# Automated Stable Isotope Sampling of Gaseous Elemental Mercury (ISO-GEM): Insights into GEM Emissions from Building Surfaces

Martin Jiskra, Nicolas Marusczak, Kin-Hung Leung, Lucas Hawkins, Eric Prestbo, Jeroen Sonke

► To cite this version:

Martin Jiskra, Nicolas Marusczak, Kin-Hung Leung, Lucas Hawkins, Eric Prestbo, et al. Automated Stable Isotope Sampling of Gaseous Elemental Mercury (ISO-GEM): Insights into GEM Emissions from Building Surfaces. Environmental Science and Technology, 2019, 53 (8), pp.4346-4354. 10.1021/acs.est.8b06381. hal-04843843

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# Automated stable isotope sampling of gaseous elemental mercury (ISO-GEM) – Insights into GEM emissions from building surfaces

Martin Jiskra, Nicolas Marusczak, Kin-Hung Leung, Lucas Hawkins, Eric M. Prestbo, and Jeroen E. Sonke

*Environ. Sci. Technol.*, **Just Accepted Manuscript** • DOI: 10.1021/acs.est.8b06381 • Publication Date (Web): 22 Mar 2019

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This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in *Environmental Science & Technology*, copyright © American Chemical Society after peer review. To access the final edited and published work see: https://pubs.acs.org/doi/10.1021/acs.est.8b06381
1 Automated stable isotope sampling of gaseous elemental mercury

2 (ISO-GEM) – Insights into GEM emissions from building surfaces.

3

4 Martin Jiskra¹,²*, Nicolas Marusczak¹, Kin-Hung Leung³, Lucas Hawkins⁴, Eric Prestbo⁴,

5 Jeroen E. Sonke¹,*

6

7 ¹Observatoire Midi-Pyrénées, Laboratoire Géosciences Environnement Toulouse,

8 CNRS/IRD/Université de Toulouse, 31400 Toulouse, France

9 ²Environmental Geosciences, University of Basel, 4056 Basel, Switzerland

10 ³Tekran Instruments Corp., M1P 2P4 Toronto, ON, Canada

11 ⁴Tekran Instruments Corp., 98125 Seattle, WA, USA

12

13 *martin.jiskra@unibas.ch, jeroen.sonke@get OBS-MIP.fr

14
15 **Abstract**

16 Atmospheric monitoring networks quantify gaseous elemental mercury (GEM)
17 concentrations, but not isotopic composition. Here, we present a new method for
18 automated and quantitative stable isotope sampling of GEM (ISO-GEM) at the outlet of a
19 commercial Hg analyzer. A programmable multi-valve manifold selects Hg at the analyzer
20 inlet and outlet based on specific criteria (location, time, GEM concentration, auxiliary
21 threshold). Outlet Hg recovery was tested for gold traps, oxidizing acidic solution traps,
22 and activated carbon traps. We illustrate the ISO-GEM method in an exploratory study on
23 the effect of building walls on local GEM. We find that GEM concentrations directly at the
24 building surface (wall inlet) are significantly enhanced (mean 3.8 ± 1.8 ng/m³) compared
25 to 3 m from the building wall (free inlet) (mean 1.5 ± 0.4 ng/m³). GEM δ²⁰²Hg (-1.26 ± 0.41
26 ‰, 1sd, n=16) and Δ¹⁹⁹Hg (-0.05 ± 0.10 ‰, 1sd, n=16) at the wall inlet were different
27 from ambient GEM δ²⁰²Hg (0.76 ± 0.09 ‰, 1sd, n=16) and Δ¹⁹⁹Hg (-0.21 ± 0.05 ‰, 1sd,
28 n=16) at the free inlet. The isotopic fingerprint of GEM at the wall inlet suggests that GEM
29 emission from the aluminum building surface affected local GEM concentration
30 measurements. These results illustrate the versatility of the automated Hg isotope
31 sampling.
## Introduction

Mercury (Hg) is a global pollutant that is predominantly emitted to- and transported through the atmosphere as gaseous elemental mercury (Hg⁰, GEM).¹, ² Atmospheric dry deposition by vegetation uptake of GEM,³ or wet deposition as Hg¹¹ in rainfall and snowfall represent the dominant Hg source to aquatic and terrestrial ecosystems.² Local GEM concentrations are controlled by primary emission, deposition, in situ production from Hg(II) and re-emission processes.³ Understanding the sources and processes that affect atmospheric GEM is essential to predict future ecosystem exposure to Hg. Current understanding of GEM dynamics and ecosystem loading is mainly based on concentration data from global Hg monitoring networks, but does not include source/process specific molecular or isotopic tracers.

Hg stable isotope measurements of GEM (which we use here interchangeably with TGM, total gaseous Hg) are a new tool to identify GEM emission sources and transformation processes and thus better understand atmospheric GEM dynamics.⁴⁻⁹ Hg currently has 5 useful isotope signatures that represent mass dependent isotope fractionation (MDF: δ²⁰²Hg) and odd and even mass independent isotope fractionation (MIF: Δ¹⁹⁹Hg, Δ²⁰⁰Hg, Δ²⁰¹Hg, Δ²⁰⁴Hg). GEM sampled at terrestrial background sites far from anthropogenic emission sources is characterized by positive δ²⁰²Hg values and slightly negative Δ¹⁹⁹Hg values.⁵⁻⁷, ¹⁰, ¹¹ Air affected by recent anthropogenic emissions exhibits mostly negative δ²⁰²Hg values and Δ¹⁹⁹Hg values around zero, ranging from 0.2 to -0.2 ‰.⁴, ⁵, ⁸, ⁹, ¹² These values are similar to the Hg isotope composition of coal,¹³⁻¹⁵ the dominant source of anthropogenic Hg emissions, illustrating the potential of Hg stable isotopes to fingerprint Hg sources.

