Seasonal source variability of carbonaceous aerosols at the Rwanda Climate Observatory

Abstract

Abstract. 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.

Atmospheric Black Carbon Loadings and Sources over Eastern Sub-Saharan Africa Are Governed by the Regional Savanna Fires

“…), and is a site in the network of the Advanced Global Atmospheric Gases Experiment (AGAGE; ) . Site characteristics and meteorology are detailed elsewhere. , A high-volume sampler (flowrate at 30 m 3 h –1 ; model DH-77, DIGITEL Elektronik AG, Switzerland) installed at the station, 5 m above ground level, was used to collect PM 2.5 aerosols on pre-combusted quartz fiber filters (400 °C for 5 h to remove organic matter; 15 cm in diameter). In this study, night-time-only (01.00–06.00 h) filter samples were collected over a 7-day period, between May 2014 and April 2016.…”

“…Sub-Saharan Africa (SSA) is a global hotspot for aerosol emissions. − A large but poorly constrained contribution is from large-scale regional fires, mainly lit by humans and occasionally triggered by lightning strikes . The region accounts for 70% of the global burned area and over 50% of the global carbonaceous aerosol budget. − Aerosols generally have a short atmospheric residence time (days-week); hence, there is large spatio-temporal variability in aerosol concentrations.…”

“…The region accounts for 70% of the global burned area and over 50% of the global carbonaceous aerosol budget. − Aerosols generally have a short atmospheric residence time (days-week); hence, there is large spatio-temporal variability in aerosol concentrations. Consequently, the associated impact on climate, human health, and biogeochemical cycles is primarily regional. − Despite the fact that the SSA region is one of the world’s largest sources of aerosol emissions, the aerosol characteristics and sources in the SSA region are poorly constrained in comparison with other geographical regions, largely owing to limited ground-based observations. , …”

“…The carbon-14 signature of BC (often reported as Δ 14 C) allows differentiation between biomass burning and fossil fuel combustion with high precision and specificity. The Δ 14 C may be combined with the stable carbon signature (δ 13 C) to further resolve biomass and/or fossil sources into more detailed categories. − Isotope characterization of BC allows refinement of model studies and emission inventories, while also providing complementary and unambiguous source information from observational data. , So far, the number of applications of carbon isotope techniques in Africa has been limited to one study of BC in urban Nairobi and to bulk carbonaceous aerosols (i.e., unresolved total carbon) at the Rwanda Climate Observatory. , Therefore, isotope-based source constraints for BC at a background location in SSAcurrently unavailablewould substantially improve the understanding of BC aerosol emissions on a regional scale.…”

Stable carbon isotopic composition of biomass burning emissions – implications for estimating the contribution of C3 and C4 plants

“…δ 13 C signatures of BB products are widely used in several scientific fields. Since the 1980s, stable carbon isotopes measured in air pollution have been used to distinguish emissions from BB and other sources (Aguilera and Whigham, 2018;Andersson et al, 2020;Cachier et al, 1985;Gromov et al, 2017;Kawashima and Haneishi, 2012). Paleo-ecologists use pyrolyzed carbon signatures in soils to trace vegetation changes (Hall et al, 2008;Masi et al, 2013;Wynn et al, 2020).…”