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Organic aerosol is a major constituent of atmospheric fine particles, especially over continental regions. These particles adversely affect human health and global climate. A significant fraction of organic aerosol is considered to be from the oxidation products of ozone and volatile organic compounds, which are called secondary organic aerosol (SOA). To study the formation mechanisms of secondary organic aerosol, it is important to characterize their molecular composition. The composition of secondary organic aerosol is very complex including thousands of species with molecular weight up to over a thousand Dalton. Methods utilized for the identification of these oxidation products involve advanced mass spectrometry techniques. In this dissertation, three mass spectrometry techniques were developed to study the molecular composition of organic aerosol. Firstly, online nano-aerosol sample deposition methods for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry was developed to incorporate matrix particles directly with analyte particles onto a conventional MALDI plate. Secondly, a microsampling and analysis technique was developed in order to collect microgram samples and analyze them with high performance mass spectrometry. With this technique, the molecular composition of particle phase SOA at a low mass loading can be elucidated, which provides information about SOA formation at the early stages. A species with the (neutral molecule) formula C 17 H 26 O 8 (MW 358) increased substantially in intensity relative to other products as the mass loading decreased. Tandem mass spectrometry (MS n) of this species showed it to be a dimer of C 9 H 14 O 4 and C 8 H 12 O 4, most likely pinic acid and terpenylic acid, respectively. This species is likely to be critical at the early stages of SOA formation. Thirdly, ambient secondary electrospray ionization (ESI) source was designed to characterize the molecular composition of both gas and particle phases SOA online. This ion source was demonstrated to be applicable to a wide range of mass spectrometers having an ambient inlet. This technique provides a tool to acquire detailed information about possible SOA nucleation agents. A species with the (neutral molecule) formula C 20 H 36 O 6 (MW 372) was found in the gas-phase products of SOA, which could be critical for the new particle formation of SOA. Tandem mass spectrometry (MS n) of this species showed it to be a dimer of an organic hydroperoxide C 10 H 18 O 3, which is likely formed via OH-initiated oxidation pathway.
The guiding question to this research is: To what extent and by what mechanisms do biogenic volatile organic compounds contribute to atmospheric aerosol mass? To address this question we need to understand the chemistry that produces condensable vapors which when in the presence of particles may partition onto the aerosol surface depending on their chemical and physical properties. I developed an insitu gas and aerosol sampling system, the FIGAERO (Filter Inlet for Gases and AEROsol) to speciate gas and particle phase organics derived from photochemical reactions with biogenic volatile organic compounds under both field and laboratory conditions. By coupling the FIGAERO to a High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-TOF-CIMS) I am able to elucidate chemical pathways by identifying elemental compositions and in some cases functional groups present in the detected molecular ions. The coupling of the FIGAERO to the HR-TOF-CIMS also allows the estimation of effective vapor pressures of the aerosol components and this information can be used to improve vapor pressure models and test associated partitioning theories and parameterizations. The approach also provides hundreds of speciated chemical tracers that can be correlated with traditional environmental and chemical measurements (e.g AMS, NOx, SO2, SMPS, VOC) to help derive sources and sinks and to constrain the mechanisms responsible for the formation and growth of organic aerosol. Measurements obtained across a wide range of conditions and locations allowing connections and contrasts between different chemical systems, providing insights into generally controlling factors of secondary organic aerosol (SOA) and its properties.
Aerosol Mass Spectrometry (AMS) has emerged as the principal tool for aerosol analysis and soft ionization techniques are an important subset particularly suited for chemical speciation investigations. This dissertation presents the use of soft ionization AMS for the analysis of complex organic aerosols in laboratory studies, both as models for Primary and Secondary Organic Aerosol (POA and SOA). An established technique (PERCI-AMS) is used to identify the resonance capture mechanisms for cysteine containing peptide residues. This technique is then applied to investigate the ozonolysis of mixed particles representative of biogenic rich organic aerosol as is common for marine aerosols. In order to apply soft ionization AMS to lower concentrations of particle mass more representative of ambient aerosol loadings and thus allow SOA investigations, an entirely new AMS technique was developed and is presented utilizing Near-Infrared Laser-Desorption-Ionization (NIR-LDI-AMS). The advantages this technique are exemplified by an analysis of limonene ozonolysis SOA whereby first generation homogeneous oxidation products are delineated from second generation heterogeneous oxidation products.
