Background: Ambient air pollution is a public and environmental health concern in Africa. Despite the belief that ambient air pollution is not problem, recent information show that it requires attention. However, sources and contributions are not clearly identified yet.

Objective: This study, therefore, aims to identify the major sources of - and contributions to- ambient aerosols (PM_2.5), and to better understand the season pattern in Addis Ababa, Ethiopia.

Methods: We collected 24-hr samples of aerosols (PM_2.5) for 61 days in one-in-every 6-day schedule, from Nov. 2015-Nov. 2016. The samples were analyzed for mass, elemental carbon, and organic carbon, water-soluble organic carbon, secondary inorganic ions, and particle-phase organic tracers. The monthly composite samples were analyzed for organic tracers, which were used for source apportionment using a Chemical Mass Balance (CMB) receptor model. Source contribution to Organic carbon (OC) estimated in terms of organic molecular markers and a CMB receptor model.

Results: It is known from current data that PM2.5 mass in Addis Ababa is more than 5-times higher than the WHO annual AQG. The highest mean monthly concentration of PM2.5 was observed during the major rain season (June–September).  This study finds that Elemental carbon and Organic carbon are higher while secondary inorganic ions are less in Addis Ababa. Organic Matter (OC times 1.4) represents 55.5±11.6% of the PM2.5 reaching the highest during the main wet season. We found that vehicles are the predominant sources for OC and total PM2.5 mass, accounting more than a quarter for total PM2.5. Biomass burning (18.3%) and dust (17.4%) along with vehicles represent about two-third of the total PM2.5 mass. Sulfate and residential charcoal burning together represent more than 10% of the total PM2.5 mass. The seasonal variation observed for the major sources to PM2.5 mass during major rain (June-September) and minor rain (February-April) seasons, respectively were: averaging (31.0±2.6%) vs. (24.7±1.2%) from motor vehicles; (21.5±5%) vs. (14±2%) from biomass burning; and (11±6.4%) vs. (22.7±8.4%) from dust.

Conclusion: CMB estimates show that five sources contributed a total of 74% of the PM2.5 mass. Vehicles, biomass burning and dust are the three major sources of ambient PM2.5 in Addis Ababa, Ethiopia with distinct seasonality. Policy measures focusing transport and biomass burning, including solid waste suggested to reduce air pollution and benefit city residents’ health.