Dissertation Abstract

Nitrogen oxides (NOx = NO + NO2) play a key role in atmospheric chemistry, contributing to production of ground-level ozone, aerosol chemistry, and acid deposition through the conversion to nitrate and nitric acid. In order to evaluate the sources and chemistry of NOx, models often compare to observed nitrate concentrations, but many questions remain about the balance of NOx sources. The isotopes of nitrate provide more information than concentration alone, potentially allowing for a quantitative measure of the influence of NOx sources on nitrate deposition. Ice core records of nitrate preserved in snow may provide a record of past NOx emissions, however, nitrate is reversibly deposited to snow and interpretation over time requires understanding the degree of preservation of nitrate. The preservation of nitrate in snow at Summit, Greenland was investigated by measurement of nitrate concentration and isotopic composition at high temporal-resolution in surface snow. A strong, linear relationship was found between the oxygen isotopes of nitrate that cannot be explained in the presence of significant post-depositional alteration of nitrate. Additionally, a suite of atmospheric constituents was measured and none show correlation with the isotopes or concentration of nitrate in the snow, indicating that the nitrate is regionally, rather than locally, derived. When the nitrogen isotopes of nitrate are compared to the oxygen isotopes, the data show a mixing of three isotopically distinct nitrate sources, currently characterized best as: 1) influenced by the stratosphere, 2) derived from mid-latitude sources and 3) local anthropogenic pollution from the field station. Using the nitrogen isotopes of nitrate to definitively distinguish these sources is difficult, however, until a better inventory of the nitrogen isotopes of NOx from various emissions sources exists. To that end, a new method was developed to collect NOx for isotopic evaluation. This method is the first NOx isotopic method to be tested in the laboratory for consistency under a variety of conditions. This method was tested in the field for reproducibility based on the collection of urban air in Providence, RI, and in a laboratory-based experiment to obtain the first ever measurements of the nitrogen isotopes of NOx produced by biomass burning.