Acquiring high-quality data of concentration and quantifying sources of atmospheric Fe based on field measurements are the basis and critical step to assess its impacts on marine phytoplankton and ocean ecosystem. However, previous studies with measurements mainly relied on chemical analysis of very limited number of filter samples, making it difficult to gain high-resolution spatial distribution of Fe concentration and sources in marine aerosol. In addition, anthropogenic emissions have been identified as significant contributors to atmospheric soluble Fe in recent years. However, the conclusion was mainly based on model results, and the verification based on observational evidence is still lacking and needed, especially in the Northwest Pacific where both dust and anthropogenic emissions have strong impacts.
In this study, an online multi-element analyzer (Xact 625) was used to measure atmospheric Fe concentration on board, with a time resolution of 0.5 to 4 hours (time resolution improved by 10 times compared to traditional offline filter sampling). The receptor model, Positive Matrix Factorization (PMF), was applied to quantitatively distinguish between dust and anthropogenic sources (e.g., coal combustion, biomass burning, and maritime transport) of Fe. The source apportionment results of Fe were further combined with Fe solubility from different emission sources to quantify the sources of soluble Fe in marine aerosol for the first time. Moreover, the source results of Fe with the method developed in this study were compared with other source apportionment methods such as iron isotope analysis and model simulation, which including Community Multiscale Air Quality (CMAQ) model and Goddard Earth Observing System-Chemical transport (GEOS-Chem) model, to examine the difference among different methods.

Based on four cruises in the Northwest Pacific from 2015 to 2022, it was found that dust was the main source to total Fe in aerosol in this region, especially in the open ocean, with an average contribution over 70%. However, anthropogenic emission was dominant source of soluble Fe, with an average contribution over 80% in different sea areas and seasons. Among anthropogenic sources, the contribution of shipping source alone to soluble Fe in marginal seas could reach 61-77% in summer, indicating the potential importance of ship emissions, as land-based anthropogenic aerosol emissions are being controlled and reduced.

atmospheric iron, online measurement, source apportionment, anthropogenic emissions