An assessment of emission characteristics of Northern Hemisphere cities using spaceborne observations of CO2, CO an NO2

Intensified anthropogenic activities and high levels of energy consumption in cities have led to an increase in emissions of CO2 and air pollutants, impacting air quality and calling for better measures to monitor and reduce emissions. This study assesses the emission characteristics of cities in the Northern Hemisphere during two winter seasons from December 2018 to March 2020 using a combination of CO2 observations from the Orbiting Carbon Observatory-2 (OCO-2) along with CO and NO2 observations from the Sentinel-5 Precursor TROPOspheric Monitoring Instrument (S-5P TROPOMI). We calculate the estimates of urban enhancement concentrations of CO2, CO, and NO2 from anthropogenic emissions in cities. Moreover, using ratios of CO/CO2 and NO2/CO2according to each city, we analyze the relationship between CO2 and air pollutants and find that cities show distinct emission patterns according to where they are located in different regions of the world. Our analysis shows that a positive relationship exists in the comparisons of CO/CO2 and NO2/CO2 ratios to city population and GDP. However, when cities in the Northern Hemisphere are divided into “developed” and “developing” regions, a difference in pattern is found where most developed cities show a smaller increase of both CO/CO2 and NO2/CO2 ratios with the increase of GDP, while developing cities show a larger increase of ratios per increase of GDP. Especially, developing cities facing rapid economic development like Mumbai and Tianjin show high emission ratios that almost match that of developed cities. The higher incline of increase in emission ratios in developing cities can be attributed to the low combustion efficiency of fossil fuels along with less stringent pollution control measures. This implies that pollution control measures are crucial for effective management of air quality degradation with economic advancement within these developing cities. The analysis also highlights the importance of urban geological features in cities such as that of Tehran and Milan which can trap air pollution under appropriate meteorological conditions. Despite the short period of analysis, our study demonstrates that satellite observations of CO2 and air pollutants together can provide clues about factors controlling the co-emission of air pollutants and carbon in cities and suggests that the application of the proposed methods with longer periods of observation will support coupled air quality and climate mitigation strategies.