Chemical characterization and source apportionment of fine particulate matter in Kigali, Rwanda, using aerosol mass spectrometry

Abstract

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 ± 15 µg m−3, 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 emissions and 40 % from biomass burning emissions. Tracer ions detected by the aerosol mass spectrometer suggest that photochemistry plays a significant role in the formation of secondary OA during the daytime (06:00 a.m.–06:00 p.m.), while primary OA dominates in the morning and evening due to increased anthropogenic activity and shallower boundary layer height. PM1 OA 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 OA during the rainy and dry seasons, while primary OA (POA: BBOA + HOA) contributed 53 % and 59 %. Short-term interventions that limit traffic activity on some Saturdays and Sundays had a mixed impact on ambient PM1 and PM2.5 concentrations, with car-free Sundays reducing PM1 concentrations throughout most of the day.