Secondary Organic Aerosol Composition Studies Using Mass Spectrometry

Trace volatile organic compounds emitted by biogenic and anthropogenic sources into the atmosphere can undergo extensive photooxidation to form species with lower volatility. By equilibrium partitioning or reactive uptake, these compounds can nucleate into new aerosol particles or deposit onto already-existing particles to form secondary organic aerosol (SOA). SOA and other atmospheric particulate matter have measurable effects on global climate and public health, making understanding SOA formation a needed field of scientific inquiry. SOA formation can be done in a laboratory setting, using an environmental chamber; under these controlled conditions it is possible to generate SOA from a single parent compound and study the chemical composition of the gas and particle phases. By studying the SOA composition, it is possible to gain understanding of the chemical reactions that occur in the gas phase and particle phase, and identify potential heterogeneous processes that occur at the surface of SOA particles. In this thesis, mass spectrometric methods are used to identify qualitatively and qualitatively the chemical components of SOA derived from the photooxidation of important anthropogenic volatile organic compounds that are associated with gasoline and diesel fuels and industrial activity (C12 alkanes, toluene, and o-, m-, and p-cresols). The conditions under which SOA was generated in each system were varied to explore the effect of NO_x and inorganic seed composition on SOA chemical composition. The structure of the parent alkane was varied to investigate the effect on the functionalization and fragmentation of the resulting oxidation products. Relative humidity was varied in the alkane system as well to measure the effect of increased particle-phase water on condensed-phase reactions. In all systems, oligomeric species, resulting potentially from particle-phase and heterogeneous processes, were identified. Imines produced by reactions between (NH₄)₂SO₄ seed and carbonyl compounds were identified in all systems. Multigenerational photochemistry producing low- and extremely low-volatility organic compounds (LVOC and ELVOC) was reflected strongly in the particle-phase composition as well.

Investigation of Fundamental Processes Governing Secondary Organic Aerosol Formation in Laboratory Chambers

Our understanding of the processes and mechanisms by which secondary organic aerosol (SOA) is formed is derived from laboratory chamber studies. In the atmosphere, SOA formation is primarily driven by progressive photooxidation of SOA precursors, coupled with their gas-particle partitioning. In the chamber environment, SOA-forming vapors undergo multiple chemical and physical processes that involve production and removal via gas-phase reactions; partitioning onto suspended particles vs. particles deposited on the chamber wall; and direct deposition on the chamber wall. The main focus of this dissertation is to characterize the interactions of organic vapors with suspended particles and the chamber wall and explore how these intertwined processes in laboratory chambers govern SOA formation and evolution.

A Functional Group Oxidation Model (FGOM) that represents SOA formation and evolution in terms of the competition between functionalization and fragmentation, the extent of oxygen atom addition, and the change of volatility, is developed. The FGOM contains a set of parameters that are to be determined by fitting of the model to laboratory chamber data. The sensitivity of the model prediction to variation of the adjustable parameters allows one to assess the relative importance of various pathways involved in SOA formation.

A critical aspect of the environmental chamber is the presence of the wall, which can induce deposition of SOA-forming vapors and promote heterogeneous reactions. An experimental protocol and model framework are first developed to constrain the vapor-wall interactions. By optimal fitting the model predictions to the observed wall-induced decay profiles of 25 oxidized organic compounds, the dominant parameter governing the extent of wall deposition of a compound is identified, i.e., wall accommodation coefficient. By correlating this parameter with the molecular properties of a compound via its volatility, the wall-induced deposition rate of an organic compound can be predicted based on its carbon and oxygen numbers in the molecule.

Heterogeneous transformation of δ-hydroxycarbonyl, a major first-generation product from long-chain alkane photochemistry, is observed on the surface of particles and walls. The uniqueness of this reaction scheme is the production of substituted dihydrofuran, which is highly reactive towards ozone, OH, and NO3, thereby opening a reaction pathway that is not usually accessible to alkanes. A spectrum of highly-oxygenated products with carboxylic acid, ester, and ether functional groups is produced from the substituted dihydrofuran chemistry, thereby affecting the average oxidation state of the alkane-derived SOA.

The vapor wall loss correction is applied to several chamber-derived SOA systems generated from both anthropogenic and biogenic sources. Experimental and modeling approaches are employed to constrain the partitioning behavior of SOA-forming vapors onto suspended particles vs. chamber walls. It is demonstrated that deposition of SOA-forming vapors to the chamber wall during photooxidation experiments can lead to substantial and systematic underestimation of SOA. Therefore, it is likely that a lack of proper accounting for vapor wall losses that suppress chamber-derived SOA yields contribute substantially to the underprediction of ambient SOA concentrations in atmospheric models.

