Headline: Understanding Air Quality in East Africa. Estimating Biomass Burning and Anthropogenic Influence with Long-Term Measurements

Air pollution is largely unstudied in sub-Saharan Africa, resulting in a large 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) on the summit of Mt. Mugogo (1.586°S, 29.566°E, 2500 m above sea level) since May 2015. Rwanda is a small, mountainous, and densely populated country in equatorial East Africa currently undergoing rapid development. The location and meteorology of Rwanda is such that 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 are higher at Mt. Mugogo, a rural site, than in major European cities. Higher BC baseline concentrations at Mugogo are correlated with fire radiative power data for the region acquired with MODIS satellite instrument. Spectral absorption of aerosol measured with a dual-spot aethalometer also varies seasonally, likely due to change in 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. The higher ozone concentration in air masses from the south could be indicative of more anthropogenic influence as Rwanda is downwind of major East African capitals in this season. During the rainy seasons, local emitting activities (e.g., cooking, driving, trash burning) remain steady, regional biomass burning is low, and transport distances are shorter as rainout of pollution occurs regularly, which allows estimation of local pollution during this time period. Urban PM2.5 measurements in the capital city of Kigali and from the neighboring city of Kampala, Uganda were compared to the observations at Mugogo. 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.