Quantifying nitrogen oxide emissions in the Eastern Mediterranean - Middle East region using TROPOMI observations

Urban areas and industrial facilities are major sources of air pollutants, as they concentrate a large part of human activity and industrial production. For most of these pollutants, emission inventories are highly uncertain, especially in developing countries. In this context, satellite observations can be used to observe column densities of chemical species to reduce uncertainties in inventories. Here, we use three years of TROPOMI daily nitrogen dioxide (NO2) retrievals to map nitrogen oxide (NOx) emissions at high resolution in Egypt, Qatar and Cyprus. We use a flux-divergence scheme, which expresses NOx emissions as the sum of a wind transport term and a chemical sink term representing the reaction between NO2 and hydroxyl radical (OH). The model allows to identify major NOx hotspots. Among these, heavy industrial facilities, such as cement plants and fossil-fuel fired power plants, are characterized by a predominance of the transport term over the sink term. Heavily populated urban centers can also be identified, with a predominance of the sink term. In Egypt, our model is able to detect a weekly cycle in NOx emissions, reflecting Egyptian social norms, and to quantify the drop of emissions in 2020 due to the Covid-19 pandemic. In Qatar, it is able to infer the emission factor of isolated power plants, which is consistent with reported values. In Cyprus, it is able to quantify the emissions from different power plants, with higher emissions on the north side of the island due to the use of different technologies and fuels. These results demonstrate a high potential for satellite-based emission mapping at the scale of large urbanised areas well observed by TROPOMI.