The air in Earth’s atmosphere contains 78% nitrogen by volume. Nitrogen is one of the fundamental building blocks of life on earth and is a component of many biologically important chemicals, such as proteins. In air, nitrogen is predominantly in the gaseous form of N2. This is extremely chemically stable and must be transformed into other chemical forms, called reactive nitrogen (Nr) before it can be utilized by plants and animals. However, too much reactive nitrogen can have negative consequences to both environmental and human health. Excess Nr contributes to groundwater acidification, soil contamination, and eutrophication of marine environments, and can cause respiratory ailments among certain members of the population.
While scientists know that a major source of Nr is the combustion of fossil fuels, there is still uncertainty about how seasonal atmospheric circulation patterns influence the effects of these emissions and the resulting deposition of Nr in the marine environment. Increased deposition of Nr to the ocean may impact ocean biogeochemistry, particularly in areas like the Sargasso Sea that are characterized by low nutrient concentrations and located adjacent to or downwind of Nr sources (e.g., major industrial centers).
Amy Gobel, an undergraduate student in the Department of Geosciences at Princeton University, recently investigated how marine aerosols contribute to Nr deposition to the ocean downwind of North America (a region that exhibits high anthropogenic NOx emissions). During 2010 and 2011 Gobel worked as a Princeton-BIOS Intern for Dr. Andrew Peters, Associate Scientist at BIOS, as part of a larger NSF-funded collaborative research project involving Princeton and Brown Universities and BIOS. During this time, she collected aerosol and rainwater samples at the Tudor Hill Marine Atmospheric Observatory and analyzed the samples to determine:
- The concentrations of nitrate (NO3-), a large contributor to the deposition of Nr in the marine environment;
- The nitrogen and oxygen isotope ratios of nitrate;
- Sources of various air masses (using air mass back trajectories, or AMBTs) to determine where nitrate originated.
By studying the ratios of nitrogen and oxygen isotopes in the samples, the researchers hoped to distinguish among various sources of nitrate, as well as the chemical pathways involving nitrate in the environment. Changes in the ratio of nitrogen isotopes would indicate changes in Nr sources, while changes in the ratio of oxygen isotopes would indicate a seasonal shift in how nitrate was being formed in the atmosphere.
Study results indicated that oxygen and nitrogen isotope ratios did follow seasonal trends, with samples taken during the cool season coming from air masses originating over North America, and samples taken during the warm season coming from air masses originating over the sub-tropical North Atlantic. In other words, during the cool season, continental (e.g., anthropogenic) sources of nitrate are the primary contributor to Nr deposition in the marine environment. The researchers also found that the nitrogen isotope chemistry of nitrate in the marine atmosphere may be controlled by the chemical reaction of nitrogen chemicals with sea-salt and mineral dust in the marine atmosphere.