Diel temperature effects on the exchange of elemental mercury between the atmosphere and underlying waters

Ample scientific literature demonstrates that elemental mercury evasion from natural waters displays a diel cycle; evasion rates measured during the day are typically two to three times the values observed at night. The traditional explanation for this phenomenon is that water column elemental mercury concentrations display a diel cycle, with elemental mercury concentrations occurring at their highest values during the day. The present study tests the hypothesis that diel atmospheric temperature cycles also may play a significant role in diel mercury evasion rates. A chemical potential model is used to provide a thermodynamic framework for the development of nonisothermal Henry's law constants to describe atmospheric–aqueous partitioning of elemental mercury under temperature disequilibrium conditions. The effects of temperature disequilibrium on aqueous diffusive layer transport properties also are examined. Findings suggest that under one set of real‐world temperature disequilibrium conditions, diel evasion rate variations of up to 44% can be anticipated. Given the inability to consider the effects of temperature disequilibrium on aqueous diffusive layer thicknesses, the actual effects of temperature disequilibrium on rates of mercury evasion may exceed this value. Finally, the temperature disequilibrium phenomenon may be most significant for atmospheric–aqueous exchange of trace toxicants that do not experience dynamic environmental speciation behavior.