Effects of non-equilibrium hygroscopic growth of (NH<inf>4</inf>)<inf>2</inf>SO<inf>4</inf> on dry deposition to water surfaces


Growth of hygroscopic aerosols near water surfaces is believed to enhance dry deposition rates, which are a strong function of particle size. Previous dry deposition models estimate hygroscopic growth by assuming equilibrium between aerosols and water vapor (Williams, R. M. Atmos. Environ. 1982, 16, 1933-1938). A model is presented here that combines the relative humidity profile above water surfaces with hygroscopic growth rates for (NH4)2SO4, assuming cases for a deliquescing and metastable aerosol. Model results show that particles greater than 0.1 μm in diameter do not grow to their equilibrium size before depositing to a hypothetical water surface. As a consequence, equilibrium models overpredict the effects of hygroscopic growth on deposition velocities by as much as a factor of 5. In addition, model results suggest a significant difference in the deposition velocities of metastable and deliquencing aerosols. Based on measured (NH4)2SO4 size distributions, overall deposition velocities calculated from a thermodynamic equilibrium model, a mass transfer limited non-equilibrium model with a deliquescing aerosol, and a mass transfer limited non-equilibrium model with a metastable aerosol are 0.11, 0.055, and 0.040 cm/s, respectively.