Statistical characterization of urban CO emission signals observed by commercial airliner measurements.

Cities are responsible for the largest anthropogenic CO emissions and are key to effective emission reduction strategies. Urban CO emissions estimated from vertical atmospheric measurements can contribute to an independent quantification of the reporting of national emissions and will thus have political implications. We analyzed vertical atmospheric CO mole fraction data obtained onboard commercial aircraft in proximity to 36 airports worldwide, as part of the Comprehensive Observation Network for Trace gases by Airliners (CONTRAIL) program. At many airports, we observed significant flight-to-flight variations of CO enhancements downwind of neighboring cities, providing advective fingerprints of city CO emissions. Observed CO variability increased with decreasing altitude, the magnitude of which varied from city to city. We found that the magnitude of CO variability near the ground (~1 km altitude) at an airport was correlated with the intensity of CO emissions from a nearby city. Our study has demonstrated the usefulness of commercial aircraft data for city-scale anthropogenic CO emission studies.