Seasonal variation of aerosol iron solubility in coarse and fine particles at an inland city in northwestern China

This work investigated seasonal variation of aerosol iron (Fe) solubility for coarse (>1 μm) and fine (<1 μm) particles at Xi’an, a megacity in northwestern China impacted by anthropogenic emission and desert dust. Total Fe concentrations were lowest in summer and similar in other seasons for coarse particles, while lowest in summer and highest in spring for fine particles; for comparison, dissolved Fe concentrations were higher in autumn and winter than spring and summer for coarse particles, while highest in winter and lowest in spring and summer for fine particles. Desert dust aerosol was always the major source of total Fe for both coarse and fine particles in all the four seasons, but it may not be the dominant source for dissolved Fe. Fe solubility was lowest in spring for both coarse and fine particles, and highest in winter for coarse particles and in autumn for fine particles. In general aerosol Fe solubility was found to be higher in air masses originating from local and nearby regions than those arriving from desert regions after long-distance transport. Compared to coarse particles, Fe solubility was similar for fine particles in spring but significantly higher in the other three seasons, and at a given aerosol pH range Fe solubility was always higher in fine particles. Aerosol Fe solubility was well correlated with relative abundance of aerosol acidic species, implying aerosol Fe solubility enhancement by acid processing; moreover, such correlations were better for coarse particles than fine particles in all the four seasons. Fe solubility was found to increase with relative humidity and acid acidity for both coarse and fine particles at Xi’an, underscoring the importance of aerosol liquid water and aerosol acidity in regulating Fe solubility via chemical processing.