Abstract:Abstract. Seasonal biomass burning (BB) over West Africa is a globally significant source of carbonaceous particles in the atmosphere, which have important climate impacts but are poorly constrained. Here, the evolution of smoke aerosols emitted from flaming-controlled burning of agricultural waste and wooded savannah in the Senegal region was characterized over a timescale of half-day advection from source during the MOYA-2017 (Methane Observation Yearly Assessment-2017) aircraft campaign. Plumes from such fi… Show more
“…Barker et al [42] found these fires had mean emission factors units (in g per kg of dry fuel) of 1.8 ± 0.6 for CH 4 , 1630 ± 21.4 for CO 2 and 67 ± 14 for CO, with a mean combustion efficiency of 0.94 ± 0.01, and obtained a δ 13 C CH 4 value of about −34‰ from all regional sources. Wu et al [43] provide further details about the FAAM flights and sampling for the MOYA project, and report aerosol measurements and chemical transformations in biomass burning plumes sampled in the region.…”
“…Wu et al . [ 43 ] provide further details about the FAAM flights and sampling for the MOYA project, and report aerosol measurements and chemical transformations in biomass burning plumes sampled in the region. Figure 1.…”
Section: Resultsmentioning
confidence: 99%
“…Flights along the coast of Senegal, Gambia, Guinea-Bissau and Guinea intersected multiple smoke plumes in the prevailing easterly wind, demonstrating that regional pollution was present, with very widespread smoke plumes in the boundary layer and lower free troposphere. Transport times of sampled smoke plumes ranged from a few minutes (in one plume overflown at low altitude over an active fire front (electronic supplementary material, figure SI 4)), to 9–12 h for plumes sampled over the ocean [ 43 ]. As there was no recent lightning from thunderstorms, fires were presumably human-lit, whether accidentally or deliberately.…”
We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane
δ
13
C
CH
4
isotopic signatures were in the range −55 to −49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely −60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar
δ
13
C
CH
4
signatures were measured over the Amazonian wetlands of NE Bolivia (around −59‰) and the overall
δ
13
C
CH
4
signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was −59 ± 2‰. These results were more negative than expected. For African cattle,
δ
13
C
CH
4
values were around −60 to −50‰. Isotopic ratios in methane emitted by tropical fires depended on the C3 : C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal,
δ
13
C
CH
4
values were around −28‰. By contrast, African C4 tropical grass fire
δ
13
C
CH
4
values were −16 to −12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had
δ
13
C
CH
4
around −37 to −36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric
δ
13
C
CH
4
values predicted by global atmospheric models are highly sensitive to the
δ
13
C
CH
4
isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa.
This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.
“…Barker et al [42] found these fires had mean emission factors units (in g per kg of dry fuel) of 1.8 ± 0.6 for CH 4 , 1630 ± 21.4 for CO 2 and 67 ± 14 for CO, with a mean combustion efficiency of 0.94 ± 0.01, and obtained a δ 13 C CH 4 value of about −34‰ from all regional sources. Wu et al [43] provide further details about the FAAM flights and sampling for the MOYA project, and report aerosol measurements and chemical transformations in biomass burning plumes sampled in the region.…”
“…Wu et al . [ 43 ] provide further details about the FAAM flights and sampling for the MOYA project, and report aerosol measurements and chemical transformations in biomass burning plumes sampled in the region. Figure 1.…”
Section: Resultsmentioning
confidence: 99%
“…Flights along the coast of Senegal, Gambia, Guinea-Bissau and Guinea intersected multiple smoke plumes in the prevailing easterly wind, demonstrating that regional pollution was present, with very widespread smoke plumes in the boundary layer and lower free troposphere. Transport times of sampled smoke plumes ranged from a few minutes (in one plume overflown at low altitude over an active fire front (electronic supplementary material, figure SI 4)), to 9–12 h for plumes sampled over the ocean [ 43 ]. As there was no recent lightning from thunderstorms, fires were presumably human-lit, whether accidentally or deliberately.…”
We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane
δ
13
C
CH
4
isotopic signatures were in the range −55 to −49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely −60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar
δ
13
C
CH
4
signatures were measured over the Amazonian wetlands of NE Bolivia (around −59‰) and the overall
δ
13
C
CH
4
signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was −59 ± 2‰. These results were more negative than expected. For African cattle,
δ
13
C
CH
4
values were around −60 to −50‰. Isotopic ratios in methane emitted by tropical fires depended on the C3 : C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal,
δ
13
C
CH
4
values were around −28‰. By contrast, African C4 tropical grass fire
δ
13
C
CH
4
values were −16 to −12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had
δ
13
C
CH
4
around −37 to −36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric
δ
13
C
CH
4
values predicted by global atmospheric models are highly sensitive to the
δ
13
C
CH
4
isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa.
