Comment on acp-2021-49

doi:10.5194/acp-2021-49-rc2

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 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 Ã ngstrÃ¶m exponent â AAE â and mass absorption coefficients â MACs) also evolved with ageing. In this study, brown carbon (BrC) was a minor fractional component of the freshly emitted BB aerosols (<â0.5âh), but the increasing AAE with particle age indicates that BrC formation dominated over any loss process over the first â¼â12âh of plume transport. Using different methods, the fractional contribution of BrC to total aerosol absorption showed an increasing trend with time and was â¼â18â%â31â% at the optical wavelength of 405ânm 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.

Highlights

Biomass burning (BB) of agricultural waste and savannah in the sub-Sahelian regions of West Africa during the dry season (November to February) is a strong contributor to the global aerosol burden every year (Roberts et al, 2009; Andreae, 2019)
We selected smoke plumes that were sampled over the Atlantic Ocean, and Numerical Atmospheric Modelling Environment (NAME) back trajectories showed that these plumes in flights C006 and C007 were mainly transported from a similar fire region as that associated with sampling in flight C005
In the MOYA-2017 campaign, we investigated the evolution of smoke aerosols emitted from flaming-controlled BB in West Africa over the first half-day following emission

Summary

Introduction

Biomass burning (BB) of agricultural waste and savannah in the sub-Sahelian regions of West Africa during the dry season (November to February) is a strong contributor to the global aerosol burden every year (Roberts et al, 2009; Andreae, 2019). Weak southerly advection over the land surface can drive air to move northward, and the warm BB plumes tend to be lifted over the cooler and drier Saharan air. Under this latter scenario, the upper-level circulation plays a role in transporting these lifted plumes southward toward the Atlantic Ocean (Haywood et al, 2008). The upper-level circulation plays a role in transporting these lifted plumes southward toward the Atlantic Ocean (Haywood et al, 2008) These BB aerosols have an important impact on the regional climate by scattering and absorbing solar radiation, and they interact with clouds. The overall climate effects of these BB aerosols are a combination of interacting warming and cooling effects, depending on the aerosol vertical distribution and their relative locations with respect to clouds, as well as their chemical, physical and optical properties and evolution with transport time (e.g. Boucher et al, 2013)

Results

Discussion

Conclusion