Field and Laboratory Studies of Atmospheric Organic Aerosol

This thesis is the culmination of field and laboratory studies aimed at assessing processes that affect the composition and distribution of atmospheric organic aerosol. An emphasis is placed on measurements conducted using compact and high-resolution Aerodyne Aerosol Mass Spectrometers (AMS). The first three chapters summarize results from aircraft campaigns designed to evaluate anthropogenic and biogenic impacts on marine aerosol and clouds off the coast of California. Subsequent chapters describe laboratory studies intended to evaluate gas and particle-phase mechanisms of organic aerosol oxidation.

The 2013 Nucleation in California Experiment (NiCE) was a campaign designed to study environments impacted by nucleated and/or freshly formed aerosol particles. Terrestrial biogenic aerosol with > 85% organic mass was observed to reside in the free troposphere above marine stratocumulus. This biogenic organic aerosol (BOA) originated from the Northwestern United States and was transported to the marine atmosphere during periodic cloud-clearing events. Spectra recorded by a cloud condensation nuclei counter demonstrated that BOA is CCN active. BOA enhancements at latitudes north of San Francisco, CA coincided with enhanced cloud water concentrations of organic species such as acetate and formate.

Airborne measurements conducted during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) were aimed at evaluating the contribution of ship emissions to the properties of marine aerosol and clouds off the coast of central California. In one study, analysis of organic aerosol mass spectra during periods of enhanced shipping activity yielded unique tracers indicative of cloud-processed ship emissions (m/z 42 and 99). The variation of their organic fraction (f₄₂ and f₉₉) was found to coincide with periods of heavy (f₄₂ > 0.15; f₉₉ > 0.04), moderate (0.05 < f₄₂ < 0.15; 0.01 < f₉₉ < 0.04), and negligible (f₄₂ < 0.05; f₉₉ < 0.01) ship influence. Application of these conditions to all measurements conducted during E-PEACE demonstrated that a large fraction of cloud droplet (72%) and dry aerosol mass (12%) sampled in the California coastal study region was heavily or moderately influenced by ship emissions. Another study investigated the chemical and physical evolution of a controlled organic plume emitted from the R/V Point Sur. Under sunny conditions, nucleated particles composed of oxidized organic compounds contributed nearly an order of magnitude more cloud condensation nuclei (CCN) than less oxidized particles formed under cloudy conditions. The processing time necessary for particles to become CCN active was short ( < 1 hr) compared to the time needed for particles to become hygroscopic at sub-saturated humidity ( > 4 hr).

Laboratory chamber experiments were also conducted to evaluate particle-phase processes influencing aerosol phase and composition. In one study, ammonium sulfate seed was coated with a layer of secondary organic aerosol (SOA) from toluene oxidation followed by a layer of SOA from α-pinene oxidation. The system exhibited different evaporative properties than ammonium sulfate seed initially coated with α-pinene SOA followed by a layer of toluene SOA. This behavior is consistent with a shell-and-core model and suggests limited mixing among different SOA types. Another study investigated the reactive uptake of isoprene epoxy diols (IEPOX) onto non-acidified aerosol. It was demonstrated that particle acidity has limited influence on organic aerosol formation onto ammonium sulfate seed, and that the chemical system is limited by the availability of nucleophiles such as sulfate.

Flow tube experiments were conducted to examine the role of iron in the reactive uptake and chemical oxidation of glycolaldehyde. Aerosol particles doped with iron and hydrogen peroxide were mixed with gas-phase glycolaldehyde and photochemically aged in a custom-built flow reactor. Compared to particles free of iron, iron-doped aerosols significantly enhanced the oxygen to carbon (O/C) ratio of accumulated organic mass. The primary oxidation mechanism is suggested to be a combination of Fenton and photo-Fenton reactions which enhance particle-phase OH radical concentrations.

Organic carbon in the sediments of the lower reaches of Periar River

Sediments are indicators of the quality of water overlying them and hence, useful in the assessment of environmental pollution. Temporal and spatial variations in sediment characteristics and organic carbon content from 9 stations in the lower reaches of Periyar River an area in Cochin Backwater, India which is polluted from different sources were studied for one year during 1981. Variations in colour and texture of sediments were brought about by changes in the grain size and state of oxidation of organic matter. The colour of the sediment varied from greyish black at stations 1 and 2, brownish at station 3, black at stations 4 to 8 and reddish at station 9. Organic carbon and sediment texture showed a direct relationship at all stations except at station 9 where organic carbon content showed an irregular pattern. Overall range of organic carbon content was between 1.19 and 29.6 mg.g super(-1). The mean organic carbon of the stations ranged between 6.8 mg.g super(-1) (station 5) and 20.8 mg.g super(-1) (station 9). On the whole temporal variations were considerable with high values at station 9 and low values at station 5. Fluctuations were more at stations 6, 7 and 8.

