Source regions contributing to excess reactive nitrogen deposition in the Greater Yellowstone Area (GYA) of the United States

Research has shown that excess reactive nitrogen (Nr) deposition in the Greater Yellowstone Area (GYA) of the United States has passed critical load thresholds and is adversely affecting sensitive ecosystems in this area. To better understand the sources causing excess Nr deposition, the Comprehensive Air Quality Model with extensions (CAMx), using Western Air Quality Study (WAQS) emission and meteorology inputs, was used to simulate Nr deposition in the GYA. CAMx's Particulate Source Apportionment Technology (PSAT) was employed to estimate contributions from agriculture (AG), oil and gas (OG), fire (Fire), and other (Other) source sectors from 27 regions, including the model boundary conditions (BC) representative of international contributions, to the simulated Nr for 2011. Emissions from the AG and Other source sectors are predominantly from reduced N and oxidized N compounds, respectively. The model evaluation revealed a systematic underestimation in ammonia (NH3) concentrations by 65 % and overestimation in nitric acid concentrations by 108 %. The measured inorganic N wet deposition at National Trend Network sites in the GYA was overestimated by 31–49 %, due at least partially to an overestimation of precipitation. Source apportionment results showed that the AG sector was the single largest contributor to the GYA total Nr deposition, contributing 34 % on an annual basis. Seventy-four percent of the AG contributions originated from the Idaho Snake River valley, with Wyoming, California, and northern Utah contributing another 7 %, 5 %, 20 and 4 % respectively. Contributions from the OG sector were small at about 1 % over the GYA, except in the southern Wind River Mountain Range during winter where they accounted for more than 10 %, with 46 % of these contributions coming from OG activities in Wyoming. Wild and prescribed fires contributed 18 % of the total Nr deposition, with fires within the GYA having the highest impact. The five largest source area contributions to the annual total Nr deposition in the GYA were 1) the Snake River valley (3 8 % with AG 68 %, OG 2 %, Fire 15 %, and Other 16 %); 2) BC (21 %); 3) Wyoming (12 % with AG 19 %, OG 5 %, Fire 38 %, and Other 39 %); 4) California (7 % with AG 26 %, OG 1 %, Fire 14 %, and Other 59 %); and 5) northern Utah (6 % with AG 25 %, OG 2 %, Fire 10 %, and 25 Other 63 %). These results suggest that Nr deposition over the GYA, especially in the western region, was above the critical loads for sensitive ecosystems, and 22 AG from the Snake River valley was the largest contributor. Distant source regions were also important, with large contributions from the BC, i.e., international source regions.