THE EFFECT OF RIMING ON THE ION CONCENTRATIONS OF WINTER PRECIPITATION .1. A QUANTITATIVE-ANALYSIS OF FIELD-MEASUREMENTS

Although riming has been demonstrated to influence precipitation chemi stry, its relative importance as a mechanism for the transfer of trace species into snow is not well known. To overcome the qualitative meas ure of riming in past studies, the mass fraction of snow originating f rom accreted cloud droplets (rimed mass fraction) is estimated indirec tly in this study from replicas of collected snow crystals and flakes. The rimed mass fraction is then compared with the concentrations of v arious ionic trace species in collected snow and cloud water samples a t a prealpine site in Switzerland from 10 case studies. The ratio of t he precipitation to the cloud water concentration for a given ion, whi ch indicates the transfer efficiency of the species from clouds to sno w, was found to be correlated weakly to the rimed mass fraction for al l sample periods investigated. However, much of the variability in the relationship came from sampling periods when the precipitation intens ity was low. The relative importance of riming and other scavenging pr ocesses was estimated from the sample set, although this estimate is c omplicated by differences in concentration between cloud droplets coll ected by snow and those sampled at the site and by case to case variab ility. Under conditions of higher precipitation intensities, riming ap pears to be the dominant transfer mechanism for the incorporation of t he major ionic species into snow.