Contributions of natural and anthropogenic sources to ambient ammonia in the Athabasca Oil Sands and north-western Canada

• Main Authors: C. H. Whaley, P. A. Makar, M. W. Shephard, L. Zhang, J. Zhang, Q. Zheng, A. Akingunola, G. R. Wentworth, J. G. Murphy, S. K. Kharol, K. E. Cady-Pereira
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-02-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/2011/2018/acp-18-2011-2018.pdf
• ISSN: 1680-7316; 1680-7324

Atmospheric ammonia (NH₃) is a short-lived pollutant that plays an important role in aerosol chemistry and nitrogen deposition. Dominant NH₃ emissions are from agriculture and forest fires, both of which are increasing globally. Even remote regions with relatively low ambient NH₃ concentrations, such as northern Alberta and Saskatchewan in northern Canada, may be of interest because of industrial oil sands emissions and a sensitive ecological system. A previous attempt to model NH₃ in the region showed a substantial negative bias compared to satellite and aircraft observations. Known missing sources of NH₃ in the model were re-emission of NH₃ from plants and soils (bidirectional flux) and forest fire emissions, but the relative impact of these sources on NH₃ concentrations was unknown. Here we have used a research version of the high-resolution air quality forecasting model, GEM-MACH, to quantify the relative impacts of semi-natural (bidirectional flux of NH₃ and forest fire emissions) and direct anthropogenic (oil sand operations, combustion of fossil fuels, and agriculture) sources on ammonia volume mixing ratios, both at the surface and aloft, with a focus on the Athabasca Oil Sands region during a measurement-intensive campaign in the summer of 2013. The addition of fires and bidirectional flux to GEM-MACH has improved the model bias, slope, and correlation coefficients relative to ground, aircraft, and satellite NH₃ measurements significantly.<br><br>By running the GEM-MACH-Bidi model in three configurations and calculating their differences, we find that averaged over Alberta and Saskatchewan during this time period an average of 23.1 % of surface NH₃ came from direct anthropogenic sources, 56.6 % (or 1.24 ppbv) from bidirectional flux (re-emission from plants and soils), and 20.3 % (or 0.42 ppbv) from forest fires. In the NH₃ total column, an average of 19.5 % came from direct anthropogenic sources, 50.0 % from bidirectional flux, and 30.5 % from forest fires. The addition of bidirectional flux and fire emissions caused the overall average net deposition of NH_<i>x</i> across the domain to be increased by 24.5 %. Note that forest fires are very episodic and their contributions will vary significantly for different time periods and regions.<br><br>This study is the first use of the bidirectional flux scheme in GEM-MACH, which could be generalized for other volatile or semi-volatile species. It is also the first time CrIS (Cross-track Infrared Sounder) satellite observations of NH₃ have been used for model evaluation, and the first use of fire emissions in GEM-MACH at 2.5 km resolution.