###### Abstract  
This is the first report investigating the transformation of gaseous elemental mercury (GEM), the major form of airborne mercury, into oxidized mercury in bulk liquid, a possible sinking pathway of atmospheric GEM in clouds, fog, rain droplets and ocean spray. A 100-150 ng m\({}^{-3}\) GEM standard gas, a 50-150 times higher concentration than the typical atmospheric concentration, was introduced into a 2.5 L rectangular glass vessel, at the bottom of which a 0.5 L uptake solution of pure water (pH 6-7), weakly acidified pure water with sulfuric or nitric acid (pH 3.2-3.6) or seawater (pH 8) was resting. The standard gas was introduced into the space above the solution in the vessel at the rate of 0.82 L min\({}^{-1}\) and exited from the opposite end of the vessel, which was open to the room's pressure. After exposing the solution to the gas for 0.5-4 h, a portion of the uptake solution was sampled, and the dissolved elemental mercury (Hg\({}^{0}\)aq) and dissolved oxidized mercury (Hg\({}^{2+}\)aq) in the solution were analyzed by the conventional trapping method, followed by cold vapor atomic fluorescent spectrometer measurements. The results showed that the quantities of total dissolved mercury (THg\({}_{\text{aq}}=\text{Hg}^{0}\)aq + Hg\({}^{2+}\)aq) in the pure water and seawater were compatible, but those were slightly lower than the equilibrated Hg\({}^{0}\)aq concentrations estimated from Henry's law, suggesting non-equilibrium throughout the whole solution. In contrast, the quantity of Hg\({}^{2+}\)aq and THg\({}_{\text{aq}}\) in the acidified pure water with sulfuric acid was significantly enhanced. Over the 4 h exposure, the THg\({}_{\text{aq}}\) concentrations were two times higher than the equilibrated Hg\({}^{0}\)aq concentration. This was due to the slow oxidation reaction of Hg\({}^{0}\)aq by the sulfuric acid in the bulk phase. Using the collision rate of GEM with the surface of the solution and the observed uptake, the estimated uptake coefficient of GEM with this uptake was (\(5.5\pm 1.6\)) \(\times\) 10\({}^{-6}\). Under the typical atmospheric concentration, this magnitude results in an atmospheric lifetime of 4970 years, negligibly small compared with other atmospheric oxidation processes. gaseous elemental mercury; dissolved elemental mercury; dissolved oxidized mercury; heterogeneous uptake; acid cloud and fog droplet; acid rain; oxidation of mercury +
Footnote †: journal: Applied Sciences  
GEM [5], comprising 81-96% of atmospheric mercury [6], which is sparingly reactive and soluble to water [7; 8; 9]. Once GEM is oxidized and transformed to GOM, it is adsorbed and absorbed by either atmospheric droplets and particulate matter or aerosols and deposits on the Earth's surface, consequently being efficiently scavenged from the atmosphere. Thus, the oxidation reaction of GEM in the gas phase is a key step to determining the fate of atmospheric GEM [10]. Based on the rate laws of GEM oxidation reactions with the major atmospheric oxidants, such as OH radicals, ozone and bromine radicals [11], the estimated atmospheric lifetime of GEM is half a year to one year [12]. However, there are unidentified processes that may significantly affect the fate of atmospheric mercury [13]. For example, [PERSON] et al. [14] reported the comparison of their model studies with the results from real observations for atmospheric mercury concentrations, and they found a discrepancy between the model and the observations; the actual concentration decrease of atmospheric mercury was faster than the estimated rate. They concluded that this could imply an unidentified chemical reaction or reactions involved, possibly the reaction with the chlorine radical in the marine atmosphere. Among the atmospheric mercury oxidations thought to occur in the natural environment [13; 15], there are more unevaluated processes for GEM, and the reactive heterogeneous uptake by acid water is one of them.