A review of current knowledge concerning size-dependent aerosol removal

The status of current knowledge on size-dependent aerosol removal by dry and wet processes, including dry deposition and impaction and nucleation scavenging, is reviewed. The largest discrepancies between theoretical estimations and measurement data on dry deposition and below-cloud scavenging are for submicron particles. Early dry deposition models, which developed based on chamber and wind tunnel measurements, tended to underestimate dry deposition velocity (Vd) for submicron particles by around one order of magnitude compared to recent field measurements. Recently developed models are able to predict reasonable Vd values for submicron particles but shift unrealistically the predicted minimum Vd to larger particle sizes. Theoretical studies of impaction scavenging of aerosol particles by falling liquid drops also substantially underestimate the scavenging coefficients for submicron particles. Empirical formulas based on field measurements can serve as an alternative to the theoretical scavenging models. Future development of size-resolved impaction scavenging models needs to include more precipitation properties (e.g., droplet surface area) and to be evaluated by detailed cloud microphysical models and available measurements. Several recently developed nucleation scavenging parameterizations for in-cloud removal of interstitial aerosol give comparable results when evaluated against parcel models; however, they need to be verified once suitable field measurements are available. More theoretical and field studies are also needed in order to better understand the role of organic aerosols in the nucleation scavenging process.