Quantifying NOx emissions from a city and a small country with the TROPOMI and OMI satellite instruments

The TROPOMI and OMI satellite sensors provide an exciting perspective on the sources, dispersion, and fate of air pollution, and in particular on nitrogen dioxide (NO₂). Yet it is still difficult to relate satellite observations of tropospheric NO₂ columns to the underlying NO_x emissions and their trends. Robust interpretation of satellite data relies on a good understanding of the accuracy and representativeness of the satellite data itself, but also on the relationship between NO_x emissions and the observable NO₂ amount. This relationship is influences by local chemistry, mixing and dispersion, and by the NO₂ amount in the free troposphere. We address these issues via two examples:

(1) Direct estimation of NO_x emissions from the satellite-observed build-up of pollution over the city of Paris. After validating NO₂ measurements from TROPOMI over the heart of Paris, we analyse the observed build-up of NO₂ pollution over the city along with the wind. Over the city, recently emitted NO_x has been oxidized to limited degree, facilitating the use of TROPOMI data to directly determine the strength and distribution of emissions from the city. From the observed build-up of NO₂ pollution, we find highest NO_x emissions on cold weekdays in February 2018, and lowest emissions on warm weekend days in spring 2018.

(2) Trends in NO₂ over The Netherlands. We use the QA4ECV OMI NO₂ record to investigate trends in tropospheric NO₂ columns over Europe, and in particular over The Netherlands between 2005 and 2018. In spite of the differences in metrics and sampling techniques, the NO₂ measured in the Dutch atmosphere from space and from the ground follows a trend that is consistent with predictions by emission inventories. Surface NO₂ is reduced by 32% in 2018 relative to 2005, OMI NO₂ by 35%, and NO_x emissions by 32%-38% depending on the inventory. Interestingly, the Dutch surface concentrations reveal an upward trend in the NO₂:NO ratio in line with O₃ increases. This suggests that the NO₂ makes up an increasing share of the NO_x in the lower atmosphere as NO_x emissions decline. This needs to be accounted for when interpreting NO₂ trends as proxy for NO_x trends.