EFFECTS OF NONEQUILIBRIUM HYGROSCOPIC GROWTH OF (NH4)(2)SO4 ON DRY DEPOSITION TO WATER SURFACES

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