Source category attribution of urban methane emissions using Positive MatrixFactorization (PMF) with long-term trace gas measurements

Cities emit methane (CH4) and have a role to play in mitigating the climate impacts of their emissions. Research suggests that CH4 emissions from most cities have large contributions from natural gas distribution and end use. In this work, we examine long-term measurements of CH4, CO, NOx, and VOCs from an urban air monitoring station in Calgary, Canada to resolve key contributors to CH4 enhancements. Using Positive Matrix Factorization (PMF), we identified four primary CH4 emissions source categories: natural gas – fugitives, natural gas – incomplete combustion, waste/biogenic, and petroleum product processing. Results from PMF modeling indicate that the bulk of CH4 emissions in Calgary are from natural gas fugitives and incomplete combustion (81% ± 35%). This is much higher than the proportion derived from available emissions inventories. The CH4 emissions from natural gas sources increase in winter and may be related to increased natural gas use for heating; results further supported by a land-use analysis. Emissions from waste/biogenic sources were the next largest contributor, which doubled in warmer months, consistent with temperature-driven microbial activity. Overall, these findings underscore the need for targeted mitigation strategies focused on the natural gas supply chain, while also highlighting the influence of seasonal dynamics on urban CH4 emissions.