Abstract : Organic aerosol affects human health and climate. These effects are largely determined by the composition of the organic aerosol, which is a complex mixture of species. Understanding the complexity of organic aerosol is critical to determining its effect on human health and climate. In this study, long range transported organic aerosol collected at the Pico Mountain Observatory was analyzed using ultrahigh resolution mass spectrometry. Organic aerosol transported in the free troposphere had an overall lower extent of oxidation than aerosol transported in the boundary layer. It was hypothesized that the lower oxidation was related to a more viscous phase state of the aerosol during transport. The results suggest that biomass burning organic aerosol injected into the free troposphere are more persistent than organic aerosol in the boundary layer. A sample was also analyzed using tandem FT-ICR MS/MS fragmentation, providing information about the functional group composition in the aerosol sample. This was done using a segmented scan approach, which revealed an unprecedented molecular complexity of unfragmented precursor ions. In addition to the expected CO2 and H2O neutral losses, neutral losses corresponding to carbonyl functional groups (C2H4O, CO) were observed. The abundance of carbonyl functional groups suggests a slower rate of aging in the atmosphere. Analysis of nitrogen and sulfur containing neutral losses highlighted a surprising abundance of reduced nitrogen and sulfur loss (NH3 and SH2). This further supports the hypothesis of slower aging in the free troposphere. Additional research was done to develop an R software package (MFAssignR) to perform molecular formula assignment with improved decision-making transparency, noise estimation, isotope identification, and mass recalibration. MFAssignR was found to assign the same molecular formula as other molecular formula assignment methods for the majority (97-99%) of mass peaks that were assigned a molecular formula by the compared methods. Additionally, MFAssignR was more effective at assigning molecular formulas to low intensity peaks relative to the other methods tested, leading to more overall molecular formula assignments. MFAssignR is available via GitHub and is the first open source package to contain a full pipeline of functions for data preparation and analysis for ultrahigh resolution mass spectrometry.
Aerosol particles are ubiquitous throughout the atmosphere and play an important role in human health, climate, and the chemistry of the atmosphere. A significant mass fraction of these particles is composed of organic species, which remain poorly characterized due to the number and diversity of species present. This thesis describes the development and characterization of two versions of a new single particle mass spectrometer with a 3D ion trap for organic aerosol studies. Version I combines CO2 laser desorption and electron impact ionization in an ion trap. Mass spectra obtained for four species are comparable to NIST EI spectra. Tandem mass spectrometry studies are also demonstrated. The effects of vaporization energy, ionization delay time, and electron pulse width on the mass spectra and fragmentation patterns are examined. The detection limit of the instrument is found to be ~1x108 molecules (350 nm diameter particle) for 2,4-dihydroxybenzoic acid. Version II integrates CO2 laser desorption and tunable VUV ionization in an ion trap and was used for a detailed study of oleyl alcohol, oleic acid and mixtures thereof. Both the degree of fragmentation in the mass spectra and the translational energy of the vaporized molecules are found to vary as a function of desorption energy in the pure particles and as a function of composition in the mixed particles. These changes can be described by the energy absorbed per particle during desorption. We show that these effects hinder the quantitative response of the instrument and have important implications for other two step laser desorption/ionization systems. The final part of this thesis presents preliminary results from atmospherically relevant particles. Mass spectra of cigarette sidestream smoke, fulvic acid, meat cooking, and ammonium bisulfate aerosols are collected using both versions of the instrument. The two step desorption/ionization process only worked for two types of aerosols, while CO2 only mass spectra w.
Ionization Methods in Organic Mass Spectrometry is a basic practical guide for scientists of all disciplines who wish to analyse samples by organic mass spectrometry. Concentrating on instrumental operation, this book gives step-by-step instructions on how to set up, and how to achieve the best results, using a range of ionization methods, including atmospheric pressure chemical ionization, electrospray ionization and matrix assisted laser desorption ionization. Ionization Methods in Organic Mass Spectrometry will enable a beginner, or practitioner with limited experience, to choose the most appropriate ionization technique in application areas such as biomolecules, drugs and metabolites, pesticides, polymers and many other organic compounds. It will be a valuable practical guide for technicians, graduates, students or researchers - or indeed anyone new to practical organic mass spectrometry.