Subtropical North Atlantic surface ocean lower atmosphere nitrogen cycle
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. We conducted a two-year 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, and are considered representative of the open ocean marine atmosphere.
Marine Source of Organic N
Global models indicate that the human-derived nitrogen emissions that reach the ocean through atmospheric transport and deposition directly impact biology and the oceanic carbon dioxide (CO2) sink. In a paper just accepted for publication in the Proceedings of the National Academy of Sciences, we find that the organic nitrogen in marine aerosols derives predominantly from biological production in the surface ocean rather than from pollution on land. The figure above shows aerosol organic N concentrations ([WSON]) as a function of a) gross primary productivity (r2=0.69), b) chlorophyll-a concentration (r2=0.25), and c) particulate organic nitrogen concentration (r2=0.05) in the surface ocean at the Bermuda Atlantic Time Series Study site. Wind speed is indicated by the color shading.
Our previous work has shown significant anthropogenic influence on North Atlantic nitrate deposition whereas ammonium cycles dynamically between the upper ocean and lower atmosphere (discussed below). Collectively, these findings indicate that the ocean is not a passive recipient of anthropogenic nitrogen deposition, as it has previously been considered (Duce et al., 2008). This implies that the contribution of atmospheric nitrogen deposition to ocean fertility, oceanic CO2 removal, and nitrous oxide emissions has been overestimated (Figure below).
Stable isotopic composition of nitrogen in marine aerosols and rain
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 and aerosols have 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.