A I N Meteorology on Precipitation Chemistry at Low and High Elevations of the Colorado Front Range, U.s.a

We explored the seasonal characteristics in wet deposition chemistry for two sites located at different elevations along the east slope of the Colorado Front Range in Rocky Mountain National Park. Seasonally separated precipitation was stratified into highly concentrated (high salt), dilute (low salt), or acid-dominated precipitation groups. These groups and unstratified precipitation data were related to mean easterly or westerly zonal winds to determine direction of local transport. Strong acid anion associations were also determined for the stratified and unstratified precipitation data sets. We found that strong acid anions, acidity, ammonium, and high salt concentrations originate to the east of Rocky Mountain National Park, and are transported via up-valley funneling winds or convective instability from differential heating of the mountains and the plains to the east. These influence the composition of precipitation at Beaver Meadows, the low elevation site, throughout the year, while their effect on precipitation at Loch Vale, the high elevation site, is felt most strongly during the summer. During the winter, Loch Vale precipitation is very dilute, and occurs in conjunction with westerly winds resulting from the southerly location of the jet stream. Key word index: Colorado, deposition chemistry, precipitation, sulfate, nitrate, ammonium, Rocky Mountains, wind. 1. I N T R O D U C T I O N It has been demonstrated in the eastern United States of America that the increased precipitation associated with mountains has contributed directly to increased deposition of strong acid anions on high elevation ecosystems (Lovett and Kinsman, 1990). Although western U.S. mountains do not receive nearly as much acidic deposition as their eastern counterparts (Young et al., 1988), this same argument has been extrapolated to the western U.S.A. Many have concluded from this that some high elevation western U.S.A. aquatic ecosystems are at great risk from wet-deposited SO 2and NO 3 (Landers et al., 1987; Baron, 1992; Turk and Spahr, 1991; Lewis et al., 1984). Precipitation increases with elevation in mountainous terrain due to two distinct processes, convective instability and large-scale orographic lifting. Convective instability is brought about by differential heating of low vs high elevation air masses. This results in uplift through the funneling effects of valleys on airstreams, and also enhances thunderstorm activity in the mountains (Bossert et al., 1989; Toth and Johnson, 1985; Banta and Schaaf, 1987). Large-scale orographic lifting occurs when synoptic scale flow is forced to rise as it encounters mountain barriers. As the rising air cools and condenses, precipitation is enhanced (Barry and Chorley, 1978). Along the eastern slopes of the Front Range of Colorado these processes are seasonally and directionally different (Barry, 1973). Precipitation associated with convective instability occurs most often during the summer months, bringing air masses in from the southeast and southwest, while precipitation of cyclonic origin dominates winter precipitation processes and originates from the northwest (Hansen et al., 1978; Barry, 1973). Surface winds can differ dramatically from geostrophic winds (Barry and Chorley, 1976), making it difficult to understand the nature of atmospheric transport in complex terrain (Bossert et al., 1989). In mountainous terrain, however, extensive work has shown that regional-scale air flows are dominated by diurnal cycles (Toth and Johnson, 1985; Bossert et at., 1989; Defant, 1951). Winds are topographically constrained to flow up-valley in the daytime, forced by solar heating on the plains. They reverse and flow down-valley at night, as a result of nocturnal radiative cooling that occurs rapidly on mountaintops. This diurnal pattern draws agricultural and urban air up to high elevations (Parrish et al., 1990; Langford and 2337 2338 J. BARON and A. S. DENNING Fehsenfeld, 1992). Thunders torm development is related to the diurnal inflow and outflow from mountains. As the uplifted air condenses to form cloud droplets, it releases latent heat, creating updrafts that form the nuclei of thunderstorms (Barry and Chorley, 1976). These updrafts accelerate as long as the given air mass retains a lower density than that of the surrounding air. At this point, an abrupt reversal in air flow occurs, and strong downdrafts modify the patterns of air flow (Bossert et al., 1989; Barry a n d Chorley, 1976; Banta and Schaaf, 1987). We explored the seasonal characteristics in wet deposition chemistry for two sites located at different elevations along the east slope of the Colorado Fron t Range in the Rocky Mounta in Nat ional Park. Our interest was in determining whether or not the same air masses were responsible for chemical composit ion of precipitation at both sites. This has a practical application, since many mountainous areas are sensitive to acidic deposit ion (Eilers et al., 1986; Landers et al., 1987), yet do not have year-round deposition measurement stations due to winter access problems. This makes it difficult to predict the effects of current or increased loading of strong acid anions. A recent paper by Warren et al. (1991) suggests that strong correlations of weekly SO~from six high and low elevation pairs of precipitation sampling sites in the southern Rockies has two possible causes. Either similar air masses affect precipitation composit ion at both sets of sites, or the weekly sampling interval allows sufficient averaging of individual storms to mask different air mass sources. If the precipitation chemistry and wind-related chemical composit ion indicate both sites are influenced by the same air masses, it could be due to one or both of two factors: (a) a regionally uniform background mixture of solutes that is deposited proportional to the amount of wet deposition (Oppenheimer et al., 1985; Epstein et al., 1986); or (b) both sites influenced by the same local sources of emissions and soil materials that are deposited proport ional to the amount of wet deposition (Lewis et al., 1984; Warren et al., 1992). If the sites are composit ionally and meteorologically different, it could be due to complex meteorological or atmospheric chemical behavior that brings air parcels of differing chemical composit ion to low and high elevations (Parrish et al., 1990; Lewis et al., 1984). Complex meteorology is an argument against predicting high elevation deposit ion from lower elevation, more easily accessible, monitor ing sites.