Headline: Seasonal and diurnal variability in air pollutants and short-lived climate forcers measured at the Rwanda Climate Observatory

Air pollution is still largely unstudied in sub-Saharan Africa, resulting in a gap in scientific understanding of emissions, atmospheric processes, and impacts of air pollutants in this region. The Rwanda Climate Observatory, a joint partnership between MIT and the government of Rwanda, has been measuring ambient concentrations of key long-lived greenhouse gases and short-lived climate-forcing pollutants (CO2, CO, CH4, BC, O3) with state-of-the-art instruments on the summit of Mt. Mugogo (1.586° S, 29.566° E, 2590 m above sea level) since May 2015. Rwanda is a small, mountainous, and densely populated country in equatorial East Africa, currently undergoing rapid development but still at less than 20 % urbanization. The position and meteorology of Rwanda is such that the emissions transported from both the northern and southern African biomass burning seasons affect BC, CO, and O3 concentrations in Rwanda. Black carbon concentrations during Rwanda's two dry seasons, which coincide with the two biomass burning seasons, are higher at Mt. Mugogo than in major European cities. Higher BC baseline concentrations at Mugogo are loosely correlated with fire radiative power data for the region acquired with MODIS satellite instrument. Spectral aerosol absorption measured with a dual-spot Aethalometer also varies in different seasons, likely due to change in types of fuel burned and direction of pollution transport to the site. Ozone concentration was found to be higher in air masses from southern Africa than from northern Africa during their respective biomass burning seasons. These higher ozone concentration in air masses from the south could be indicative of more anthropogenic emissions mixed with the biomass burning emissions from southern Africa as Rwanda is downwind of major East African capital cities in this season. During the rainy season, local emitting activities (e.g., cooking, transportation, trash burning) remain steady, regional biomass burning is low, and transport distances are shorter as rainout of pollution occurs regularly. Thus local pollution at Mugogo can be estimated during this time period. Understanding and quantification of the percent contributions of regional and local emissions is essential to guide policy in the region. Our measurements indicate that air pollution is a current and growing problem in equatorial East Africa that deserves immediate attention.