ALBERT

Description: Sub-Saharan Africa (SSA) is a global hot spot for aerosol emissions, which affect the regional climate and air quality. In this paper, we use ground-based observations to address the large uncertainties in the source-resolved emission estimation of carbonaceous aerosols. Ambient fine fraction aerosol was collected on filters at the high-altitude (2590 m a.s.l.) Rwanda Climate Observatory (RCO), a SSA background site, during the dry and wet seasons in 2014 and 2015. The concentrations of both the carbonaceous and inorganic ion components 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, which approach ∼95 % for carbonaceous aerosols. An isotopic mass-balance estimate shows that the amount of the carbonaceous aerosol stemming from savanna fires may increase from 0.2 µg m−3 in the wet season up to 10 µg m−3 during the dry season. Based on these results, we quantitatively show that savanna fire is the key modulator of the seasonal aerosol composition variability at the RCO.

Description: Air pollution is understudied in sub-Saharan Africa, resulting in a gap in the 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 the short-lived climate-forcing pollutants CO2, CO, CH4, black carbon (BC), and 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. Black carbon concentrations during Rwanda's two dry seasons (December–January–February, DJF, and June–July–August, JJA), which coincide with the two regional biomass burning seasons, are higher at Mt. Mugogo than in major European cities with daily values (24 h) during the dry season of around 5 µg m−3 (daily average concentrations ranging from less than 0.1 to over 17 µg m−3 for the entire measurement period). BC baseline concentrations during biomass burning seasons are loosely correlated with fire radiative power data for the region acquired with a MODIS satellite instrument. 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. Spectral aerosol absorption measured with a dual-spot Aethalometer varies seasonally due to changes in types of fuel burned and the direction of pollution transport to the site. Ozone concentrations peaked during Rwanda's dry seasons (daily measured maximum of 70 ppbv). The understanding and quantification of the percent contributions of regional and local (beyond large-scale biomass) emissions is essential to guide policy in the region. During the rainy seasons, 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 and was found to account for up to 35 % of annual average BC measured. Our measurements indicate that air pollution is a current and growing problem in equatorial East Africa.

Description: 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, 2590m 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.