Atmospheric Chemistry and Physics | 2019
In-cloud formation of secondary species in iron-containing particles
Abstract
Abstract. The increase in secondary species through cloud processing potentially\nincreases aerosol iron (Fe) bioavailability. In this study, a ground-based\ncounterflow virtual impactor coupled with a real-time single-particle aerosol\nmass spectrometer was used to characterize the formation of secondary species\nin Fe-containing cloud residues (dried cloud droplets) at a mountain site in\nsouthern China for nearly 1 month during the autumn of 2016. Fe-rich,\nFe-dust, Fe-elemental carbon (Fe-EC), and Fe-vanadium (Fe-V) cloud residual\ntypes were obtained in this study. The Fe-rich particles, related to\ncombustion sources, contributed 84\u2009% (by number) to the Fe-containing\ncloud residues, and the Fe-dust particles represented 12\u2009%. The remaining\n4\u2009% consisted of the Fe-EC and Fe-V particles. It was found that above\n90\u2009% (by number) of Fe-containing particles had already contained sulfate\nbefore cloud events, leading to no distinct change in number fraction (NF) of\nsulfate during cloud events. Cloud processing contributed to the enhanced NFs\nof nitrate, chloride, and oxalate in the Fe-containing cloud residues.\nHowever, the in-cloud formation of nitrate and chloride in the Fe-rich type\nwas less obvious relative to the Fe-dust type. The increased NF of oxalate in\nthe Fe-rich cloud residues was produced via aqueous oxidation of oxalate\nprecursors (e.g., glyoxylate). Moreover, Fe-driven Fenton reactions likely\nincrease the formation rate of aqueous-phase OH, improving the conversion of\nthe precursors to oxalate in the Fe-rich cloud residues. During daytime, the\ndecreased NF of oxalate in the Fe-rich cloud residues was supposed to be due\nto the photolysis of Fe-oxalate complexes. This work emphasizes the role of\ncombustion Fe sources in participating in cloud processing and has important\nimplications for evaluating Fe bioavailability from combustion sources during\ncloud processing.