Comment on acp-2021-1051

Abstract

Urban areas and industrial facilities, which concentrate most human activity and industrial production, are major sources of air pollutants, with serious implications for human health and global climate. For most of these pollutants, emission inventories are often highly uncertain, especially in developing countries. Spaceborne observations from the TROPOMI instrument, onboard the Sentinel-5 Precursor satellite, are used to measure nitrogen dioxide (NO2) slant column densities with a high spatial resolution. Here, we use two years of TROPOMI retrievals to map nitrogen oxides (NOx = NO + NO2) emissions in Egypt with a top-down model based on the continuity equation in steady state. Emissions are expressed as the sum of a transport term and a sink term representing the three-body reaction comprising NO2 and OH. This sink term requires information on the lifetime of NO2, which is calculated with the use of CAMS near-real-time temperature and hydroxyl radical (OH) concentration fields. The applicability of the OH concentration field is evaluated by comparing the lifetime it provides with the lifetime inferred from the fitting of NO2 line density profiles with an exponentially modified Gaussian function. This comparison, which is conducted for 39 samples of NO2 patterns above the city of Riyadh, provides information on the reliability of the CAMS near-real-time OH concentration fields; It also provides the location of the most appropriate vertical level to represent typical pollution sources in industrial areas and megacities in the Middle East. In Egypt, total derived emissions of NOx are dominated by the sink term. However, they can be locally dominated by wind transport, especially along the Nile where human activities are concentrated. Megacities and industrial regions clearly appear as the largest sources of NOx emissions in the country. Our top-down model produces emissions whose annual variability is consistent with the national electricity consumption. It is also able to detect lower emissions on Fridays, which are inherent to the social norm of the country, and to quantify the drop in emissions due to the COVID-19 pandemic. Overall, our indications of NOx emissions for Egypt are found to be 25.0 % higher than the CAMS-GLOB-ANT_v4.2 inventory, but significantly differ in terms of seasonality.