The scavenging of two different types of marine aerosol particles calculated using a two-dimensional detailed cloud model

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Our 2-D dynamic model including spectral microphysics and scavenging has been evaluated for a warm precipitating convective cloud at Day 261 (18 September 1974) of the GATE campaign. Two different chemical species ((NH4)2SO4 and NaCl) of aerosol particles were followed in the air, inside the drops in the cloud, and inside the drops reaching the ground. Concerning the dynamics and microphysics, as well as the scavenging and wet deposition, the model results agree quite well with available observations. The cloud rained after 19 min of cloud life time. For the considered aerosol loading of the atmosphere, rough estimates are derived for the total material processed by such a warm convective cloud as input for larger scale models. In particular, the following conclusions could be drawn for the situation considered. (1) If a drop spectrum forms on an aerosol spectrum where the small particles consist of (NH4)2SO4 and the large ones of NaCl, the resulting small drops also mainly consist of (NH4)2SO4 and the larger drops of NaCl. Collision and coalescence causes a redistribution of the chemical species such that the precipitation sized drops consist of NaCl to about 70%. (2) The mixing ratio of aerosol material in the drops is a function of the age of the drops and their history and therefore the variation of the mixing ratio with drop size depends on the entrainment and evoluion of the relative humidity. The mixing ratio decreased with increasing drop radius at almost all grid points due to continuous activation of fresh particles. (3) Assuming that the sulfate aerosol would not consist of (NH4)2SO4 particles but instead consist of NH4 HSO4 particles the acidic cloud water has a pH of 4.7 which agrees with observations of marine precipitation. (4) The scavenging efficiency of the cloud considered is closely related to its precipitation efficiency (both near 40%). About 90% of the total amount of aerosol material scavenged is incorporated into the cloud water through nucleation scavenging. About 30% of the aerosol material in the rain at the ground is due to scavenging below cloud base. (5) The scavenging coefficient of the considered storm was on the order of 10-4 s-1. Differences between two commonly used formulations for calculating the scavenging coefficient Λ can be explained in terms of humidity changes of the atmosphere.DOI: 10.1034/j.1600-0889.1991.t01-2-00004.x
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