A NEW PARAMETERIZATION OF H2SO4 H2O AEROSOL COMPOSITION - ATMOSPHERICIMPLICATIONS/

Recent results from a thermodynamic model of aqueous sulfuric acid are used to derive a new parameterization for the variation of sulfuric a cid aerosol composition with temperature and relative humidity. This f ormulation is valid for relative humidities above 1 % in the temperatu re range of 185 to 260 K. An expression for calculating the vapor pres sure of supercooled liquid water, consistent with the sulfuric acid mo del, is also presented. We show that the Steele and Hamill [1981] form ulation underestimates the water partial pressure over aqueous H2SO4 s olutions by up to 12% at low temperatures. This difference results in a corresponding underestimate of the H2SO4 concentration in the aeroso l by about 6 % of the weight percent at approximately 190 K. In additi on, the relation commonly used for estimating the vapor pressure of H2 O over supercooled liquid water differs by up to 10 % from our derived expression. The combined error can result in a 20 % underestimation o f water activity over a H2SO4 solution droplet in the stratosphere, wh ich has implications for the parameterization of heterogeneous reactio n rates in stratospheric sulfuric acid aerosols. The influence of aero sol composition on the rate of homogeneous ice nucleation from a H2SO4 solution droplet is also discussed. This parameterization can also be used for homogeneous gas phase nucleation calculations of H2SO4 solut ion droplets under various conditions such as in aircraft exhaust or i n volcanic plumes.