Abstract
Abstract. The increase in secondary species through cloud processing potentially increases aerosol iron (Fe) bioavailability. In this study, a ground-based counterflow virtual impactor coupled with a real-time single-particle aerosol mass spectrometer was used to characterize the formation of secondary species in Fe-containing cloud residues (dried cloud droplets) at a mountain site in southern China for nearly 1 month during the autumn of 2016. Fe-rich, Fe-dust, Fe-elemental carbon (Fe-EC), and Fe-vanadium (Fe-V) cloud residual types were obtained in this study. The Fe-rich particles, related to combustion sources, contributed 84 % (by number) to the Fe-containing cloud residues, and the Fe-dust particles represented 12 %. The remaining 4 % consisted of the Fe-EC and Fe-V particles. It was found that above 90 % (by number) of Fe-containing particles had already contained sulfate before cloud events, leading to no distinct change in number fraction (NF) of sulfate during cloud events. Cloud processing contributed to the enhanced NFs of nitrate, chloride, and oxalate in the Fe-containing cloud residues. However, the in-cloud formation of nitrate and chloride in the Fe-rich type was less obvious relative to the Fe-dust type. The increased NF of oxalate in the Fe-rich cloud residues was produced via aqueous oxidation of oxalate precursors (e.g., glyoxylate). Moreover, Fe-driven Fenton reactions likely increase the formation rate of aqueous-phase OH, improving the conversion of the precursors to oxalate in the Fe-rich cloud residues. During daytime, the decreased NF of oxalate in the Fe-rich cloud residues was supposed to be due to the photolysis of Fe-oxalate complexes. This work emphasizes the role of combustion Fe sources in participating in cloud processing and has important implications for evaluating Fe bioavailability from combustion sources during cloud processing.
Highlights
Iron (Fe)-containing particles were frequently detected in the atmosphere, with concentration ranging from 10 ng m−3 over remote marine environments to 28 μg m−3 near desert areas (Zhang et al, 2003; Fomba et al, 2013)
By manually combining similar clusters, four primary types of Fe-containing aerosols were obtained, including Fe-rich, Fe internally mixed with mineral dust species (Fe-dust), Fe internally mixed with elemental carbon (EC) (Fe-EC), and Fe internally mixed with V (Fe-V)
These findings indicate the contribution of cloud processing to the formation of oxalate precursors
Summary
Iron (Fe)-containing particles were frequently detected in the atmosphere, with concentration ranging from 10 ng m−3 over remote marine environments to 28 μg m−3 near desert areas (Zhang et al, 2003; Fomba et al, 2013). The chemical properties of the Fe-containing particles depend on its emission sources and could be modified by the formation of secondary species during atmospheric processes (Zhang et al, 2014; Dall’Osto et al, 2016; Lin et al, 2017). In European urban areas, Dall’Osto et al (2016) found that the Fe-containing particles were internally mixed with nitrate rather than sulfate and were most likely associated with urban traffic activities. The above observations about atmospheric processes of the Fe-containing particles were mainly performed in environments with low aerosol liquid water content. The impact of cloud processing on the formation of sulfate, nitrate, chloride, ammonium, oxalate precursors, and oxalate in various Fe-containing particle types was addressed
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