Nitrogen oxides (NO and NO(2)) pollution in the Accra metropolis: Spatiotemporal patterns and the role of meteorology

Economic and urban development in sub-Saharan Africa (SSA) may be shifting the dominant air pollution sources in cities from biomass to road traffic. Considered as a marker for traffic-related air pollution in cities, we conducted a city-wide measurement of NO(x) levels in the Accra Metropolis and examined their spatiotemporal patterns in relation to land use and meteorological factors. Between April 2019 to June 2020, we collected weekly integrated NO(x) (n=428) and NO(2) (n=472) samples at 10 fixed (year-long) and 124 rotating (week-long) sites. Data from the same time of year were compared to a previous study (2006) to assess changes in NO(2) concentrations. NO and NO(2) concentrations were highest in commercial/business/industrial (66 and 76 μg/m(3), respectively) and high-density residential areas (47 and 59 μg/m(3), respectively), compared with peri-urban locations. We observed annual means of 68 and 70 μg/m(3) for NO and NO(2), and a clear seasonal variation, with the mean NO(2) of 63 μg/m(3) (non-Harmattan) increased by 25-56% to 87 μg/m(3) (Harmattan) across different site types. The NO(2)/NO(x) ratio was also elevated by 19-28%. Both NO and NO(2) levels were associated with indicators of road traffic emissions (e.g. distance to major roads), but not with community biomass use (e.g. wood and charcoal). We found strong correlations between both NO(2) and NO(2)/NO(x) and mixing layer depth, incident solar radiation and water vapor mixing ratio. These findings represent an increase of 25 – 180% when compared to a small study conducted in two high-density residential neighborhoods in Accra in 2006. Road traffic may be replacing community biomass use (major source of fine particulate matter) as the prominent source of air pollution in Accra, with policy implication for growing cities in SSA.