![]() Satellite remote sensing of additional air pollutants has been greatly expanded with the launch of the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor (S5P) satellite on 13 October 2017 ( Veefkind et al., 2012). The Orbiting Carbon Observatory-3 (OCO-3) aboard the International Space Station (ISS) since May 2019 ( Eldering et al., 2019) has provided additional observations of CO 2 in urban areas ( Kiel et al., 2021). Since the launch of the Orbiting Carbon Observatory-2 (OCO-2) in July 2014 ( Crisp et al., 2004), further studies have characterized emissions from urban regions (e.g., Wu et al., 2018 Reuter et al., 2019). ![]() ( 2012) used observations from the Greenhouse Gases Observing Satellite (GOSAT), launched in January 2009, to measure enhancements of atmospheric carbon dioxide ( CO 2) over megacities. The expansion of the constellation of Earth-observing satellites taking measurements of greenhouse gases and air pollutants has led to observations over urban regions with unprecedented spatiotemporal coverage. ![]() Atmospheric measurements have been shown to be useful as part of top-down approaches in validating and refining these emissions inventories ( McKain et al., 2012 Duren and Miller, 2012). These inventories are constructed using bottom-up approaches: information on socio-economic activity is used alongside expected emissions factors for these activities to model emissions ( Gurney et al., 2012 Janssens-Maenhout et al., 2019). Emissions inventories provide information about the distribution and sources of air pollution and greenhouse gas emissions as well as their trends over time. Improving air quality and reducing greenhouse gas emissions are focuses of environmental policy from global to municipal levels ( Gurney et al., 2018 a). Additionally, we use high-resolution CO 2 inventories for two cities (Los Angeles and Indianapolis) to estimate emissions of CO and NO 2 using our calculated enhancement ratios and find good agreement with both a previous modelling study for the megacity of Los Angeles and California Air Resources Board (CARB) inventory estimates. We further demonstrate that the calculation and intercomparison of enhancement ratios of multiple tracers can help to identify the underlying biases leading to disagreement between observations and inventories. We find that these global inventories underestimate CO emissions in many North American and European cities relative to our observed enhancement ratios, while smaller differences were found for NO 2 emissions. Enhancement ratios between species are calculated and compared to emissions ratios derived from four globally gridded anthropogenic emissions inventories. We apply this method to observations taken over or downwind of 27 large (population of >1 million) urban areas from around the world. Using co-located space-based measurements of carbon dioxide ( CO 2) from the Orbiting Carbon Observatory-2 and Orbiting Carbon Observatory-3 (OCO-2/3) and carbon monoxide ( CO) and nitrogen dioxide ( NO 2) from the TROPOspheric Monitoring Instrument (TROPOMI), we calculate total column enhancements for observations influenced by anthropogenic emissions from urban regions relative to clean background values.
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