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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.

The Bakken formation contains billions of barrels of oil and gas trapped in rock and shale. Horizontal drilling and hydraulic fracturing methods have allowed for extraction of these resources, leading to exponential growth of oil production in the region over the past decade. Along with this development has come an increase in associated emissions to the atmosphere. Concern about potential impacts of these emissions on federal lands in the region prompted the National Park Service to sponsor the Bakken Air Quality Study over two winters in 2013–2014. Here we provide an overview of the study and present some initial results aimed at better understanding the impact of local oil and gas emissions on regional air quality. Data from the study, along with long term monitoring data, suggest that while power plants are still an important emissions source in the region, emissions from oil and gas activities are impacting ambient concentrations of nitrogen oxides and black carbon and may dominate recent observed trends in pollutant concentrations at some of the study sites. Measurements of volatile organic compounds also definitively show that oil and gas emissions were present in almost every air mass sampled over a period of more than four months.

Decreasing trends of elemental carbon (EC) have been reported at US Interagency Monitoring of PROtected Visual Environments (IMPROVE) network from 1990 to 2004, consistent with the phase-in of cleaner engines, residential biomass burning technologies, and prescribed burning practices. EC trends for the past decade are examined due to an upgrade of IMPROVE carbon instruments and the thermal/optical analysis protocol since 2005. Filter reflectance (τR) values measured as part of the carbon analysis were retrieved from archived data and compared with EC for 65 sites with more complete records within 2000–2009. EC–τR relationships suggest minor changes of EC quantified by the original and upgraded instruments for most IMPROVE samples. EC and τR show universal decreasing trends across the US. The EC and τR trends are correlated, with national average downward rates (relative to the 2000–2004 baseline medians) of 4.5% yr−1 for EC and 4.1% yr−1 for τR. The consistency between independent EC and τR measurements adds to the weight of evidence that EC reductions are real rather than an artifact of changes to the measurement process.

Observations at national parks and other remote sites show that average elemental carbon and fine particle mass concentrations in the United States both decreased by over 25% between 1990 and 2004. Percentage decreases in elemental carbon were much larger in winter than in summer. These data suggest that emissions controls have been effective in reducing particulate concentrations not only in polluted areas but also across the United States. Despite the reduction in elemental carbon, the simultaneous decrease in non-absorbing particles implies that the overall radiative forcing from these changes was toward warming. The use of a 2005 instead of 1990 as a baseline for climate-relevant emissions from the United States would imply a significantly lower baseline for aerosol emissions. The use of older data will overestimate the possibility for future reductions in warming due to black carbon controls.