Direct observations of atmospheric oxidized mercury speciation in polar areas

Domain: neuroscience
Published: 2026-04-07
Editorial outcome: minor_revision

Abstract
Mercury is a persistent pollutant with significant public health impacts in polar regions where fish consumption drives human exposure. Atmospheric oxidation pathways control where mercury deposits globally, but the lack of molecular-level observations of oxidized mercury products has hindered the validation of proposed chemical mechanisms. Here, we show the in-situ online detection of individual mercuric halides (HgCl2, BrHgCl, HgBr2, ClHgI, BrHgI, and HgI2) in the polar boundary layer using atmospheric pressure chemical ionization mass spectrometry. Our observations identify HgBr2 as the dominant oxidized mercury species at both poles, while HgCl2 and other halides were also observed in Antarctica. The observed speciation diverges from current model predictions, which favor HgCl2 and HOHgBr as dominant oxidized forms. Our results show that real-time molecular measurements can substantially advance global mercury monitoring and improve the chemical models used to assess environmental policies and predict deposition patterns.

Introduction
Understanding mercury speciation in the atmosphere is crucial for identifying the lifecycle of mercury contamination in the environment. Atmospheric elemental mercury (Hg0) is relatively inert and insoluble, allowing it to persist (lifetime of 6–20 months) and be transported globally1,2. Hg0 oxidation by halogen and hydroxyl radicals3,4 converts it to soluble gaseous oxidized mercury (HgII, collectively known as GOM) that is rapidly washed out by precipitation3,5. GOM drives roughly half of all atmospheric mercury deposition5, with models estimating that up to 80% of HgII deposition occurs over the ocean2,6. Once deposited, Hg0 methylates in aquatic systems to form neurotoxic methylmercury that further bioaccumulates in fish and wildlife7. Polar regions are hotspots for this cycle, especially during spring when photochemical activation of saline snowpack releases reactive bromine, triggering ozone-depletion events and rapid Br radical oxidation of Hg0 to HgII within hours8. This intense halogen chemistry leads to seasonal GOM deposition pulses, which, upon incorporation into marine food webs, threatens Arctic communities dependent on seafood8–11. However, an ongoing uncertainty in polar Hg budgets, as well as an understanding of global Hg cycling in general, is the lack of methods to directly measure the molecular speciation of oxidized mercury.

The balance between atmospheric Hg0 and HgII is driven by complex redox chemistry that remains highly uncertain4,12. This hinders our understanding of the major reaction pathways of atmospheric Hg and predicting Hg inputs to ecosystems. Much of the current knowledge about this chemistry derives from quantum chemical calculations4,12 and chemical-transport models3. Despite chemical-transport models now tracking 20–30 GOM tracers3,13, their predictions rest on theoretical rate constants and pre-concentrated denuder data that are subject to various biases14. Laboratory kinetic experiments, as well as direct, real-time molecular field measurements, have remained mostly out of reach due to the extremely low ambient concentrations (pg m⁻³) and toxicity of Hg15,16, leaving key reaction pathways unvalidated. Conventional GOM measurement methods rely on hours-to-days of pre-concentration on KCl denuders17 or cation-exchange/nylon membranes14,16,18–21 followed by offline analysis, which limits molecular detail and time resolution and hinders direct molecular-level validation.

In this work, we present direct in-situ observations of several mercuric halides in the polar regions, using the nitrate-based Chemical Ionization—Atmospheric Pressure interface—Time of Flight (CI-APi-TOF) mass spectrometry technique together with ambient anion composition measurements (APi-TOF)22,23. In Antarctica, measurements ran over the 2014–2015 austral summer at the Finnish research station Aboa (73°03′S, 13°25′W)24. In the central Arctic, the data from the spring leg (April–June 2020, >80°N) of the MOSAiC expedition were analyzed alongside collocated O₃, BrO, and Hg0 observations25,26. Further details are provided in the methods section. Our observations reveal unexpected GOM speciation that challenges existing models of Hg redox chemistry and provides critical data to refine global mercury deposition patterns.