GEM isotope signatures are very different from Hg¹¹ in wet deposition, which has strongly positive Δ¹⁹⁹Hg and Δ²⁰⁰Hg.⁷, ¹¹ Studies show that vegetation and soil Hg have
57 $\Delta^{199}$Hg and $\Delta^{200}$Hg values that resemble more closely the isotope signatures of GEM than
58 those of wet deposition.$^{7, 10, 11, 16, 17}$ Hg isotope mass balance of global soils and vegetation
59 data suggest GEM plant uptake to be 3-4x larger than Hg wet deposition.$^{7}$ This has led to
60 a re-appraisal of the foliar GEM uptake mechanism and the idea that the ‘vegetation Hg
61 pump’ controls diurnal and seasonal GEM dynamics over terrestrial surfaces.$^{3}$

62 Precise GEM isotope analysis, with 2σ uncertainty <0.1 ‰, requires 1000-fold
63 more Hg (10 nanograms) than GEM concentration analysis (10 picograms). An additional
64 challenge is that GEM isotope sampling recovery must be near-quantitative, to avoid
65 isotope fractionation artifacts during sampling. Manual GEM isotope sampling methods
66 thus far have used classical Hg sorbents, such as gold$^{4, 5, 11, 18}$ or various activated carbon
67 powders,$^{6, 7, 10, 19, 20}$ and accumulated GEM over periods of 1 – 30 days. Such manual
68 sampling complicates the study of short-lived (hours) pollution or chemical reactivity
69 events, diurnal GEM dynamics, or spatial Hg isotope gradients. In addition, manual
70 sampling only makes marginal use of atmospheric Hg monitoring network infrastructure.

71 The broader goal of this study was to design an automated stable isotope sampling
72 application for GEM (ISO-GEM) able to resolve pollution or transformation events. The
73 application should direct GEM isotopes onto multiple traps, based on predefined criteria,
74 such as time (diurnal, other), space (multiple inlets), meteorology (temperature,
75 humidity, wind direction) or pollution events (Hg or CO concentration triggers). The
76 device ideally had to be compatible with the most common Hg analyzer used in Hg
77 monitoring networks. More specifically, we focused on recovering analyzed Hg at the
78 instrument detection cell outlet which has the advantage that Hg isotope recovery
79 efficiency can be assessed from the GEM monitoring data. We tested different Hg trapping
80 materials for GEM isotope sampling, and illustrate re-emission of GEM from building
81 surfaces using the novel ISO-GEM sampling device.
82

83

## Materials & Methods

84

### Instrumentation

85

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The Tekran® 2537X was developed in 2012 and expanded electronic command and control capabilities compared to earlier 2537 Models. The enhanced communication capabilities and modular firmware design provides the ability to develop research-specific tools such as the ISO-GEM application used in this research. The 2537X analyzer also provides remote controlled access to its operating software and firmware plug-in modules. The 2537X analyzer pumps ambient air at 1.5 L/min over a gold cartridge where the GEM is amalgamated and quantitatively trapped. During each 5 min collection cycle approx. 11 pg GEM are trapped on a gold cartridge if ambient air contains ~1.5 ng/m³ GEM.²¹ The trapped GEM is then thermo-desorbed and analyzed by atomic fluorescence spectroscopy (AFS) in high purity, dry argon (80 mL/min). After analysis the GEM is exhausted by the cell vent at the back of the 2537X, where it is re-captured for Hg stable isotope analysis. A dual pair of gold cartridges allows continuous measurement, one cartridge collects GEM while the second one is analyzed. With the added capabilities of the 2537X, Tekran designed and manufactured a programmable multi-port sampling system (Tekran® 1115i) specifically adapted for automated GEM isotope sampling (ISO-GEM). The firmware plug-in controls the function of the Tekran® 1115i multi-valve manifold, determining both how the 2537X analyzer collects sample and the post-analysis flow path of the mercury measured by the instrument. The 1115i plug-in controls all valve positions, which are synchronized with the 2537X 5-min analysis cycles. The 1115i multi-valve manifold was outfitted with 3 large 3-way valves (NResearch 648T032 with PFA 1/4” connectors), and 5 small 3-way valves (NResearch 225T032 with PFA 1/8” connectors). The 3 large valves permit regular flow rate (1.5 L min⁻¹) sampling of GEM at
up to 4 different physical locations (e.g. gradient, inside vs. outside a building, etc.). The 5 smaller valves are used to direct exhausted Hg at the 2537X cell vent, in a high-purity, dry argon flow rate of 80 mL min⁻¹, to the dedicated Hg isotope traps. Inlet lines to the 2532X Tekran consisted of 1/4" PFA tube and exhaust lines consisted of 1/8" PFA tube. The original brass cell vent of the Tekran® 2537X was replaced by a PTFE cell vent to prevent potential sorption of GEM.

Atmospheric Temperatures were obtained for Toulouse-Blagnac (43.62°N, 1.39°E) close to the GET Laboratory building through the French Meteo services (www.infoclimat.fr).

