JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. D6, PAGES 11,517-11,527, JUNE 20, 1995 The effect of riming on the ion concentrations of winter precipitation 1. A quantitative analysis of field measurements RoyW. Dixon, • Lukas Mosimann, Beat Oberholzer, Johannes Staehelin, and Albert Waldvogel Institute for Atmospheric Physics, Eidgenassische Technische Hochschule, Zurich, Switzerland JeffreyL. Collett Jr. Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado Abstract. Although riminghas been demonstrated to influence precipitation chemistry, its relative importance asa mechanism for thetransfer of trace species into snow is not well known. To overcome the qualitative measure of rimingin past studies, themass fraction of snow originating fromaccreted cloud droplets (rimed mass fraction) is estimated indirectly in thisstudy from replicas of collected snow crystals and flakes.Thetimed mass fraction is then compared with theconcentrations of various ionic trace species in collected snow andcloud watersamples at a prealpine site in Switzerland from 10 case studies. The ratioof theprecipitation to the cloud water concentration for a given ion, which indicates thetransfer efficiency of thespecies from clouds to snow, was found to be correlated weaklyto therimedmass fraction for all sample periods investigated. However, much of the variability in therelationship came from sampling periods when theprecipitation intensity was low. The rdative importance of rimingandother scavenging processes wasestimated from the sample set, although thisestimate is complicated by differences in concentration between cloud droplets collected by snow andthose sampled at the site andby case to case variability. Underconditions of higher precipitation intensifies, riming appears to be the dominant transfer mechanism for the incorporation of themajorionic species into snow. 1. Introduction The removal of acidic trace species from the atmosphere through precipitation is important because of their influence on precipitation acidity [National Research Council, 1983] and because wet deposition is a major sink for numerous compounds present as gases or residing in aerosol particles. From numerous field and numerical studies, various processes have been indicated to affect the chemical concentration of precipitation. In addition to changes in scavenging processes, air mass changes are known to lead to changes in precipitation concentration [e.g., Collett et al., 1991]. Processes contributing to incorporation of trace species into precipitation can be divided into processesoccurring in and below clouds. Numerous field studies have investigatedbelow- cloud scavengingof gases and aerosol particles [e.g., Isaac and Daum, 1987; Oberholzer et al., 1992b, 1993; Zinder et al., 1988] and have found that these processesare not the only important processes in determining the ground level precipitation concentrations of various trace species. • Now at Chemistry Department, New MexicoInstitute of Mining and Technology, Socorro, NM 87801. Copyright 1995 by the American Geophysical Union. Paper number 94JD03321. 0148-0227/95/94JD-033 21 $05.00 'l'hus processes occurring in clouds also must be responsible for the transfer of pollutants from the air to precipitation. In cold clouds (clouds with an ice phase present), water may be transferred to snow by two different processes: by diffusion of water vapor from the gas phase at the expense of existing supercooled cloud droplets (also known as the Bergeron process) or by the collection of supercooled cloud droplets by falling snow crystals in a process known as riming. Theoretical considerations long have shown that timing, by allowing the transferof pollutants presentin cloud droplets to precipitation, can increase the concentration of pollutants in precipitation over that in precipitation forming through diffusional growth [Scott, 1978]. The increasedefficiency of trace species removal with rimed snow, trends of greater timing and more similar cloud and precipitation concentrations, simultaneously observed increases of riming and precipitation concentrations, and have been reported by Scott [1981], by Borys et al. [1988] and Mitchell and Lamb [1989], and by Collett et al. [1991], respectively. In a recent study, an attempt was made to separatesnow crystals on the basis of physical properties related to the amount of riming, but this study showed no significant differences between unrimed and timed snow collected simultaneously [Mitchell and Borys, 1992]. Although some of the above studies have shown a connection between the degree of timing and the transfer of species from cloud water to snow [Borys et al., 1988; Mitchell and Lamb, 1989], the extent to which timing influences precipitation concentrations has not been investigated in a 11,517