Cloud chemistry at Mt Rigi, Switzerland: Dependence on drop size and relationship to precipitation chemistry

The chemical composition of winter and spring cloud water sampled at 1620 masl elevation on Mt Rigi in central Switzerland was dominated by NO3-, SO42-, NH4+ and H+. A wide range of concentration levels was observed, with maxima of 3700, 1800 and 4600 micronormal for NO3-, SO42- and NH4+, respectively. Concentrations at a lower elevation (1030 masl) site on the mountain were higher due to lower cloud liquid water contents and higher pollutant levels at that site. The lowest pH observed was 2.95; large concentrations of NH3 in the region prevented pH values from falling even lower. A comparison of simultaneously sampled cloud water and precipitation revealed much higher concentrations for most species in the cloud water, except in one case of extreme precipitation riming when the concentrations in the two phases converged. An exception to the pattern was H+; at times the precipitation was more acidic than the cloud water. The chemical composition of the cloud drops varied with drop size. Drops smaller than 10 μm diameter were enriched in NO3-, SO42- and NH4+ relative to larger drops. Since the larger drops are the ones most efficiently captured by snow crystals, knowledge of their composition is essential to understanding the chemical implications of accretional growth of precipitation.