Description
- Abstract:
- There has been a steady rise in land-based emissions of reactive nitrogen (N) over the last several decades, which is subsequently deposited into systems like the ocean. While N is a critical nutrient, in excess it can cause algal blooms, reduced biodiversity, and shifts in ecosystem structures. Deposition rates, source origins, and distance traveled of excess N, especially ammonium (an inorganic form of N), are not agreed upon in the literature and are especially unresolved for the North Pacific Ocean (NPO). Aerosol samples were collected and analyzed for two “endmember” locations: (1) Changdao Island, China, a coastal site heavily influenced by anthropogenic pollution, and (2) Oahu, Hawaii, an open ocean site dominated by marine sources. Using a suite of tools, including ion concentration measurements, N isotope analyses, air mass back trajectory calculations, and isotope mixing models, ammonium concentrations near the China coast were observed to vary seasonally (high in winter = 2,800 ± 1,100 ng/m3, low in autumn = 700 ± 600 ng/m3), predominantly due to agricultural emissions (>65% year-round). In the open ocean environment (Oahu), ammonium concentrations were not seasonally different and annually averaged 55 ± 37 ng/m3, with a main source of oceanic emissions (~65%). The discovery of a new external source of N, seabird guano emissions, was isotopically identified and found to account for ~35% of the atmospheric ammonium. When inorganic N deposition (ammonium + nitrate) was upscaled to the NPO it resulted in 1.2 Tg N/yr from Oahu and 93.5 Tg N/yr from Changdao Island. Calculations of airmass back trajectories indicated that continental emissions travel further into the ocean during the winter than the summer, allowing for the seasonal determination of coastal vs open ocean area. With Oahu calculations representing the open ocean and Changdao Island calculations representing the coastal ocean, average total deposition rates were estimated at ~61 Tg N/Yr in the winter and ~47 Tg N/Yr in the summer. This work provides new inorganic N observations and isotopic constraints on inorganic N deposition rates in oceanic environments and advances future research aimed at the biogeochemical impacts of atmospheric N deposition to the open ocean.
- Notes:
- Thesis (Ph. D.)--Brown University, 2025
Citation
MacFarland, Alexandra,
"Atmospheric Ammonium Deposition to the North Pacific Ocean: Source Origins and Implications"
(2025).
Earth, Environmental and Planetary Sciences Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://repository.library.brown.edu/studio/item/bdr:wpm44th6/
Relations
Collection:
-
Earth, Environmental and Planetary Sciences Theses and Dissertations
Theses and Dissertations for the Earth, Environmental and Planetary Sciences department....