The effect of metastable equilibrium states on the partitioning of nitrate between the gas and aerosol phases

With the aid of three atmospheric aerosol equilibrium models, we quantify the effect of metastable equilibrium states (efflorescence branch) in comparison to stable (deliquescence branch) on the partitioning of total nitrate between the gas and aerosol phases. On average, efflorescence branch concentrations of aerosol nitrate are 11% greater than those of the deliquescence branch at low aerosol nitrate concentrations (<8 μg m−3), whereas for higher aerosol nitrate concentrations (>8 μg m−3), deliquescence branch concentrations are 3% greater. In the low aerosol nitrate range, approximately 40% of the time deliquescence and efflorescence branch concentrations of aerosol nitrate have differences greater than 20% implicating the importance of considering both branches of aerosol behavior in this region. The largest differences between the two equilibrium states occur at several sets of conditions: at temperatures above 295 K and mid-range rh (60%), at mid-range temperatures (290–300 K) and low rh (<40%), and for sulfate-to-aerosol nitrate molar ratios of less than 0.5 and greater than 1 at low rh (<40%). In these two regions, average differences of 1–2 μg m−3 between deliquescence and efflorescence branch concentrations of aerosol nitrate are estimated. The potential existence of efflorescence branch aerosols in Southern California, where pollutant levels are high, appears to have a small effect on total nitrate partitioning. However, for areas characterized by moderate-to-low pollutant levels such as the Northeastern US, a significantly larger effect is predicted. The implications of these findings for modeling studies are discussed.