Quantification of Methane Emissions from Large Sources in Southern Ontario by Mobile Mass Balance Method

Accurate estimation of methane by top-down measurements is often a challenge to make. Yet, top-down measurements that validate emission inventories of greenhouse gases (GHG) is important as recent studies have shown that the bottom-up inventories frequently underestimate the emissions, especially from the oil and gas industry. As an alternative to costly aircraft campaigns for top-down estimates, this study uses the surface level mobile mass balance method for measuring emissions of methane from large sources.

The study was conducted at Greenlane landfill in Ontario between 2021-2022. Ground level mixing ratios of CH4, CO2, and CO were measured employing a Picarro Cavity Ringdown Spectrometer mounted in a Tesla vehicle. The enhancement of CH4 downwind of the source relative to the background was integrated across the width of the plume at distances sufficiently far from the source to assume homogeneity in vertical mixing ratio of methane and wind profile within the planetary boundary layer. The mixed boundary layer height was obtained by a ceilometer (CL51) located in Windsor, and meteorological data from London (ON) international Airport was used for wind speed and direction.

The result of the mass balance calculation showed average CH4 emission rates of 3600 ± 870 kghr-1 from the landfill which is equivalent to 31.5 kt/year under an assumption of steady seasonal emissions. A 2020 GHG emissions summary for Ontario claims 250kt/year CH4 emission from solid waste disposals; thus, Greenlane landfill accounts for ~13% of the landfill CH4 emission in Ontario. The analysis of the mixing ratios of CO2 and CH4 in the landfill plumes showed average linear correlation of 0.98±0.02 mole mole-1. Therefore, CO2 emissions from the landfill are estimated to be 9700 ± 2400 kghr-1.

The surface mass balance method was confirmed to be an auspicious technique for top-down quantification of GHGs. Yet, the large uncertainty in our measurement indicates further rigorous analysis of boundary meteorology is necessary for accurate quantification. Further field studies are being made in the city of Sarnia where major Petro chemical facilities are located.