Biomass burning aerosols play an important role in the chemistry and physics of the atmosphere and therefore, affect global climate. Biomass burning aerosols are generally aqueous and have a strong saccharidic component due to the combustion and pyrolysis of cellulose, a major component of foliar fuel. This class of aerosol is known to affect both the absorption and scatter of solar radiation. Also, biomass burning aerosols contribute to cloud formation through their action as cloud-condensation nuclei. Many questions exist about the chemical speciation and chemical aging of biomass burning aerosols and how this affects their atmospheric properties and ultimately, global climate. Also, knowledge of the chemical components of these aerosols is important in the search for chemical tracers that can give information about the point or regional source, fuel type, and age of a biomass burning aerosol parcel. Levoglucosan was chosen for these studies as a model compound for biomass burning aerosols because of its high measured concentrations in aerosol samples. Levoglucosan often dominates the aerosol composition by mass. In this dissertation, laboratory proxy systems were developed to study the solution-phase chemistry of levoglucosan with common atmospheric reactants found in biomass burning aerosols (i.e. H+, •OH). To mimic these natural conditions, acid chemistry was studied using sulfuric acid in water (pH=4.5). The hydroxyl radical (•OH) was produced by the Fenton reaction which consists of iron, hydrogen peroxide and acid (H2SO4) in aqueous solvent. For studies in aqueous sulfuric acid, oligomers of levoglucosan were measured by matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF-MS). A rational mechanism is proposed based on both the acid-catalyzed cationic ring-opening of levoglucosan and nucleophilic attack of ROH from levoglucosan on the hemi-acetal carbon to produce pyranose oligomers through the formation of glycosidic bonds. Oligomer formation is further supported by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Reactions of levoglucosan with •OH produced from Fenton chemistry were studied in solution. Two modes of oligomerization (2000 u) were observed for reaction times between 1 and 7 days using MALDI-TOF-MS and laser desorption ionization (LDI) TOF-MS. Single-mass unit continuum mass distributions with dominant -2 u patterns were measured and superimposed by a +176/+162 u oligomer series. This latter oligomer pattern was attributed to a Criegee rearrangement (+14 u) of levoglucosan, initiated by •OH, forming a lactone (176 u). The acid-catalyzed reaction of any ROH from levoglucosan (+162 u) forms an ester through transesterification of the lactone functionality, whereupon propagation forms polyesters. Proposed products and chemical mechanisms are suggested as sources and precursors of humic-like substances (HULIS), which are known to possess a large saccharic component and are possibly formed from biomass burning aerosols. These products could also serve as secondary tracers, giving further information on the source and age of the aerosol.

Biomass burning aerosol can perturb the atmospheric energy budget and hence regional and global climates via interactions with solar radiation and cloud microphysics. Furthermore, there are significant deleterious effects on human and ecosystem health. The magnitude and nature of these impacts is driven by the aerosols physiochemical properties and their vertical distribution. However the drivers of these are poorly characterised, especially in the tropics where widespread biomass burning is co-located with complex cloud fields and processes, high levels of solar insolation and rapid land use change. In this work the key drivers determining the geographic, vertical, meteorological and temporal variability of biomass burning haze in tropical South America are identified and quantified. This is based on an analysis of simultaneous and vertically resolved measurements of aerosol burden, aerosol intrinsic properties (composition, size, hygroscopicity and optics), gas phase mixing ratios and atmospheric thermodynamics. These novel in-situ measurements were undertaken during research flights as part of the South America Biomass Burning Analysis (SAMBBA) campaign in September/October 2012. A clear difference is observed between the two distinct fire regimes in tropical South America. Cerrado (deforestation) regimes in the east (west) are found to be characterised by more flaming (smouldering) combustion, leading to a contrast in emissions with relatively more (less) refractory black carbon to organic aerosol and smaller (larger) aerosol sizes. This results in a population which absorbs (scatters) more incoming solar radiation. Furthermore, the aerosol vertical distribution differs between regimes: in the east (west) biomass burning aerosol of a similar loading is distributed from the surface to ~4 km (~2 km). This is driven by contrasting thermodynamics, in particular convective mixing, and plume injection to greater altitudes in the east. This work is the first demonstration of a contrast between these two regions from in-situ measurements. The additional atmospheric heating from biomass burning aerosol, calculated from in-situ measurements for the first time in the tropics, is significant in both fire regimes, but especially so in the eastern Cerrado where it is greater than that from molecular absorption. Heating also increases with altitude in the east, owing to the prevalence of flaming combustion which is observed to inject more absorbing emissions to higher altitudes. Models do not consider this process, nor do they capture (via emissions factors) the regional difference identified. As such, the associated effects on atmospheric stability, cloud formation and large scale dynamics may not be adequately considered in model simulations and thus predictions may not be representative. To contextualise the in-situ measurements, satellite derived climatologies of fire and aerosol properties are presented for the past decade. In the west the aerosol and trace gas burden has significantly declined, in association with deforestation rates, total fire count and fire intensity. In the east, a small increase in aerosol and trace gas burden is coupled to decreasing single scattering albedos and increasing absorption at near-UV wavelengths, fire intensity and relative fire occurrence. The findings presented in this work offer new insight into the nature of tropical biomass burning aerosols: on how and why fire regimes result in contrasting physiochemical properties; on how the population is vertically distributed and why this varies between regimes; and on the significant additional heating biomass burning aerosol transfers to the atmosphere. In tropical South America specifically, the heating rate is greatest in the eastern Cerrado regions, co-located with increases in fire count and intensity and thus likely to have an increasingly significant impact on weather and climate in the region.

