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The Aqueous Formation of Secondary Organic Aerosol from Biomass-Burning Organic Compounds with Hydroxyl Radical, Organic Triplet Excited States, and Singlet Molecular Oxygen
Get PDFBiomass burning (BB) is an important source of fine particulate matter and reactive volatile gases that affect air quality, human health, and climate. Phenols and furans are two classes of organic gases from BB that react with various gas-phase oxidants to form secondary organic aerosol (SOA). These gases can also partition into aqueous phases including clouds, fog drops, and aerosol liquid water (ALW) and undergo reaction with aqueous photooxidants, such as hydroxyl radical (●OH), triplet excited organic states of organic compounds (3C* or triplets), and singlet molecular oxygen (1O2*). While a significant amount is known about SOA formation in the gas phase, less is known about the reactions of phenols and furans in the aqueous phase. To investigate this, we studied the aqueous-oxidation kinetics of these two classes of organics and evaluated the potential for both gas- and aqueous-phase reactions of phenols and furans to form SOA across a wide range of liquid water contents.In chapter 2, we investigated the oxidation kinetics of six highly substituted phenols with ●OH since these reactions might be an important source of SOA in ALW. The second-order reaction rates are fast, with rate constants in the range (1.9 – 14) × 109 M-1 s-1 at pH 2 and (14 – 20) × 109 M-1 s-1 at pH 5 and 6. ALW solutes have a range of impacts on phenol oxidation by •OH: a BB sugar and some inorganic salts were found to suppress oxidation, while a nitrate salt and transition metals were observed to enhance oxidation. Our results suggest formation of aqueous-SOA (aqSOA) is significant across clouds/fog drops and ALW, although gas-phase •OH becomes dominant under ALW conditions.
In chapter 3, we evaluated how aqueous reactions of phenols with ●OH and 3C* might be a source of light-absorbing organic carbon, i.e., brown carbon (BrC). Past work found these reactions form low-volatility, high-molecular-weight products, but the light absorption and lifetimes of the BrC formed are poorly understood. To study this, we monitored the formation and loss of light absorption by the aqSOA. While the parent phenols absorb very little tropospheric sunlight, they are oxidized to aqSOA that can absorb significant amounts of sunlight. The extent of light absorption by the aqSOA depends on both the ArOH precursor and oxidant: more light-absorbing aqSOA is formed from more highly substituted phenols and from triplet reactions rather than ●OH. Estimated lifetimes of light-absorbing phenolic aqSOA range from 3 to 17 hours in cloud/fog drops, where ●OH is the major sink, and from 0.7 to 8 hours in ALW, where triplet excited states are the major sink.
In chapter 4, we measured the oxidation kinetics of 17 furans with singlet oxygen. Second-order rate constants for furans with 1O2* widely vary with chemical substitution, ranging from 3.7 × 103 to 5.9 × 108 M-1 s-1. Inorganic species have a range of impacts on singlet oxygen oxidation kinetics, with ammonium sulfate greatly suppressing oxidation, especially at high concentrations. We estimated the rate of SOA formation from the reaction of 2-methylfuran-3,4-dicarboxylic acid, which suggest that aqueous reactions of highly substituted furans with 1O2* could be a significant source of aqSOA but that triplet excited states and hydroxyl radical can also be significant oxidants
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Photooxidants in Aerosol Liquid Water: Organic Triplet Excited State, Hydroxyl Radical, and Singlet Molecular Oxygen
Get PDFAtmospheric waters – including fog/cloud drops and aerosol liquid water (ALW) – are important sites for the transformations of atmospheric species and the formation of aqueous secondary organic aerosols (aqSOA). These chemical processes largely occur through reactions with photoformed oxidants such as hydroxyl radical (●OH), singlet molecular oxygen (1O2*), and oxidizing triplet excited states of organic matter (3C*). Despite this, there are few measurements of these photooxidants, especially in extracts of ambient particles. Moreover, ALW – which has high solute concentrations – has great potential to produce aqSOA, but the chemistry in ALW is poorly understood. To address this gap, in this work we measured photooxidant concentrations and formation kinetics in dilute aqueous particle extracts and extrapolated the results to ALW conditions. We also studied the kinetics of triplet reactions with biomass burning phenols under ALW conditions.In chapter 2, we investigated kinetics of six highly substituted phenols that are potentially important in ALW with the triplet excited state of 3,4-dimethoxybenzaldehyde. Second-order rate constants at pH 2 are all fast, (2.6-4.6) × 10^9 M-1 s-1, while values at pH 5 are 2-5 times smaller. Rate constants are reasonably described by a quantitative structure-activity relationship with phenol oxidation potentials. Triplet-phenol kinetics are unaffected by ammonium sulfate, sodium chloride, galactose (a biomass-burning sugar), or Fe(III). In contrast, ammonium nitrate increases the rate of phenol loss by making ●OH, while Cu(II) inhibits phenol decay. Our results suggest that phenols with high KH can be an important source of aqSOA in ALW, with 3C* typically the dominant oxidant.
