Dissertation Abstract
Investigating anthropogenic impacts on reactive nitrogen chemistry using isotopic composition of Central Greenland ice-core nitrate
Geng, Lei 2012
Chemistry & Biochemistry, South Dakota State University (United States), 164 pp.
Atmospheric reactive nitrogen (NOx, nitrate, etc.) chemistry has implications for atmospheric environment as the influence of NOx cycling on tropospheric ozone and OH radicals which determines atmospheric oxidation capacity. The atmospheric oxidation capacity determines the rates of production/destruction of secondary aerosols/trace gases (e.g., CO, CH4, VOCs), and is thus an important matrix of the atmospheric environment. In addition, the deposition of nitrate, which is the oxidation production of NOx, disturbs terrestrial ecosystems as nitrate is a biologically available nitrogen and a major component of acid rain. Chronological records of nitrate derived from polar ice cores have been sought to evaluate past changes in atmospheric NOx and nitrate deposition patterns, and potentially to provide past information on atmospheric oxidation capacity. However, the interpretation of ice core nitrate records has been hampered, mainly by 1) the effects of post-depositional processing of nitrate in surface snow which disturbs the preservation of nitrate in ice cores, and 2) there has been lack of proxies providing insights into the chemical dynamics which determine the production of nitrate in the atmosphere and are directly related to the oxidation capacity of the atmosphere.
Recently, the oxygen isotopic composition of nitrate, in particular, its oxygen-17 excess has been suggested to reflect the formation mechanisms of nitrate and is indicative of the oxidation capacity of the atmosphere at the time of nitrate formation. In addition, the nitrogen isotopic ratio of snow/ice core nitrate (15N/14N) has been suggested to be indicative of the source of NOx and/or the degree of post-depositional processing of snow nitrate. In this study, I use a combination of nitrate concentration, and its nitrogen and oxygen isotopic composition from central Greenland snow and ice core samples to assess how well nitrate is preserved in central Greenland snow with respect to the effects of post-depositional processing, and to evaluate changes in atmospheric reactive nitrogen chemistry and the related oxidation capacity of the atmosphere in response to anthropogenic forcing since the beginning of the Industrial Revolution.
Preliminary conclusions from this study include, 1) in addition to direct pollution transport, the springtime nitrate peak observed occasionally in some post-1950 snow layers is associated with a low overhead column ozone which enhanced surface photo-chemistry through elevating surface UV-B radiation levels; 2) post-depositional processing of snow nitrate at Summit is active, but unable to erase atmospheric oxidation information that nitrate preserves during its formation in the source region(s); 3) mainly through emissions of SO2, human activities altered the atmospheric acidity, which in turn affected the partitioning of atmospheric nitrate between gas- and aerosol-phase and left fingerprints in 15N/14N of ice-core nitrate.
Further work involving coupled chemistry-climate model will be done in order to interpret the oxygen isotope data, and to obtain information qualitatively and potentially quantitatively on changes in atmospheric oxidation capacity since the beginning of the Industrial Revolution.
Recently, the oxygen isotopic composition of nitrate, in particular, its oxygen-17 excess has been suggested to reflect the formation mechanisms of nitrate and is indicative of the oxidation capacity of the atmosphere at the time of nitrate formation. In addition, the nitrogen isotopic ratio of snow/ice core nitrate (15N/14N) has been suggested to be indicative of the source of NOx and/or the degree of post-depositional processing of snow nitrate. In this study, I use a combination of nitrate concentration, and its nitrogen and oxygen isotopic composition from central Greenland snow and ice core samples to assess how well nitrate is preserved in central Greenland snow with respect to the effects of post-depositional processing, and to evaluate changes in atmospheric reactive nitrogen chemistry and the related oxidation capacity of the atmosphere in response to anthropogenic forcing since the beginning of the Industrial Revolution.
Preliminary conclusions from this study include, 1) in addition to direct pollution transport, the springtime nitrate peak observed occasionally in some post-1950 snow layers is associated with a low overhead column ozone which enhanced surface photo-chemistry through elevating surface UV-B radiation levels; 2) post-depositional processing of snow nitrate at Summit is active, but unable to erase atmospheric oxidation information that nitrate preserves during its formation in the source region(s); 3) mainly through emissions of SO2, human activities altered the atmospheric acidity, which in turn affected the partitioning of atmospheric nitrate between gas- and aerosol-phase and left fingerprints in 15N/14N of ice-core nitrate.
Further work involving coupled chemistry-climate model will be done in order to interpret the oxygen isotope data, and to obtain information qualitatively and potentially quantitatively on changes in atmospheric oxidation capacity since the beginning of the Industrial Revolution.