Estimating NOx Emissions and NO2 Lifetimes in West African Cities Using TROPOMI Observations

Abstract:

Poor air quality is a growing issue in West Africa. Nitrogen oxides (NO_x = NO + NO₂) play a key role in urban air pollution, affecting the atmospheric oxidation capacity and the production of other pollutants including ozone and secondary particulate matter. NO_x emissions in West African cities are uncertain, owing in part to the region’s rapid urbanization, more diverse traffic, and energy sources. Here, we use observations from the TROPOspheric Monitoring Instrument (TROPOMI) to infer NO_x emission rates and NO₂ lifetimes in major cities in West Africa using the established technique of fitting plume line densities. We quantify and compare emissions in different cities separately on workdays and rest days. We advance the plume fitting method to study overlapping plumes, focusing on Dakar, Senegal, which is located adjacent to the large NO_x sources of the Industrial Zone and airport, and where, because the prevailing north-south winds during the dry season, we are also able to quantify the NO₂ lifetime over the ocean.