In chapter 3, we evaluated how dissolved organic matter (DOM) and Cu(II) inhibit 3C* probe decay, which can cause an underestimation of 3C* concentrations. Our overarching goal is to find a triplet probe that has low inhibition by DOM and Cu(II), and low sensitivity to 1O2*. We tested 12 potential probes from a variety of compound classes and found that (phenylthiol)acetic acid (PTA) seems well suited for ALW conditions, with mild inhibition and fast rate constants with triplets. We evaluated the performance of both PTA and syringol (SYR) as triplet probes in several aqueous extracts of particulate matter. While PTA is less sensitive to inhibition than SYR, it measures lower triplet concentrations, possibly because it is less sensitive to weakly oxidizing triplets.
In chapter 4, we measured photooxidant concentrations in illuminated aqueous particle extracts as a function of particle dilution and used the resulting oxidant kinetics to extrapolate to ALW conditions. Extracts were prepared from winter (WIN) and summer (SUM) particles from Davis, California. As particle mass gets more concentrated, ●OH concentrations in WIN remain relatively unchanged, while they increase in SUM. In both winter and summer samples, 3C* concentrations increase rapidly with particle mass concentrations and then plateau under more concentrated conditions. 1O2* in WIN increases linearly with particle mass while in SUM it approaches a plateau. Under ambient aerosol liquid water conditions we predict a ratio of concentrations of 1O2*: 3C*: ●OH of 10^3 – 10^2: 10^2: 1 with ●OH concentrations on the order of 10^-14 M (including mass transport from the gas phase). Although ●OH is often considered the main sink for organic compounds in the aqueous phase, the much higher concentrations of 3C* and 1O2* in aerosol liquid water suggest these photooxidants are more important sinks for many organics in particle water.
In chapter 5, we investigate the seasonal variation of photooxidants by measuring their concentrations in dilute aqueous extracts of ambient particles. Based on UV/Vis and aerosol mass spectrometer data, we categorized our 18 samples into four groups: Winter & Spring (Win-Spr) influenced by residential wood combustion; Summer & Fall (Sum-Fall) without wildfire influence; summer fresh biomass burning particles from wildfires (FBB), and aged biomass burning plumes (ABB). The concentration ratio of 1O2*: 3C*: ●OH in dilute particle extracts is (150 – 1500) : (10 – 300) : 1, respectively, while this ratio in ALW is predicted to be (100 – 1000): (10 – 150): 1. FBB has the highest mass absorption coefficient (MAC) and highest average 1O2* concentration but lowest average quantum yields (Φ) for formation of all three photooxidants. Win-Spr and ABB have similar MAC and higher average Φ of oxidants, indicating aging tends to enhance quantum yields. Sum-Fall has the lowest 1O2* due to low DOC, but highest Φ1O2*. 1O2* and 3C*determined by SYR are linearly correlated with DOC and appear to be independent of sample type
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Exploring the Effects of Organic Photosensitizers on Nitrate Photolysis
No full textReactive nitrogen species (NOy) including nitrogen oxides (NOx) and nitrous acid (HONO) are important atmospheric gases that contribute to ozone concentrations, particulate matter formation, and the overall oxidative capacity in the troposphere. Studies have revealed that current models underpredict the measured environmental HONO and NOx concentrations. Researchers have theorized that nitrate photolysis, a process that generates NOy, is enhanced in the environment and accounts for this discrepancy. Nitrate photolysis in actinic sunlight (wavelengths > 290 nm) proceeds through 2 channels, the first channel produces NO2 and •O– , and the second produces nitrite (NO2 – ) and an oxygen atom (O(3P)). It has been shown that a variety of chemicals can impact nitrate photolysis in a variety of ways i.e., increasing the production rate of the photolysis products or converting either NO2 to NO2 – or NO2 – to NO2. These chemicals include inorganic ions as well as light-absorbing organic chemicals, commonly termed “brown carbon”. However, the ability to identify specific chemical enhancers, as well as the enhancement mechanisms, remains poorly understood. To address this gap, in this work we develop a method that allows one to measure both nitrate photolysis channels in aqueous solution and propose mechanisms for brown carbon species enhancement of nitrate photolysis. In Chapter 2, we develop a method that allows us to measure both photo-channels for nitrate photolysis in aqueous solution by converting NO2 to NO2 – . The addition of sulfite, S(IV), to solutions has been shown to remove NOx from flue gas in industrial settings by converting NO2 to NO2 – . We demonstrate how to utilize this conversion method experimentally and determine how much NO2 is converted to NO2 – based on the concentration of S(IV) added to solution. By adding 1.5 mM S(IV) to the illumination containers after illuminating the solution, we completely convert 200 nM NO2 to NO2 – . This method is then used to show how the addition of hydroxyl radical scavengers, i.e., 2-propanol, does not enhance the overall nitrate photolysis rate, but prevents the conversion of NO2 – to NO2. By preventing this conversion, researchers are able to assess the actual amounts of NO2 and NO2 – produced from nitrate photolysis, without secondary chemistry impacting their measurements.In Chapter 3, we investigate the enhancement to nitrate photolysis caused by brown carbon. Our primary goal is to identify potential mechanisms for enhancement and determine what types of brown carbon are able to enhance nitrate photolysis. We tested brown carbon species that can generate triplet excited states, singlet molecular oxygen, and solvated electrons. Of the chemicals with known photochemical properties, solvated-electron producing brown carbon were the only subset that enhanced nitrate photolysis. However, comparison of experimental results with kinetic models shows that solvated electrons do not account for all of the observed enhancement. Utilizing computational calculations, we show that charge transfer reactions are likely responsible for the remaining observed enhancement
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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