Rapid transformation of ambient absorbing aerosols from West African biomass burning
Authors: Wu Huihui , Taylor Jonathan W. , Langridge Justin M. , Yu Chenjie , Allan James D. , Szpek Kate , Cotterell Michael I. , Williams Paul I. , Flynn Michael , Barker Patrick , Fox Cathryn , Allen Grant , Lee James , Coe Hugh 
Published: June 2021

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 the 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
– MACs) 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 h 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 the 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 hours. MOYA-2017 provides novel field results. The comparisons between MOYA-2017 and previous field studies imply that the evolution
of absorbing aerosols (BC and BrC) after emission varies with source combustion conditions. Different treatments of absorbing aerosol properties from
different types of fires and their downwind evolution should be considered when modelling regional radiative forcing. These observational results
will be very important for predicting climate effects of BB aerosol in regions controlled by flaming burning of agricultural waste and savannah-like
biomass fuels.