Reducing organic substances from anaerobic decomposition of hydrophytes

Oxidation-reduction properties of surface sediments are tightly associated with the geochemistry of substances, and reducing organic substances (ROS) from hydrophytes residues may play an important role in these processes. In this study, composition, dynamics, and properties of ROS from anaerobic decomposition of Eichhornia crassipes (Mart.) Solms, Potamogenton crispus Linn, Vallisneria natans (Lour.) Hara, Lemna trisulca Linn and Microcystis flos-aquae (Wittr) Kirch were investigated using differential pulse voltammetry (DPV). The type of hydrophytes determined both the reducibility and composition of ROS. At the peak time of ROS production, the anaerobic decomposition of M. flos-aquae produced 6 types of ROS, among which 3 belonged to strongly reducing organic substance (SROS), whereas there were only 3-4 types of ROS from the other hydrophytes, 2 of them exhibiting strong reducibility. The order of potential of hydrophytes to produce ROS was estimated to be: M. flos-aquae > E. crassipes > L. trisulca > P. crispus approximate to V. natans, based on the summation of SROS and weakly reducing organic substances (WROS). The dynamic pattern of SROS production was greatly different from WROS. The total SROS appeared periodic fluctuation with reducibility gradually weakening with incubation time, whereas the total WROS increased with incubation time. Reducibility of ROS from hydrophytes was readily affected by acid, base and ligands, suggesting that their properties were related to these aspects. In addition to the reducibility, we believe that more attention should be paid to the other behaviors of ROS in surface sediments.

Sorptive interaction between goethite and strongly reducing organic substances from anaerobic decomposition of green manures

Strongly reducing organic substances (SROS) and iron oxides exist widely in soils and sediments and have been implicated in many soil and sediment processes. In the present work, the sorptive interaction between goethite and SROS derived from anaerobic decomposition of green manures was investigated by differential pulse voltammetry (DPV). Both green manures, Astragaltus sinicus (Astragalus) and Vicia varia (Vicia) were chosen to be anaerobically decomposed by the mixed microorganisms isolated from paddy soils for 30 d to prepare different SROS. Goethite used in experiments was synthesized in laboratory. The anaerobic incubation solutions from green manures at different incubation time were arranged to react with goethite, in which SROS concentration and Fe(II) species were analyzed. The anaerobic decomposition of Astragalus generally produced SROS more in amount but weaker in reducibility than that of Vicia in the same incubation time. The available SROS from Astragalus that could interact with goethite was 0.69 +/- 0.04, 0.84 +/- 0.04 and 1.09 +/- 0.03 cmol kg(-1) as incubated for 10, 15 and 30 d, respectively, for Vicia, it was 0.12 +/- 0.03, 0.46 +/- 0.02 and 0.70 +/- 0.02 cmol kg(-1). One of the fates of SROS as they interacted with goethite was oxidation. The amounts of oxidizable SROS from Astragalus decreased over increasing incubation time from 0.51 +/- 0.05 cmol kg(-1) at day 10 to 0.39 +/- 0.04 cmol kg(-1) at day 30, but for Vicia, it increased with the highest reaching to 0.58 +/- 0.04 cmol kg(-1) at day 30. Another fate of these substances was sorption by goethite. The SROS from Astragalus were sorbed more readily than those from Vicia, and closely depended upon the incubation time, whereas for those from Vicia, the corresponding values were remarkably less and apparently unchangeable with incubation time. The extent of goethite dissolution induced by the anaerobic solution from Vicia was greater than that from Astragalus, showing its higher reactivity. (c) 2008 Published by Elsevier Ltd.

Trichloroisocyanuric acid: A convenient oxidation reagent for phase-transfer catalytic epoxidation of enones under non-aqueous conditions

Trichloroisocyanuric acid (TCCA) is a cheap, safe and readily available alternative to the commonly used hydrogen peroxide and hypochlorite for the phase-transfer catalytic epoxidation of alpha,beta-enones under non-aqueous conditions. A variety of chalcone derivatives give the corresponding epoxides with quantitative conversion and satisfactory yields in just a few hours under mild conditions. An asymmetric variant of the epoxidation can be carried out in the presence of chiral N-anthracenylmethylcinchonidine bromide catalyst giving 73-93% ees and 76-94% yields.