Results
Molecular identification of mercuric halides
Using the CI-APi-TOF, we directly detected neutral mercuric halides in ambient air at both polar sites, confirming each species by exact mass-to-charge ratios and natural abundance isotope patterns (Table S1). In Antarctica, six mercuric halides were identified: HgCl2, BrHgCl, HgBr2, ClHgI, BrHgI, and HgI2 (Fig. 1), all observed exclusively as NO3− clusters (e.g. as HgBr2·NO3−). In the central Arctic, HgBr2 was the only detected neutral species in April (Fig. 2), and by June the signal was below limit of detection (LOD, Fig. S1). Other GOM species were either below the LOD or masked by local ship-emitted interferences (cf. “Methods”).

In Antarctica, we also observed naturally charged ions corresponding to HgCl2NO3−, BrHgClNO3−, and HgBr2NO3− (Fig. S2). In the central Arctic, these naturally charged species were not detected, likely due to low concentrations and a different inlet configuration that increased losses (cf. Methods). These naturally charged ions are likely formed by ambient NO3− ions clustering with mercuric halides (Fig. S3). Similar experimental evidence of ambient nitrate ion clustering has been observed with highly oxygenated molecules (HOM) and sulfuric acid27. The concurrent detection of neutral and naturally charged ions provides a robust validation of our findings of mercuric halides in the ambient atmosphere.

We use theoretical calculations to further support the detection of neutral mercuric halide clustering with NO3− (cf. “Methods” for details). Ab initio calculations of the XHgY + NO3− ↔ XHgY·NO3− equilibria (X,Y=OH, Cl, Br, I) under typical inlet conditions (1 bar, residence time τres ≈ 300 ms, [NO3−] ≈ 107 cm⁻³) yield equilibrium ratios > 1 and slow dissociation (Table 1) when assuming near kinetic limit charging of XHgY species with NO3− (k=10−10 cm3 molecule−1 s−1). This suggests selective clustering of pure mercuric halides and a lack of sensitivity toward species such as Hg(OH)2 or HOHgBr3. The absence of these signals in the CI-APi-TOF reflects the selectivity of the chemical ion-chemistry employed, rather than the true absence of these compounds in ambient air. However, the observations are consistent with the APi-TOF measurements, where neither anion nor cation measurements in Antarctica showed any indication of Hg(OH)2 or HOHgBr ion cluster signals in ambient air, which suggests either the absence of the compounds at our measurement location or inefficient charging of these compounds to form ions.

Abundance and comparison with conventional methods
We quantified the mercuric halide concentrations by utilizing the gas-phase, kinetic limit calibration used with gaseous H2SO428, which ionizes with NO3− mainly by proton transfer. This approach has been applied previously to quantify HIO329, chlorine oxyacids30 and highly oxygenated organic molecule concentrations31. Ab initio calculations of the XHgY + NO3− ↔ XHgY·NO3− equilibria yield large binding enthalpies (Table 1) and cluster lifetimes τdiss » τres (~300 ms). However, the equilibrium ratios [XHgY·NO3−]/[XHgY] (≈ 1–5) are not » 1, implying lower ionization efficiencies. Thus, the concentrations reported here should be viewed as lower limits.

