A new parameterization of H2SO4/H2O aerosol composition: Atmospheric implications

Recent results from a thermodynamic model of aqueous sulfuric acid are used to derive a new parameterization for the variation of sulfuric acid aerosol composition with temperature and relative humidity. This formulation is valid for relative humidities above 1% in the temperature range of 185 to 260 K. An expression for calculating the vapor pressure of supercooled liquid water, consistent with the sulfuric acid model, is also presented. We show that the Steele and Hamill 1981 formulation underestimates the water partial pressure over aqueous H 2 SO 4 solutions by up to 12% at low temperatures. This difference results in a corresponding underestimate of the H 2 SO 4 concentration in the aerosol by about 6% of the weight percent at approximately 190 K. In addition, the relation commonly used for estimating the vapor pressure of H 2 O over supercooled liquid water differs by up to 10% from our derived expression. The combined error can result in a 20% underestimation of water activity over a H 2 SO 4 solution droplet in the stratosphere, which has implications for the parameterization of heterogeneous reaction rates in stratospheric sulfuric acid aerosols. The influence of aerosol composition on the rate of homogeneous ice nucleation from a H 2 SO 4 solution droplet is also discussed. This parameterization can also be used for homogeneous gas phase nucleation calculations of H 2 SO 4 solution droplets under various environmental conditions such as in aircraft exhaust or in volcanic plumes.