Characteristics of Particulate Emissions from Low Temperature Combustion and Renewable Fuels : Aerosol Mass Spectrometry of Refractory Carbonaceous Particles
Sammanfattning: Particulate air pollution is one of the major causes of premature death in the world, and combustion-derived soot emissions contribute strongly to the particulate pollution to which humans are exposed. Black carbon (BC) is one such emission and denotes soot with strong light absorption in the ultraviolet to infrared spectrum. Combustion can also generate brown carbon (BrC) particles, with absorption confined to shorter wavelengths than BC and absorption Ångström exponents (AAEs) significantly higher than 1. When emitted to the atmosphere, BC and BrC can accelerate global warming by absorbing incoming solar radiation. The overall aim of this thesis is to improve the understanding of relationships between combustion conditions, physicochemical soot properties, and parameters which are of relevance for adverse health effects and climate impact.Soot emissions were studied from a miniCAST soot generator, a heavy-duty diesel engine, and from traditional and modern biomass based cook stoves. The soot particles were characterized for their optical properties, chemical composition, size, and carbon nanostructure (soot maturity). Soot formation and oxidation processes were studied by extracting particles from the cylinder of a heavy-duty diesel engine. The diesel engine was equipped with an exhaust gas recirculation system, and used either Swedish MK1 fossil diesel, a rapeseed methyl ester (RME) biodiesel, or a renewable hydrotreated vegetable oil (HVO) fuel. Immature soot was characterized by short and amorphous nanostructures, BrC absorption, high polycyclic aromatic hydrocarbon (PAH) fractions, and refractory organic carbon that partially formed pyrolytic carbon during thermal-optical analysis. Mature soot was characterized by ordered nanostructures, BC absorption, low PAH mass fractions, and mass dominated by elemental carbon. A novel methodology was introduced to investigate differences in soot maturity using a soot particle aerosol mass spectrometer (SP AMS). Mature soot, characterized by long fringe lengths, generated mainly low molecular weight carbon cluster fragments (C1-5+). In addition to C1-5+, immature soot with shorter fringe lengths produced signals from midcarbon and fullerene carbon clusters (C≥6+). The new methodology and interpretation can improve methods that use aerosol mass spectrometry for the source apportionment of combustion emissions. It can also aid in the development of new emission mitigation strategies, for example, with respect to the soot oxidation reactivity of relevance for diesel particulate filters. The results show that low temperature combustion conditions result in soot with immature characteristics, while higher temperatures result in more mature soot. The elevated AAEs and a major fraction of the BrC absorption were assigned to refractory soot components. Specifically, the analysis suggested that the progression from BrC to BC absorption as soot maturity increased was caused primarily by the growth of refractory aromatic units, which are the soot building blocks. This description of how combustion conditions may control soot properties can improve our understanding of processes related to light absorption in the atmosphere.The renewable HVO and RME fuels reduced particulate matter and BC emissions; the RME, in addition, reduced PAH emissions compared to fossil diesel. The formation of reactive oxygen species (ROS) is an important mechanism in particle-induced toxicity. The ability of soot particles to form ROS increased with increasing combustion temperatures. It was hypothesized from the analysis of soot properties that the diesel soot potential to form ROS with increasing combustion temperature in the first step increased due to more mature soot nanostructures, and in the second step due to increased oxidation and altered surface oxygen functional groups. This hypothesis can form a basis for future evaluations of drivers of soot particle toxicity that are of relevance for global air pollution problems.
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