Ambient air pollution poses a significant threat to public health, particularly in low and middle-income countries, where detailed data on particulate matter (PM) mass and composition are scarce. We conducted a year-long study on PM composition and sources in Eastern Africa (Kigali, Rwanda). The annual mean concentration of PM₁ was 31 μg/m³, with slightly higher concentrations during the dry season. Organic aerosols (OA) contributed 73 % of the observed PM₁ mass, black carbon (BC) 16 %, nitrate 6 %, sulfate and ammonium 2 % each, and chlorine 1 %. BC is approximately 60 % due to fossil fuel and 40 % from biomass burning emissions. Tracer ions detected by the mass spectrometer suggest that photochemistry plays a significant role in the formation of secondary OA during the daytime (6:00 am to 6:00 pm), while primary OA dominates in the morning and evening due to increased anthropogenic activity and shallower boundary layer height. PM₁ in Kigali is primarily composed of Oxygenated Organic Aerosols (OOA, 45 %), Hydrocarbon-like OA (HOA, 32 %), and Biomass Burning OA (BBOA, 23 %). Secondary organic aerosol (SOA) accounted for 47 % and 41 % of PM₁ during the wet and dry seasons, respectively, while primary OA (POA: BBOA + HOA) contributed 53 % and 59 %. This suggests that seasonal changes in PM₁ mass in Kigali are primarily driven by deposition rather than shifts in emissions, chemical processing, or source strengths.