Elevated methylmercury in inland fog and rain linked to anthropogenic emissions and dust

Fog and rain play crucial roles in the regional biogeochemical cycles of mercury (Hg) as they link atmosphere inputs with terrestrial and aquatic ecosystems. Methylmercury (MeHg), which composes a small fraction of total Hg (THg) in the atmosphere, is easily overlooked considering the uncertainties of its formation and role as an input to inland areas. We quantified MeHg species and their drivers in fog and rain from inland China. Elevated MeHg is observed in inland urban fog and rain, with the fraction of THg as MeHg higher than values from remote areas. Particulate Hg (PHg) is the dominant form (>50%) of THg, and dissolved MeHg (DMeHg) is the dominant form of total MeHg in inland fog. Chemical equilibrium modeling simulations suggest that Cl^- complexes dominates MeHg speciation, which are highly pH dependent. Continuous observations of remote fog show that MeHg species are altered by THg concentrations. A positive correlation of MeHg/THg with the dissolved Hg (DHg) fraction of THg is observed. MeHg shows varied correlations with organic acids (formate, acetate and oxalate) and secondary ions (NH₄⁺, NO₃^- and SO₄^2-) in inland fog and rain, suggesting diverse sources of MeHg. We find that anthropogenic emissions and dust are dominant drivers of MeHg in inland fog, while methyl donors (e.g., low molecular weight organic acids) play more important roles in rainwater. As MeHg photodemethylation can occur in atmospheric waters, elevated MeHg and its diverse drivers in fog and rain suggest that further investigation of atmospheric MeHg species should be conducted in inland areas.