Inverse Dispersion Modeling (IDM) has been successfully used as an alternative method to flux chamber measurements to estimate fugitive greenhouse gas (GHG) emissions from area sources at the Horizon Oil Sands facility for regulatory reporting since 2015. The IDM-CALPUFF approach combines ambient GHG monitoring with Ultraportable GHG analyzers (which significantly reduce noise and improve accuracy compared to open-path lasers) around each of the area sources; meteorological monitoring with 2D and 3D sonic anemometers; meteorological and dispersion modeling with WRF, CALMET, and CALPUFF; and a Bayesian statistical inversion technique. By relying on net concentrations, linked to emissions from all parts of the tailings pond and mine, over several days, the method delivers a spatially and temporally representative survey of GHG sources. The IDM-CALPUFF approach enables a detailed and accurate spatial breakdown of GHG fugitive emissions over the tailings pond and the complex terrain of the mine, including areas that cannot safely be sampled by in situ measurements, detected by smaller footprint or shorter range methods, or differentiated by remote sensing surveys. By doing so, the approach has informed targeted mitigation strategies and subsequently confirmed their effectiveness. Methane emission estimates with IDM-CALPUFF are very robust, with only 5% uncertainty for the measurement period, showed active mine zone flux intensities consistent over the years, and detected unexpected hot spots. Carbon dioxide fugitive emissions from the mine have been shown to be negligible, compared to methane fluxes and mobile emissions, while the tailings pond was identified as a sink of CO_2 at times. Owing to meteorological and operational variations, it is imperative to assess emissions throughout the year, at least on a seasonal and ideally on a continuous basis, for the most accurate annual GHG reporting and targeted mitigation. The IDM-CALPUFF offers that opportunity. In 2018-2019, the field campaigns were extended beyond the single regulatory summer sampling period to seasonal campaigns, in conjunction with the three-year Emission Reduction Alberta (ERA) Methane Challenge study ongoing at Horizon. In 2018, the measurement campaign occurred in the spring, shortly after the tailings pond thawed. Winter, summer, and fall campaigns are taking place in 2019.
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