Photo-sensitized oxidation in natural water via .OH radicals

A method is presented for determining production and consumption rates of .OH radicals produced photochemically in natural surface waters. It is based on the determination of the kinetics by which the concentration of a specified trace compound decreases during irradiation. In samples from Lake Greifensee (Switzerland) low production rates for .OH limit its possible effects. In addition, fast consumptions by the natural dissolved organic solutes and by the bicarbonate protect organic micropollutants from oxidation by .OH. Neither direct nor indirect H2O2 photolysis was a significant source of .OH in the lakewater studied lacking iron, whereas nitrate photolysis could have been a source. Comparison with reaction kinetic formulations allows generalizations for other types of waters.

Controlled synthesis of organic-inorganic hybrid nanofibers by a wet-chemical route

Organic-inorganic hybrid nanofibers are successfully synthesized by incorporating 3,3 ',5,5 '-tetramethylbenzidine (TMB) and H2PtCl6 at room temperature. The morphology and size can be simply controlled by tuning the molar ratio and initial concentration of reactants. A possible formation mechanism was suggested on the basis of the experimental results. The optical properties were investigated and the as-obtained product displays a strong fluorescence emission at room temperature that may be promising for applications in the fabrication of photoelectric materials. (C) 2008 Elsevier B.V. All rights reserved.

Selective oxidation of cyclohexane with hydrogen peroxide in the presence of copper pyrophosphate

Liquid phase oxidation of cyclohexane was carried out under mild reaction condition over copper pyrophosphate catalyst in CH3CN using hydrogen peroxide as an oxidant at the temperature between 25 and 80 degrees C. The copper pyrophosphate catalyst was characterized by means of XRD, FT-IR and water contact angle measurement. It was found that appropriate surface hydrophobicity is the key factor for the excellent performance of the catalyst. In addition, a significant improvement for the cyclohexane conversion in the presence of organic acid was observed.

The enhancement effect of Eu3+ on electro-oxidation of ethanol at Pt electrode

It is reported for the first time that the slow electrochemical kinetics process for the electro-oxidation of ethanol can be promoted by changing the electrochemical environment. The electro-oxidation of ethanol at a Pt electrode in the presence of Eu3+ cations was studied and an enhancement effect was exhibited. Cyclic voltammetry experiment results showed that the peak current density for the electro-oxidation of ethanol was increased in the presence of EU3+ in the ethanol solution. A preliminary discussion of the mechanism of the enhancement effect is given. This is based on a CO stripping experiment, which shows that either the onset potential or the peak potential of CO oxidation is shifted negatively after adding Eu3+ to the solution.

Preparation of Pt/C catalyst with a new and simple organic sol method

It is reported for the first time that the Pt/C catalyst can be prepared with a new and simple organic sol method using SnCl2 as the reductant. It was found that the average size of the Pt particles in the Pt/C catalysts could be controlled with controlling the preparation conditions. The effect of the average sizes of the Pt particles in the Pt/C catalysts obtained with this method on the electrocatalytical activity of the oxidation of methanol was investigated.

Kinetics of atomic force microscope-based scanned probe oxidation on an octadecylated silicon(111) surface

Atomic force microscope (AFM)-based scanned probe oxidation (SPO) nanolithography has been carried out on an octadecyl-terminated Si(111) surface to create dot-array patterns under ambient conditions in contact mode. The kinetics investigations indicate that this SPO process involves three stages. Within the steadily growing stage, the height of oxide dots increases logarithmically with pulse duration and linearly with pulse voltage. The lateral size of oxide dots tends to vary in a similar way. Our experiments show that a direct-log kinetic model is more applicable than a power-of-time law model for the SPO process on an alkylated silicon in demonstrating the dependence of oxide thickness on voltage exposure time within a relatively wide range. In contrast with the SPO on the octodecysilated SiO2/silicon surface, this process can be realized by a lower voltage with a shorter exposure time, which will be of great benefit to the fabrication of integrated nanometer-sized electronic devices on silicon-based substrates. This study demonstrates that the alkylated silicon is a new promising substrate material for silicon-based nanolithography.

Scanned probe oxidation on an octadecyl-terminated silicon (111) surface with an atomic force microscope: kinetic investigations in line patterning

Scanned probe oxidation (SPO) nanolithography has been performed with an atomic force microscope (AFM) on an octadecyl-terminated silicon (111) surface to create protuberant oxide line patterns under ambient conditions in contact mode. The kinetic investigations of this SPO process indicate that the oxide line height increases linearly with applied voltage and decreases logarithmically with writing, speed. The oxide line width also tends to vary with the same law. The ambient humidity and the AFM tip state can remarkably influence this process, too. As compared with traditional octadecylsilated SiO2/Si substrate, such a substrate can guarantee the SPO with an obviously lowered voltage and a greatly increased writing speed. This study demonstrates that such alkylated silicon is a promising silicon-based substrate material for SPO nanolithography.