Timeseries of Antarctic mercuric halides (Fig. 1, right column) are reported in pg Hg m⁻³ at standard temperature and pressure (STP) to facilitate comparison with prior GOM studies. Mercuric halides are abundant and chemically diverse in Antarctica, exhibiting periods when HgBr2 and HgCl2 peak at several hundred pg Hg m⁻³. On January 11, three mercuric iodide species were identified: HgI2, BrHgI, and ClHgI. All compounds show values above the LOD, which is estimated between 25 and 37 pg Hg m⁻³ for 60 min time resolution (Table S2 and “Methods” for LOD determination). Note that the LOD is determined using three times the standard deviation of the measured baseline during the whole campaign; this baseline changed with environmental conditions such as temperature inside the measurement cabin. Thus, the given LOD values should be taken as conservative values. In Antarctica, the abundance of HgBr2 is potentially controlled by the presence of atmospheric bromine (Fig. S4), as evidenced by the moderate to strong correlation (r=0.72) between the HgBr2 signal and the Br− signal (78.9189 Th, Thomson, likely from HBr30).

In the central Arctic (Fig. 2), HgBr2 reached ~80 pg Hg m⁻³ during April but fell below the LOD by June (Fig. S5). Other halides, neutral or naturally charged, were undetected or spectrally unresolved throughout the observation period. Although ship emissions introduced an abundance of interfering peaks, CI-APi-TOF measurements of H2SO4, CH3SO3H, HIO3 were mostly unaffected, indicating negligible effect on the mass spectrometric measurements32. Hence, we can infer that stack emissions during MOSAiC and pollution from the Aboa station are not the source of GOM (Figs. 1, S5). Collocated Hg0 data33–35 reveal that HgBr2 maxima coincide with Hg0 depletion events, mirroring ozone variability and underscoring the tight coupling of mercury speciation to bromine photochemistry35–37.

The total concentration of mercuric halides in Antarctica (interquartile range, IQR, 29–110 pg Hg m⁻³, full range: <LOD–1177 pg Hg m⁻³) lies well within the 5–705 pg Hg m⁻³ range reported across Antarctica by pre-concentration methods that lumped together all HgII compounds38 (Fig. 3). In the central Arctic, springtime HgBr2 concentrations during MOSAiC (IQR 17–49 pg Hg m⁻³) mirror reactive gaseous mercury Tekran measurements at the Canadian Alert station in April 2020 (IQR 29–70 pg Hg m⁻³) and at the Svalbard Zeppelin station in April 2019 (IQR 6–24 pg Hg m⁻³)39,40. Because Tekran measurements have been suggested to underestimate total GOM concentration14, we also compared our data to reactive mercury (GOM + particulate Hg) values from cation-exchange membrane detection at Zeppelin (IQR 37–86 pg Hg m⁻³)40,41. We note, however, that the reactive mercury measurements also include also a contribution from particulate Hg39, which would not be measured by CI-APi-TOF.

Comparison with model simulations
The close agreement across methods indicates that HgBr2 constitutes a major fraction of spring GOM in the polar regions, contradicting current atmospheric models and our understanding of Hg redox chemistry. GEOS-Chem, a Hg chemical-transport model, predicts that oxidized mercury is dominated globally by HgCl2 (49%), followed by particulate Hg (22%), Hg(OH)2 (19%), and HOHgBr (9%)3, reflecting oxidation via the Br, OH, and Cl pathways, which all involve second-step oxidation by O34,12,15. The WACCM chemistry-climate model also reports HgCl2 to be the dominant atmospheric oxidized mercury species, with a minor contribution (<0.1%) of HgBr2 globally42. Photochemical model simulations of the polar springtime period predict that HgCl2 and HOHgBr are the major components of GOM, with other HgII species contributing minor amounts (<0.3%) (Fig. S6). Compared to these modeling estimates, the CI-APi-TOF observations reveal that a much broader range of HgII species contribute to the total observed GOM mass concentration in the polar regions. In the central Arctic spring, HgBr2 was the only GOM species detected above the LOD, accounting for essentially all of the CI-APi-TOF–measured GOM mass (Fig. 2), directly contradicting model predictions. In fact, the only point of agreement between existing models and our measurements in Antarctica is the presence of HgCl2 as an important oxidized Hg species. Because CI-APi-TOF does not detect HOHgBr or Hg(OH)2, their true ambient levels remain unknown, but the agreement and similarity of the measured mercuric halide concentrations to previous GOM measurements suggest that the species detected here are major contributors to the GOM mass. Therefore, the discrepancy between modelled and detected species points strongly to missing reaction channels in current chemistry schemes, including the role of iodine in oxidizing mercury.