## Hg trapping methods

Two trapping methods have been previously used to collect nanogram quantities of GEM for isotopic analysis: (i) amalgamation of GEM on gold coated quartz beads,⁴, ⁵, ¹¹, ¹⁸ and (ii) activated carbon traps impregnated with halogens (Cl, I)⁶, ⁷, ¹⁰, ¹⁹, ²⁰ or sulfur.²², ²³ A third method that is commonly used to trap GEM, not during ambient GEM sampling, but as part of laboratory pre-concentration methods, consists of purging GEM through a strongly oxidizing, acidic aqueous solution.¹³, ²⁴ There are important differences between direct manual sampling of GEM onto gold or activated carbon, and the ISO-GEM application: 1. Direct sampling is done in air at 1-2 L min⁻¹ flow rates, while 2537X cell vent trapping is done in low flow (80 mL/min) high purity, dry argon. 2. Direct sampling onto multiple, parallel gold traps is done for at most 1-3 days, while 2537X cell vent trapping in ISO-GEM applications will typically last 1-6 weeks in line with routine instrument maintenance schedules. The ISO-GEM trapping method therefore has to be robust and free of Hg breakthrough over prolonged sampling times. We tested several GEM trapping methods, namely gold-quartz traps, acid-traps and activated carbon traps impregnated with iodine (I-AC) and sulfur (HGR-AC). As the goal of this project was to
132 develop an optimal ISO-GEM system, we chose an iterative development scheme, which
133 may result in the fact that not all conditions have been tested for all trapping materials.

134 Gold coated quartz bead traps (Ref. # 35-26510-00), and solid gold matrix traps
135 (Ref. # 35-25500-00) were obtained from Tekran® Instruments. Iodated activated
136 carbon powder (I-AC) was obtained from Brooks Rand. Sulfur impregnated activated
137 carbon (Calgon HGR-AC, product no 2300, Calgon Carbon Corp.) was provided by F.
138 Wania at the University of Toronto at Scarborough. I-AC (125mg) and HGR-AC (400 mg)
139 powder was weighed and traps were prepared manually in 12cm long Pyrex tubes (7
140 mm OD, 4 mm ID); the powder was held in place by a diameter constriction of the tube
141 and two quartz wool plugs. Halogen sorbent traps, used to eliminate volatile iodine,
142 were obtained in the form of LECO AMA-254 catalyst tubes (part No. 614-822-105) from
143 Symalab (France) and manually prolonged by 20 cm. Bi-distilled HNO₃ and HCl was
144 produced in-house, and high purity oxygen and ultra high purity dry (UHP) argon gas
145 was obtained from Air Products, France.

146 Hg concentration analysis

147 All analyses were performed at the temperature controlled (21±1°C) Geoscience
148 Environnement Toulouse laboratory. Both gold bead and gold matrix traps were
149 desorbed and analyzed by dual amalgamation atomic fluorescence spectrometry (AFS,
150 Brooks Rand Model III, USA). The AFS was calibrated using manual GEM vapor injections
151 from an in-house thermostatted liquid/vapor Hg source.

152 I-AC and HGR-AC traps were processed using protocols adapted from Fu et al.
153 2014.¹⁹ The method is based on dual-tube furnace combustion in an AMA-254 halogen
154 trap. I-AC and HGR-AC was combusted in a first oven under a continuous flow of Hg-free
155 O₂ (75 mL/min) over 6h by ramping temperature to 680°C (ramp 1: 5°C/min to 120°C,
ramp 2: 1°C/min to 250°C, ramp 3: 2°C to 680°C). The I-AC and HGR-AC powders were placed in 10 cm quartz tubes closed with quartz wool. To increase the life-time of the catalyst tube and to reduce wall sorption of GEM the activated carbon traps were embedded with 1g of additional trapping material on each side (Figure S1). For I-AC a halogen scrubber (mixture of MnO₂ (70% w/w), CoO (20 % w/w) and CaO (10% w/w, pre-baked at 680°C for 2h) was used following the SDS 06112 of the AMA-245 (Leco) and for HGR-AC sodium bicarbonate (Na₂CO₃, pre-baked at 500°C for 2h) was used following McLagan et al. 2017.²⁵ The second oven housed the halogen scrubber section, which was constantly kept at 680°C. After the combustion oven GEM was introduced into a 8 mL oxidizing solution trap consisting of 40 volume% inverse aqua regia (iAR, 4.2 N HNO₃, 1.2 N HCl) in a 15mL Falcon tube. A custom-made type#3 (16-40 µm) porosity glass bubbler tube was used to generate small diameter bubbles.

Total Hg concentrations in 40vol%iAR solutions were measured after dilution following the USEPA 1631 method.²⁶ Aliquots of 0.2 - 2 mL were analyzed in duplicate on a semi-automatic cold vapor atomic fluorescence spectrometry (CV-AFS, Brooks Rand Model III, USA) using a single gold trap.

Hg levels recovered from different traps and measured by AFS were compared to GEM measurements of a Tekran® 2537X analyzer to determine sampling yields. The analyzer was regularly (23h) calibrated using its internal GEM permeation source, and was monthly calibrated by manual GEM vapor injections from the external GEM vapor source. All manual calibrations agreed within 5% of the permeation source calibration over the 2 years of research.

Blank levels of the I-AC and HGR-AC powders were measured using a DMA-80 (Milestone) mercury analyzer.

