The use of stable isotopes and particulate matter in the investigation of local and regional atmospheric chemistry
The chemical composition of particulate matter (PM), a known contributor to air pollution, is highly variable, and elemental analysis reveals information about local and regional sources, as well as how air masses and climate influence PM compositions. Seasonal changes in climate, such as temperature, amount of daylight, or meteorological patterns influence source emissions (increased residential heating activities, decreased natural soil emissions) and the relative importance of certain chemical pathways in the atmosphere. Since the magnitude of these seasonal changes are highly dependent on location, each sampling site is unique and the chemical composition of PM provides valuable insight into local and regional atmospheric chemistry. Elemental analysis was used to evaluate local atmospheric chemistry at four sites in Southern California (Chula Vista, El Cajon, El Centro, and Brawley) and in Whangarei, New Zealand. PM in Southern California sites revealed seasonal trends, but also how emissions from the 2007 wildfire season impacted local chemistry, producing elevated PM and trace gas concentrations and low O3 concentrations. Analysis of PM collected in Whangarei, New Zealand revealed that local atmospheric chemistry is heavily influenced by marine air masses, seasonal shifts in source contributions (e.g. residential heating activities), and changes in boundary layer height. Stable isotope ratios are often applied as tracers of sources and local chemistry, which is extremely useful for deciphering PM. As the main NO x sink, the stable isotope composition of NO3- reflects NOx sources contributions, oxidation pathways, and other processes that effect the isotope distribution (e.g. equilibrium exchange). However, the use of N isotopes (δ15N) as a tracer is usually split between two schools of thought: the source hypothesis and the chemistry hypothesis. The source hypothesis claims that the δ 15N value of NO3- is solely determined by NOx source δ15N values, and observed variations are due to shifts in source emissions. Alternatively, the chemistry hypothesis argues that the δ15N value of NO3- is impacted by source contributions and chemical reactions occurring in the atmosphere. Here, variations in observed δ15N values are attributed to changes in reaction pathway contribution, as well as shifts in source emissions. Stable isotope analysis of NO3- collected in Southern California and Whangarei, New Zealand was used to evaluate these hypotheses. Using source emission data, known δ15N values of NOx sources, and observed δ15N values of NO3- collected in Chula Vista, CA, isotope mass balance suggests that the source δ15N value is not conserved, requiring a NOx source with an unreasonably large δ 15N value (~ 280‰) to explain observed values. Isotope exchange equilibrium was found to explain observed δ15N values well, but deviations did exist, particularly in the winter. These deviations are likely due to shifts in the importance of this exchange and additional fractionation effects associated with reaction pathways. Additionally, the inverse correlation between δ15N and solar radiation observed in Whangarei further supports the chemistry hypothesis. The research presented in this dissertation is the first known evaluation of these two stable isotope hypotheses, with the results strongly support the chemistry hypothesis. While the oxidation of NO2 is well understood, the mechanism of the oxidation of NO to NO2 is highly uncertain, and so stable isotopes were utilized to determine this reaction mechanism. Laboratory studies found that the remaining O2 became depleted relative to the O2, and followed a strict mass dependent relationship. Complimented by kinetic modeling, results strongly suggest that this reaction proceeds in two steps, with the formation of a peroxynitrate intermediate being favored due to the observed mass dependent relationship. This research is the first to offer support to the peroxynitrate intermediate, whereas previous works favored the energetically more stable nitrogen trioxide form.
Michalski, Purdue University.
Atmospheric Chemistry|Analytical chemistry
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