Investigation of coastal ammonium aerosol sources in the Northwest Pacific Ocean

Abstract

Determining the magnitude and origins of nitrogen (N) deposition in the open ocean is vital for understanding how anthropogenic activities influence oceanic biogeochemical cycles. Excess N in the North Pacific Ocean (NPO) is suggested to reflect recent anthropogenic atmospheric deposition from the Asian continent, changes in nutrient dynamics due to marine N-fixation, and/or lateral transport of nutrients. We investigate 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). This study site is used as a proxy for continental emissions that can be exported and subsequently deposited to the ocean. The NH4+ concentration of aerosol samples varied seasonally (p < 0.05), with a higher average value in winter (2.8 ± 1.1 μg/m3) and spring (1.9 ± 0.8 μg/m3) compared to autumn (0.7 ± 0.6 μg/m3) and summer (1.4 ± 0.4 μg/m3). The isotopic composition of aerosol NH4+ 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 NH4+ are 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 find that anthropogenic activities, mainly agricultural practices (e.g., volatilization, fertilizer, animal husbandry), are the primary sources of NH4+ deposited to the NPO.