We detect and quantify NOx point sources from the divergence of the horizontal NOx flux based on the continuity equation.

The analysis steps are:

- The NOx flux is determined for each TROPOMI (TROPOspheric Monitoring Instrument) orbit by upscaling the TROPOMI tropospheric NO2 column to NOx and multiplying it with horizontal wind fields from ECMWF (300m above ground).

- The NOx fluxes are averaged for 2018-2019.

- The divergence, i.e. spatial derivative, of the mean NOx flux is calculated, which is particularly sensitive for point sources.

- NOx point sources are detected in the divergence map by an automated search algorithm for local maxima, and quantified by fitting a Gaussian function to these maxima. Ambiguous cases are skipped.

TROPOMI is the satellite instrument on board of the Copernicus Sentinel-5 Precursor satellite.

The approach of deriving emission information from the divergence of the NOx flux is described in Beirle et al., 2019:

Beirle, S., Borger, C., Dörner, S., Li, A., Hu, Z., Liu, F., Wang, Y. and Wagner, T.: Pinpointing nitrogen oxide emissions from space, Science Advances, 5(11), eaax9800, doi:10.1126/sciadv.aax9800, 2019.

The details and modifications made for the automated detection of NOx point sources on global scale are provided in Beirle et al., 2020:

Beirle, S., Borger, C., Dörner, S., Eskes, H., Kumar, V., de Laat, A., and Wagner, T.: Catalog of NOx emissions from point sources as derived from the divergence of the NO2 flux for TROPOMI, to be submitted to Earth System Science Data, 2020.