A 50-year record of NOx and SO2 sources in precipitation in the Northern Rocky Mountains, USA

The chemical quality of snowfall deposited in high-elevation areas in the Rocky Mountain region can be affected by energy generation and associated population growth [1,2]. High elevation areas in the Wind River Range (WRR) of Wyoming (figure 1) exceed 4 km above sea level and are adjacent to areas of accelerating energy development [3]. For example, over 3,000 natural gas wells are being installed in the Green River Basin, directly west of the WRR. Full development of the Jonah gas field could result in the production of 1,480 metric tons/yr of NOx and 25.7 metric tons/yr of SO2 [3].Thin soils and dilute surface-water systems in high-elevation areas have limited capacities to buffer increased acidity associated with the airborne contaminants of NOx and SO2. Trends in precipitation chemistry at NADP/NTN sites in the western United States have indicated an increase in total N deposition and a decrease in deposition from 1981-1998 [4]. In addition to monitoring trends in N and S deposition, the isotopic composition of snow, firn, and ice has been used to differentiate natural and anthropogenic solute sources. Stable isotope ratios of sulfur in , expressed as δ34S, were monitored in bulk snowpack samples collected from a network of 52 high-elevation sites in the Rocky Mountains from 1993 to 1999 [5]. The δ34S values indicated that snowpack S in high-elevation areas is primarily derived from anthropogenic sources [5].Changes in δ34S values in firn and ice-core samples have been used to reconstruct changes in sulfate sources to central Asia, Greenland, and Antarctica. The variation in δ34S values in a firn core from central Asia allowed for the identification of S derived from marine evaporites (+15 ‰) during high dust deposition events and anthropogenic emissions (+5.4 ‰) [6]. Preindustrial δ34S signatures in Greenland ice cores were comprised of marine biogenic emissions, continental dust sources, background volcanism, and continental biota [7]. Beginning in 1870 A.D.,