This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.
“…At shorter wavelengths, a parameterization that results in lower BrC absorption at low BC:OA (WE‐CAN) and higher BrC absorption at higher BC:OA (ORACLES‐2016/CLARIFY) would better match these observations. It is possible that we are not representing another potential explanation for the different absorption of BrC in the two regions: recent work has shown that under conditions with very high primary BC:OA ratios from biomass combustion (such as savanna burning in Africa), a first stage of BrC enhancement happens after emission, owing to formation of highly absorbing SOA, and before whitening (the whitening may result from less‐absorbing SOA formation in those experiments) (Cappa et al., 2020; Wu et al., 2021).…”
Section: Impact Of Alternate Brc Absorption Parameterizationsmentioning
Biomass burning (BB) produces large quantities of carbonaceous aerosol (black carbon and organic aerosol, BC and OA, respectively), which significantly degrade air quality and impact climate. BC absorbs radiation, warming the atmosphere, while OA typically scatters radiation, leading to cooling. However, some OA, termed brown carbon (BrC), also absorbs visible and near UV radiation; although, its properties are not well constrained. We explore three aircraft campaigns from important BB regions with different dominant fuel and fire types (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen [WE‐CAN] in the western United States and ObseRvations of Aerosols above CLouds and their intEractionS and Cloud‐Aerosol‐Radiation Interactions and Forcing for Year downwind of southern Africa) and compare them with simulations from the global chemical transport model, GEOS‐Chem using GFED4s. The model generally captures the observed vertical profiles of carbonaceous BB aerosol concentrations; however, we find that BB BC emissions are underestimated in southern Africa. Our comparisons suggest that BC and/or BrC absorption is substantially higher downwind of Africa than in the western United States and, while the Saleh et al. (2014, https://doi.org/10.1038/ngeo2220) and FIREX parameterizations based on the BC:OA ratio improve model‐observation agreement in some regions, they do not sufficiently differentiate absorption characteristics at short wavelengths. We find that photochemical whitening substantially decreases the burden and direct radiative effect of BrC (annual mean of +0.29 W m−2 without whitening and +0.08 W m−2 with). Our comparisons suggest that whitening is required to explain WE‐CAN observations; however, the importance of whitening for African fires cannot be confirmed. Qualitative comparisons with the OMI UV aerosol index suggest our standard BrC whitening scheme may be too fast over Africa.
“…A low-pressure water-based condensation particle counter (WCPC model 3786-LP) was connected to a TSI 3081 differential mobility analyser (DMA). Following the schemes developed by Zhou (2001), the SMPS data were inverted based on a ~1 min averaging time when aerosol mass concentrations from the AMS and SP2 varied less than 30%.…”
Seasonal biomass burning (BB) over West Africa is a globally significant source of carbonaceous particles in the atmosphere, which have important climate impacts but are poorly constrained. Here, the evolution of smoke aerosols emitted from flaming-controlled burning of agricultural waste and wooded savannah in the Senegal region was characterized over a timescale of half-day advection from source during the MOYA-2017 (Methane Observation Yearly Assessment-2017) aircraft campaign. Plumes from such fire types are rich in black carbon (BC) emissions. Concurrent measurements of chemical composition, organic aerosol (OA) oxidation state, bulk aerosol size and BC mixing state reveal that emitted BB submicron aerosols changed dramatically with time. Various aerosol optical properties (e.g., absorption Ångström exponent (AAE), and mass absorption coefficients (MAC)) also evolved with ageing. In this study, brown carbon (BrC) was a minor fractional component of the freshly emitted BB aerosols (< 0.5 h), but the increasing AAE with particle age indicates that BrC formation dominated over any loss process over the first ~12 hours of plume transport. Using different methods, the fractional contribution of BrC to total aerosol absorption showed an increasing trend with time and was ~ 18-31 % at the optical wavelength of 405 nm after half-day transport. The generated BrC was found to be positively correlated with oxygenated and low-volatility OA, likely from the oxidation of evaporated primary OA and secondary OA formation. We found that the evolution of BrC with particle age was different in this region compared with previous BB field studies that mainly focused on emissions from smouldering fires, which have shown a high contribution from BrC at source and BrC net loss upon ageing. This study suggests an initial stage of BrC absorption enhancement during the evolution of BB smoke.Secondary processing is the dominant contributor to BrC production in this BB region, in contrast to the primary emission of BrC previously reported in other BB studies. The total aerosol absorption normalized to BC mass (MACmeas-BC) was also enhanced with ageing, due to the lensing effect of increasingly thick coatings on BC and the absorption by BrC. The effect of ageing on aerosol absorption, represented by the absorption enhancement (EAbs-MAC), was estimated over timescales of
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