PHYSICAL FACTORS INFLUENCING WINTER PRECIPITATION CHEMISTRY

Two case studies of winter precipitation events highlight the roles of transport and snow crystal riming (the capture of supercooled cloud droplets by snow crystals) in determining precipitation chemistry. In one case, passage of a cold front leads to a change in the air mass producing precipitation over the monitoring site. A simultaneous decrease in precipitation ion concentrations is observed. Correlations of the ion concentrations with the pseudoequivalent potential temperature, which serves as an air mass identifier, suggest that the decrease in ion concentrations is caused by the air mass change, rather than by washout of aerosols and gases from the atmosphere. In the second case, evidence is presented indicating that an increase in precipitation ion concentrations results from significant capture of polluted cloudwater droplets by the snow crystals. Influences on precipitation chemistry from both processes, transport and riming, can be large. In order to study other processes influencing precipitation chemistry that occur on similar time scales (minutes to hours), such as aerosol and gas scavenging or aqueous-phase oxidation, it is important to evaluate possible confounding effects of transport and riming.