Secondary organic aerosols (SOA) affect visibility, health and global climate. Current chemical transport models cannot represent SOA in the free troposphere. Fog/cloud processing, which is the dominant source of atmospheric sulfate, has been recognized as a missing source of SOA globally. Aqueous photooxidation of water-soluble products (e.g., glyoxal and methylglyoxal) of gas-phase photochemistry yields low-volatility compounds including oxalic acid. When this chemistry takes place in clouds and fogs followed by droplet evaporation (or if this chemistry occurs in aerosol water) then products remain in part in the particle phase, forming SOA. However, current aqueous SOA formation mechanism has not shown how the starting concentrations of precursors and presence of acidic sulfate affect product formation. Aqueous phase photochemical batch reactions were conducted with glyoxal and methylglyoxal at cloud relevant concentrations, using hydrogen peroxide photolysis as the hydroxyl radical (OH) source. Experiments were repeated at higher concentrations and with/without sulfuric acid. Precursors and products were investigated using ion chromatography (IC), electrospray ionization mass spectrometry (ESI-MS), and IC-ESI-MS. Products included carboxylic acids and higher molecular weight compounds, which are major constituents of aerosols. Sulfuric acid shows little effect on product formation. Dilute aqueous chemistry models successfully reproduced product formation for glyoxal and methylglyoxal at cloud relevant conditions, but measurements deviated from predictions from predictions at elevated concentrations. Higher molecular weight products become increasingly important as precursor concentration increases. Aqueous radical-radical reactions provide explanations for observed higher molecular weight products. Additionally, acetic acid is identified as an SOA precursor for the first time. This work provides an improved understanding of aqueous phase dicarbonyl oxidation mechanism and the overall significance of aqueous SOA formation. Kinetic data are made available to regional and global atmospheric models, and the mechanism described in this work will help people to mitigate adverse aerosol effects.
Secondary Organic Aerosol (SOA) can have significant impacts on visibility, human health, and global climate, and a more detailed understanding of the roles of both gas-phase and heterogeneous/multiphase chemistry is needed to develop air quality models that accurately represent the formation of SOA from the oxidation of aromatic hydrocarbons. The objective of this dissertation is to investigate the mechanisms and products of SOA formation from the OH radical-initiated reaction of aromatics in an environmental chamber. This is done using a combination of thermal desorption particle beam mass spectrometry, functional group and CHON elemental analysis, and UV spectroscopy. Chapter 2 investigates the variability of SOA yields measured for reactions of m-xylene and other methylbenzenes as a function of humidity, seed particle, OH source, NO x concentration, light intensity, and mass loading. The most significant factor that determined SOA yields was the amount of m -xylene reacted. The chapter concludes with a discussion of a series of experiments conducted to isolate the contribution to SOA formation of specific primary gas-phase products of the m -xylene reaction. Chapter 3 examines the formation of SOA from the oxidation of 3-methylfuran, which produces among other compounds an [Alpha, Beta]-unsaturated dicarbonyl that is also a major product of the oxidation of m -xylene. We have determined that SOA forms from the heterogeneous/multiphase oligomerization of primary reaction products to form esters, hemiacetals, and acetals, and not through second-generation reactions. Chapter 4 discusses the chemical composition of SOA formed from the reaction of m -xylene and how the variables detailed in Chapter 2 affect the composition. Experiments were carried out with deuterated m-xylene to confirm that SOA is dominated by hemiacetals formed from C8 ring-opened primary products and their second-generation products. Finally, Chapter 5 shows that SOA formed from the oxidation of benzaldehyde in the absence of NOx is largely composed of oligomeric products formed through heterogeneous/multiphase reactions involving benzoic acid, peroxybenzoic acid, phenol, and benzaldehyde.