These discrepancies have profound implications for deposition estimates. Each HgII species exhibits unique photolysis rates, particle uptake efficiencies, and solubilities12,13,43,44. These are the properties that govern atmospheric lifetime and hence transport distance. For example, HgBr2 photolyzes roughly three orders of magnitude faster than HgCl245. Thus, if HgBr2 is a more dominant GOM species than HgCl2, as our measurements suggest, this would enhance the photoreduction of GOM and reformation of Hg0, increasing the possibility of long-range transport of Hg. Refining the Hg chemistry schemes in chemical transport models to better represent measured HgII speciation could lead to strong shifts in regional deposition patterns, with direct consequences for ecosystem exposure assessments.

Methods
Sampling location in Antarctica
This study conducted observations during two campaigns in polar regions. In Antarctica, measurements took place at the Finnish research station Aboa (73°03’S, 13°25’W), in Queen Maud Land, located ~130 km inland from the coast at 480 meters above sea level. Spanning the austral summer months with midnight sun conditions, the campaign lasted from November 2014 to February 2015. The aerosol laboratory utilized in the campaign was situated ~200 m upwind from the main station and any pollution sources24. Polluted periods when the wind was blowing from the main station (wind direction 210-270o)50 were occasionally observed, but do not have any significant effect on the measurements (Fig. 1). Data availability extends over the whole austral summer, but only a period after 21 December was considered in this study due to a suspected leak in the mass spectrometric system prior to this date. For all other campaign measurements (meteorological parameters, aerosol number concentrations, and aerosol precursor concentrations) refer to Jokinen et al.24.

Sampling location in the Arctic
In the central Arctic, observations were conducted as part of the ship-based Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign on R/V Polarstern25. The CI-APi-TOF was located in the bow of the ship in the “Swiss container” and the APi-TOF in the container behind the Swiss container. This Arctic campaign unfolded from October 2019 to the end of September 2020 in the central Arctic region25. We selected a springtime period (April 2020) for further investigation due to the event known as a “bromine explosion” in the springtime Arctic atmosphere. In addition, we selected another period in late spring-early summer (June 2020) to understand the relevance of oxidants (ozone, bromine) to our observations. The location of the ship during the spring leg3 can be visualized from Fig. 2 in Shupe et al25. The disadvantage of executing measurements on a drifting vessel is that the pollution from the ship’s exhaust can be pushed towards the sampling location with the wind. The wind direction was often unfavorable for measuring pollution-free air masses during spring. Due to this, we used a pollution mask that was developed for detecting particle pollution from the ship’s stack51. The effects of the pollution in our measurements, particularly on the identification of peaks in the mass spectra, is discussed below.

Chemical ionization—atmospheric pressure interface—time-of-flight mass spectrometer (CI-APi-TOF)
In both campaigns, a nitrate-based CI-APi-TOF mass spectrometer (Tofwerk, Switzerland, “HTOF”, with finite resolving power of approximately m/Δm ≈ 4000–4500) with a so-called “Eisele” type CI-inlet was employed23. In short, the CI-APi-TOF consists of three core parts: the chemical ionization inlet, atmospheric pressure interface, and the time-of-flight analyzer. In the CI-inlet, charger ions are produced by exposing concentrated nitric acid (HNO3, 3–5 ml pm) contained in a sheath flow (20–30 lpm) to a soft X-ray source. The total flow of the system was 30–40 lpm in order to achieve the best isokinetic flow dynamics. The difference between the total and sheath flow is equal to the sample flow of approximately 10 lpm, which was checked weekly. The two flows inside the CI-inlet are coaxial to eliminate any interaction with the walls. The drift time (reaction time) between the sample and charger ions is ~300 ms, which prevents ion-induced nucleation occurring inside the inlet. Sample ionization in the CI system occurs at ambient pressure for better sensitivity than low pressure systems. The charger ions (NO3−, (HNO3)1-2NO3−) produced are pushed into the sample flow using an electric field (ion source -125 V, drift tube –100 V). Sample charging happens through proton transfer between nitrate ions (or its multimers) and the sample molecule or via clustering of the sample compound (e.g. in the case of HgBr2 or oxygenated organic molecules) with the charger ion. All reported mercury-containing compounds in this study were detected with a NO3− ion (e.g. as HgBr2·NO3−).

