Isotopic studies of nitrate and nitrogen dioxide: Atmospheric and biosphere nitrogen cycling

Nitrogen oxides play a critical role in atmospheric chemistry. In the troposphere they directly and catalytically produce ozone that in turn regulates the oxidative capacity of the troposphere. In the stratosphere, they catalytically destroy ozone, which limits the penetration of harmful ultraviolet radiation. Nitrogen oxides are removed from the atmosphere after they are chemically converted to nitric acid/nitrate particles and deposited to the Earth's surface either directly or through wet deposition. Once deposited, nitrate is a source of bio-available nitrogen and is recycled through the biologic nitrogen cycle including processes such as denitrification and nitrification processes.; A method was developed to measure the total oxygen isotopic composition of nitrate (16O, 17O, 18O). This method was then used to analyze the δ18O and Δ 17O composition of nitrate aerosols collected at several locations. The results showed that there is a large mass-independent isotopic anomaly in photochemically produced nitrate (Δ17O ∼ 20–30‰). The source of this anomaly and its seasonal variation was explained by using a mass transfer mechanism in reactions involving ozone coupled to a model that traced the homogenous and heterogeneous HNO₃ formation pathways.; The unique Δ17O signature of atmospherically produced nitrate was then used to trace NO₃− deposition in a southern California coastal sage scrub habitat. It was shown that both biologic and atmospheric NO₃− components were present suggesting that N deposition can lead to leaching and sequestering of available N. The Δ17O signal was also used to determine the source of the renowned nitrate deposits of the Atacama Desert in Chile: atmospheric dry deposition spanning 100,000's of years.; Nitrogen dioxide is also an excellent gas for testing of the more recent theories of mass-independent isotopic fractionations. Suggestions that the affect is due to more complete intramolecular vibration redistribution in asymmetric molecules relative to symmetric molecules were tested by LIF spectroscopy on six isotopologues of NO₂. The results of this experiment yielded precise dissociation energies and density of states near the dissociation threshold. The observed higher density in the asymmetric isotopologue suggests there is validity in the recent theoretical framework, but a definitive answer will require additional experiments.