Award: OCE-1851343

Anthropogenic nitrogen (N) emissions to the atmosphere as well as N deposition have increased exponentially since preindustrial times. Global modeling studies have suggested that as much as 80% of total N deposition to the oceans is anthropogenic in origin, and the magnitude of input to the global oceans rivals estimates of biological N fixation. The impacts associated with this increased N deposition are clear in both terrestrial and coastal systems, but the implications on open ocean biogeochemistry remain uncertain.

 

We studied the influence of both terrestrial/anthropogenic and marine/natural sources on reactive N deposition (nitrate, ammonium, organic N) in the North Pacific Ocean (NPO). This was done via aerosol and rainwater samples collected year-round at two sites: (1) Chang-Dao Island, China (assumed high anthropogenic N inputs); and (2) Oahu, Hawaii (assumed dominant marine N input).  Our overall goals were to: characterize the atmospheric composition and sources of inorganic (ammonium and nitrate) and organic N with an emphasis on seasonality; diagnose the influence of air-sea exchange versus anthropogenic sources of nitrogen on atmospheric deposition; and determine the isotopic composition of gaseous and particulate inorganic N in the marine boundary layer via ship-based collections in the NPO. Using concentration and isotopic measurements of reactive N species, aerosol composition, and transport and chemical box modeling, we characterized reactive N in the atmosphere in two locations with very different source influences. 

 

Results from the coastal China site revealed important influences of anthropogenic activities, as expected. Seasonal patterns in the isotopic composition of ammonium were investigated based on correlations of aerosol chemical species, seasonal shifts in transport patterns, partitioning of ammonia/ammonium between the gas and particle phase, and continental versus marine sources of ammonia. Agricultural practices (e.g., volatilization, fertilizer, animal husbandry) are the primary sources of ammonium deposited to the NPO, making this a potentially important external source of N to the coastal and open ocean (published in the journal of Atmospheric Environment). Upon application of methods used in the coastal China study to the Hawaii-based samples, it was found that the current chemical oxidation methodologies commonly utilized for characterizing isotopic values of ammonium samples suffered from interferences. This led to method development work to extract and concentrate ammonium from samples through use of a solid phase extraction technique involving cation exchange resins (published in ACS Earth and Space Chemistry). The sample pretreatment methodology was evaluated using the atmospheric aerosol samples previously measured from China for d15N-NH4+, which indicated an excellent match between sample pretreatment and no treatment (R2 = 0.99). This method then allowed for analyses of the samples collected in Oahu. Interpretation of the Oahu isotopic evidence supported that ammonium in this region of the NPO is a result of direct ocean emissions via the oceanic nitrogen cycle as well as from seabird guano emissions. It was further concluded that the ocean source contributed 65% of the ammonium emissions to this region and seabirds contributed 35%, which supplies a natural external nitrogen source to the ocean and is not typically considered in models and nitrogen budgets (in review at Marine Chemistry.

 

Combining the results from the two end-member studies allows us to comprehensively consider atmospheric N deposition to the NPO. Overall, the rates of external inputs to the NPO range from on the order of 1 TgN/yr at Oahu to 67 TN N/yr near the coastal regions. Much of the change in the biogeochemistry in the NPO has been suggested to be linked to anthropogenic atmospheric deposition. However, the rates of deposition of inorganic N are only high enough near the coasts to create this disruption, and overall the rates based on this observational study are considerably lower than models suggest. Finally, a motivating factor for these studies is to also consider what emissions reductions from the continent would be needed to improve air quality and N deposition. During our sampling, the Coronavirus (COVID-19) lockdown took place, providing a unique opportunity to identify chemical changes in atmospheric N production with greatly reduced emissions due to the lockdown orders. We found that significant reductions in nitrogen oxide emissions primarily from reduced coal burning and vehicle traffic led to less ozone loss (ie ozone titration) during the lockdown and an increase in the involvement of peroxy and hydroxy compounds in the oxidation of reactive N in the winter. Overall this implies that simple emissions reductions may not have the desired air quality improvements for ozone in China (in prep. For ACP). This project provided support for two graduate students, technical staff an assistant professor and an associate professor at two different institutions. This work also gave valuable opportunities for communication of results and implications with undergraduate students, graduate students and the general public. The professional development of the graduate students and professors contributes to broadening participation in STEM and a talented STEM workforce.

Last Modified: 03/05/2025
Modified by: Meredith Hastings