Effects of Equilibrium and Non‐Equilibrium Adsorption States on the Photoreduction and Isotope Fractionation of Hg(II) in Urban Aerosols

Abstract Mercury (Hg) photoreduction on urban aerosols plays a critical role in atmospheric Hg cycling, yet the effects of the adsorption state on its kinetics and isotopic fractionation remain poorly constrained. This study investigates Hg(II) photoreduction under equilibrium (EAC) and non‐equilibrium adsorption conditions (NAC) using a flow‐through semi‐batch suspension reactor. NAC resulted in photoreduction rates double those under EAC, attributable to a higher fraction of readily reducible, soluble Hg(II). A two‐compartment kinetic model confirmed faster aqueous‐phase than solid‐phase reduction, with efficiency enhanced at higher relative humidity (RH). Pronounced mass‐dependent fractionation (MDF) occurred: Hg(0) was enriched in light isotopes (δ 202 Hg ≤ −2.02‰), while the remaining Hg(II) became increasingly enriched in heavy isotopes. MDF was stronger under NAC (ε 202 Hg = 1.55–2.02‰) than EAC (1.30‰ at 68% RH), consistent with adsorption experiments showing preferential retention of heavy isotopes in the aqueous phase under EAC (δ 202 Hg up to 0.75‰). Mass‐independent fractionation (MIF) signals indicated odd‐isotope enrichment in the remaining Hg(II) (Δ 199 Hg up to 0.87‰) and depletion in Hg(0), consistent with magnetic isotope effects. Critically, instantaneous isotope fractionation trends revealed kinetic control: MDF decreased as the reaction slowed, whereas MIF peaked mid‐reaction before declining, supporting the two‐compartment model. By coupling the adsorption state to photoreaction kinetics and isotopic fractionation, this study provides a mechanistic basis for understanding Hg redox processes in urban aerosols. These findings underscore that adsorption history and RH jointly regulate Hg(II) reactivity and isotopic signatures—essential for refining atmospheric Hg models, particularly in urban environments with dynamic aerosol and moisture conditions.