Biomass burning aerosols from African Wildfires: Assessing aerosol properties, ageing process and effects on regional clouds

  • Huihui Wu

Student thesis: Phd


African biomass burning (BB) during the dry seasons is one of the largest sources of global carbonaceous particles and also significantly contributes to aerosol precursors. These BB aerosols are expected to have significant impacts on regional/global climate system by directly scattering and absorbing solar radiation, as well as perturbing cloud microphysical properties and distributions. The climate effects of these BB aerosols are dependent on their vertical distributions and relative locations with respect to clouds, as well as their properties and evolution during lifetime. Nevertheless, the characterisation of African BB aerosols and their interaction with clouds are limited, in particular in-situ observations in remote regions after long range transport. The systematic observations of optical properties for African BB aerosols during lifetime are also lacking. This project addresses these issues by conducting aircraft measurements in West Africa during the Methane Observation Yearly Assessment-2017 (MOYA) campaign to observe the emissions and evolution of African BB aerosol, and also in the remote transport region over the southeast Atlantic during the Cloud-Aerosol-Radiation Interactions and Forcing for Year 2017 (CLARIFY) campaign, to investigate highly aged BB aerosols transported from southern Africa. A series of online aerosol or/and cloud instrumentation were employed during the campaigns. The observed African wildfire smoke plumes from two campaigns were consistently controlled by flaming-phase burning of wooded savannah and agricultural residue at source, which are rich in black carbon (BC) emissions. A broad-scale picture of African BB aerosols and their properties can be derived from these campaigns. Some specific features of the evolution of African BB aerosol were observed. The main finds are that brown carbon (BrC) makes a minor contribution at source and experiences an initial stage of BrC net enhancement which is followed by a decrease to initial levels. The observations indicate that different treatments of aerosol properties from different types of fires and their downwind evolution should be considered when modelling regional radiative forcing. Another important finding is that highly aged African BB aerosols remain strongly absorbing across wide transport region, which are more absorbing than currently represented in climate models. This suggests an underestimation of absorption for aged African BB aerosols in current studies. A persistent feature of vertical variation in BB aerosol properties, i.e. aerosol chemical composition and single scattering albedo, is found when southern African BB plumes are transported to the southeast Atlantic, as well as an essential separation between the free troposphere (FT) and marine boundary layer (MBL). Future work should consider the impact of this vertical variability on climate models. The transport and entrainment history of southern African BB aerosols over the southeast Atlantic were also investigated. The analysis shows that efficient entrainment of FT smoke into the MBL could happen multiple days before getting to Ascension Island. The region of efficient entrainment is found to be considerably further west than previously predicted. Aircraft measurements around Ascension Island show that the entrained BB aerosols resulted in a substantially enhanced cloud droplet number concentration but decreased cloud effective radius in the polluted MBL compared to clean cases. These findings presented in this project provide new insight into the transport history and properties of African BB aerosols, as well as their interaction with clouds, which are unique constraints on aerosol and cloud parameterisations used in modelling regional transport and radiation interactions over the important African BB-impacted region.
Date of Award1 Aug 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorJames Allan (Supervisor) & Hugh Coe (Supervisor)

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