STABLE NITROGEN AND SULFUR ISOTOPES IN ATMOSPHERIC CHEMISTRY
Creators: name: Li, Jianghanyang
Publisher: name: Purdue University Graduate School
Year: 2021
Subjects: Atmospheric aerosols
Atmospheric sciences not elsewhere classified
Other environmental sciences not elsewhere classified
Atmospheric composition, chemistry and processes
Isotope geochemistry

description: SO<sub>2</sub> and NO<sub>x</sub> (NO+NO<sub>2</sub>) are important trace gases in the atmosphere as they adversely affect air quality and are precursors to sulfate and nitrate aerosols in the atmosphere. However, there are significant uncertainties in the emission inventories and the atmospheric chemistry processes of both gases. Addressing these uncertainties will help us to 1) better regulate their emissions from anthropogenic activities, 2) understand the formation mechanism of aerosol pollution events, during which rapid accumulation of nitrate and sulfate aerosols are commonly observed, and 3) better constrain the impact of SO<sub>2</sub>, NO<sub>x</sub>, sulfate aerosols and nitrate aerosols to the global radiation balance. Stable isotopes of nitrogen and sulfur are useful tools in understanding both the origins and chemistry of SO<sub>2</sub> and NO<sub>x</sub> since different emission sources usually display distinct sulfur and nitrogen isotopic compositions, and different SO<sub>2</sub> and NO<sub>x</sub>oxidation pathways fractionate sulfur and nitrogen isotopes differently. In this dissertation, five studies are conducted to 1) use sulfur isotopes to investigate the sources and chemistry of atmospheric sulfur, and 2) improve our understanding of the isotopic fractionation processes associated with the atmospheric chemistry of reactive nitrogen. Using stable sulfur isotopes, we first analyzed the sources of sulfate aerosols collected at Baring Head, New Zealand and atmospheric deposition at the Atacama Desert. At Baring Head, we found that the secondary sulfate, i.e., sulfate formed from atmospheric oxidation of SO<sub>2</sub>, is mainly observed in fine aerosols (&lt;1 µm) while the sulfate in coarse aerosols (&gt;1 µm) is mostly sea salt sulfate. 73-77% of the secondary sulfate is sourced from biogenic emissions by ocean phytoplankton, and the rest is originated from anthropogenic activities. The sulfate deposition across the Atacama Desert, on the other hand, is a mixture of sea salt sulfate (only near the coast), anthropogenic SO<sub>2</sub> emissions, local soil, and lake salts. Then, sulfur isotopes were used to investigate the formation chemistry of sulfate aerosols collected during a strong winter haze episode in Nanjing, China, where the sources of SO<sub>2</sub> were well-understood. We found that, although the sources of sulfur remain unchanged during the haze episode, the sulfur isotopic compositions of sulfate vary significantly, suggesting isotopic fractionation occurred during the formation of sulfate aerosols. We interpreted the variation using a Rayleigh distillation model to evaluate the contribution of sulfate formation pathways. The model suggested that the Transition Metal Ion catalyzed O<sub>2</sub> oxidation pathway contributed 49±10% of the total sulfate production, while the O<sub>3</sub>/H<sub>2</sub>O<sub>2</sub> oxidations accounted for the rest. Next, we conducted experiments in an atmospheric simulation chamber to determine the isotopic fractionations between NO and NO<sub>2</sub>. This isotopic fractionation is controlled by a combination of two factors: 1) the equilibrium isotopic exchange between NO and NO<sub>2</sub> molecules, and 2) the kinetic isotope effects of the NO<sub>x</sub> photochemical cycle, namely the Leighton Cycle Isotope Effect (LCIE). Our experiments showed that the fractionation factor during the isotopic exchange is 1.0289±0.0019, and the fractionation factor of LCIE is 0.990±0.005. A model was constructed to assess the relative importance of the two factors, showing the isotopic exchange should be the dominant factor when NO<sub>x</sub> &gt;20 ppb, while LCIE should be more important at low NO<sub>x</sub> concentrations (&lt;1 ppb) and high rates of NO<sub>2</sub>photolysis. Last, we quantified the overall nitrogen isotopic fractionation during the formation of nitrate aerosols collected at Baring Head, New Zealand. Our results showed that significant and variable (0-15‰) isotopic fractionations occurred during the formation of nitrate aerosols. The isotopic fractionation factors are lower in the summer and higher in the winter, which is mainly caused by seasonal variations in nitrate formation pathways. Overall, this dissertation first applied stable sulfur isotopes in aerosol samples collected in different environments, demonstrating that isotopes are excellent tools in identifying the origins and chemistry of atmospheric sulfur. Then, we investigated the isotopic fractionation processes during the atmospheric nitrogen chemistry, which can be useful for future studies aimed at understanding the origins and chemistry of atmospheric nitrogen using stable nitrogen isotopes.