CONTRIBUTIONS OF CLOUD PROCESSES TO PRECIPITATION CHEMISTRY IN MIXED-PHASE CLOUDS

The role clouds play as processors of atmospheric aerosols and trace gases was studied along the slope of Mt. Rigi in central Switzerland. Upon cloud formation many aerosols and trace gases are efficiently scavenged by cloud drops. The cloud drops can enhance removal of pollutants from the atmosphere by transferring them to snow or rain which falls rapidly to the ground. This often occurs through a process known as riming, where falling ice crystals capture cloud drops. When ice crystals are grown primarily via water vapor deposition, without significant capture of cloud drops, however, the cloud drops isolate atmospheric pollutants from the precipitation process, thereby inhibiting their deposition. Increased riming results in increased precipitation ion concentrations. The extent of ice crystal riming at times exhibits spatial inhomogeneities with greater riming apparent near the mountain summit. Variations in cloud chemistry with drop size indicate that bulk cloudwater composition is not an accurate predictor for the composition of cloud drops captured by the ice crystals.