Mass Accommodation Coefficient of H2SO4Vapor on Aqueous Sulfuric Acid Surfaces and Gaseous Diffusion Coefficient of H2SO4in N2/H2O

The experimental determination of the mass accommodation coefficient of H 2 SO 4 vapor on aqueous sulfuric acid and the gas-phase diffusion coefficient of H 2 SO 4 vapor in N 2 /H 2 O at 303 K is reported. The measurements were carried out under laminar flow conditions in a coated wall tubular flow reactor coupled to a chemical ionization mass spectrometer for gas-phase detection. Wall loss rates of H 2 SO 4 vapor, from which both the mass accommodation coefficient and the gas diffusion coefficient were determined, were measured as a function of total reactor pressure, water vapor concentration, and sulfuric acid vapor concentration. The observed wall loss rate coefficient depends linearly on the inverse of the total reactor pressure (0.54−10 Torr) and is independent of the aqueous sulfuric acid composition over the range 73−98 wt %, which was varied by the addition of water vapor. A kinetic model based on the additivity of kinetic resistances that couples gas-phase diffusion and mass accommodation to the measured H 2 SO 4 vapor loss rate has been applied to the data. The model yields a lower limit of 0.43 with a best fit value of 0.65. The mass accommodation coefficient is independent of the liquid H 2 SO 4 /H 2 O composition over the range investigated. The gas-phase diffusion coefficient for H 2 SO 4 vapor in N 2 /H 2 O (H 2 O mixing ratio ≤0.32) was determined to be 66.8 ± 1.1 Torr cm 2 s -1 . The resistance model agrees well with a more rigorous approximate solution to the full continuity equation describing mass transport and kinetics. The atmospheric implications of the reported results are discussed.