Award: OCE-1851318

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 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 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. Two studies are nearing publication in peer reviewed literature, with plans for at least one additional manuscript. The first manuscript (Atmospheric Environment) is titled Investigation of Ammonium Aerosol Sources in the Northwest Pacific Ocean and summarizes the impact of anthropogenic and marine sources on reactive N deposition in the NPO, with a focus on ammonium (NH4+), an important bioavailable nutrient. Using aerosol samples (n=108) collected off the coast of China (Changdao Island), we found that the ammonium concentration of aerosol samples varied seasonally, with a higher average value in winter (2.77 ± 1.06 μg/m3) and spring (1.93 ± 0.80 μg/m3) compared to autumn (0.74 ± 0.55 μg/m3) and summer (1.43 ± 0.41 μg/m3). While industry was initially expected to play a major role in aerosol NH4+, we were able to utilize the isotopic composition of aerosol ammonium to detail this further. The nitrogen isotopic composition of ammonium varied seasonally, with higher averages in spring (13.3 ± 7.9 ‰) and summer (15.6 ± 6.2 ‰) compared to autumn (3.2 ± 2.5 ‰) and winter (3.8 ± 11.4 ‰). These 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. We found that anthropogenic activities, mainly agricultural practices (e.g., volatilization, fertilizer, animal husbandry), are the primary sources of ammonium deposited to the NPO. A second manuscript, titled Atmospheric NO3- in the North Pacific atmosphere and the unexpected chemical formation shifts associated with the COVID-19 lockdown is to be submitted in summer 2023. This study provides a comprehensive, seasonal analysis of concentration and the isotopic composition of NO3- (d15N-NO3-, d18O-NO3-, and D17O-NO3-) in the NPO atmosphere, via bulk aerosol samples collected year-round (Sep 22, 2019–Sep 5, 2020) on Tuoji Island (Chang-Dao County), China (38.17 degrees N, 120.76 degrees E). The complete isotopic composition of nitrate all showed strong seasonality, with highest values in the winter and lowest values in the summer. We find that emissions from coal combustion dominated in the winter and biogenic soil emissions played a major role in the summer. The isotopic composition of nitrate was further utilized to investigate isotopic changes during long-range transport (which leads to lower values of (d15N-NO3-). The oxygen isotopic composition allows for details regarding the chemical processes in the atmosphere that produce nitrate from gaseous emissions of nitrogen oxides. Higher values of δ18O- and Δ17O-NO3- support increased ozone interaction in the winter and OH interaction in the summer, following the expectation that photolysis plays an important role in nitrate formation. During our sampling, the Coronavirus (COVID-19) lockdown took place, providing a unique opportunity to identify chemical changes in nitrate production with greatly reduced emission due to lockdown orders. We found that reductions in ozone titration during the lockdown resulted in an increase in OH, O2/RO2/HO2, and H2O involvement in nitrate formation during the winter season. To our knowledge, this is the first study compiling the complete isotopic composition for Northern China before, during, and after the COVID-19 lockdown. This project also provided support for two graduate students, 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 (all women), broadens participation in STEM and contributes to a richer STEM workforce. Last Modified: 06/15/2023 Submitted by: Hayley Schiebel