Aerosol and hydrometeor concentrations and their chemical composition during winter precipitation along a mountain slope — III. size-differentiated in-cloud scavenging efficiencies

Abstract

Abstract During three winter seasons a mountain top station at 1620 m a.s.l. in central Switzerland (Mount Rigi) instrumented with an electrical aerosol analyzer, an optical particle counter, a cascade impactor, meteorological sensors, and snow collectors was operated with high temporal resolution. With the obtained detailed data set size-dependent in-cloud scavenging efficiencies were determined. In-cloud scavenging could thus be explained as a combination of nucleation scavenging (for particles with a diameter of 0.02–2 μm) and collection scavenging (by diffusive processes for particles smaller than 0.2 μm, and by impaction processes for those larger than 2 μm). The case-to-case variability of determined efficiencies was very large, being the result of the influence of simultaneous precipitation, the solubility of the aerosols, the number concentration of fine particles ( d a μ m), and the maximum supersaturation reached during cloud formation. The experimentally determined efficiencies agree, at least qualitatively, with current nucleation theories, e.g. by Hanel (1987, Beitr. Phys. Atmosph. 60, 321–339). Field experiments reported by other authors reconfirm the mean total efficiency of 70% (by aerosol volume) found in this study. The inorganic ion concentrations found in aerosols and precipitation indicate that the precipitation first falling contained overproportional amounts of coarse, Ca 2+ -enriched aerosol particles, advocating that the transformation of cloud water to precipitation water favors droplets with pronounced cloud condensation nuclei characteristics. The submicron particles consisting mainly of NO 3 − , SO 4 2− , and NH 4 + are readily incorporated into cloud water, but much less efficiently transformed into precipitation than super-micron particles.