Silver Onyango1, Crystal M. North  This email address is being protected from spambots. You need JavaScript enabled to view it.2,3,4, Hatem A. Ellaithy2, Paul Tumwesigye5, Choong-Min Kang3, Vasileios Matthaios3,6, Martin Mukama5, Nuriat Nambogo5, J. Mikhail Wolfson3, Stephen Ferguson3, Stephen Asiimwe1,2, Lynn Atuyambe7, Data Santorino1,5, David C. Christiani2,3,4, Petros Koutrakis

1 Mbarara University of Science and Technology, Mbarara, Uganda
2 Massachusetts General Hospital, Boston, MA, USA
3 Department of Environmental Health, Harvard T.H. Chan School of Public Health, Boston, MA, USA
4 Harvard Medical School, Boston, MA, USA
5 Consortium for Affordable Medical Technologies, Uganda
6 School of Geography Earth and Environmental Science, University of Birmingham, UK
7 Makerere School of Public Health, Kampala, Uganda

These authors contributed equally to this work.

Received: August 30, 2023
Revised: December 15, 2023
Accepted: January 3, 2024

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

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Onyango, S., North, C.M., Ellaithy, H.A., Tumwesigye, P., Kang, C.M., Matthaios, V., Mukama, M., Nambogo, N., Wolfson, J.M., Ferguson, S., Asiimwe, S., Atuyambe, L., Santorino, D., Christiani, D.C., Koutrakis, P. (2024). Ambient PM2.5 Temporal Variation and Source Apportionment in Mbarara, Uganda. Aerosol Air Qual. Res.


  • Daily ambient PM2.5 levels were compared at semi-rural and urban sites in Uganda.
  • Ambient PM2.5 source identification was done with positive matrix factorization.
  • PM2.5 levels exceeded health standards on most days at semi-rural and urban sites.
  • Leading PM2.5 contributors were traffic and biomass/secondary aerosol sources.


Air pollution is the leading environmental cause of death globally, and most mortality occurs in resource-limited settings such as sub-Saharan Africa. The African continent experiences some of the worst ambient air pollution in the world, yet there are relatively little African data characterizing ambient pollutant levels and source admixtures. In Uganda, ambient PM2.5 levels exceed international health standards. However, most studies focus only on urban environments and do not characterize pollutant sources. We measured daily ambient PM2.5 concentrations and sources in Mbarara, Uganda from May 2018 through February 2019 using Harvard impactors fitted with size-selective inlets. We compared our estimates to publicly available levels in Kampala, and to World Health Organization (WHO) air quality guidelines. We characterized the leading PM2.5 sources in Mbarara using x-ray fluorescence and positive matrix factorization. Daily PM2.5 concentrations were 26.7 µg m-3 and 59.4 µg m-3 in Mbarara and Kampala, respectively (p < 0.001). PM2.5 concentrations exceeded WHO guidelines on 58% of days in Mbarara and 99% of days in Kampala. In Mbarara, PM2.5 was higher in the dry as compared to the rainy season (30.8 vs. 21.3, p < 0.001), while seasonal variation was not observed in Kampala. PM2.5 concentrations did not vary on weekdays versus weekends in either city. In Mbarara, the six main ambient PM2.5 sources identified included (in order of abundance): traffic-related, biomass and secondary aerosols, industry and metallurgy, heavy oil and fuel combustion, fine soil, and salt aerosol. Our findings confirm that air quality in southwestern Uganda is unsafe and that mitigation efforts are urgently needed. Ongoing work focused on improving air quality in the region may have the greatest impact if focused on traffic and biomass-related sources.

Keywords: Air pollution, Air quality, Africa, Biomass, Resource-limited setting

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