Photoreduction and origin of dissolved and particulate mercury in cloud water: Insights from stable mercury isotopes

The photoreduction of mercury (Hg) in clouds is crucial for determining global Hg cycling. The recently-developed isotope approach provides new insight into the fate of atmospheric Hg, however, limited data have been reported on the dynamics of Hg isotopes in clouds. This study presented the isotopic compositions of dissolved mercury (DHg) and particulate mercury (PHg) in cloud water collected at Mt. Tai (1545 m a.s.l.) in eastern China during summer 2021. Both DHg and PHg exhibited positive mass-independent fractionation of odd isotopes (odd-MIF, denoted as Δ¹⁹⁹Hg), with averaged Δ¹⁹⁹Hg values of 0.83 ± 0.34‰ and 0.20 ± 0.11‰, respectively. This high odd-MIF likely resulted from aqueous photoreduction in clouds, with DHg being more susceptible to photolysis than PHg. Our findings indicated that the photoreduction was promoted by sunlight and influenced by the chemical compositions of cloud water that controlled the Hg(II) speciation. The isotope mixing model estimation revealed that particulate-bound Hg and reactive gaseous Hg constituted the principal sources of Hg in cloud water, accounting for 55% to 99% of the total, while gaseous element Hg also made a notable contribution. Additionally, cloud water samples with faster reduction rates of Hg(II) were located outside of the isotope mixing models, which indicated an enhanced photoreduction process in cloud water.