Characterizing space-time patterns of nitrogen oxides (NO 2 and NO) pollution in Kigali, Rwanda

Kigali, like many cities in sub-Saharan Africa (SSA), must balance rapid urban growth and the provision of essential services with the need to curb environmental pollution and protect public health in the context of its unique topography. Although the city has implemented policies aimed at reducing emissions from multiple sectors, systematic data on oxides of nitrogen (NO _X ; NO ₂ and NO), key markers of combustion-related urban air pollution, have been limited. We applied a standardized measurement protocol previously used in Accra, Ghana, to characterize city-scale spatial and temporal patterns of NO _X pollution in Kigali. Between November 2022 and December 2023, we deployed Ogawa passive samplers to collect weekly integrated NO ₂ (n = 630) and NO (n = 630) samples across 130 sites (10 year-long and 120 rotating week-long locations) representing diverse land-use types and source characteristics. Weekly NO ₂ and NO concentrations ranged from approximately 2 to 62 µg/m3 (mean [SD]: 13.9 [11.4]) and approximately 1 to 49 µg/m³, respectively. Although nearly all background sites recorded NO ₂ concentrations below the World Health Organization (WHO) annual guideline of 10 µg/m3, exceedances were common in more urbanized settings, occurring in 39% of samples from sparsely residential areas, 89% from commercial, business, and industrial (CBI) areas, and 99% from densely populated residential areas. Mean NO ₂ concentrations were significantly higher in urban compared with rural neighborhoods (18.2 vs. 6.3 µg/m³; p < 0.001), at sites located within 200 m of primary roads compared with those farther away (19.9 vs. 11.6 µg/m3; p < 0.001), and at lower compared with higher elevations (15.4 vs. 9.0 µg/m3; p < 0.001). The levels were higher and exceeded the WHO annual guideline in the more densely populated and urbanized districts of Kicukiro and Nyarugenge, compared with the more rural Gasabo district. Similar spatial patterns were observed for NO. Overall, NO ₂ and NO concentrations across Kigali were strongly patterned by land use, traffic proximity, population density, and topography, with the highest levels observed in traffic-dominated, densely populated, low-elevation areas. These city-wide measurement data provide critical evidence to inform land-use planning, air quality management, and regulatory strategies in a rapidly urbanizing, landlocked city characterized by complex topography.