180
<table><caption>Table 1: Summary of the performance of different tested trapping materials for Hg stable isotope sampling. *Samples collected at the exit of the Tekran 2537X cell vent in an ultra high purity, dry (UHP) argon matrix. This does not represent trapping efficiency in ambient air. BDL stands for below detection limit, numbers in brackets represent range. **including data reported in Ref. 10</caption><thead><tr><th>Trap material</th><th>gold-trap*</th><th>acid-trap*</th><th>I-AC</th><th>HGR-AC</th></tr></thead><tbody><tr><td>description</td><td>gold surface or sand beads coated with gold</td><td>40% inverse Aqua regia</td><td>Iodine impregnated (10% w/w) activated carbon</td><td>Sulfur impregnated activated carbon</td></tr><tr><td>Matrix</td><td>UHP Argon</td><td>UHP Argon</td><td>ambient air</td><td>ambient air</td></tr><tr><td>trap flow (mL/min)</td><td>80</td><td>80</td><td>~300</td><td>~300</td></tr><tr><td>Time sampled</td><td>1-70 h</td><td>24-210 h</td><td>48 - 62 d</td><td>48 - 62 d</td></tr><tr><td>Hg amount (ng)</td><td>0.2 - 12</td><td>6-13</td><td>11-32</td><td>21-39</td></tr><tr><td>Recovery (%)</td><td>66% ± 27% (n=15)</td><td>94% ± 9% (n=13)</td><td>97 ± 39% (n=10)**</td><td>95% ± 4% (n=4)</td></tr><tr><td>Breakthrough</td><td>11% (2-19, n=12)</td><td>&lt;2% (n=1)</td><td>BDL</td><td>BDL</td></tr><tr><td>Blank (ng/trap)</td><td>BDL</td><td>0.1 ± 0.04</td><td>0.32 ± 0.1</td><td>0.06 ± 0.03</td></tr><tr><td>advantage</td><td>Simple handling and good recovery for short sampling periods (&lt;16h)</td><td>Direct analysis of the trap by MC-ICPMS</td><td>Good performance</td><td>Very good performance</td></tr><tr><td>disadvantage</td><td>Passivation of gold traps at sampling times exceeding</td><td>i) Handling acids requires training</td><td>i) Additional oven</td><td>(i) Additional oven</td></tr></tbody></table>
<table><tr><td rowspan="3">16h (at 1.5 L/min<br/>sampling rate)</td><td>and safety<br/>measures.</td><td>combustion<br/>step</td><td>combustion<br/>step</td></tr><tr><td>ii) Acid trap<br/>might lead to an<br/>increase in pressure<br/>in the Tekran<br/>measurement cell</td><td>(ii) Matrix effects<br/>possible on the<br/>MC-ICPMS if<br/>the halogen<br/>scrubber did<br/>not work</td><td></td></tr><tr><td>iii) Needs cooling<br/>for longer trapping<br/>periods</td><td>efficiently</td><td></td></tr></table>

186

187

## Hg stable isotope analysis

189 Hg isotope ratios were analyzed by cold vapor multi-collector inductively coupled plasma 190 mass spectrometry (MC-ICP-MS, Thermo-Finnigan Neptune) following published 191 protocols.²⁴ Hg isotope composition is reported in delta notation (δ) in per mil (‰) by 192 referencing to the bracketed NIST 3133 Hg standard:

$$ \delta^{\text{xxx}}\text{Hg}=(\text{xxx}/198\text{Hg}_{\text{sample}}/\text{xxx}/198\text{Hg}_{\text{NIST3133}}-1) \times 10^3 \quad (1) $$

193 where ‘xxx’ refers to measured isotope masses: 199, 200, 201, 202 and 204. MIF is 194 reported in capital delta (Δ) notation (‰), which is defined as the difference between the 195 measured δ¹⁹⁹Hg, δ²⁰⁰Hg, δ²⁰¹Hg and δ²⁰⁴Hg and those predicted from δ²⁰²Hg using the 196 kinetic MDF law:

$$ \Delta^{\text{xxx}}\text{Hg}= \delta^{\text{xxx}}\text{Hg} - \beta_{\text{xxx}} \times \delta^{202}\text{Hg} \quad (2) $$

198 where the mass-dependent scaling factor βxxx is 0.252 for ¹⁹⁹Hg, 0.502 for ²⁰⁰Hg, 0.752 for ²⁰¹Hg and 1.493 for ²⁰⁴Hg. The long-term uncertainty was evaluated by repeated 200 measurement of the ETH-Fluka Hg standard, which yielded values of -1.45±0.19‰, ²⁰¹ 0.08±0.09‰, 0.02±0.09‰, 0.03±0.09‰, -0.03±0.2‰ (2σ, n=10) for δ²⁰²Hg, Δ¹⁹⁹Hg, ²⁰²
203 $\Delta^{200}\text{Hg}$, $\Delta^{201}\text{Hg}$ and $\Delta^{204}\text{Hg}$, respectively, in agreement with the published values<sup>17,27</sup>. The
204 2$\sigma$ uncertainties of isotope compositions for ETH-Fluka were taken as the typical analytic
205 uncertainties of isotope compositions for samples. If the 2$\sigma$ uncertainties of isotope
206 compositions for samples with multiple measurements were larger than the typical 2$\sigma$
207 uncertainties, then the 2$\sigma$ uncertainties of samples applied.

208

209 **Results & Discussion**

210 *Cell vent trapping efficiency on gold in argon*

211 Initially gold bead traps were directly connected to the 2537X cell vent using 1/8"
212 FEP tubing, and without presence of the 1115i valve module. GEM from the cell vent was
213 loaded onto the traps for time periods ranging from 1 to 70 hours. Two traps were placed
214 in series to monitor potential Hg breakthrough. Trapping recoveries were measured by
215 manual gold trap desorption with AFS detection and were variable, from 10-88%, with 4
216 out of 6 recoveries considered as low, <85% (Figure 1, Table S1). The 85% cut-off is
217 defined by the combined analysis uncertainty of sampled Hg by the Tekran® 2537X (15%,
218 2$\sigma$), and of recovered Hg by AFS (15%, 2$\sigma$). All secondary, in-series, gold traps showed
219 signs of breakthrough and did not recover all GEM vapor lost by the first trap, suggesting
220 that the secondary traps too suffered from Hg breakthrough (Table S1).