In Aboa, the sample was drawn in via an unheated ~60 cm ¾” polished stainless-steel tube. In the MOSAiC campaign, we used an insulated but unheated “New Particle Formation” NPF-inlet that was specifically designed to minimize diffusionallosses with high inlet total flow of about 70 lpm. The NPF-inlet was ~1.3 m long, with a diameter of 10.2 cm, connected to the ¾” inlet tube of the CI-inlet with a flange. Even when operated at extreme conditions in polar regions, the inlets do not frost due to high flow rates and no salt depositions were observed. The residence time of the sample air in the sampling line is approximately 1 s (60 cm tube with 10 lpm flow), which is likely too fast to cause any significant wall effects on the ambient samples.

Atmospheric pressure interface time of flight mass spectrometer (APi-TOF)
The APi-TOF consists of an Atmospheric Pressure interface (APi) that allows sampling from ambient air directly to the time-of-flight (TOF) mass spectrometer22. The TOF analyser is identical with the CI-APi-TOF (Tofwerk, Switzerland, “HTOF”, with finite resolving power of approximately m/Δm ≈ 4000–4500). The sampling flow to the instrument is ~0.8 lpm through a 0.3 mm diameter pinhole. The ambient air sample is drawn to the pinhole at 10–20 lpm using a vacuum pump. The inlet in Aboa was a polished stainless-steel tube, 10 mm in diameter, with a 10 lpm flow rate and approximately 1 min length. In MOSAiC, we had to opt for a vertical inlet line, again made of 10 mm stainless-steel, through the roof of the container. This inlet was ~2 min length with a core sampling style inlet sampling at 20 lpm to decrease diffusional losses. The length of the inlet might have caused some decrease in the ion signal.

In Aboa and MOSAiC, most of the campaign was sampled using negative polarity settings to sample anions. This is to target observations of mostly acidic and highly oxygenated organic compounds and molecular clusters, connected to new particle formation events. In Aboa, we sampled two days of cations in positive polarity between 26 and 29 January and a week during MOSAiC at the end of July.

The observations represent the naturally charged molecules and clusters produced mainly by cosmic radiation. The decay of radon also produces ions, but the concentration is extremely low over the ocean and in Antarctica. Ion collisions with neutral molecules will transfer the charge in energetically favourable situations. In the case of anions, the most abundant signals in daytime are bisulfate, iodate and their clusters, and nitrate ions during nighttime. In positive polarity, strong bases such as pyridines dominate the spectra22.

Quantification of mercuric halides
The data analysis for this study was conducted using tofTools software version 613 built on Matlab22. The analysis employed an average time of 180 min for MOSAiC data and 60 min for Aboa data. The original data time resolution was 5 s. Both CI-APi-TOF datasets were baseline corrected. Mass calibration was performed utilizing the known compounds in the spectra. The Aboa spectra were mass calibrated using the 2-parameter model with 3 most favorable options from NO3−, HNO3NO3−, (HNO3)2NO3−, (HNO3)3NO3−, HgCl2NO3− and HgBr2NO3−. The mean mass accuracy was 5.3 ppm and always < 15 ppm. MOSAiC spectra were mass calibrated using the same 2-parameter model and 3 most favorable options from NO3−, NO3[18O]−, HNO3NO3[18O]−, (HNO3)2NO3[18O]−, C5F10O2−. The mean mass accuracy was 2.9 ppm and always < 5 ppm.

