Investigation of open ocean ammonium aerosol sources in the North Pacific Ocean (Oahu, Hawaii)

Abstract

Anthropogenic nitrogen (N) deposition entering the ocean from the atmosphere has increased over time. Ammonia (NH₃), a precursor to ammonium (NH₄⁺), is released into the atmosphere via both natural sources (e.g., ammonification, biomass burning, waste products, surface ocean emissions) and anthropogenic sources (e.g., agriculture, industry, sewage, vehicle emissions). Studies disagree on the quantity of anthropogenic N deposition to the ocean, as well as the consequences this excess N poses to the biogeochemistry of the open ocean, particularly in the Pacific Ocean. Understanding the current role that the open ocean is playing in the N cycle and budget is essential to determine the sources of N (e.g., internal or external, recycled or excess) and to further distinguish the relationships between atmospheric and oceanic N. Therefore, in the current study, ion concentrations and ammonium isotope values (δ¹⁵N-NH₄⁺) were measured for aerosol samples collected on the coast of Oahu, Hawaii from 2021 to 2022 (n = 67). This location was chosen based on low anthropogenic activity, access to the open ocean, and the premise of a dominant marine signal. Particulate NH₄⁺ concentrations ([NH₄⁺]) averaged 3.7 ± 7.3 ng/m³, with no distinct seasonality (p > 0.05). The δ¹⁵N-NH₄⁺ values also did not exhibit distinct seasonality (p > 0.05), but did fall into three unique clusters (using a K-means clustering analysis): cluster one = 19.0 ± 3.9 ‰ (n = 8), cluster two = 4.1 ± 2.3 ‰ (n = 31), and cluster three = −3.3 ± 2.4 ‰ (n = 13). Cluster three was best explained as an ocean emissions signature, which was determined via a phase partitioning model that incorporated N isotope fractionation associated with NH₃ conversion to NH₄⁺. Cluster one had higher than average [NH₄⁺] along with air mass origins from two seabird sanctuaries and was hypothesized to be a seabird emission signature. Cluster two was considered a mix of these two sources (67 % marine, 33 % seabird emissions). 6 % of aerosol inorganic N (NH₄⁺ + nitrate; annual dry inorganic N = 5.5 ng/m³) is from [NH₄⁺], with 35 % resulting from seabird emissions. The annual inorganic N (IN; NO₃⁻ + NH₄⁺) dry deposition value for this site was 1.2 ± 1.1 Tg N·y⁻¹.