Effects of non-equilibrium hygroscopic growth of (NH sub(4)) sub(2)SO sub(4) on dry deposition to water surfaces

• Published in: Environmental science & technology Vol. 32; no. 5; pp. 584 - 590
• Main Authors: Zufall, MJ, Bergin, M H, Davidson, C I
• Format: Journal Article
• Language: English
• Published: 01.03.1998
• Subjects: Brackish; Freshwater; Marine
• ISSN: 0013-936X

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. A model is presented here that combines the relative humidity profile above water surfaces with hygroscopic growth rates for (NH sub(4)) sub(2)SO sub(4), assuming cases for a deliquescing and metastable aerosol. Model results show that particles greater than 0.1 mu m 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 deliquescing aerosols. Based on measured (NH sub(4)) sub(2)SO sub(4) 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.