Deriving NOx emissions of cities in Africa using the space-based TROPOMI instrument

Surface concentrations of nitrogen oxides (NOx) are increasing at rates of up 10% per year in cities in Africa, as inferred with trends in long-term satellite observations of tropospheric columns of nitrogen dioxide (NO2). This is being driven by rapid population growth and urbanization in the absence of air quality policies. Models needed to inform air quality policies use out-of-date inventories for cities in Africa due to absence of detailed temporal and spatial information about emission and activity factors of sources unique to African cities. Here we apply a recently improved method of deriving city NOx emissions from satellite observations of NO2 that builds on the well-established rotation of each city NO2 plume along a consistent direction and selecting a single sampling area around the city centre to fit a modified Gaussian to calculate emissions. The improved method uses a more efficient fit routine and multiple sampling areas to eliminate subjective area selection and increase the success of deriving annual emissions from ~50% to 100%. Such an approach is ideal for African cities that have wide-ranging sizes due to the different development stages of countries and urban centres in Africa. Work is underway to quantify NOx emissions for more cities in Africa than has been achieved with global studies using 2019 observations of NO2 from the TROPOspheric Monitoring Instrument (TROPOMI). The resultant NOx emissions will then be compared to emissions estimates from widely used global (EDGAR, CEDS, HTAP) and regional (DACCIWA, DICE-Africa) inventories to assess shortcomings in inventories and the influence of these on designing evidence-based air quality policies and regulations.