Inequalities in urban air pollution in sub-Saharan Africa: an empirical modeling of ambient NO and NO2 concentrations in Accra, Ghana

Abstract

Road traffic has become the leading source of air pollution in fast-growing sub-Saharan African cities. Yet, there is a dearth of robust city-wide data for understanding space-time variations and inequalities in combustion related emissions and exposures. We combined nitrogen dioxide (NO _2 ) and nitric oxide (NO) measurement data from 134 locations in the Greater Accra Metropolitan Area (GAMA), with geographical, meteorological, and population factors in spatio-temporal mixed effects models to predict NO _2 and NO concentrations at fine spatial (50 m) and temporal (weekly) resolution over the entire GAMA. Model performance was evaluated with 10-fold cross-validation (CV), and predictions were summarized as annual and seasonal (dusty [Harmattan] and rainy [non-Harmattan]) mean concentrations. The predictions were used to examine population distributions of, and socioeconomic inequalities in, exposure at the census enumeration area (EA) level. The models explained 88% and 79% of the spatiotemporal variability in NO _2 and NO concentrations, respectively. The mean predicted annual, non-Harmattan and Harmattan NO _2 levels were 37 (range: 1–189), 28 (range: 1–170) and 50 (range: 1–195) µ g m ^−3 , respectively. Unlike NO _2 , NO concentrations were highest in the non-Harmattan season (41 [range: 31–521] µ g m ^−3 ). Road traffic was the dominant factor for both pollutants, but NO _2 had higher spatial heterogeneity than NO. For both pollutants, the levels were substantially higher in the city core, where the entire population (100%) was exposed to annual NO _2 levels exceeding the World Health Organization (WHO) guideline of 10 µ g m ^−3 . Significant disparities in NO _2 concentrations existed across socioeconomic gradients, with residents in the poorest communities exposed to levels about 15 µ g m ^−3 higher compared with the wealthiest ( p < 0.001). The results showed the important role of road traffic emissions in air pollution concentrations in the GAMA, which has major implications for the health of the city’s poorest residents. These data could support climate and health impact assessments as well as policy evaluations in the city.

Keywords

• air pollution
• nitrogen dioxide (NO2)
• nitrogen oxides (NOx)
• sub-Saharan Africa
• Ghana
• air pollution inequality