## 2 Method and Materials
### TGM Analysis  
The analysis of trapped TGM by BAuT is described elsewhere [24]. Briefly, BAuT samples, including front and backup samples, were brought to the laboratory, and the captured TGM in each BAuT was pre-concentrated in a commercially available gold-coated sand trap. The pre-concentration was made by heating the BAuT to 873 K for 1 h under 0.5 mL min\({}^{-1}\) flow of mercury-free dried air, then the released gaseous elemental mercury  
Figure 3: Sampling setup for GEM and PM from Aso open grass field burning.  
Figure 2: Photos of “susuki” (_Miscanthus sinensis_, **left**) and “nezasa” (_Pleiblastus chino_ var. _viridis_, **right**).  
(GEM) was trapped again using the gold trap. The pre-concentration step was necessary for isotope analysis so that nearly all the captured TGM in the front BAuT was efficiently transferred into a 2L Tedler bag, and the TGM sampled using the conventional mercury trap and the mini-pump was analyzed for atmospheric concentration by a cold vapor-atomic fluorescent spectrometer (WA-5F, Nippon Instruments Corp.). This pre-concentration step was taken for the backup BAuT samples as well. The pre-concentrated mercury sample from the front BAuT was attached to a 2L Tedler bag with a single stopcock (AS ONE, Osaka, Japan), then heated to 873 K again to transfer the mercury into the bag. Prior to the transfer, 5 mL trapping acid solution (40% reversed aqua regia) was pre-loaded in the Tedler bag. After the transfer, the bag was left for a week to convert GEM to Hg\({}^{2+}\) in the solution, which was validated elsewhere [24]. The acid sample solution was then subjected to isotope measurement. Stable mercury isotope ratios were measured by a cold vapor generator (HGX-200, Teledyne CETAC Technologies, Omaha, NE, USA) coupled with a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS; Neptune Plus, Thermo Fisher Scientific GmbH, Bremen, Germany). Measurement artifacts (unfavorable isotope fractionations) occurring in ICP sections were corrected by analyzing the simultaneously introduced SRM 997 thallium aerosol produced by an aerosol generator (Aridus II, Teledyne CETAC Technologies). The delta expression of the stable mercury isotope ratio is defined as follows:  
\[\delta^{\mathrm{x}}\mathrm{Hg}\left(\%_{\mathrm{o}}\right)=\left[\frac{ \left(\frac{\mathrm{{}^{3}H_{\mathrm{H}}}}{\mathrm{{}^{18}H_{\mathrm{g}}}} \right)_{\mathrm{sample}}}{\left(\frac{\mathrm{{}^{3}H_{\mathrm{H}}}}{ \mathrm{{}^{8}H_{\mathrm{g}}}}\right)_{3133}}-1\right]\times 1000 \tag{1}\]  
where x stands for the stable mercury isotope with mass x, and the bracketed isotope ratios with subscripts \"sample\" and \"3133\" indicate the stable mercury isotope ratios of mass x relative to mass 198 for the sample and SRM 3133 (NIST), respectively.  
Overall recovery and isotope measurement tests were performed by introducing the GEM standard from the NIST 8610 standard reference material to the double BAuT setup (Supplementary Figure S1). The recovery tests were conducted with a carrier gas of mercury-free dried air during the GEM transfer from the BAuT to the conventional trap. Further recovery tests were conducted using GEM from a standard box (MB-1, Nippon Instruments Corp.) to find the difference in recovery yields using either mercury-free dried air or pure nitrogen (Fukuoka Sanso) during the GEM transfer.  
Pre-concentrated TGM from backup BAuTs and TGM samples collected with the conventional gold traps were quantitatively analyzed by the CV-AFS. The detection limit was on the order of sub-picograms, and the typical precision based on its calibration was 3% or better.