This project is designed to characterize the sources, composition, and interrelationships among nitrogen (N) species in rainwater and aerosols deposited into the North Atlantic Ocean. The island of Bermuda will be used for sampling due to its location in the western North Atlantic, its history as a key sampling location for studies of the marine atmosphere, and the fact that seasonal changes in transport allow for study of both anthropogenically and primarily marine influenced air masses. The intellectual merit of this project includes contribution to a fundamental understanding of the sources and composition of inorganic and organic N in the marine atmosphere. Results from this work will also have implications for diagnosing how much new, bioavailable N is entering the marine biogeochemical system. Estimates today suggest that atmospheric deposition can account for approximately a third of the ocean's external N supply. However, deposition fluxes to the ocean have typically been interpreted simply as N inputs, whereas recent work suggests that the ocean may influence and even contribute to the reactive N cycle found in the overlying atmosphere. This work aims to clarify both the natural and human-impacted atmospheric N cycle and its links to the ocean.
Broader impacts include the education and training of undergraduates in the classroom, laboratory and field; training and career development of a graduate level research technician; the education, training and development of a post-doctoral researcher; contribution to the development of the research program of an early career scientist; and dissemination of results within the scientific community, as well as to a broader audience. The work will be highly collaborative and interdisciplinary, involving groups at Brown University, Princeton University and the Bermuda Institute for Ocean Sciences (BIOS). A Princeton undergraduate will participate in field and laboratory work, as part of the Princeton-BIOS summer internship program. A research technician will be dedicated to the project and based at BIOS for 18 months. A post-doctoral scholar will spend time at all three collaborating institutions and will coordinate the research of the undergraduate and a Brown graduate student. The results of the research will be disseminated at scientific conferences and through peer-reviewed publications. This research and its implications will be shared with a broader audience through three specific efforts: a public lecture in Bermuda, a presentation to K-12 teachers as part of Brown's NSF supported GK-12 program, and development of a module to be taught as part of an undergraduate environmental studies course at Brown entitled "Analysis and Resolution of Environmental Problems."
The emission of anthropogenic nitrogen (i.e., nitrogen from human activities) has increased ten-fold since the industrial revolution, primarily driven by nitrogen oxide (NOx) emissions from fossil fuel combustion, and ammonia emissions from agriculture. The impacts of the resulting atmospheric nitrogen deposition (i.e., rainwater and aerosol particles that contact the Earth’s surface) are well documented in terrestrial and coastal systems, and include harmful algal blooms and acidification of natural waters. However, the total magnitude of anthropogenic and natural nitrogen deposition to the ocean, and its potential impact(s) on ocean biology and climate remain poorly constrained due to a lack of observations in the marine environment. This project was designed to characterize the sources (anthropogenic vs. natural) and composition of nitrogen species in rainwater and aerosols deposited into the North Atlantic Ocean. We conducted a field campaign on the island of Bermuda, an ideal sampling location located in the subtropical North Atlantic Ocean, 1000 km downwind of North America. It experiences both polluted air masses from North America as well as clean air masses that originate over the ocean. Daily rainwater samples and weekly aerosol samples were collected for two years, and are considered representative of the open ocean marine atmosphere. Throughout this project, three Princeton University undergraduate interns conducted field research in Bermuda, collecting samples from the NSF-supported AEROCE tower and participating in oceanographic research cruises. One intern conducted her senior thesis based on her internship fieldwork, winning the Arthur F. Buddington award for excellence in the Geosciences and publishing in Geophysical Research Letters (Gobel et al., 2013). Nitrate is the ultimate sink for atmospheric NOx and is produced through both hydroxyl radical (OH) and ozone (O3) driven pathways. At Bermuda, rain and aerosols from marine vs. continental air masses are different; marine rain has lower nitrate concentrations, higher average nitrogen isotope ratios (15N/14N), and lower average oxygen isotope ratios (18O/16O) (Altieri et al., 2013, Gobel et al., 2013), where 15N/14N is a tracer for the nitrogen source, and 18O/16O is a tracer for the chemistry that formed the nitrate. As expected due to the large anthropogenic sources of NOx and O3 over North America, the data indicate that continental air masses contribute anthropogenic nitrate from North America formed via the O3 pathway to the subtropical surface ocean, whereas nitrate associated with marine air masses has a different NOx source and is formed via OH from the marine atmosphere. One remarkable finding is that the nitrate concentration does not differ greatly in rain from marine air masses vs. continental air masses. This implies a natural source of nitrate from the atmosphere over the open ocean. We expected a similar dynamic for the ammonium concentration and isotopes, as global ammonia emissions are dominated by human activities including agriculture and animal husbandry. However, we observe no clear relationship in ammonium concentration or 15N/14N with air mass history at Bermuda (Altieri et al., 2014). The lack of a trend could be due to i) large anthropogenic sources that persist in the marine atmosphere regardless of air mass history, ii) changes in sources or processes that coincidentally yield similar isotopic values, or iii) a predominance of marine sources. Measuring the isotopic composition of ammonium is very challenging; thus, there are no measurements from the marine atmosphere available for comparison with our data. We developed a mathematical model to investigate whether the surface ocean can be the dominant source of ammonia to the marine atmosphere given the observed concentrations and isotope ratios. The model demonstrates that this is indeed feasible. While 90% of the ammonium deposition to the global ocean has previously been attributed to anthropogenic sources (Duce et al., 2008), the evidence from Bermuda suggests that the anthropogenic contribution is likely much smaller (Altieri et al., 2014). In addition to nitrate and ammonium, the atmosphere also contains organic nitrogen, a complex mixture of compounds that contribute 20-80% of total nitrogen in rain and aerosols in both the polluted and marine atmosphere. Despite this, there are very few measurements of atmospheric organic nitrogen. Via ultra-high resolution mass spectrometry, we identified >2000 organic nitrogen compounds in rainwater from Bermuda (Altieri et al., 2012). Statistical analyses show a clear chemical distinction between the suite of organic nitrogen compounds in rain from marine vs. continental air masses. This, in conjunction with patterns in the ratio of oxygen to carbon and nitrogen, suggest that when rain comes from the continent, the organic nitrogen has a mixture of anthropogenic and marine sources, while organic nitrogen in rain from clean air masses has a marine source. These findings indicate that, although the concentration and percent contribution of organic nitrogen to total nitrogen is fairly consistent across diverse geographic areas, the chemical composition of organic nitrogen differs across regions and is impacted by the local atmosphere.
