aerosol equilibrium model is used to investigate the response of inorganic particulate matter (PM) concentrations with respect to the precursor concentrations of sulfuric acid, ammonia, and nitric acid over a range of temperatures and relative humidities. Diagrams showing regions of PM response to precursor concentrations are generated, thus allowing the qualification of assumptions concerning the response of PM to sulfate and overall sensitivity to ammonia and nitric acid availability. The PM concentration level responds nonlinearly to sulfate and shows overall sensitivity to ammonia and nitric acid availability for specific atmospheric conditions and precursor concentrations. The generated diagrams are applied as a means of approximating the PM response to precursor concentrations for two urban polluted areas. In both cases, reductions in ammonia emissions have the most significant impact on the total PM level. However, such a reduction will result in significant increases in atmospheric acidity.

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.

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The hygroscopic nature of atmospheric aerosol has generally been associated with its inorganic fraction. In this study, a group contribution method is used to predict the water absorption of secondary organic aerosol (SOA). Compared against growth measurements of mixed inorganic−organic particles, this method appears to provide a first-order approximation in predicting SOA water absorption. The growth of common SOA species is predicted to be significantly less than common atmospheric inorganic salts such as (NH4)2SO4 and NaCl. Using this group contribution method as a tool in predicting SOA water absorption, an integrated modeling approach is developed combining available SOA and inorganic aerosol models to predict overall aerosol behavior. The effect of SOA on water absorption and nitrate partitioning between the gas and aerosol phases is determined. On average, it appears that SOA accounts for approximately 7% of total aerosol water and increases aerosol nitrate concentrations by approximately 10%. At high relative humidity (≥85%) and low SOA mass fractions (<20% of total PM2.5), the role of SOA in nitrate partitioning and its contribution to total aerosol water is negligible. However, the water absorption of SOA appears to be less sensitive to changes in relative humidity than that of inorganic species, and thus at low relative humidity (∼50%) and high SOA mass fraction concentrations (∼30% of total PM2.5), SOA is predicted to account for approximately 20% of total aerosol water and a 50% increase in aerosol nitrate concentrations. These findings could improve the results of modeling studies where aerosol nitrate has often been underpredicted.