The concentration of mercuric halides was calculated as:
[mercuric halide]neutral = [mercuric halide·NO3− peak area / Reagent ion peak area] x C (1)

where [mercuric halide]neutral is the calculated concentration (in molecules/cm3) of the neutral measured mercuric halide, mercuric halide·NO3− peak area is the measured signal including all detected isotopic peaks as integrated peak area. The reagent ion peak areas are the sum of the observed nitrate signals (fitted peak areas): NO3−, HNO3NO3− and (HNO3)2NO3−. C is a calibration factor. C was obtained from a sulfuric acid calibration in molecules cm-3 which is commonly used in aerosol precursor observations. For Aboa, C=1.195×1010 molecule cm-3 and MOSAiC C=6×109 molecule cm–3. Both calibrations were done using the original inlet tubing, so the calibration factor includes all diffusional losses.

The conversion from molecule cm−3 to mass concentration (pg Hg m−3) was calculated using a constant factor of 3.333×10–4 (=200.59 g mol−1 / 6.02×1023 molecule mol−1 × 1012 pg g−1 × 106 cm3 m−3) to better compare with previous total GOM measurements.

Detection limit for mercuric halides
The sensitivity of the CI-APi-TOF instrument is relatively high with low detection limits, only obscured by the baseline signal due to the constant feed of charger ion-containing gas into the system, and electronic noise. In our processing workflow (tofTools), background correction is always applied before further analysis. The limit of detection (LOD) of the instruments used at Aboa (60 min time resolution, observations in January 2015) and the MOSAiC (180 min time resolution, observations in April 2020) campaign are given in Table S2.

For the present study, the limit of detection (LOD) was determined separately for the Aboa and MOSAiC datasets because the baseline characteristics and the presence of interfering peaks significantly differed between the sites (Fig. S8).

Aboa: At Aboa, a 24 h period at the end of the campaign, between 26 and 27 Jan 2015, showed no detectable peaks at the m/z values of any mercury-halide species. The LOD was calculated by adding three times the standard deviation of the unit mass resolution signal (so called signal integration region, SIR, 0.4 Th width) on the average baseline deducted signal during this clean interval. This provided a stable and physically meaningful baseline (LOD), independent of short-term variability in temperature or counting statistics.

MOSAiC: At the MOSAiC site, the mercury-halide m/z values always contained contributions from overlapping organic ions (double or partially resolved peaks, Fig. S8). Consequently, a clean baseline interval comparable to Aboa did not exist. Instead, we estimated the noise from the inter-peak signal, i.e., the local minima between transient organic peaks. This region corresponds to the Noise Integration Region (NIR) in tofTools.

Because the NIR represents the lowest ion counts within the same Δm/z extraction window used for quantifying the analyte (the Signal Integration Region, SIR), it provides the most accurate and internally consistent estimate of the noise when neighboring peaks are present. The NIR can include low-level contributions from unresolved ions, which results in slightly higher LODs compared to a clean SIR baseline; however, this approach is standard for TOF mass spectrometers and necessary for masses where true zero-signal periods do not exist. LODs for MOSAiC were computed as 3σ of this NIR signal for each m/z.

Estimating bromine content
We use the CI-APi-TOF and the APi-TOF to provide proxies for atmospheric bromine content. In the CI-APi-TOF, Br− signal is a trace of e.g. neutral HBr. It could also originate from the Br− fragment from other bromine-containing molecules that charge with the aforementioned charger ions. The identification of Br- is based on both the exact mass and isotopic pattern. The peak areas normalized by charger ions (CI-APi-TOF) and anion signal (APi-TOF) reported in Fig. S3 (Aboa) corresponds to Br− (78.9189 Th). For the Br- signal recorded during MOSAiC, refer to Tham et al30.

