Satellite Observation of Spatio-temporal Variations in Nitrogen Dioxide over West Africa and Implications for Regional Air Quality

BACKGROUND. Nitrogen dioxide (NO(2)) is known to affect human health, causing heart and cardiovascular diseases, and it has been shown that locations with long term NO(2) pollution recorded a high number of fatalities due to the COVID-19 pandemic. There are no ground stations monitoring emissions of NO(2) over West Africa. The present study aimed to use satellite observations to examine pollutant trends over this region. OBJECTIVE. To examine the trend of NO(2) over the entire West Africa sub region in relationship to contributions to environmental emissions using satellite-derived data. This enables the assessment of West Africa regional air pollution hot spots in relationship to enhancing atmospheric factors. The results from this study will also be useful guidance for setting air quality standards for air pollution controls to minimize health hazards. METHODS. The present study examined thirteen years of average monthly values of nitrogen dioxide (NO(2)) to determine the spatio-temporal variation of this pollutant over West Africa. Satellite data for NO(2) between 2005 and 2017 were used to determine the variation in pollution levels over West Africa. Correlations between NO(2) and meteorological variables (wind speed, rainfall and air temperature) were obtained to explain the influence of West African weather on the region's pollution accumulation. RESULTS. The present study observed that NO(2) concentrations varied from place to place and from season to season. Nitrogen dioxide concentrations during the dry season were higher (sometimes 200% higher) than values observed in the wet season which ranged between 0.5 and 6×10(15) molec/cm(2). Nitrogen dioxide north-south oscillation during the course of a year is largely controlled by the inter-tropical discontinuity (ITD) zone as high concentrations of NO(2) are found in the vicinity of the ITD where wind speeds and horizontal vorticity approaches zero. Correlation analysis between NO(2) and some atmospheric variables indicated NO(2) concentrations are well influenced by atmospheric variables showing bipolar signals depending on the season. An increasing trend of NO(2) was also found over selected cities of the region. This indicated that regional air quality is gradually deteriorating. CONCLUSIONS. The implications of worsening regional air quality were examined in the light of the prevailing COVID-19 pandemic. The dominant atmospheric factor determining pollution episodes in the region is the inter-tropical discontinuity line which marks the meeting point between the two wind regimes over the region. Densely populated areas are characteristically prone to elevated pollution and have experienced high fatalities during the COVID-19 pandemic. COMPETING INTERESTS. The authors declare no competing financial interests.