Some approximations for the wet and dry removal of particles and gases from the atmosphere

Abstract

Semi-empirical formulae are presented which can be used to estimate precipitation scavenging and dry deposition of particles and gases. The precipitation scavenging formulae are appropriate both for in-and below-cloud scavenging and comparisons with data indicate the importance of accounting for aerosol particle growth by water vapor condensation and attachment of the pollutant to plume or cloud particles. It is suggested that both wet and dry removal of gases is usually dictated by other than atmospheric processes. Dry deposition of particles to a canopy is shown to depend on canopy height, biomass, vegetative type and mean wind. Two large-scale practical problems are addressed dealing with the relative importance of wet and dry deposition and with the sources which contribute to deposition in a specific location.Essentially all air pollution is eventually cleansed from the atmosphere by the natural processes generally referred to as precipitation scavenging and dry deposition. The purpose of this report is to present formulae which can be used to approximately describe these cleansing processes. Within the details of the development of these formulae, some unifying features may not be apparent and it may be worthwhile to mention them here. One is that in all cases, whether the removal is by rain, snow, grass, leaves, water surfaces or whatever, the efficiency with which the pollutant is removed from an air stream by an obstacle must be considered. This collection efficiency is usually written as the product of a collision efficiency, defined in Figure 1, multiplied by a retention efficiency. In most cases, out of ignorance, the retention efficiency will be taken to be unity although some comments will be made about the retention of aerosol particles by vegetation and the desorption of gases from liquids. A second unifying feature is that after a collection efficiency has been obtained, it is necessary to sum over all collecting elements to obtain an overall removal rate and usually these integrals must be rather severely

Scavenging of ultrafine particles by rainfall at a boreal site: observations and model estimations

“…In this case, the model predicted L eff is significantlly smaller than L o . 3) based on Scott (1982) and consistent with the results of Slinn (1977). Based on the temperature distribution near the ground for the samples used, the number of cases with possible snow precipitation in-cloud is quite limited and the overall influence on L o is low as well.…”

“…The physics of this process is the following: as UFP are drawn into the cloud, they are activated as CCN and grow to a cloud droplet size with a diameter ∼10 µm. The collection efficiency E I C for cloud droplets of 10 µm in diameter varies between 0.5 and 0.8 when the collectors are raindrops with diameter D p ∼0.2−2 mm (Slinn, 1977). These considerations apply also for highly soluble aerosol, or aerosol attached to material that is highly soluble, such that particles can grow to a droplet size.…”

Aerosol data assimilation in the chemical transport model MOCAGE during the TRAQA/ChArMEx campaign: aerosol optical depth

“…The sedimentation is implemented as described in Seinfeld and Pandis (1998). For the wet deposition, the model uses Giorgi and Chameides (1986) for the implementation of the in-cloud scavenging, and Slinn (1977) for the rain and snowfall below-cloud scavenging.…”

Ultrafine particle scavenging coefficients calculated from 6 years field measurements

“…Dingle and Hardy, 1962;Cataneo, 1973;Harju and Jatila, 1973) where the rain intensity and the rain droplet number density were simultaneously measured. Several distributions (Slinn, 1977;Mircea and Stefan, 1997) have been fitted to these measurements, the most widely used is the MarshallPalmer (MP) distribution (Marshal and Palmer, 1948). However, rain properties can show much variation (Slinn, 1983), so the MP-distribution is valid only for average conditions.…”

“…Several attempts (Slinn, 1977;Fenton, 1980;Levine and Schwartz, 1982) to measure or estimate collision efficiencies have been made but the difference between theory and measurements is still one or two orders of magnitude in the submicron range (Volken and Schumann, 1993). The main phenomena affecting collision efficiency between falling droplets and aerosol particles are inertial impaction (Pruppacher and Klett, 1978), Brownian diffusion, phoresis caused by thermal or concentration gradients, turbulent effects and electrical forces.…”