The importance of new collection efficiency values including the effect of rear capture for the below-cloud scavenging of aerosol particles

A numerical study is presented to evaluate the possible impact of recently measured collection scavenging efficiencies of aerosol particles by raindrops on the calculation of wet removal of pollution plumes by rain. These new collection efficiencies cover the previously undocumented range between 0.3 mu m and 3.5 mu m diameter for the aerosol particles and 2 to 2.6 mm diameter for the drops. They show for particles between 03 mu m and 0.7 mu m a region of an important increase of the efficiencies with decreasing particle size probably due to the capture of particles in the rear vortex developing behind the falling drop. This hypothesis was motivated by measurements using particle image velocimetry (PIV). Also for the particles larger than 1.5 mu m the efficiencies exceed old approximations by up to a factor of two. Typical continental and maritime aerosol particle distributions were used for background and plume distributions and each time the deposited particle mass was calculated with the old and the new efficiencies for the different resulting precipitation rates. In the simulations the new efficiencies increased the calculated wet removal rate of pollution plume particles between 5% and 17%, with respect to the simulations with the old efficiencies, whereby one third of this increase could be attributed to rear capture. This phenomenon has not been observed in these size ranges before. The study also highlights the weakness of certain methods to determine the scavenging coefficient of a pollution plume by rain. Instead of linking the scavenging coefficient to the decrease of the particle spectrum in the air, an operational approach, e.g. in case of accidental releases, needs to be developed that links it to the rainfall intensity, as well as other variables like meteorological parameters, cloud size and plume characteristics. Considering the still persisting gaps of measurements in the collection efficiencies more laboratory measurements of collection scavenging efficiencies between aerosol particles and raindrops are also necessary. (C) 2013 Elsevier B.V. All rights reserved.