Accurate space-based NO_x emission estimates with the flux divergence approach require fine-scale model information on local oxidation chemistry and profile shapes

Abstract. The flux divergence approach (FDA) is a popular technique for deriving NO_X emission estimates from tropospheric NO₂ columns measured by the TROPOMI satellite sensor. An attractive aspect of the FDA is that the method simplifies three-dimensional atmospheric chemistry and transport processes into a two-dimensional (longitude-latitude) steady-state continuity equation for columns that balances local NO_X emissions with the net outflow and chemical loss of NO_X. Here we test the capability of the FDA to reproduce known NO_X emissions from synthetic NO₂ column retrievals generated with the LOTOS-EUROS chemistry transport model over the Netherlands at high spatial resolution of about 2x2 km during Summer. Our results show that the FDA captures the magnitude and spatial distribution of the NO_X emissions to high accuracy (absolute bias <9 %), provided that the observations represent the NO₂ column in the boundary layer, that wind speed and direction are representative for the boundary layer (PBL) column, and that the high resolution spatiotemporal variability of the NO₂ lifetimes and NO_X:NO₂ ratio is accounted for in the inversion, instead of using single fixed values. The FDA systematically overestimates NO_X emissions by 15–60 % when using tropospheric NO₂ columns as the driving observation, while using PBL NO₂ columns largely overcomes this systematic error. This merely reflects that the local balance between emissions and sinks of NO_X occurs in the boundary layer, which is decoupled from the NO₂ in the free troposphere. Based on the recommendations from this sensitivity test, we then applied the FDA using observations of NO₂ columns from TROPOMI, corrected for contribution from free tropospheric NO₂, between 1 June and 31 August 2018. The NO_X emissions derived from the default TROPOMI retrievals are biased low over cities and industrialized areas. However, when the coarse 1x1 degree TM5-MP NO₂ profile used in the retrieval is replaced by the high-resolution profile of LOTOS-EUROS, the TROPOMI NO_X emissions are enhanced by 22 % and are in better agreement with the inventory for the Netherlands. This emphasizes the importance of using realistic high-resolution a-priori NO₂ profile shapes in the TROPOMI retrieval. We conclude that accurate quantitative NO_X emissions estimates are possible with the FDA, but that they require sophisticated, fine-scale, corrections for both the NO₂ observations driving the method, as well as the estimates of the NO₂ chemical lifetime and NO_X:NO₂ ratio. This information can be obtained from high-resolution chemistry transport model simulations, at the expense of the simplicity and applicability of the FDA.