Distinct Spatiotemporal Patterns of Atmospheric Total and Soluble Iron from Three Sources Revealed by Shipboard Online Observations in the Northwest Pacific

Abstract. Non-dust emissions have been increasingly recognized as important contributors to atmospheric iron (Fe), influencing marine productivity through enhanced bioavailable Fe inputs. However, accurately quantifying the contributions and spatiotemporal variability of non-dust sources remains challenging due to relatively low time-resolution of traditional filter-based analytical methods. In this study, the contributions of non-dust emissions to atmospheric total and soluble Fe in the Northwest Pacific were quantified based on online measurements from three ship-based observation campaigns in 2021–2022. A Positive Matrix Factorization (PMF) model was applied for source apportionment. Results showed non-dust emissions contributed substantially to atmospheric Fe, accounting for 21 %–48 % of total Fe across different regions and seasons. Importantly, their contributions to soluble Fe were significantly higher, reaching 79 %–98 % and largely dominating the bioavailable Fe supply in the study area. Among non-dust sources, land anthropogenic emissions contributed significant portion of both total and soluble Fe, whereas ship emissions contributed small portion to total Fe but was major source of soluble Fe, particularly in coastal regions. In summer, ship emissions over coastal waters even exceeded land anthropogenic sources, becoming the dominant contributor to soluble Fe. Additionally, Fe from non-dust sources exhibited stronger spatial variability than dust source. The concentrations of land anthropogenic Fe differed by 3–5 times between coastal and open-ocean areas during the same cruises, while ship-derived Fe varied by an order of magnitude or more. This study offers critical observational evidence to advance understanding of how diverse emission sources shape atmospheric composition in Asian continental outflow regions.