Monitoring megacity CO2 emissions using long-term atmospheric observations from commercial airliners

Cities are the major contributor to global anthropogenic CO₂ emissions. Given that major international airports are often located close to megacities, observations from commercial airliners can uniquely supplement existing ground networks and satellites for more timely and accurate CO₂ emission estimates. Here we prototype a novel urban CO₂ monitoring system that utilizes atmospheric CO₂ data collected as part of the Comprehensive Observation Network for Trace gases by Airliners (CONTRAIL) project. We examine relationship between inventory-based CO₂ emission estimates in the Greater Tokyo Area (GTA) and long-term (2006-) atmospheric CO₂ data over Tokyo Narita International Airport (NRT; located ~60 km east of the central GTA). Synoptic-scale variability in atmospheric CO₂ is frequently observed at lower altitudes (~1 km altitude) over NRT, particularly linked to elevated CO₂ levels with relatively weak wind speed downwind of the GTA. To quantify the magnitude of such synoptic CO₂ variability, we calculate the standard deviation (SD) of atmospheric CO₂ after adjusting long-term and seasonal variations. We then reveal a correlation between the annual SD values at 1 km altitude and the interannual changes in CO₂ emissions in GTA. For example, our analysis shows good correspondence between the decrease in the annual SD and significant emission reductions during the global financial crisis in 2009 and the COVID-19 pandemic (2020-). Transport model simulations also confirms that contributions from other non-anthropogenic CO₂ fluxes, such as terrestrial and oceanic fluxes and biomass burning emissions, have minor impact on the interannual changes in SD over NRT. These results suggest regularly collected aircraft CO₂ data has the potential to inform early changes in megacity CO₂ emissions, supporting subnational climate mitigation policies and actions in a timely manner.