Seasonal source variability of carbonaceous aerosols at the Rwanda climate Observatory

Abstract. Sub-Saharan Africa (SSA) is a global hotspot for aerosol emissions, affecting regional environmental sustainability. In this paper we use atmospheric observations to address one of the major uncertainties of the, e.g., climate and health impact of SSA aerosols: the quantitative contributions from different emissions sources. Ambient fine fraction aerosol (PM2.5) were collected on filters at the high altitude (2590 m a.s.l.) Rwanda Climate Observatory (RCO), an SSA background site, during dry and wet seasons in 2014 and 2015. The concentrations of both carbonaceous aerosols and inorganic ions show a strong seasonal cycle, with highly elevated concentrations during the dry season. Source marker ratios, including carbon isotopes, show that the wet and dry seasons have distinct aerosol compositions. The dry season is characterized by elevated amounts of biomass burning products, approaching ~ 95 % for carbonaceous aerosols. An isotopic mass-balance estimate shows that the amount of the carbonaceous aerosols stemming from savanna fires may increase from ~ 0.6 μg/m3 in the wet season up to ~ 10 μg/m3 during the dry season. Taken together, we here quantitatively show that savanna fire is the key modulator of the seasonal aerosol composition variability at the RCO, an SSA background site.