Organic particulate matter in the atmosphere plays an important role in climate forcing, visibility, and adverse health effects. Atmospheric organic aerosol is predominantly of secondary origin, formed in the atmosphere. Laboratory photooxidation experiments, atmospheric aerosol measurements below vs. above clouds and at increasing humidity, and modeling studies all suggest that secondary organic aerosol (SOA) forms from water-soluble gases through aqueous chemistry in clouds and wet aerosols (aqSOA). Previous laboratory experiments are simple compared to the atmospheric water media (single compound deionize water solutions), thus a more realistic approach is needed for the understanding of SOA formation through aqueous chemistry. We conducted batch photooxidation experiments with three different rainwater samples from Camden and Pinelands, NJ and hydroxyl radicals (formed from 150 æM H2O2 + UV radiation). We used rainwater (RW) as a surrogate for cloud water in these experiments. SOA precursors and products were identified by real-time Electrospray Ionization -- Mass Spectrometry (ESI-MS, continuous online sampling) and by Ion Chromatography (discrete samples). Precursors were found predominantly in the positive mode, suggesting the presence of aldehydes, alcohols and organic peroxides, and products were found predominantly in the negative mode, suggesting the presence of organic acids. A decrease in the abundance of ions with the same unit mass-to-charge ratio as standards of glyoxal, methylglyoxal and glycolaldehyde and an increase in the abundance of ions associated with organic acids (e.g., oxalic and pyruvic acid) suggest that these aldehydes were present and reacting. The evidence is strongest for methylglyoxal (three RW samples). Glyoxal oxidation appears to occur in two RW samples; evidence for glycolaldehyde is not as strong. Other potential contributors to SOA formation (precursor and products) were identified based on their percentage of change and absolute change in ion abundance across the reaction.
Organic aerosols (OA) are a dominant fraction of particulate mass in the atmosphere, and much is secondary in nature. Secondary organic aerosol (SOA) is formed in the atmosphere from volatile organic compound precursors. Traditional SOA formation pathways involve primarily gas-phase processes: Oxidation reactions of organic gases result in low-volatility products that condense to the particulate phase, increasing aerosol mass. However, in recent years heterogeneous processes, including aqueous reactions, have gained more attention as gas-phase processes often fail to accurately predict observed mass loadings of aerosol in the atmosphere. Aqueous SOA formation is the result of a volatile organic species partitioning to the aqueous phase (clouds, fogs, aqueous aerosols), where they are chemically converted into a non-volatile species that remains in the particulate phase upon water evaporation. In this work we explore the aqueous chemical reaction kinetics and the SOA formation potential of phenols, which are released in large quantities from biomass combustion. Phenols are a broad class of organic compounds with intermediate volatilities (102 - 106 [mu]g m−3 at 20°C) and moderate to high Henry's Law Constants (103 - 109M atm−1), indicating significant partitioning to atmospheric aqueous phases. We begin in chapters 2 and 3 by investigating the aqueous oxidation of the compounds phenol (compound with formula C6H5OH), guaiacol (2-methoxyphenol), syringol (2,6-dimethoxyphenol), and three dihydroxybenzenes (catechol, resorcinol, hydroquinone). For each phenol we examined reactions with two oxidants: hydroxyl radical (*OH) and the triplet excited state of 3,4-dimethoxybenzaldehyde, which is also emitted from biomass combustion. Triplet excited states (3C*) have been widely studied in surface waters (oceans and lakes) but are a novel oxidation pathway in atmospheric aqueous phases. The precursors for triplet excited states are essentially brown carbon: organic molecules high amoutns of conjugation (or nitrogen hetero atoms) that can absorb solar radiation, resulting in an excited molecule with a high oxidative potential. We find that the 3C*-mediated aqueous oxidations of phenols are rapid and can dominate over *OH at low pH (
Secondary organic aerosol (SOA) is formed and transformed in atmospheric aqueous phases (e.g., cloud and fog droplets and deliquesced airborne particles containing small amounts of water) through a multitude of chemical and physical processes. Understanding the formation and transformation processes of SOA via aqueous-phase reactions is important for properly presenting its atmospheric evolution pathways in models and for elucidating its climate and health effects. Phenolic compounds, which are emitted in significant amounts from biomass burning, can undergo fast reactions in atmospheric aqueous phases to form secondary organic aerosol (aqSOA). In this study, we investigate the formation and evolution of phenol (C6H6O), guaiacol (C7H8O2; 2-methoxyphenol) and syringol (C8H10O3; 