Anthropogenic Dominance and Secondary Processes Drive Aerosol Iron Solubility in Asian Continental Outflow: Insights from Spring Qingdao, China

Soluble aerosol iron (FeS) plays a critical role in marine primary productivity, yet the relative contributions of primary emissions versus secondary atmospheric processes to its abundance remain poorly quantified. Qingdao is a coastal city in eastern China, located in the Asian continental outflow, where iron constitutes one of the major mineral species in fine particles (PM2.5). Using positive matrix factorization (PMF) analysis of PM2.5 composition data (139 samples collected during the spring of 2017 and 2018) from Qingdao, we quantified the sources of total iron (FeT) and its water-soluble fraction (i.e., FeS) and the iron solubility (%FeS = FeS/FeT×100%) enhancement mechanisms. Our results revealed that while mineral dust contributed 71.1% of FeT, anthropogenic sources dominated FeS, accounting for 63.6% of FeS. During dust events, aged dust particles contributed about half of the FeS, with %FeS enhanced by up to 2.2-fold through atmospheric aging processes. In non-dust periods, anthropogenic sources accounted for 84.8% of FeS, with the %FeS mainly modulated by ship emissions, secondary aerosols, and aging of industrial particles. Notably, ship emissions exhibited exceptionally high %FeS, while secondary processes involving organic ligands and acid-processed industrial aerosols significantly enhanced %FeS. These findings quantitatively demonstrate the significance of anthropogenic air pollutants and secondary processes in FeS occurrence in Asian continental outflow, with important implications for understanding iron biogeochemical cycling in downwind marine ecosystems.