## 2 Materials and Method  
### Uptake Experiment  
Uptake experiments of GEM by bulk solutions were conducted using a rectangular glass vessel (50 cm length \(\times\) 10 cm width \(\times\) 5 cm depth, COSMOS VID, Fukuoka, Japan)under room lighting, temperature and pressure conditions. A three-eighths inch o.d. glass tubing was fused to the both ends of the vessel (Figure 1) so that a GEM standard gas could flow into and out of the vessel via the three-eighths inch tubing. Prior to use, the vessel was thoroughly cleaned in the following manner to reduce the background of mercury: 30% of inversed aqua regia was filled fully and left for at least 2 days, followed by rinsing with tap water at least 6 times and with Milli-Q water 6 times, and then drying with mercury-free air, which was prepared using compressed room air (super oil free BEBICON 0.4 LE-8 SB, Hitachi Industrial Equipment Systems Co., Ltd., Tokyo, Japan), a dryer stuffed with silica gel (Kanto Chemical Co., Inc., Tokyo, Japan) and a mercury trap stuffed with activated charcoal (Hokuetsu MA-HG, Ajinomoto Fine Techno Co., Inc., Yokohama, Japan). The background mercury in the mercury-free air, analyzed by the method utilizing conventional gold trapping and a cold vapor atomic fluorescent spectrometer or CV-AFS (WA-5F, Nippon Instruments Corp.), was 7 pg m\({}^{-3}\) or below. This whole cleaning procedure was repeated again using 20% inversed aqua regia and rinsing with tap and Milli-Q water at least 15 times prior to use for the uptake experiments. The uptake solutions used were either fresh pure water (ultrapure water, Kanto Chemical Co. Inc.), acidified water prepared by mixing 600 mL of fresh pure water with a drop of sulfuric (ACS grade, Nacalai Tesque Inc., Kyoto, Japan) or nitric acid (ACS grade, Kanto Chemical Co. Inc.) right before the experiments or sea water sampled from the coast of Akune city, Kagoshima, Japan (31.6\({}^{\circ}\) N, 130.1\({}^{\circ}\) E), which is located northeast of the East China Sea. The sea water was filtered in advance with 47 mm PTFE filters (Omnipore, 0.45 mm PTFE membrane, Merck Millipore Ltd., Watford, UK). It should be noted that the typical volumes of an acid droplet, according to the mass measurements using a mass balancer (XPES02, Mettler-Toledo GmbH, Greifensee, Switzerland), are \(9\pm 1\) mL and \(13\pm 1\) mL for sulfuric and nitric acid, respectively. The background concentrations of Hg\({}^{0}_{\text{aq}}\) and Hg\({}^{2+}_{\text{aq}}\) were 23 \(\pm\) 7 pg L\({}^{-1}\) and 106 \(\pm\) 66 pg L\({}^{-1}\) for the ultrapure water (\(n\) = 6, average \(\pm\) SD (standard deviation)) and 60 pg L\({}^{-1}\) and 81 pg L\({}^{-1}\) (\(n\) = 1) for the non-purged seawater, respectively. The seawater was purged for two days with mercury-free air for the reduction of background mercury prior to use for the uptake experiments. However, the background concentrations of Hg\({}^{0}_{\text{aq}}\) and Hg\({}^{2+}_{\text{aq}}\) in the purged seawater remained the same. The pH of the uptake solution was measured before and after the GEM exposure using a pH meter (D-71, HORIBA Advanced Techno Co. Ltd., Kyoto, Japan), which was calibrated every time it was used. The glass vessel, in which two PTFE-coated magnet stirrers were placed, was placed on a weighing balancer (KERN and SOHN GmbH, Balingen, Germany), and 500 g of the uptake solution was introduced gently into the glass vessel using a polypropylene long nose funnel (FWC220, AS ONE Co., Ltd., Osaka, Japan) so that the solution was filled from the bottom of the vessel. The vessel with the uptake  
Figure 1: Schematic of the uptake experiment of gaseous elemental mercury (GEM) by a bulk solution. solution was weighed before and after the uptake experiments to check the evaporation loss of the uptake solution during the experiments. The vessel was kept horizontal during the exposure.