Abundance and fractional solubility of aerosol iron during winter at a coastal city in northern China

Aerosol deposition is a major source of soluble Fe in open oceans, affecting marine biogeochemistry and primary production. However, Fe fractional solubility, a key parameter in estimating deposition fluxes of soluble aerosol Fe, is still highly uncertain. Abundance and fractional solubility of aerosol Fe in fine and coarse particles was measured at Qingdao (a coastal city in northern China) in November-December 2019. Average concentrations of total and soluble Fe were found to be 798±466 and 7.7±14.5 ng/m³ in coarse particles, and 801±534 and 7.3±7.6 ng/m³ in fine particles. Total Fe was well correlated with total Al for both fine and coarse particles, whereas soluble Fe was correlated with total Al for coarse particle but not for fine particles. Fe solubility was significantly lower in coarse particles (average: 0.80±1.03%) than fine particles (average 1.29±1.41%), and inverse relationship was observed between Fe solubility and total Fe concentration for fine particles but not for coarse particles. Compared to clean days, total Fe concentration was substantially increased during dust and haze days; however, Fe solubility was significantly reduced in dust days and elevated in haze days. Primary emission and secondary formation both contributed significantly to enhanced Fe solubility for both fine and coarse particles. Higher Fe solubility (>1%) in fine and coarse particles was usually observed at high aerosol acidity (pH<4) and high RH (>60%), suggesting critical roles of aerosol acidity and RH in regulating aerosol Fe solubility.