Quantifying seasonal urban NO2 emissions using satellite observations

The tropospheric NO₂ vertical column density (VCD) values measured by the Tropospheric Monitoring Instrument (TROPOMI) were used to study the NO₂ variability and estimate urban NO₂ emissions for 261 major cities worldwide. The used algorithm isolated three components in tropospheric NO₂ data: background NO₂, NO₂ from urban sources, and from industrial point sources, and then each of these components was analyzed separately. The method is based on fitting satellite data by a statistical model with empirical plume dispersion functions driven by a meteorological reanalysis. Unlike other similar studies that studied plumes from emission point sources, this study included the background component as a function of the elevation in the analysis and separated urban emissions from emissions from industrial point sources. Population density and surface elevation data as well as coordinates of industrial sources were used in the analysis. Differences between workday and weekend emissions were also studied. Urban emissions on Sundays (or Fridays for some countries) are typically 20%-50% less than workday emissions for all regions except China. No significant difference in urban emissions between seasons was found. In contrast, the background component does not show any significant differences between workdays and weekends suggesting that background NO₂ has a substantially longer lifetime compared to that in the plumes.