Evaluating urban methane emissions from space using TROPOMI methane and carbon monoxide observations

Anthropogenic methane emissions from urban centers are important and addressable, yet remain poorly characterized, and the representativeness of studies from individual cities is unknown. A satellite-based approach provides a pathway to tackle this challenge on a national or global scale. Here we present a space-based method that uses the simultaneous, daily observation of methane and carbon monoxide from the TROPOMI satellite to estimate urban methane emissions. We assess and validate the method and demonstrate that using these simultaneous observations enables robust assessment of methane emissions from urban centers without relying on atmospheric transport models. Initial assessments with this approach in eight United States cities suggest emission inventory underestimates previously discovered in older East Coast cities are more broadly representative, with aggregated emissions totaling 1.47 (0.56, 3.19, 95% confidence interval) Tg CH4/year, compared to the Environmental Protection Agency estimate of 0.52 Tg CH4/year. We show this data driven approach provides a pathway to study urban methane emissions across the globe and track how emissions respond if urban mitigation measures are implemented by investigating three additional megacities outside the United States.