221 Next, we investigated gold matrix trap performance by loading 1-70 hours of cell
222 vent GEM in argon onto the traps during regular 2537X operation (Figure 1). During 4 out
223 of 9 tests, in series gold matrix traps were used to assess breakthrough. Recoveries were
224 found to range from 38-98%, with 3 out of 5 long loading experiments having recoveries
225 <80%. Again, in-series traps in 2nd position showed significant Hg breakthrough in the
226 argon matrix (Table S1).
Finally, gold matrix traps were loaded with controlled amounts of GEM vapor from the 2537X internal permeation source, also in high purity argon matrix. During 4, 7, 13 and 16 hours, 1-4 ng of permeation source GEM vapor was loaded onto the gold traps. Recoveries measured by manual trap desorption with AFS were 86-99% (Figure 1), which is within the combined analysis uncertainty of the methods.

In summary, we observed that gold traps placed at the 2537X cell vent showed good recovery, >80%, of loaded GEM over time periods <16 hours. For atmospheric GEM loading times from 17-70 hours recoveries were incomplete. Overall the performance of gold traps over longer time periods at the cell vent was unexpected and likely related to the gradual passivation of the gold surface. After heating the gold traps for analysis, they performed normally, indicating no degradation of the trap. Adding a soda-lime trap prior to the gold trap has been shown to increase the performance of gold traps.¹⁹ We stress that our findings by no means question the use of gold traps in standard GEM vapor analyzers sampling ambient air, including the Tekran® 2537X used here, with very short Hg loading times generally ranging from 2 to 10 minutes.

242

243

### Cell vent trapping efficiency in oxidizing solution in argon

We evaluated the acid traps consisting of 40vol% iAR oxidizing solution traps by directing variable amounts (6-14 ng) of ambient GEM or 2537X permeation source GEM to the traps, without presence of the 1115i valve manifold (Table S2). Solution Hg concentrations were analyzed by CV-AFS and recoveries found to be good, in the range of 85-104%. We subsequently connected the 1115i valve module, with its multiple tees and connectors. Thirteen ambient GEM tests, loading 4-18 ng of Hg over 22-70 hours, were performed with good recoveries ranging from 85-111% (Figure 1).

244

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249

250
We observe a 6-22 % volumetric loss of the 40 % iAR solutions after 25-70 hours of Hg trapping at ambient laboratory temperature for the first four samples, 1-4. This volumetric loss did not an effect on the Hg trapping efficiency, as recoveries were 89-110 % for samples 1-4. Indicating soda lime traps at the solution trap outlet did not show any color change suggesting that the weight loss is mostly water, rather than acid. Nevertheless, a loss of 22 % over 3 days can potentially become problematic for longer sampling periods (7-14 day). To reduce the volume loss, we tested trapping with a manual cooling system that maintained a temperature of 5-6°C. The solution traps were placed inside a polystyrene cooler, filled with ice-packs that were changed every 2 days. This system effectively reduces the volume loss by evaporation to <6%.

Carry-over of GEM from the permeation source calibration cycles to sample traps was tested. We connected the sample inlet of the Tekran® 2537X to a zero-air cartridge and collected cell-vent GEM for 48-72h, after routing calibration pulses of the permeation source (once every 2h) to a separate trap. We found that less than 2% (n=2) of perm-cell Hg was transferred to subsequent samples and conclude that carry-over from the permeation source to the following sample did not cause any significant bias in ISO-GEM measurements.

We tested the Hg from the permeation cell calibration unit as internal standard for Hg stable isotope measurements by trapping the calibration cycle GEM pulse on a dedicated valve position and found very consistent δ<sup>202</sup>Hg and Δ<sup>199</sup>Hg values, over multiple (n=5) test days (Figure 2). Please note that we tested only one instrument and that the uniformity of the permeation source Hg isotope composition from other instruments has to be confirmed.

In summary, acid traps based on 40%-iAR provide a good cell vent trapping with high recovery, low break-through and low blanks. A major advantage is the direct
276 measurement of the (2x diluted) acid on the MC-ICPMS without further pre-enrichment
277 steps. Acid traps however have the drawback of acid being handled in the field and during
278 shipment which can cause higher costs/efforts for logistics and personal safety.

279

280

trapping efficiency on activated carbon in ambient air

281 We evaluated the efficiency of iodine activated carbon (I-AC) and sulfur-impregnated
282 activated carbon (HGR-AC) traps by pumping ambient air over the traps for prolonged
283 periods (48 – 62 days) at a flow rate of 0.3 L/min. We expect a similar performance of I-
284 AC and HGR-AC traps in 80 mL/min Argon at the cell vent of the Tekran® 2532X analyzer
285 compared to the 4 times higher flowrate of ambient air tested here.

286 Breakthrough of I-AC and HGR-AC traps was measured after two months of
287 continuous sampling of at 0.3 L/min, by connecting the traps to a Tekran® 2537X analyzer
288 and pumping ambient air over the traps at 1 L/min. For both carbon traps, I-AC and HGR-
289 AC the Hg concentration measured after the trap was below the detection limit (<0.1
290 ng/m³), indicating that there was no measurable breakthrough.

291 Both, I-AC and HGR-AC showed good recoveries, however the reproducibility for
292 HGR-AC (94 ± 4 %, n=4) was better than for I-AC (97 % ± 39 %, n=10) (including data from
293 Ref.¹⁰). The blank for I-AC was 2.6 ± 0.8 ng/g and for HGR-AC was 0.16 ± 0.06 ng/g.
294 Considering the different amounts of powder used for the traps, this resulted in absolute
295 Hg amounts of 0.32 ng and 0.06 ng per trap for I-AC and HGR-AC, respectively.

