A global analysis of biomass burning organic aerosol

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Abstract

Organic aerosols represent one of the main sources of uncertainty affecting attempts to quantify anthropogenic climate change. The diverse physical and chemical properties of organic aerosols and the varied pathways involved in their formation and aging form the basis of this uncertainty, preventing extensive and accurate representation within regional and global scale models. This inability to constrain the radiative forcings produced by organic aerosols within the atmosphere consequently acts as a limitation to the wider objective of providing reliable projections of future climate. Biomass burning constitutes one of the main anthropogenic contributions to the global atmospheric organic aerosol (OA) burden, particularly in tropical regions where the potential for perturbations to the climate system is also enhanced due to higher average levels of solar irradiance. Emissions from biomass burning have been the subject of an intense research focus in recent years, involving a combination of field campaigns and laboratory studies. These experiments have aimed to improve the limited understanding of the processes involved in the evolution of biomass burning organic aerosol (BBOA) and contribute towards the development of more robust parameterisations for climate and chemical transport models. The main objective of this thesis was to use datasets acquired from several different global regions to perform a broad analysis of the BBOA fraction, with the extensive temporal and spatial scales provided by such measurements enabling investigation of a number of key uncertainties, including regional variability in emissions and the role of secondary organic aerosol (SOA) formation in aging smoke plumes.Measurements of BBOA mass concentration obtained using Aerodyne Research Inc. Aerosol Mass Spectrometers (AMS) were used to calculate characteristic ΔOA/ΔCO ratios for different environments, accounting for the effects of dilution and contrasting fire sizes to give a proportional representation of OA production. High levels of variability in average ΔOA/ΔCO were observed both between and within different regions. The scale of this variability consistently exceeded any differences between plumes of different ages, while a widespread absence of any sustained increase in ΔOA/ΔCO with aging indicates that SOA formation does not provide a net increase in OA mass. Despite this lack of OA enhancement, increasing proportions of oxygenated OA components in aged plumes highlight the chemical transformations occurring during the evolution of BBOA, and the additional influence of OA loss through evaporation or deposition.Potential drivers of variability in ΔOA/ΔCO at source, such as changes in fuel types and combustion conditions, were investigated for controlled fires carried out within a combustion chamber. These laboratory experiments revealed a number of complex relationships between BB emissions and source conditions. Although ΔOA/ΔCO was shown to be influenced by both fuel properties and transitions between flaming and smouldering combustion phases, the extent of these effects was limited, while variability between fires exceeded levels observed for ambient measurements. These findings emphasise the complexity of the BBOA lifecycle and the need to address the extensive uncertainties associated with its various constituent processes, in order to improve understanding of eventual climate impacts from biomass burning.

Keyword(s)

aerosol aging; aerosol mass spectrometer; aircraft measurements; biomass burning; combustion chamber; emission ratios; field campaigns; organic aerosol

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