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

Soluble aerosol iron (Fe_S) 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 (PM_2.5). Using positive matrix factorization (PMF) analysis of PM_2.5 composition data (139 samples collected during the spring of 2017 and 2018) from Qingdao, we quantified the sources of total iron (Fe_T) and its water-soluble fraction (i.e., Fe_S) and the iron solubility (%Fe_S = Fe_S/Fe_T×100%) enhancement mechanisms. Our results revealed that while mineral dust contributed 71.1% of Fe_T, anthropogenic sources dominated Fe_S, accounting for 63.6% of Fe_S. During dust events, aged dust particles contributed about half of the Fe_S, with %Fe_S enhanced by up to 2.2-fold through atmospheric aging processes. In non-dust periods, anthropogenic sources accounted for 84.8% of Fe_S, with the %Fe_S mainly modulated by ship emissions, secondary aerosols, and aging of industrial particles. Notably, ship emissions exhibited exceptionally high %Fe_S, while secondary processes involving organic ligands and acid-processed industrial aerosols significantly enhanced %Fe_S. These findings quantitatively demonstrate the significance of anthropogenic air pollutants and secondary processes in Fe_S occurrence in Asian continental outflow, with important implications for understanding iron biogeochemical cycling in downwind marine ecosystems.