CHEMICAL CHARACTERIZATION OF SOURCE-SPECIFIC ATMOSPHERIC ORGANIC AEROSOL VIA MASS SPECTROMETRY

Atmospheric fine aerosol (PM2.5) is important to Earth’s biogeochemical cycles and climate, and adversely impacts air quality and human health. Specifically, organic aerosol (OA) substantially contributes to PM2.5 mass but remains poorly characterized due to the diversity and complexity of its sources, formation mechanisms, and chemical composition. A suite of state-of-the-art mass spectrometry (MS) methods were deployed and optimized to analyze distinct sets of source-specific OA samples collected from field studies and laboratory experiments in order to chemically characterize OA constituents at the molecular level. First, over 200 archived marine aerosol samples collected from Cape Grim, Tasmania, Australia under remote background conditions from 1991-2015, were analyzed using ultra-performance liquid chromatography interfaced to a high-resolution quadrupole time-of-flight mass spectrometer equipped with an electrospray ionization source (UPLC/ESI-HR-QTOFMS) and gas chromatography interfaced to a quadrupole mass spectrometer equipped with electron ionization source (GC/EI-MS). Several ng m-3 of biogenic (e.g., isoprene- and monoterpene-derived) secondary organic aerosol (SOA) tracers were quantified from 29 summer and winter seasons. Biogenic SOA tracers were enhanced during summer seasons and had moderate-to-strong correlations with marine bioactivity indicators such as methanesulfonic acid and chlorophyll-a. In addition, UPLC/ESI-HR-QTOFMS coupled with inline diode array detection (DAD) were utilized to assess light-absorbing brown carbon (BrC) OA from over 100 systematically-performed laboratory-simulated primary and aged wildfire emissions during the 2016 Fire Influence on Regional and Global Environments Experiment (FIREX) at the US Forest Service Fire Science Lab in Missoula, Montana. 37 solvent-extractable BrC constituents were characterized in terms of their composition, contributions to PM2.5 mass, emission factors, light absorbance, and evolution due to photochemical aging. Furthermore, we developed and optimized a versatile hydrophilic interaction liquid chromatography (HILIC)/ESI-HR-QTOFMS method that can efficiently resolve and measure the major isoprene epoxydiols (IEPOX)-derived and several other water-soluble SOA constituents with enhanced resolution of separation, sensitivity of detection, and accuracy of measurements. These findings provide detailed chemical composition of the atmospheric OA constituents that originate from varying sources, and thus, can serve as inputs for future studies on atmospheric analytical chemistry, earth science modeling, environmental management, and health effects of PM2.5.Doctor of Philosoph

Secondary organic aerosol formation from alpha-pinene and toluene: laboratory studies examining the role of pre-existing particles, relative humidity and oxidant type

Secondary organic aerosol (SOA) is a major fraction of fine particulate matter (PM2.5), thus impacting air quality and climate. In the first section of this thesis, experiments were performed to investigate SOA formation from both ozonolysis and hydroxyl radical (OH)-initiated oxidation (so-called photooxidation) of α-pinene under conditions with varying relative humidity (RH) and seed aerosol acidity at the UNC dual outdoor smog chamber facility. Formation of dimer esters was observed only in SOA derived from α-pinene ozonolysis with increased concentrations at high RH, indicating these compounds could serve as tracers for SOA enhanced by anthropogenic pollution. In the second section of this thesis, toluene photooxidation experiments in presence of nitric oxide were conducted to examine the effect of the pre-existing titanium dioxide (TiO2) seed aerosol, as an instance of engineered metal oxide nanomaterials, considering their unique photocatalytic properties. Results indicate that TiO2 aerosol enhanced and accelerated SOA formation from toluene.Master of Scienc

Brown Carbon from Photo-Oxidation of Glyoxal and SO2 in Aqueous Aerosol

Aqueous-phase dark reactions during the co-oxidation of glyoxal and S(IV) were recently identified as a potential source of brown carbon (BrC). Here, we explore the effects of sunlight and oxidants on aqueous solutions of glyoxal and S(IV), and on aqueous aerosol exposed to glyoxal and SO2. We find that BrC is able to form in sunlit, bulk-phase, sulfite-containing solutions, albeit more slowly than in the dark. In more atmospherically relevant chamber experiments where suspended aqueous aerosol particles are exposed to gas-phase glyoxal and SO2, the formation of detectable amounts of BrC requires an OH radical source and occurs most rapidly after a cloud event. From these observations we infer that this photobrowning is caused by radical-initiated reactions as evaporation concentrates aqueous-phase reactants and aerosol viscosity increases. Positive-mode electrospray ionization mass spectrometric analysis of aerosol-phase products reveals a large number of CxHyOz oligomers that are reduced rather than oxidized (relative to glyoxal), with the degree of reduction increasing in the presence of OH radicals. This again suggests a radical-initiated redox mechanism where photolytically produced aqueous radical species trigger S(IV)–O2 auto-oxidation chain reactions, and glyoxal-S(IV) redox reactions especially if aerosol-phase O2 is depleted. This process may contribute to daytime BrC production and aqueous-phase sulfur oxidation in the atmosphere. The BrC produced, however, is about an order of magnitude less light-absorbing than wood smoke BrC at 365 nm