296 Based on our experience, IC powder is more challenging to combust as it liberates
297 large amounts of volatile iodine compounds that need to be removed from the combustion
298 carrier gas using halogen traps. If not, the iodine will form strong Hg-iodide complexes in
299 the oxidizing solution trap, which may affect Hg isotope analyses by an incomplete
300 reduction of Hg(II)-iodide during cold-vapor generation. Sulfur generates gaseous SO₂,
301 which partly becomes sulfuric acid in the oxidizing solution trap and does not interfere
302 later on with Hg(II) reduction by Sn(II).

303 In summary, we found both 40vol%iAR oxidizing solution traps and activated
304 carbon based I-AC and HGR-AC traps to recover GEM quantitatively at the 2537X cell vent
305 over prolonged periods of sampling. The three types of traps have different advantages
306 and disadvantages (Table 1). Solution traps require no further processing, other than
307 dilution to 20vol%iAR, before direct Hg isotope measurement. The solution traps need to
308 be cooled however, to avoid evaporation of the acidic solution, and shipping acidic
309 solutions from sampling site to laboratory requires strict safety precautions. Handling
310 and shipping of I-AC and HGR-AC traps is more convenient, however they require further
311 processing by dual tube furnace combustion methods to recover trapped Hg for isotopic
312 analysis. The choice of trap type has to be made on an individual basis depending on the
313 research question and logistical settings (e.g. long-term vs. short-term campaign).

314

315 **ISO-GEM multi-valve manifold configuration**

316 The Tekran® 1115i multi-valve manifold we tested is equipped with 5 small valves
317 and 3 large valves (Figure S2). The small valves can be connected in a parallel or tree
318 configuration to the 2537X cell vent (Figure S3) and guide GEM vapor to 4 or 5 different
319 GEM isotope traps. The three large valves can be configured either at the 2537X inlet to
320 sample different physical sources of Hg (outside/inside; night/day; etc.) or be used at the
321 cell vent for additional GEM trapping capacity.

322 One GEM isotope trap is exclusively dedicated for the mercury coming from the
323 permeation source in the aim to not have a contribution to ambient air samples and bias
324 in the Hg isotopic ratios. Permeation source GEM also serves as internal Hg isotope
325 standard.
The 1115i multi-valve manifold is directly programmed by the intuitive plugin developed by Tekran® (Figure S4). This plugin allows to program the 1115i according to different criteria chosen by the user. For each event (e.g. inside building during nighttime) a series of valve positions (open vs. closed) are defined, determining which air mass is analyzed (e.g. inside vs. outside) and directing the GEM to a designated isotope trap after analysis (Figure 3A and Figure S5 – S7). This allows for the automated sampling of air collected under pre-defined conditions (e.g. time, location, Hg amount). The plugin monitors the status and timing in the programming and actuates the solenoid valves according to the program synchronized with the measurement cycle of the 2537X. The user will also assign any programmed event-flags in the sample data recorded by the 2537X. These flags provide positive indication of 1115i operations performed during each sampling period and allow to calculate trapping yields on individual traps.

The following events can be programmed by the Tekran® 1115i plugin.

(i) **A/B- cycling.** This event switches the valve position every A/B cycle (usually 10 min) between customized valve positions, to e.g. quasi-simultaneously measuring and sampling at different locations (e.g. gradient, inside/outside, etc.) (Figure S5).

(ii) **time event.** For this event, a start time and an end time must be defined in the 1115i plugin. In this example, we program a nighttime period and a daytime period on separate ports (valves) (Figure S6).

(iii) **GEM threshold event.** This event will be a function of the Hg concentration measured by the 2537X. In the plugin we specify a threshold of GEM concentration allowing to switch a valve and load GEM to a different trap. Any GEM concentration higher than the specified value will trigger a set of valve state and flag (Figure 3A).
351 (iv) **ADC trigger input event.** The ISO-GEM application can be configured with an
352 external trigger from an auxiliary sensor (e.g. O₃ or CO analyzers) or program.
353 An analog signal (up to 5V) can be sent by an external source to the 1115i
354 plugin and the valve positions are switched when a certain pre-defined
355 threshold is reached. Note that we did not have time to test this feature. (Figure
356 S7)

357 **ISO-GEM application to building emissions**

358 In an exploratory study we used ISO-GEM to investigate the GEM emission from a building
359 surface and its effect on spatial GEM concentrations. In different settings we programmed
360 ISO-GEM to quasi-simultaneously measure inside and outside the building (i – A/B
361 cycling), to distinguish between day and night (ii – time event) and to distinguish between
362 GEM concentration levels (arbitrary GEM threshold: >3 ng/m³ and <3 ng/m³, iii – Hg
363 amount event) (Results in Table S3). Furthermore, we sampled GEM at two different
364 locations outside the GET laboratory building, close to the building surface (wall-inlet, 1.2
365 m above ground) and 3m away from the building (free-inlet, 2m above ground) (Figure
366 4A). The building wall consists of powder coated aluminum siding.

