Analyzing the nitrogen dioxide spatiotemporal trends in Dakar, Senegal using Pandora measurements

Nitrogen dioxide (NO2) is an urban air pollutant largely emitted as a combustion product from vehicles, powerplants, and industrial processes. NO2 is harmful to human health both directly and through its role in the production of near-surface ozone and particulate matter. There is a general lack of ground-based observational data in West African cities, which has limited our understanding of the sources, relationships with human activities, and meteorological controls over NO2. Because of this, we installed a Pandora NO2 spectrometer on the rooftop of the Simeon Fongang Atmosphere and Ocean Physics Laboratory (LPAOSF), located on the western corniche of Dakar, Senegal. In this study, we analyze two years of these NO2 measurements and compare the Pandora data with those of the TROPOMI satellite and the surface concentrations recorded in the coastal station of the Air Quality Management Center (CGQA). We also report spatially-resolved MAX-DOAS Pandora NO2 observations across the city of Dakar, which we compare with CGQA measurements at multiple monitoring stations. We report diurnal, day-of-week, and seasonal patterns, which reflect anthropogenic emissions patterns, especially from local traffic, and show that NO2 levels generally decreased during the COVID-19 shutdown period. We present a correlation analysis of Pandora and TROPOMI columns, with results showing good agreement (R = 0.8), but with Pandora measuring a factor of two higher NO2 than observed from space. We assess the spatial variability in this correlation using MAX-DOAS observations over the city center, along the coast, and over the ocean. We also discuss column-surface correlations using the CGQA NO2 network.