Annual wet and dry deposition of sulfur and nitrogen in the snowy range, Wyoming

The collocation of three national networked programs NADP, EPAʹs CASTNET, and the Forest Serviceʹs IMPROVE Module A, within a few hundred meters of each other in the pristine Medicine Bow forest of Wyoming has made it possible to assess the total amount of sulfur and nitrogen deposition, both wet and dry for this alpine/subalpine ecosystem. Additional sites within a few kilometers add spatial depth to this study. Wet deposition assessed using NADP data accounts for 1 to a little over 3 kg ha−1 yr−1 for both nitrogen and sulfur; however, annual trends for the two species differ. Dry deposition assessed using both CASTNET (a.k.a. NDDN) and IMPROVE (for sulfur) indicates 1–2 kg ha−1 yr−1 for nitrogen but less than 1 kg ha−1 yr−1 for sulfur. The overall trend of wet plus dry for nitrogen has been downward from 5 kg ha−1 yr−1 in 1989 to 3.6 kg ha−1 yr−1 in 1994, while varying between 2 and under 4 kg ha−1 yr−1 for sulfur. This paper introduces the sites and presents the three programs and the analysis approach. Spatial comparisons between sites are investigated. Weekly data are analyzed from three NADP sites, separated horizontally 6.8 and 2.4 km and vertically 430 and 98 m from the highest elevation site. The site comparisons demonstrate that winter season data requires careful analysis due to the vagaries of inefficient precipitation collection during high winds and snow fall.