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

Atmospheric ammonia (NH 3 ) is a short-lived pollutant that plays an important role in aerosol chemistry and nitrogen deposition. Dominant NH 3 emissions are from agriculture and forest fires, both of which are increasing globally. The Alberta Oil Sands region has relatively low ambient NH 3 concentrations because of its remote location in northern Canada; however, a previous attempt to model NH 3 in the region showed a substantial negative bias compared to satellite column and aircraft observations. Known missing sources of NH 3 in the model were re-emission of NH 3 from plants and soils (bidirectional flux), and forest fire emissions, but the relative impact of these sources on NH 3 concentrations and column totals 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 natural (bidirectional flux of NH 3 and forest fire emissions) and anthropogenic (Oil Sands operations, combustion of fossil fuels, and agriculture) sources on ammonia concentrations, 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 has improved the model bias, slope and correlation coefficients relative to ground, aircraft, and satellite measurements significantly. By running the GEM-MACH 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 NH 3 came from direct anthropogenic sources, 56.6 % (or 1.24 ppbv) from bidirectional flux (reemission from plants and soils), and 20.3 % (or 0.42 ppbv) from forest fires. In the NH 3 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 flux of NH 3 across the domain to be positive (upward). It also increased the NH + 4 wet deposition by nearly a factor of three during the period simulated. Note that forest fires are very episodic and their contributions will vary significantly for different time periods and regions.