Acid dew and the role of chemistry in the dry deposition of reactive gases to wetted surfaces

Abstract

Using the resistance model for mass transfer, a formulation is derived for the dry deposition rate of a reactive, soluble gas depositing on a wetted, vegetative surface. The deposition velocity is expressed in terms of a dynamical resistance, which is a function of the physical state of the atmosphere, and a surface resistance, which is a function of the chemical and physical properties of the surface. This formulation is then used in a model to simulate the generation of acidic dew from the dry deposition of HNO3 and SO2, as well as the SIV oxidants, H2O2 and O3. Dewdrop pHs of about 4 are calculated by the end of the night, however these pHs can rapidly fall to potentially toxic levels soon after sunrise as the dewdrops evaporate. The deposition velocities to the dew for highly soluble species such as HNO3 and H2O2 are found to be entirely determined by the dynamical resistance; for the conditions adopted here a value of 0.4 cm s−1 is calculated for these species. However, much smaller deposition velocities are predicted for SO2 and O3 because of their lower solubilities and hence larger surface resistances. In the case of SO2, a nocturnally averaged deposition velocity of only about 0.03 cm s−1 is calculated for the standard model. Because the chemical lifetime of SO2 in the dew is influenced by the atmospheric levels of H2O2, O3, and SO2, the SO2 deposition velocity is found to be a strong function of these species' atmospheric abundances. These results imply that the deposition velocities of species such as SO2 to wetted surfaces may be influenced by the chemical as well as the physical state of the atmosphere; the assumption typically adopted that dry deposition velocities are independent of the species' atmospheric abundance may not always be appropriate.