367 The air sampled at the wall inlet was characterized by relatively high GEM
368 concentrations (3.8 ± 1.8 ng/m³, mean and 1σ on 1h means) and Hg stable isotope
369 signatures showed negative δ²⁰²Hg values (-1.26 ± 0.41 ‰, 1sd, n=16) and circum-zero
370 Δ¹⁹⁹Hg values (-0.05 ± 0.10 ‰, 1sd, n=16). GEM concentrations measured at the free inlet
371 of 1.5 ± 0.4 ng/m³ (mean and 1σ on 1h means) were similar to Northern Hemispheric
372 background levels of ~1.5 ng/m³.²¹ Hg stable isotope signatures measured at the free
373 inlet (δ²⁰²Hg = 0.77 ‰ ± 0.08 ‰, Δ¹⁹⁹Hg = -0.22 ‰ ± 0.04 ‰, mean and 1σ, n= 7) agreed
374 well with measurements reported for background sites in France and the US (δ²⁰²Hg =
375 0.53 ‰ ± 0.37 ‰, Δ¹⁹⁹Hg = -0.22 ‰ ± 0.05 ‰, mean and 1σ, n= 59)(Figure 2).⁵⁻⁷,¹⁰ The
similar Hg isotope signatures and Hg concentrations measured at the free inlet in suburban Toulouse compared to Northern Hemispheric background sites suggests that GEM was dominated by long-range transport background GEM and that the contribution from local anthropogenic sources and local building emission was minor.

Using the time event (ii) function, we collected the air at the wall inlet sampled during daytime and nighttime on two different traps. Daytime air was characterized by higher GEM concentrations (3.5 ± 0.8 ng/m³, n=3) than nighttime air (2.6 ± 0.3 ng/m³, n=3). The higher GEM concentration of daytime air was concomitant with more negative δ²⁰²Hg values (-1.82 ‰ ± 0.37 ‰, n=3) than nighttime air (-1.36 ‰ ± 0.17 ‰, n=3). Δ¹⁹⁹Hg values were similar between daytime (-0.03 ‰ ± 0.09 ‰, n=3) and nighttime air (-0.03 ‰ ± 0.11 ‰, n=3). The higher GEM concentrations during daytime observed at the wall inlet is opposite to observations from other urban sites, where highest GEM concentrations were observed in the early morning when the nocturnal boundary layer was most stable.²⁸,²⁹ This suggests that the high GEM concentrations measured at the wall inlet were affected by local emissions from photo-chemical or temperature related processes (see discussion below).

Using the GEM threshold event (iii) function we separated the outside air of the wall inlet on two different traps using a threshold of 3 ng/m³ (Figure 3). δ²⁰²Hg was more negative for the trap with higher concentration (>3 ng/m³, mean GEM = 3.9 ng/m³, δ²⁰²Hg = -1.54 ‰) compared to the trap that collected lower GEM (<3 ng/m³, mean GEM = 2.8 ng/m³, δ²⁰²Hg = -1.19 ‰).

Combining all wall and free outside measurements, we found a strong linear correlation between the δ²⁰²Hg signature and the Hg concentration of GEM expressed as 1/Hg (R² = 0.91, p<0.001, Figure 5C) as well as between Δ¹⁹⁹Hg and 1/Hg (R² = 0.57, p<0.001, Figure 5D). This strong correlation suggests that outdoors GEM can be explained
by two distinct sources, GEM emission from the local building and GEM from background air. The high variability of the GEM concentration at the wall-inlet (Figure 4 B) and the extent of mixing between GEM emission from the building with GEM from background air is likely controlled by the turbulence of air in proximity to the building.

Mercury concentrations in buildings can be highly elevated compared to outdoor ambient air concentrations.30 It has been suggested that 10% of the households in the United States exhibit indoor GEM concentrations exceeding the U.S. reference concentration of 300 ng m⁻³.30 Hg has been used as biocide in paint in between 1950 and 1990 and was thus intentionally employed in buildings.31 A second source of Hg in buildings are micro-spills through the accidental release from products containing Hg, such as e.g. thermometers or fluorescent light-bulbs.30 Using the A/B-cycle (i) sampling scheme we measured the GEM concentration quasi-simultaneously inside and outside the GET laboratory building in Toulouse. The GEM concentration inside the building was 3.61 ng/m³, whereas the outside concentration was 2.45 ng/m³ during the same period. Given that the concentration difference was relatively small (1.2 ng/m³) and all the windows of the building were permanently closed, we assume that emission from inside the building contributed insignificantly to the elevated GEM concentrations measured at the wall inlet.

Elevated GEM concentrations outside buildings have been reported for several urban areas with e.g. 2.7 to 3.8 ng/m³ in New York,32 2.0 ng/m³ in Windsor, ON close to Detroit,33 1.9 ng/m³ in Toronto34 and 9.7 ng/m³ in Guiyang, China.35 Carpi and Chen suggested that elevated GEM concentrations in urban areas were related to re-emission of GEM from building and other urban surfaces.32 They suggest that the GEM originates from photochemical reduction of divalent Hg that has previously been deposited through dry deposition.32 Preliminary measurements of divalent Hg in precipitation at the Toulouse site (data not reported here in detail; Δ²⁰⁰Hg = 0.12 ‰ ± 0.11 ‰, n=4) are in
agreement with precipitation data measured elsewhere.⁴, ⁷, ¹¹, ³⁶, ³⁷ The Δ²⁰⁰Hg values of GEM samples at the wall inlet (-0.04 ‰ ± 0.06 ‰, n=16) was very similar to that of GEM measured at the free inlet (-0.06 ‰ ± 0.04 ‰, n=7) or at remote sites (-0.04 ‰ ± 0.03 ‰, n=39).⁵,⁶, ⁷,⁵, ¹⁰ Aluminum, the material of the building facade, can form an amalgam with Hg.³⁸ Hg in the building wall may originate from dry deposition of GEM from the atmosphere during colder winter months (i.e. through amalgamation by the aluminum building material) rather than from dry deposition of divalent Hg which would be expected to exhibit positive Δ²⁰⁰Hg anomalies. With the data presented here we cannot exclude the presence of residual Hg from the production process of the building material and a contribution to the re-emission observed. There are no experimental studies investigating Hg stable isotope fractionation during volatilization of GEM from aluminum amalgam, however the isotopic signatures at the wall inlet agree with the volatilization of GEM from liquid Hg, with negative δ²⁰²Hg and small positive Δ¹⁹⁹Hg in the vapor phase.³⁹, ⁴⁰ GEM concentrations at the wall inlet were positively correlated with the ambient air temperature (R² = 0.21, p<0.001, Figure S8) in agreement with previous findings,³² suggesting that the re-emission flux was favored by higher temperature or solar irradiation.

