Characterising dissolved nitrate in precipitation using stable nitrogen and oxygen isotopes

Lee, Vanessa Eileen (2005) Characterising dissolved nitrate in precipitation using stable nitrogen and oxygen isotopes. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (30MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Single event precipitation samples were collected at two sites in Newfoundland for stable isotopic analysis of nitrogen and oxygen of dissolved nitrate. Nitrate, the second largest acidifying component of rain, is formed during the oxidation of NO and NO₂, collectively known as NOx, as well as other nitrogen species. Both natural and anthropogenic sources exist, however fossil fuel combustion in power plants and vehicles are the main contributors of NOx to the atmosphere. Nitrogen isotopes of nitrate may give insight into the source of the atmospheric oxinitrogen species, while oxygen isotopes may provide information about the type of oxidation the NOx emissions have undergone. The isotopic signatures, in combination with chemical and meteorological data, were used to characterise the main controls of the nitrate in the precipitation. -- The two sites chosen for this study represent different types of environments: St. John's is a marine/urban location, while McIvers is a marine/rural site with no major point sources in the area. St. John's precipitation samples generally displayed characteristics of oil combustion emissions from stationary sources (high V, Ni) with smaller inputs from the ocean. Samples collected at McIvers were not as strongly affected by inputs from fossil fuel combustion, and generally had stronger marine signatures along with wood combustion and vehicle emissions. However, several samples collected at this location did display characteristics of oil combustion as well as smelting processes, confirming that the site is affected by long range transport of pollutants from North America. -- Nitrogen isotopes appear to be controlled by the source of the nitrogen emissions. Fossil fuel combustion in stationary sources results in δ¹⁵N values close to 0‰, vehicle NOx emissions range from -2 to -13‰, while natural sources such as soil and oceanic emissions can result in NOx with values as low as -20‰. A seasonality in δ¹⁵N was observed at both sites, whereby samples collected during the warmer season (Apr.-Sept.) exhibited more positive δ¹⁵N values than those collected during the cold season (Oct.-Mar.). Chemical data revealed that both sites were affected more strongly by anthropogenic emissions during the warm months, while the influence of the ocean was dominant during the cold periods. -- The oxygen isotopic composition of nitrate in this study appears to be controlled by atmospheric transformations of the oxinitrogen species, which are related to temperature. Summer samples, mainly oxidised by ozone, had more positive ¹⁸O signatures than their winter counterparts. There is no evidence that the δ¹⁸O of the nitrate in this study is controlled by the sources of the nitrate. -- This study has provided valuable isotopic measurements of atmospheric nitrate. To the author's knowledge, the method used in this study for isolating and processing dissolved nitrate in precipitation for stable isotopic analysis had not been previously applied to single rain events, making this study unique. As well, it has shown through the combination of isotopic and chemical data how nitrogen isotopes of precipitation nitrate are controlled by sources, while the oxygen isotopes of these compounds are controlled by atmospheric oxidation processes.

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