Polluted periods
In the case of MOSAiC, the mass spectra were occasionally heavily polluted with exhaust from the ship’s stack. Pollution created abundant signals on the same m/z unit masses as the studied mercuric halides, and thus we may not always separate the mercuric halide peaks (e.g. HgCl2 or BrHgCl) from overlapping and dominant peaks at the same mass. In order to separate nearby peaks with similar masses, one should have mass resolving power owning instruments. Better resolution instrumentation should to be used for mercuric halide analysis in the future.

Ab initio quantum calculations
Calculations were performed with the Gaussian 16 suite of programs52, using the hybrid density functional/Hartree−Fock B3LYP method together with Dunning’s quadruple-ζ aug-cc-pVQZ correlation consistent basis, augmented with diffuse functions53. The aug-cc-pVQZ basis set for Hg was taken from Peterson and Puzzarini54. The molecular geometries of the XHgY molecules and the XHgY·NO3− clusters (X,Y=OH, Cl, Br, I) were first optimized and their respective vibrational frequencies calculated. The geometries, rotational constants, vibrational frequencies and heats of formation (at 0 K) of the XHgY molecules and the XHgY·NO3− clusters are listed in Table S3. The geometries of the cluster ions are illustrated in Fig. S9.

Photochemical and kinetics model details
Simulations in a (photo)chemical box model with the latest mercury chemical scheme3,12,13,55 (Table S4). The model allows all the HgI,II species to vary freely throughout the simulation, with their coupled continuity equations (ordinary differential equations) solved by using a Rosenbrock integrator. The simultaneous rate equations are converted into the differential equations using the KPP 2.2.3 kinetic preprocessor56. The photolysis rates were calculated using the absorption cross-sections, quantum yields and the actinic flux vertical profile, computed using the Tropospheric Ultraviolet and Visible (TUV) radiation model57. The actinic fluxes during May at 79°N, 0°E, were used as representative of springtime condition in polar regions. The model is constrained with 1.5 ng m−3 of Hg0, 17 ppbv ozone, 90 ppbv CO, 1.95 ppmv CH4, 8 pptv NO and 45 pptv DMS. Two scenarios for high bromine (BrO peaking at 20 pptv) and low bromine (BrO peaking at 1 pptv) were run to represent bromine explosion and normal air mass events.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reports in-situ measurements of gas-phase mercuric halides in the Arctic and Antarctic using atmospheric pressure chemical ionization mass spectrometry, establishing the utility of this new method in monitoring and evaluating mercury oxidation. This is important work, as all but one prior measurement (Deeds et al 2015), have only measured non- speciated HgII. Therefore, this is an important step forward in understanding atmospheric mercury oxidation, which is critical given consequences to human and ecosystem health. The manuscript is promising and exciting, and the observational data are compelling as presented. However, as described below, the comparison to global modeling is vague and general, which is problematic given that the conclusion of “missing reaction channels” is based upon this. In addition, calculation of the method LOD and discussion of below LOD data need attention, as described below, as the authors inaccurately claim that ClHgI (and HgBr2 in the Arctic spring in the main text) was detected when the signal was below LOD. Revisions are needed for a proper model comparison and for accurate treatment and discussion of data below LOD.\nAbstract, Lines 24-26: The authors state that “previous measurements have been unable to identify specific gas-phase oxidized mercury species at ambient levels.” Interestingly, the authors cite Deeds et al. (2015, Analytical Chem), which reports ambient concentrations of HgCl2 and HgBr2 in urban air in Montreal. While I agree that the data presented in Deeds et al. is extremely limited (since the paper goal was methodological, not an ambient study), the authors statement is inaccurate as written and should be revised to take into account this prior work.\nLines 128 & 133: By definition, an analyte cannot be detected by a method below its LOD. Therefore, it is not accurate to state “…ClHgI which is seen slightly under its LOD” because, by definition, the instrument did not measure (“see”) ClHgI if below the LOD. Nor is it accurate to state “In reality, even lower concentrations can be detected than is expected by the LODs, as seen with ClHgI.” Overall, it is inaccurate to state that ClHgI was observed at Aboa.