Urban centres are an important source of anthropogenic methane emissions and the focus of recent policy efforts aimed at emissions reductions. But emissions from urban areas are poorly characterized except in the few cities with robust measurement infrastructure. Satellite measurements offer a means for monitoring urban emissions. We use co-located measurements of methane (CH4) and carbon monoxide (CO) from the TROPOspheric Monitoring Instrument (TROPOMI) and carbon dioxide (CO2) from the Orbiting Carbon Observatory 2 and Orbiting Carbon Observatory 3 (OCO-2/3) to calculate CH4:CO2, CH4:CO, and CO:CO2 enhancement ratios over 85 cities. We compare our enhancement ratios to those derived from ground-based instruments in Los Angeles and find good agreement. Then we combine our enhancement ratios with CO and CO2 inventories to calculate CH4 emissions and find good agreement with estimates from past studies. Finally, we compare our satellite-based enhancement ratios to those computed from the bottom-up globally gridded emission inventories, and we find significant differences between them. Combining results from the three enhancement ratios, we find EDGARv8 to best represent CH4 emissions in urban areas with mean errors of 30% compared to CAMS-GLOB-ANTv6.2 (based on EDGARv6.1), and CEDSv2021, with mean errors of 42% and 48%, respectively. These differences are largely driven by EDGARv8 changes in South Central and East Asia. We also find that EDGARv8 and HTAPv3 underestimate CO emissions by a factor of 2-5 for cities in Iran, Turkmenistan, Argentina. Finally, we find that CO emissions are overestimated in some cities in Europe by a factor of ∼2.

Carbon dioxide (CO2) emissions from combustion sources are uncertain in many places across the globe. Here, we estimate CO2 emission rates from a small number of collocated observations from the Orbiting Carbon Observatory-3 (OCO-3) and the Earth Surface Mineral Dust Source Investigation (EMIT), both onboard the International Space Station (ISS). These near-simultaneous measurements allow for an unprecedented comparison of two unique space-based CO2 sensors over both isolated coal-fired power plants and multi-source scenes in China. We estimate CO2 emission rates using integrated mass enhancement and a Gaussian plume model. Where validation data is available, 15 of the 19 estimated emission rates have errors less than 37%. For the multi-source scenes, EMIT can estimate emissions from individual facilities but its aggregate emissions are 33% lower than OCO-3, likely because it cannot detect small sources or diffuse emissions. OCO-3, with its excellent precision, may better constrain CO2 emissions over the entire scene.