2,6-dimethoxyphenol) and with two major aqueous phase oxidants -- the triplet excited state of an aromatic carbonyl (3C*) and hydroxyl radical (·OH) - and interpret the reaction mechanisms. In addition, given that dissolved organic matter (DOM) is an important component of fog and cloud water and that it can undergo aqueous reactions to form more oxidized, less volatile species, we further investigate the photochemical processing of DOM in fog water to gain insights into the aqueous-phase processing of organic aerosol (OA) in the atmosphere. In Chapter 2, we thoroughly characterize the bulk chemical and molecular compositions of phenolic aqSOA formed at half-life (t[subscript 1/2]), and interpret the formation mechanisms. We find that phenolic aqSOA formed at t[subscript 1/2] is highly oxygenated with atomic oxygen-to-carbon ratio (O/C) in the range of 0.85-1.23. Dimers, higher oligomers (up to hexamers), functionalized monomers and oligomers with carbonyl, carboxyl, and hydroxyl groups, and small organic acids are detected. Compared with ·OH-mediated reactions, reactions mediated by 3C* are faster and produce more oligomers and hydroxylated species at t[subscript1/2]. We also find that aqSOA shows enhanced light absorption in the UV-vis region, suggesting that aqueous-phase reactions of phenols are an important source of secondary brown carbon in the atmosphere, especially in regions impacted by biomass burning. In Chapter 3, we investigate the chemical evolution of phenolic aqSOA via aqueous-phase reactions on the molecular level and interpret the aging mechanisms. Our results indicate that oligomerization is an important aqueous reaction pathway for phenols, especially during the initial stage of photooxidation. Functionalization and fragmentation become dominant at later stages, forming a variety of functionalized aromatic and ring-opening products with higher carbon oxidation states. Fragmentation reactions eventually dominate the photochemical evolution of phenolic aqSOA, forming a large number of highly oxygenated ring-opening molecules. In addition, phenolic aqSOA has a wide range of saturation vapor pressures (C*), varying from 10−20 [mu]g m−3 for functionalized phenolic oligomers to 10 [mu]g m−3 for ring-opening species with number of carbon less than 6. The detection of abundant extremely low volatile organic compounds (ELVOC) indicates that aqueous reactions of phenolic compounds are likely an important source of ELVOC in the atmosphere. Chapter 3 investigates the molecular transformation with aging based on the characterization of three aqSOA filter samples collected at the defined time intervals of the photoreaction. However, the chemical evolution of aqSOA products with hours of illumination at a higher time resolution is largely unknown. In Chapter 4, we investigate the chemical evolution of aqSOA at a 1-min time resolution based on high-resolution aerosol mass spectrometer (AMS) analysis. This is important for understanding the continuous evolution of phenolic aqSOA with aging as well as for elucidating the formation and transformation of different generations of products. Our results suggest that dimer and higher-order oligomers (trimers, tetramers, etc.) are formed continuously during the first 1-2 hours of photoreaction but show a gradual decrease afterwards. Functionalized derivatives grow at a later time and then gradually decrease. Highly oxidized ring-opening species continuously increase over the course of reactions. Positive matrix factorization (PMF) analysis of the AMS spectra of phenolic aqSOA identifies multiple factors, representing different generations of products. The 1st-generation products include dimers, higher-order oligomers and their oxygenated derivatives. The 2nd-generation products include oxygenated monomeric derivatives. The 3rd-generation products include highly oxidized ring-opening species. In Chapter 5, we investigate the evolution of dissolved organic matter (DOM) in fog water. Our results show that the mass concentration of DOM[subscript OA] (i.e., low-volatility DOM in fog water) is enhanced over the course of illumination, with continuous increase of O/C and atomic nitrogen-to-carbon ratio (N/C). The increase of DOM[subscript OA] is due to the incorporation of oxygen- and nitrogen-containing functional groups into the molecules. The aqueous aging of DOM[subscript OA] can be modeled as a linear combination of the dynamic variations of 3 factors using PMF analysis. Factor 1 is chemically similar to the DOM[subscript OA] before illumination, which is quickly reacted away. Factor 2 is representative of an intermediate component, which is first formed and then transformed, and O/C of Factor 2 is intermediate between that of Factor 1 and Factor 3. Factor 3 represents highly oxidized final products, which is continuously formed during illumination. Fog DOM absorbs significantly in the tropospheric sunlight wavelengths, but this absorption behavior stays almost constant over the course of illumination, despite the significant change in chemical composition.
