## 2 Materials and Method
### Extraction for Dissolved Gaseous and Oxidized Mercury  
To better understand the process of GEM uptake, it is critical to separately extract \(\text{Hg}^{0}_{\text{aq}}\) and \(\text{Hg}^{2+}_{\text{aq}}\) in a solution. We used a conventional purging method with an inertial gas, nitrogen, to extract \(\text{Hg}^{0}_{\text{aq}}\) and \(\text{Hg}^{2+}_{\text{aq}}\) separately from the same solution [24; 25]. First, a sample solution in a 375 mL PFA impinger was purged with mercury-free nitrogen (nitrogen gas passed through a gold mercury trap) via a glass bubbler (G-3, COSMOS VID) at a flow rate of 0.5 L min\({}^{-1}\) for a given period of time. The \(\text{Hg}^{0}_{\text{aq}}\) in the solution was purged out and captured by a conventional gold trap (Nippon Instruments Corp.) through a soda lime that dried the outflow gas. After this extraction, a 50 cm long piece of one-quarter inch o.d. PFA tubing (Tombo, 9003-PFA, NICHIAS Corp, Tokyo, Japan) was connected to the impinger. Then, 5 mL of a 20% (\(w/w\)) tin chloride (99% purity, Kanto Chemical Co., Inc.) solution in a 10% hydrochloric acid (Kanto Chemical Co., Inc.) solution was spiked to the residual solution in the impinger via the PFA tubing and purged with mercury-free nitrogen. The \(\text{Hg}^{2+}_{\text{aq}}\) in the solution was reduced to \(\text{Hg}^{0}_{\text{aq}}\) by \(\text{Sn}^{2+}\), purged out from the solution and captured in the same manner as in the extraction of \(\text{Hg}^{0}_{\text{aq}}\). The trapped mercury was quantitatively analyzed by the CV-AFS. It should be noted that after the extraction, the glassware and containers needed to be thoroughly cleaned for the next extraction. In particular, extra care was needed not only to remove residual mercury, but to quench the residual \(\text{Sn}^{2+}\), which is a fast-reducing reagent of \(\text{Hg}^{2+}_{\text{aq}}\), but it also turned out to be a stubborn contaminant, interfering with the analysis of \(\text{Hg}^{0}_{\text{aq}}\) and \(\text{Hg}^{2+}_{\text{aq}}\) in a solution. Our cleaning procedure was as follows: the impingers and gas bubblers were soaked in a 30% (\(v/v\)) inversed aqua regia mixture for a week, rinsed thoroughly with tap water 15 times, followed by rinsing with Milli-Q water 15 times and, finally, drying in a drying oven set to 353 K (DG400, Yamato Scientific Co. Ltd., Tokyo, Japan) for at least two days. This cleaning procedure was performed twice prior to use for the next extraction. For optimization of the purging time, we used an SRM 8610 Hg\({}^{2+}\) standard solution (NIST, Gaithersburg, MD, USA). It should be noted that the reference mercury concentration of this standard material was analyzed and provided by NIST, but the value was not certified. In the impinger, 145-310 pg of Hg\({}^{2+}\) was spiked to 290 mL pure water. Then, Hg\({}^{2+}\)aq was extracted in the same manner described above with a variety of purging times. This test confirmed the unfavorable reduction of Hg\({}^{2+}\)aq by just purging the solution with the mercury-fee air, followed by the recovery yields of Hg\({}^{2+}\)aq using the tin chloride reduction method. Unfortunately, we could not directly evaluate the recovery yields for Hg\({}^{0}\)aq because there was no standard solution available for Hg\({}^{0}\)aq. However, we assumed that the optimum purging time for Hg\({}^{0}\)aq corresponded to that of Hg\({}^{2+}\)aq. This is because Sn\({}^{2+}\) very effectively and quickly converts Hg\({}^{2+}\) to Hg\({}^{0}\)aq and, therefore, the extraction or purging time for Hg\({}^{2+}\)aq, optimized using the SRM 8610, was dependent on the extraction efficiency of Hg\({}^{0}\)aq reduced from Hg\({}^{2+}\)aq by Sn\({}^{2+}\). For the purpose of quality control, a number of sample blanks were taken in the following manner: A quantity of 300 mL of the ultrapure water or filtered and purged seawater was poured into the cleaned 2.5 L glass vessel, and the vessel was thoroughly rinsed.