## Materials and Methods
### Methodology Test  
We performed tests for the sampling and plastic bag extraction in laboratory and field studies. The BAuT performance was evaluated by the collection efficiency: TGM was sampled using two BAuTs connected in series (double BAuT sampling), and the collection efficiency was then determined by comparing the mass of TGM in the backup BAuT with that in the primary BAuT. During this evaluation, the concentration of the preconcentrated TGM in the backup BAuT was directly measured by CV-AFS, and that in the primary BAuT was measured by CV-MC-ICP-MS. The extraction tests were performed by introducing 103 ng GEM prepared from SRM 8610 into the Tedlar bag. The dependence of the recovery yields on different inversed aqua regia concentrations (20%-40%) and the extraction time (3-24 h) was evaluated. The concentrations were chosen based on the preliminary tests, which showed that the use of 5% inversed aqua regia required 7 days to yield 90% GEM and that concentrations higher than 40% resulted in lower signal intensity (see subsection S1 in the Supplementary Materials).  
The overall methodology was evaluated in source, air quality, and laboratory studies. TGM from the biomass burning was sampled on 24 March and 6 April 2019 during the Aso open field burning, also referred to as Noyaki (Figure 3). The air was drawn through a 47 mm (o.d.) Teflon-coated glass fiber filter (Pallflex Emfab, Pall Corporation, Port Washington, NY, USA) placed on an open-face filter holder (NL-O-01, NILU, Kjeller, Norway) to remove the soot. The electric power for the sampling pump was supplied from an uninterruptible power supply (SURTA1500 XLJ, APC, West Kingston, RI, USA). The background concentration of TGM in the regional air was also collected on 31 March and 23 May 2019. Moreover, one outdoor and two indoor air samplings were also conducted at the institute the author affiliates. The BAuT was also tested for sampling GEM in the gas mixture used in the laboratory experiment. The experimental conditions were similar to those reported in [17; 18]. Briefly, approximately 200 ng of GEM prepared from SRM 8610, oxidant precursors, and dried zero air were introduced into a 1.5 m\({}^{3}\) photochemical reactor. Then, 366 nm UV lamps (UV lamp 4, 8 W, CAMAG,  
Figure 2: Dependence of the accuracy of stable mercury isotope measurements of SRM 8610 on the concentration. “Offset” indicates the difference between the measured and reference isotope ratios (offset = measured-reference). Error bars refer to the propagated errors estimated from the uncertainty given for SRM 8610 by NIST and the standard errors from the 50 replicate measurements.  
Muttenz, Switzerland) were switched on and off to initiate and stop the reactions with OH radicals, one of the major sinking pathways of atmospheric GEM. After the reaction, GEM in the gas mixture was sampled using the BAuT through a KCl-coated eight-channel annular denuder (URG-2000-30 CF, URG Corporation, Chapel Hill, NC, USA), which removes GOM [19]. The sampling conditions are presented in Supplementary Table S1, together with the weather conditions. The preconcentration and extraction procedures were the same as those referred to previously.