Atmospheric aerosol scavenging processes and the role of thermo- and diffusio-phoretic forces

• Survey of in-cloud and below-cloud scavenging due to ice-crystals
• Contradictory results in laboratory and field experiments concerning the role of phoretic forces.
• Open questions and suggestions for future researches

In-cloud and below-cloud scavenging due to snow crystals is reviewed, outlining the theoretical models, laboratory and field measurements which take into account also the role of phoretic forces in this process. In-cloud scavenging includes contributions from both nucleation and impaction, while below-cloud scavenging includes only impaction. Scavenging of aerosol particles by ice has been modelled only for simple shapes (planar and columnar ice crystals) and restricted size range, in view of the large variety of shapes and, consequently, the complicated flow patterns of air, water vapour and heat around the crystal. A significant feature of theoretical efficiency curves is the predominant minimum for aerosol particles of radius between 0.01 and 0.1 μm where phoretic forces are active, analogous to the particle scavenging behaviour of water drops.Experiments on aerosol particle scavenging by snow include field measurements, experiments where natural snow crystals are allowed to fall through laboratory generated aerosol, and experiments where both crystals and aerosol are generated in the laboratory. Contradictory results have been found in laboratory and field experiments concerning the role of phoretic forces. In particular, an important discrepancy arises relating to the roles of thermophoresis and diffusiophoresis in the scavenging of submicron particles by ice crystals growing in mixed-phase clouds, consisting of water vapour, supercooled liquid droplets and ice particles.A decrease in scavenging efficiency as a function of crystal diameter is reported both theoretically and experimentally. By comparing aerosol scavenging by drops and snow, most studies agree that, in terms of equal mass of precipitation, snow is more efficient at scavenging atmospheric particles than rain.