Spatiotemporal patterns of ambient NO2 and NO pollution in Kigali Rwanda

Abstract Kigali, like many cities in sub-Saharan Africa, faces rapid urban growth alongside the need to manage air pollution and protect public health. Despite its policy efforts, systematic data on nitrogen oxides (NO x : NO 2 and NO), key indicators of combustion-related pollution, have been limited. We applied a standardized protocol to characterize city-scale spatial and temporal patterns of NO x across Kigali. Between November 2022 and December 2023, we collected weekly integrated NO 2 and NO samples ( n  = 630 each) at 130 sites representing diverse land-use types. NO 2 concentrations ranged from 1.3 to 61.9 µg/m 3 (mean 13.9 µg/m 3 ), with annual-equivalent frequently exceeding the WHO annual guideline (10 µg/m 3 ) in urban areas. Exceedances occurred in 39% of sparsely residential, 89% of commercial/industrial, and 99% of densely populated residential sites. NO 2 concentrations were significantly higher in urban versus rural areas (18.2 vs. 6.3 µg/m 3 ), near major roads (19.9 vs. 11.6 µg/m 3 ), and at lower elevations (15.4 vs. 9.0 µg/m 3 ). The highest levels were observed in the densely populated districts of Kicukiro and Nyarugenge. Overall, NO 2 and NO exhibited strong spatial gradients related to land use, traffic, population density, and topography, highlighting the importance of targeted urban planning and air quality management in rapidly growing cities.