The exploratory results of the ISO-GEM application illustrate the potential of smart automated sampling strategies based on pre-defined criteria (e.g. time, location, internal (GEM concentration) and triggers from external sensors (O³, wind direction)) to maximize the information of the Hg stable Hg isotope finger printing tool. At the same time the elevated GEM concentration and distinct Hg stable isotope signatures measured at the wall inlet close to the building surface illustrate how air can be affected very locally by building emissions. This has important implications for atmospheric GEM monitoring
450 stations, where sample inlets have to be placed carefully in order to avoid measurement

451 bias from local building emission.41,42

452

453 **Financial Interests:**

454 The automated multi-valve module (1115i) at the center of the ISO-GEM application is commercially available from the Tekran® Instruments Corporation. We would like to mention that the conceptual ISO-GEM idea can be implemented by a custom-made instrumentation and control software without purchase of the 1115i module.

458

459 **Acknowledgments:**

460 This work was supported by ERC-2015-PoC_665482 grant from the European Research Council to JES, and by the Tekran® Instruments Corp. MJ received funding through the H2020 Marie Sklodowska-Curie grant agreement 657195 and Swiss National Science Foundation grant PZ00P2_174101.

464

465 **Supporting Information:**

466 Additional figures, and tables. The Supporting Information is available free of charge on the ACS Publications website at DOI:

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<figure><img src="image_2.png" /><figcaption>**Figure 1:** Sampling yields (%) of GEM recovered in high purity argon by different trap methods at the outlet of a Tekran® 2537X analyzer relative to Tekran® GEM concentration measurements. GEM from ambient air was recovered at the cell vent by gold-traps (red diamonds) and acid-traps (blue circles). GEM from the permeation source recovered from the cell vent in argon with gold-traps is shown as red triangles. The error bars represent 7.5% (1σ) uncertainty of combined AFS measurements. The dashed line represents 100 % sampling yield and the shaded area represent the interval of 85% - 115% yield that is expected acceptable for Hg stable isotope measurements.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Figure 2:** Mass-dependent (δ<sup>202</sup>Hg) vs. mass-independent (Δ<sup>199</sup>Hg) Hg isotope signatures of GEM measured by different trap types at the free inlet in Toulouse, France. For comparison, GEM measurements at remote sites<sup>5,6,7,5,10</sup> are shown as blue crosses. The isotope signatures of GEM supplied by the internal permeation source of the Tekran® 2537X collected with an acid-trap (green squares) serves as internal isotope standard for the ISO-GEM system. The error bars represent the 2 SD of replicate inhouse standard measurements.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

B

<table><thead><tr><th>Event</th><th>Port 0</th><th>Port 1</th><th>Port 2</th><th>Port 3</th><th>Flag</th></tr></thead><tbody><tr><td>GEM &lt; 3 ng/m<sup>3</sup></td><td>✓</td><td>X</td><td>X</td><td>X</td><td>1</td></tr><tr><td>GEM &gt; 3 ng/m<sup>3</sup></td><td>X</td><td>✓</td><td>X</td><td>X</td><td>2</td></tr><tr><td>Permeation source calibration</td><td>X</td><td>X</td><td>X</td><td>✓</td><td>0</td></tr></tbody></table>

C

<figure><img src="image_5.png" /></figure>

494

**Figure 3:** A: ISO-GEM configuration with a GEM threshold event. B: Valve configuration of GEM threshold event. When GEM <3 ng m⁻³, analyzed GEM at the cell vent is directed to 'port 0' (and trap 0). When GEM > 3 ng m⁻³, analyzed GEM at the cell vent is directed to 'port 1' (and trap 1). C: δ²⁰²Hg of GEM separated by concentration criteria (<3 ng m⁻³, >3 ng m⁻³).

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<figure><img src="image_6.png" /></figure>

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<figure><img src="image_8.png" /><figcaption>**Figure 4:** A) Setup of air sampling inlets with the wall inlet (red) and free inlet (blue) B) GEM concentration measured from both inlets during summer 2017 C) GEM concentration measured during fall 2017.</figcaption></figure>

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<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 5:** Hg isotope signature of GEM measured at a free inlet (blue circles) and a wall inlet (red triangles and diamonds) outside a laboratory building and inside the laboratory (green squares) in urban Toulouse, France **A)** $\Delta^{199}\text{Hg}$ vs. $\delta^{202}\text{Hg}$, **B)** $\Delta^{200}\text{Hg}$ vs. $\delta^{202}\text{Hg}$, **C)** $\delta^{202}\text{Hg}$ vs. 1/Hg, and **D)** $\Delta^{199}\text{Hg}$ vs. 1/Hg. The dashed lines represent the linear regression and the shaded area represents the 95% confidence interval of the linear regression. The error bars represent the 2 SD of replicate inhouse standard measurements.</figcaption></figure>
557 TOC ART:

Atmospheric mercury isotope
fingerprinting: ISO-GEM

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References

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