Top-down NO_x emissions of european cities based on the downwind plume of modelled and space-borne tropospheric NO₂ columns

Top-down estimates of surface NO_X emissions were derived for 23 European cities based on the downwind plume decay of tropospheric nitrogen dioxide (NO₂) columns from the LOTOS-EUROS (Long Term Ozone Simulation-European Ozone Simulation) chemistry transport model (CTM) and from Ozone Monitoring Instrument (OMI) satellite retrievals, averaged for the summertime period (April–September) during 2013. Here we show that the top-down NO_X emissions derived from LOTOS-EUROS for European urban areas agree well with the bottom-up NO_X emissions from the MACC-III inventory data (R² = 0.88) driving the CTM demonstrating the potential of this method. OMI top-down NO_X emissions over the 23 European cities are generally lower compared with the MACC-III emissions and their correlation is slightly lower (R² = 0.79). The uncertainty on the derived NO₂ lifetimes and NO_X emissions are on average ~55% for OMI and ~63% for LOTOS-EUROS data. The downwind NO₂ plume method applied on both LOTOS-EUROS and OMI tropospheric NO₂ columns allows to estimate NO_X emissions from urban areas, demonstrating that this is a useful method for real-time updates of urban NO_X emissions with reasonable accuracy.