A large fraction of organic aerosol particles are formed as secondary organic aerosol (SOA) resulting from the condensation of partially oxidized biogenic and anthropogenic volatile organic compounds (VOCs) with gas phase oxidants such as O3, OH, NOx, and NO3. An additional pathway for SOA formation is by the photochemical aqueous processing of VOC occurring inside cloud and fog droplets, followed by droplet evaporation. Once formed, SOA can age through heterogeneous oxidation and fog photochemical processes involving the hydroxyl radical (OH) as well as various other oxidants in the atmosphere. In addition to condensed phase oxidation, SOA can also age in the atmosphere upon exposure to radiation, for many of these organic compounds are photolabile and can degrade through direct photolysis, wherein the compounds absorb radiation and break into products, and indirect photolysis, wherein absorption of solar radiation initiates chemistry through the production of non-selective oxidants such as OH. These photochemical aging processes have the potential to be on time scales that are comparable to the typical lifetimes of droplets (hours) and particles (days), making them relevant to study further for both climate and health reasons. This dissertation presents a systematic investigation of the optical properties, molecular composition, and the extent of photochemical processing in different types of SOA from various biogenic and anthropogenic VOC precursors. Chamber- or flowtube-generated SOA is made and then analyzed using high-resolution mass spectrometry (HR-MS) to observe the extent of change in the molecular level composition of the material before and after aqueous photolysis. Significant differences in the molecular composition between biogenic and anthropogenic SOA were observed, while the composition further evolved during photolysis. To study the optical properties and lifetimes of organic aerosol, spectroscopy tools such as UV-Vis is utilized. Results of this study suggest that the condensed phase photolysis of SOA can occur with effective lifetimes ranging from minutes to hours, and therefore represents a potentially important aging mechanism for SOA. The outcome of this dissertation will be improved understanding of the role of condensed-phase photochemistry in chemical aging of aerosol particles and cloud droplets.
Secondary organic aerosol (SOA) is a substantial contributor to atmospheric organic particulate matter; however, its formation via aqueous oxidation reactions is only beginning to be understood. Although the aqueous organic chemistry that drives SOA formation in clouds (SOACld) has now been incorporated into a few photochemical transport models, it is yet unknown to what extent the newly formed organic material remains in the particle-phase after droplet evaporation. This work investigates SOA formation through cloud water chemistry and droplet evaporation. Aqueous hydroxyl radical oxidation and droplet evaporation experiments were conducted using precursors commonly found in cloud water: glycolaldehyde, methylglyoxal, and glyoxal. A new method was used to measure the volatility of the product mixture. The effective vapor pressure, enthalpy of vaporization, and mass yields of SOACld were determined. Aqueous oxidation produced carboxylic acids and oligomers (i.e., small polymers), which are major constituents of atmospheric aerosols. Enhanced yields (e.g., ~50-80% yields from glycolaldehyde) provide evidence for additional chemistry during droplet evaporation. The overall vapor pressure and enthalpy of vaporization of SOACld were ~1E-07 atm and ~70 kJ/mol, respectively, similar to the mix of organic acids identified. Lastly, a substantial decrease in volatility (~ 1E-08 - 1E-16 atm) was observed when glyoxal SOACld products were exposed to sufficient ammonia to form organic salts. These results provide an important insight on the effects that cloud droplet evaporation and neutralization have on SOA formation through cloud processing. This work furthers our understanding of SOACld formation, and provides measurements that are needed for accurate prediction of SOA in global and regional air quality models.
