Atraospheric Environment Vol. 19, No. 7, pp. 1199-1203, 1985 0004-6981/85 $3.00 + 0.00 Printedin Great Britain. Pergamon Press Ltd. SHORT COMMUNICATION ESTIMATES OF AVERAGE MAJOR ION CONCENTRATIONS IN BULK PRECIPITATION AT TWO HIGH-ALTITUDE SITES NEAR THE CONTINENTAL DIVIDE IN SOUTHWESTERN COLORADO MICHAEL M. REDDY and HANS C. CLAASSEN United States Geological Survey, Box 25046, MS 407, Denver Federal Center Lakewood, Colorado 80225, U.S.A. (First received 22 August 1984; in revised form 29 October 1984 and received for publication 17 December 1984) Abstract--The composition of bulk precipitation from two high-altitude sites, established in 1971 near the Continental Divide in southwestern Colorado, has been monitored by season during the past decade. Calcium ions are the predominant cationic species; sulfate is the major anionic constituent. Bulk precipitation major ion concentrations exhibit log-normal distributions. Representative mean and standard deviation values for the major inorganic ionic species present in bulk precipitation have been calculated for three years of consecutive seasons. Standard deviations for all species, except nitrate, are similar. For two years of data grouped into quarters, deviations from mean values fall well within the plus or minus two standard deviation limit. There does not seem to be a systematic deviation from the mean concentration values, with respect to either ionic component or season. Key word index: Precipitation quality, bulk deposition, high altitude precipitation chemistry. INTRODUCTION Attention has focused recently on acidic deposition and precipitation chemistry in remote areas. This report presents means and ranges of expected concentration for major chemical constituents in bulk precipitation (wet plus dry deposition) for two high-altitude locations near the Continental Divide in southwestern Colorado. Bulk precipitation chemistry has been characterized on a regional and temporal basis in the Eastern United States (see, e.g. Peters and Boneili, 1982; Peters et al., 1982). In additon, wet-only precipitation composition has been monitored on a nation-wide basis in conjunction with the National Atmospheric Deposition Program (see for example Gibson, 1985). Monitoring sites used in these studies are typically at low elevations [in the report by Wilson and Mohnen (1982) less than 625 meters above mean sea level], and are clustered in areas of high regional and local pollutant emission. Such sampling locations would seem to be less able to identify changes in long range pollutant transport than high altitude, remote locations. Thus, it seems useful to examine bulk precipitation chemical composition for high altitude sam- pling sites located in remote areas of the Western United States. The sites, two mountain passes (Fig. 1) in the San Juan Mountains of southwestern Colorado, were established to monitor atmospheric deposition of radionuclides ( Voegeli and Claassen, 1971). Chemical characterization of archived samples and currently collected samples was begun in 1980. MATERIALSAND METHODS Precipitation collectors were installed during the last week of May 1970 (Voegeli and Claassen, 1971). Since site access- ibility was a major consideration it was necessary to place collectors near highways which have full-time maintenance during winter months. Study sites were selected to minimize local pollution. Each collector was in an open area, away from dirt roads and other obvious sources of pollution. Both collectors are located in open areas on the sides of the passes. At the Red Mountain Pass site there are several trees, some rock outcrops, and a relatively thin soil cover. At Wolf Creek Pass, there are no trees nor rock outcrops in the vicinity of the collector. Barry (1981), in summarizing weather and climate for the Colorado Rocky Mountains, states that Pacific storms and westerly air circulation dominate the site locations in winter. High mean wind speeds at mountain summits during the winter half of the year are associated with this westerly air flow. Average wind speeds are lower during the summer months. Meridionai troughs and occasional deep cut-off lows draw moist air northward from the Gulf of Mexico during spring and summer. Summer precipitation is due mainly to convective storms. Near the end of each season the bulk sample was removed from the collector. Measurement of pH, conductance and volume were performed, and an aliquot was placed in a washed polyethylene bottle for transport to the laboratory. Ice and/or snow samples were kept in the polyethylene collection bags and transported in clean plastic containers for thawing and processing at the laboratory in Denver, Colorado. Filtration was done with a pre-rinsed 0.1 /~m Millipore filter (VCWP 04700) under nitrogen pressure*. One of two sample bottles was acidified with ultrapure nitric acid (J. T. Baker Chemical CO., Ultrex Nitric Acid 4801-1) for subsequent metal analyses. Chemical analysis of precipitation was instituted during 1980. Samples underwent an extensive suite of chemical and radiochemical analyses; results for major dissolved inorganic species are reported here. Metal ion analytical procedures were adapted from recommendations of the instrument manufacturer (Perkin-Elmer, Model 603, 1976). Multipoint calibration curves, with standards bracketing the sample concentration level, were used for all analyses. Numerous samples were also analyzed using the technique of Garbarino *Any use of trade names is for descriptive purposes only and does not constitute endorsement by the U. S. Geological Survey. 1199