Abundance of Organic and Inorganic Nitrogen in Marine Aerosols over the China Seas and the Pacific Ocean from Cruises to the Arctic and the Antarctic

Aerosol-bound nitrogen (N) serves as a significant external source of N nutrients for marine ecosystems. However, the measurement of aerosol N in the marine atmosphere is challenging due to difficulties in field sample collection. Specifically, the assessment of organic nitrogen (ON) abundance in marine aerosols remains largely unexplored. In this study, marine aerosols were collected from various regions, including the China Seas (Region 1), the Northwest Pacific to Southern Arctic Ocean (Region 2), and the Southwest Pacific to Antarctic (Region 3). A newly developed method enabling the simultaneous detection of inorganic N (IN) and ON in filter-based aerosol samples has been employed to determine the total quantities of IN and ON. Additionally, the concentrations of organic and elemental carbon (OC/EC) and major ions were measured.

Over the China Seas, the average aerosol IN and ON levels exhibited the following rank: Bohai & Yellow Sea (3.87 and 0.61 μg N m^-3) > East China Sea (1.50 and 0.27 μg N m^-3) > South China Sea (0.52 and 0.18 μg N m^-3). However, the average ratio of ON to total N (ON/TN) was higher in the South China Sea (27.1%) compared to the Bohai & Yellow Sea (16.1%) and East China Sea (16.0%). Notably, aerosol IN and ON showed a strong correlation with EC over the China Seas, particularly in the Bohai & Yellow Sea and East China Sea, indicating significant contributions of anthropogenic emissions to the marine aerosol N pool. In comparison, much lower aerosol N levels were observed in Region 2 and 3, with average IN concentrations of 0.064 and 0.021 μg N m^-3, and average ON concentrations of 0.049 and 0.024 μg N m^-3, respectively. On average, ON accounted for approximately half of the aerosol N in clean marine atmospheres, as observed in Region 2 and 3. Furthermore, the positive correlation between aerosol IN and ON with non-sea salt potassium (nss-K⁺) was observed in Region 2, but not in Region 3, suggesting a more prominent contribution of biomass burning emissions to marine aerosol N in the northern hemisphere compared to the southern hemisphere. Interestingly, aerosol ON exhibited a stronger correlation with secondary species such as nitrate (NO₃^-) and non-sea salt sulfate (nss-SO₄^2-) than with sodium (Na⁺) in both Region 2 and 3, indicating that secondary formation might play a dominant role in contributing to marine aerosol ON, surpassing primary sea salt emissions. Future research efforts should focus on molecular characterizations to gain a better understanding of the sources and transformations of marine ON aerosols.