<p dir="ltr">Atmospheric aerosols are solid and liquid particles emitted from a range of natural and anthropogenic sources, and that impact Earth’s climate directly by interacting with solar radiation, as well as indirectly through modifications to the properties and lifecycles of clouds. Furthermore, atmospheric particles yield substantial implications on air quality, visibility, and human health. While the impact of aerosols on the planet has been broadly defined, accurate forecasting of atmospheric particle processes remains challenging due to their complex physicochemical properties. Highly variable aerosol characteristics include size, morphology, viscosity, elemental and molecular composition, hygroscopicity, mixing state, and light-absorption. Moreover, aerosols experience transformations as they evolve during transport downwind of the emission source. Aerosol evolution is dictated (between many other factors) by ambient conditions, such as relative humidity, temperature, and sunlight activity. This dissertation aims at providing a comprehensive characterization of atmospheric particles both at the bulk and single-particle level by implementing a unique combination of offline and online instrumentation.</p><p dir="ltr">The first chapter of this dissertation describes sources of atmospheric particles, as well as aerosol properties frequently examined to quantify their impact on our planet, such as chemical composition and light absorption. The second chapter delves into the wide range of techniques implemented in this study to characterize laboratory-generated and field-sampled aerosols. Notably, online measurements of chemical composition and optical properties were acquired with aircraft-deployed mass spectrometers and a particle-into-liquid sampler. These were frequently used to complement single-particle analysis employed with offline electron and X-ray microscopy techniques.</p><p dir="ltr">The third chapter describes a systematic approach to infer the viscosity of organic particles based on their morphology. Specifically, particles deform upon impacting a rigid surface during sampling, and the degree of deformation is highly influenced by the viscoelastic properties. Highly viscous and solid particles will retain their shape after sampling, whereas liquid-like particles will flatten drastically. Hence, we expanded on a semi-quantitative approach to infer the viscosity of particles based on their measured height-to-width aspect ratios. The fourth chapter discussed bulk measurements of chemical composition of smoke plumes emitted during wildfires in Western United States. An aerosol mass spectrometer was employed to quantify the mass concentration of key chemical species and their subsequent evolution during plume transport. Analyzed samples corresponded to daytime and nighttime particulate, which provided valuable insights on the impact of photochemical reactions on the composition and evolution of biomass burning particles. The fifth chapter serves as a follow-up study for that discussed in Chapter 4. Biomass burning particles were deposited onto substrates and taken for further chemical imaging. Scanning electron microscopy coupled with X-ray microanalysis provided single-particle information on the size, morphology, and elemental composition of aerosols sampled at different locations of the smoke plume. More detailed chemical information was acquired using synchrotron-based X-ray microscopy coupled with near-edge X-ray absorption fine structure spectroscopy. This technique distinguished between organic carbon, soot, and inorganic species, while also determining the contribution of functional groups, including alkenes, aliphatic, and carboxyl groups. Chemical imaging measurements were examined with respect to real-time optical data acquired onboard research aircraft. This facilitated correlating the chemical and light-absorbing properties of particles. The sixth chapter discusses a multi-modal, novel approach to distinguish between sources of soot-containing particles. Atomic force microscopy, integrated with Raman spectroscopy, was implemented for a screening of the morphological and spectral features of individual particles. Subsequently, automated μ-Raman was used to acquire the spectra of large ensembles of particles that are considered representative of the whole particle population. Emission sources of soot particles were then determined following two curve-fitting approaches previously established.</p><p dir="ltr">Overall, the studies discussed in this dissertation provide a comprehensive understanding of aerosol characteristics at the single-particle level, which is often overlooked by atmospheric model when predicting the impact of atmospheric particles on climate.</p>