Centrality and city size effects on NO2 ground and tropospheric concentrations within European cities

Nitrogen dioxide (NO2) has been one of the constant health threats of urban citizens. But how the NO2 levels change across European cities, relate to urban population size is still unclear. Meanwhile, a theory shows that traffic-induced air pollution (e.g. NO2) decreases with the distance to the city center. But among empirical studies, whether this centrality effect is prevalent in NO2 pollution in European cities is still unknown. We therefore investigate 378 European cities to quantify the effects of centrality and population size using NO2 data from monitoring stations and the satellite Sentinel-5P. We confirm the centrality effect proposed by the theory, and we find the intra-urban NO2 profiles are very similar among cities. For a city, when the distance to the city center increases from 100 m to 500 m or from 1 km to 5 km, its ground and tropospheric NO2 concentrations decrease by 25%; when the distance to the city center increases from 500 m to 1 km or from 5 km to 10 km, its ground and tropospheric NO2 concentrations decrease by 12%. And, under the same conditions of centrality, cities with larger population size always have higher NO2 ground and tropospheric concentrations. Our results aim to inform urban planners how to best organize the population across and within cities for reducing hazardous air pollutants, and to link intra-urban structure with city size (scaling) effects towards urban sustainability.