Using atmospheric in situ mobile measurements to monitor urban methane emissions

Despite the Paris Agreement, greenhouse gas (GHG) concentrations in the atmosphere continue to increase because of the anthropogenic activities, and this is inducing catastrophic effects. Past studies revealed that urban areas are responsible of a large part of these emissions and many cities already started to implement climate actions to reduce their GHG emissions and address climate change. The effectiveness of these actions depends on accurate knowledge of the many sources of GHG in each city, so that efforts are focused on the sources whose emission reduction would be the most effective. Atmospheric measurements are useful to locate and characterize these sources and to monitor the evolution of their emissions. Different approaches have been developed during the past decades including stationary and mobile surface-based in situ measurements, remote sensing of solar absorption spectra from space and from the ground, or aircraft-based observations. All these techniques are complementary and provide information about urban GHG emissions at different scales.

In situ mobile measurements of methane mixing ratios have been performed in the two largest cities of Canada using 1) a high-precision gas analyzer providing continuous measurements, 2) a weather station measuring wind speed and direction, and 3) a GPS recording coordinates during the campaigns. These mobile surveys allow rapid screening of large areas, the revisit of specific sites to monitor the evolution of their emissions over time, and can therefore improve our understanding of the emissions at local scale. Methane emissions of the Greater Toronto Area (GTA) have been intensively investigated since 2018 with a total of 84 days of measurements corresponding to a distance of about 8,000 km. A one-week campaign has also been realized in November 2019 in Montreal corresponding to a distance of about 1,100 km. Methane enhancements observed during these surveys have been identified, classified into three categories depending on their magnitudes and areas, and attributed to potential sources, several of which are not catalogued in FLAME-GTA, the point source level inventory developed for the Toronto metropolitan area. Important methane sources in the GTA have been surveyed regularly since 2018 and their emissions have been estimated using an inverse modeling framework with a Gaussian model and compared to the inventory-based estimates of FLAME-GTA.