Aromatic organosulfates in atmospheric aerosols: Synthesis, characterization, and abundance

Aromatic organosulfates are identified and quantified in fine particulate matter (PM2.5) from Lahore, Pakistan, Godavari, Nepal, and Pasadena, California. To support detection and quantification, authentic standards of phenyl sulfate, benzyl sulfate, 3-and 4-methylphenyl sulfate and 2-, 3-, and 4-methylbenzyl sulfate were synthesized. Authentic standards and aerosol samples were analyzed by ultra-performance liquid chromatography (UPLC) coupled to negative electrospray ionization (ESI) quadrupole time-of-flight (ToF) mass spectrometry. Benzyl sulfate was present in all three locations at concentrations ranging from 4 – 90 pg m−3. Phenyl sulfate, methylphenyl sulfates and methylbenzyl sulfates were observed intermittently with abundances of 4 pg m−3, 2-31 pg m−3, 109 pg m−3, respectively. Characteristic fragment ions of aromatic organosulfates include the sulfite radical (•SO3−, m/z 80) and the sulfate radical (•SO4−,m/z 96). Instrumental response factors of phenyl and benzyl sulfates varied by a factor of 4.3, indicating that structurally-similar organosulfates may have significantly different instrumental responses and highlighting the need to develop authentic standards for absolute quantitation organosulfates. In an effort to better understand the sources of aromatic organosulfates to the atmosphere, chamber experiments with the precursor toluene were conducted under conditions that form biogenic organosulfates. Aromatic organosulfates were not detected in the chamber samples, suggesting that they form through different pathways, have different precursors (e.g. naphthalene or methylnaphthalene), or are emitted from primary sources

Strong anthropogenic control of secondary organic aerosol formation from isoprene in Beijing

Isoprene-derived secondary organic aerosol (iSOA) is a significant contributor to organic carbon (OC) in some forested regions, such as tropical rainforests and the Southeastern US. However, its contribution to organic aerosol in urban areas that have high levels of anthropogenic pollutants is poorly understood. In this study, we examined the formation of anthropogenically influenced iSOA during summer in Beijing, China. Local isoprene emissions and high levels of anthropogenic pollutants, in particular NOx and particulate SO2-4 , led to the formation of iSOA under both high- A nd low-NO oxidation conditions, with significant heterogeneous transformations of isoprene-derived oxidation products to particulate organosulfates (OSs) and nitrooxyorganosulfates (NOSs). Ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry was combined with a rapid automated data processing technique to quantify 31 proposed iSOA tracers in offline PM2.5 filter extracts. The co-elution of the inorganic ions in the extracts caused matrix effects that impacted two authentic standards differently. The average concentration of iSOA OSs and NOSs was 82.5 ngm-3, which was around 3 times higher than the observed concentrations of their oxygenated precursors (2-methyltetrols and 2-methylglyceric acid). OS formation was dependant on both photochemistry and the sulfate available for reactive uptake, as shown by a strong correlation with the product of ozone (O3) and particulate sulfate (SO2-4). A greater proportion of high-NO OS products were observed in Beijing compared with previous studies in less polluted environments. The iSOA-derived OSs and NOSs represented 0.62% of the oxidized organic aerosol measured by aerosol mass spectrometry on average, but this increased to ∼ 3% on certain days. These results indicate for the first time that iSOA formation in urban Beijing is strongly controlled by anthropogenic emissions and results in extensive conversion to OS products from heterogenous reactions

Low-NO atmospheric oxidation pathways in a polluted megacity

The impact of emissions of volatile organic compounds (VOCs) to the atmosphere on the production of secondary pollutants, such as ozone and secondary organic aerosol (SOA), is mediated by the concentration of nitric oxide (NO). Polluted urban atmospheres are typically considered to be “high-NO” environments, while remote regions such as rainforests, with minimal anthropogenic influences, are considered to be “low NO”. However, our observations from central Beijing show that this simplistic separation of regimes is flawed. Despite being in one of the largest megacities in the world, we observe formation of gas- and aerosol-phase oxidation products usually associated with low-NO “rainforest-like” atmospheric oxidation pathways during the afternoon, caused by extreme suppression of NO concentrations at this time. Box model calculations suggest that during the morning high-NO chemistry predominates (95 %) but in the afternoon low-NO chemistry plays a greater role (30 %). Current emissions inventories are applied in the GEOS-Chem model which shows that such models, when run at the regional scale, fail to accurately predict such an extreme diurnal cycle in the NO concentration. With increasing global emphasis on reducing air pollution, it is crucial for the modelling tools used to develop urban air quality policy to be able to accurately represent such extreme diurnal variations in NO to accurately predict the formation of pollutants such as SOA and ozone

