Production and degradation of isoprene-derived organic nitrates in the atmosphere
Ground-level ozone is a regulated pollutant due to its adverse effects on human health and crop yields. This pollutant is catalytically produced in the photochemical reactions that involve volatile organic compounds and nitrogen oxides. Nitrogen oxides can facilitate the formation of ozone. However, when nitric oxide is sequestered by an organic compound to form an organic nitrate, the nitric oxide is removed from the catalytic cycle that forms ozone. Hence, formation of organic nitrates can reduce the ozone production rate near the ground. However, when organic nitrates decompose and release nitrogen oxides back into the atmosphere, ozone formation is enhanced again. Therefore, detailed understanding of the production and degradation processes of organic nitrates is important to controlling ground-level ozone to a safe level. Among all the volatile organic compounds emitted into the atmosphere, isoprene is the most important non-methane organic compounds, marked by its high emission rate and high photochemical reactivity. Therefore, this work is focused on the chemical processes surrounding isoprene-derived organic nitrates. Chapter two describes the sensitivity tests on the ionization efficiency using iodide as the reagent ion to detect multi-functional organic nitrates via mass spectrometry, such as hydroxynitrate. Different organic molecules were tested using an iodide-based chemical ionization mass spectrometer. The results suggested that iodide ion is not sensitive to alkyl alcohol or alkyl nitrate, but the combination of these two functionalities, such as those in α,β-hydroxynitrates, can increase the sensitivity by five orders of magnitude, making iodide-based chemical ionization a very powerful tool to study organic nitrates. Chapter three describes the laboratory study to quantify the first-generation isoprene hydroxynitrates with the iodide-based chemical ionization mass spectrometer. Two authentic standards were synthesized for instrument calibration. For the important tertiary nitrate that cannot be synthesized, a GC-ECD/MS interface system was developed to calibrate the relative sensitivity for the nitrate isomers generated in the gas phase. A series of chamber experiments was conducted to derive the yield of the first-generation hydroxynitrates in the OH-initiated oxidation of isoprene under high NO condition. Chapter four describes a field study conducted in the summer of 2013 in rural Alabama that was focused on ambient measurements of isoprene hydroxynitrates. A distinctive diurnal profile for the concentrations of isoprene hydroxynitrates was observed. By calculating the production and loss rates of isoprene hydroxynitrates, it is inferred that the isoprene hydroxynitrates had fast production in the morning, but in the afternoon, their production was overshadowed by the oxidative loss, due to the limited availability of NO during this study. Therefore, the isoprene oxidation chemistry undergoes a transition from the high NO to the low NO regime through the course of one day, and highly oxidized products such as dihydroxy hydroxyperoxy nitrate can be expected. Chapter five describes a zero-dimensional photochemical kinetics modeling study to simulate the formation and degradation of isoprene hydroxynitrates in rural Alabama. The model includes the hydroxynitrate yield obtained in Chapter 3, as well as the most recent update in the isoprene oxidation mechanism in literature. Since the zero-dimensional model does not include factors such as transport and mixing that can significantly affect the observed concentrations of species in the atmosphere, a relative concentration, instead of the measured absolute concentration, was simulated. The modeled result was able to capture the general profile of the field observation, lending support to the hydroxynitrate yield obtained in the laboratory. However, the model-observation discrepancy suggested that vertical transport in the morning can also have ~27% influence on the observed hydroxynitrate concentrations. Chapter six describes a focused kinetics study on a synthesized carbonyl nitrate, which is a product of the NO3-initiated isoprene oxidation reaction. The NMR, IR and UV spectra of the molecule were reported for structural characterization. The rate constants for the nitrate to react with OH or to react with O3 were obtained using the relative rate method. The photolysis frequency of the nitrate was also obtained in chamber experiments and extrapolated to the ambient environment. It is estimated that for this unsaturated carbonyl nitrate, photolysis is its dominant atmospheric degradation pathway, instead of reaction with OH, which is the most important loss mechanism for most organic compounds in the atmosphere. Two nitrate products from the OH oxidation reactions were observed and quantified. Chapter seven describes future studies of organic nitrates relevant to this thesis work. More effort should be focused on understanding the isomeric nitrates produced from conjugated biogenic volatile organic compounds and performing chamber experiments in conditions that resemble the ambient environment.
Shepson, Purdue University.
Atmospheric Chemistry|Environmental science
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