Key role of NO3 radicals in the production of isoprene nitrates and nitrooxyorganosulfates in Beijing

The formation of isoprene nitrates (IsN) can lead to significant secondary organic aerosol (SOA) production and they can act as reservoirs of atmospheric nitrogen oxides. In this work, we estimate the rate of production of IsN from the reactions of isoprene with OH and NO3 radicals during the summertime in Beijing. While OH dominates the loss of isoprene during the day, NO3 plays an increasingly important role in the production of IsN from the early afternoon onwards. Unusually low NO concentrations during the afternoon resulted in NO3 mixing ratios of ca. 2 pptv at approximately 15:00, which we estimate to account for around a third of the total IsN production in the gas phase. Heterogeneous uptake of IsN produces nitrooxyorganosulfates (NOS). Two mono-nitrated NOS were correlated with particulate sulfate concentrations and appear to be formed from sequential NO3 and OH oxidation. Di- and tri-nitrated isoprene-related NOS, formed from multiple NO3 oxidation steps, peaked during the night. This work highlights that NO3 chemistry can play a key role in driving biogenic–anthropogenic interactive chemistry in Beijing with respect to the formation of IsN during both the day and night

Photooxidation of naphthalene and 2-methylnaphthalene: acidity, humidity and seed aerosol effects on chemical mechanisms

In the atmosphere, polycyclic aromatic hydrocarbons (PAHs) are ubiquitous compounds. They are mostly emitted into the troposphere by incomplete combustion processes, such as diesel exhaust, residential heating, and wood burning. PAHs may react with oxidants by either homogeneous or heterogeneous reactions to form nitro and/or oxy-PAHs known to be more carcinogenic and mutagenic than parent PAHs (IARC, 2010, 2013; Rosenkranz and Mermelstein, 1985). It has been recently shown that lighter PAHs such as naphthalene and methylnaphthalene are precursors of secondary organic aerosol (SOA) and could represent one of the missing sources of SOA in urban areas (Chan et al., 2009). The purpose of this study is to better understand the SOA formation from gaseous PAH oxidation under different experimental conditions. Photooxidation experiments were performed in the UNC outdoor smog chamber. Naphthalene and 2-methylnaphthalene were selected as the two most abundant PAHs emitted in the gas phase (Reisen and Arey, 2005). Experiments were performed in the presence of nitrogen oxides, varying the type of the seed aerosol (MgSO4 vs (NH4)2SO4), aerosol acidity and relative humidity inside the chamber, as these parameters are known to have an impact on SOA formation. A complete chemical characterization of the aerosol has been performed using three complementary high-resolution mass spectrometry techniques. The composition of the gas phase has been determined during the time course of the experiments using a chemical ionization time of flight mass spectrometer (ToF-CIMS) using iodide reagent ion chemistry and the composition of the condensed phase has been characterized using ultra-performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) and gas chromatography coupled to a high-resolution quadrupole time-of-flight mass spectrometry (GC-QTOFMS)..

Secondary Organic Aerosol Formation via 2-Methyl-3-buten-2-ol Photooxidation: Evidence of Acid-Catalyzed Reactive Uptake of Epoxides.

Secondary organic aerosol (SOA) formation from 2-methyl-3-buten-2-ol (MBO) photooxidation has recently been observed in both field and laboratory studies. Similar to the level of isoprene, the level of MBO-derived SOA increases with elevated aerosol acidity in the absence of nitric oxide; therefore, an epoxide intermediate, (3,3-dimethyloxiran-2-yl)methanol (MBO epoxide), was synthesized and tentatively proposed to explain this enhancement. In this study, the potential of the synthetic MBO epoxide to form SOA via reactive uptake was systematically examined. SOA was observed only in the presence of acidic aerosol. Major SOA constituents, 2,3-dihydroxyisopentanol and MBO-derived organosulfate isomers, were chemically characterized in both laboratory-generated SOA and in ambient fine aerosol collected from the BEACHON-RoMBAS field campaign during the summer of 2011, where MBO emissions are substantial. Our results support the idea that epoxides are potential products of MBO photooxidation leading to the formation of atmospheric SOA and suggest that reactive uptake of epoxides may explain acid enhancement of SOA observed from other biogenic hydrocarbons