\nLine 91: Similar to the above comment, the authors state here that HgBr2 was detected in June in the Arctic and refer to Fig S1. However, Figure S1 and its caption show that the signal during this period was entirely below LOD. Therefore, HgBr2 was NOT detected in June.\nRelated to the above comments, the authors state, on Lines 129-130, that “…the LOD is determined using three times the standard deviation of the measured baseline during the whole campaign; this baseline changed with environmental conditions such as temperature inside the measurement cabin.” Given this fluctuation, why don’t the authors calculate time-varying LODs to get the most out of their data (and to test their hypothesis that the ClHgI signal is real (above comment))? Please also clarify in the methods what is meant by “baseline”; is this an N2 background measurement?\nJustification for current model Hg oxidation mechanisms being inaccurate and the “discrepancy between modelled and detected species” is a general comparison to global modeling results on Lines 157-158. However, it is not clear whether these global modeling results of oxidized mercury speciation correspond to springtime in the Polar regions, since bromine chemistry, for example, is most active in the Polar regions and in springtime. If they simply correspond to a global annual average, then this is not an appropriate comparison. Reactive halogen chemistry varies significantly spatially and temporally across the globe, with the lifetimes of halogen atoms being incredibly short. Therefore, one would not expect an equal distribution of speciated oxidized Hg compounds spatially and temporally across the globe. The authors allude to potential heterogeneity on Lines 189-191 where they call for further measurements and reanalysis of existing datasets to “illuminate GOM speciation trends across seaons and environments.” This discussion is critical to the interpretation of the results herein and conclusions (e.g. “missing reaction channels”, Line 170). This discussion and comparison needs to be strengthened with a Polar springtime specific comparison in order to properly evaluate the model to measurement comparison.\nSimilar to the above comment, clarify what is meant by “incorporation of our speciation data into chemical-transport models” (Line 178; similar phrasing again on Line 186). How can this be done if there are “missing reactions channels” (Line 170) that haven’t been identified? This also points to confusion about the authors implying that HgBr2 and HgCl2 formation aren’t included in models.\nThe presence of the ambient NO3- clusters in Antarctica is noted but not related to reaction mechanisms. Further discussion of how this fits into knowledge of reaction pathways would be helpful. To support this, LODs need to be added to Figures S2 and S3, which present these data.\nAdditional Comments:\nLines 34-35: It is stated “Our results show that real-time molecular measurements are able to…guide effective emission-control strategies.” Please revise this phrasing because this manuscript does not show results evaluating emission-control strategies.\nLine 50: It is stated that Br2 and BrCl are released from “photochemical activation of sea-ice surfaces”. However, it was shown by Pratt et al. 2013 (Nat. Geosc.) that the sea ice surface is buffered and does not produce Br2 (or BrCl). Therefore, the authors should change the phrasing to “photochemical activation of saline snowpacks”.\nLine 65: I recommend splitting this long paragraph here.\nLine 76: “real-time, (sub-)hour detection” seems somewhat contradictory as phrased. Please clarify – perhaps replace with “online detection”.\nLines 78 and 80: Add instrument references.\nLine 110 and elsewhere: The authors alternate between referring to “Antarctica” and “Aboa”. For clarity, I suggest referring to Antarctica, rather than the station name, in the discussion, since the discussion refers to the “central Arctic”.\nLines 137-138: This HgBr2 seasonality is consistent with the seasonality of Arctic reactive bromine chemistry (Burd et al. 2017, JGR). Relating to this prior work would further support your finding.\nLine 147: It is not possible to measure 0 pg m-3. The range minimum should be noted as “<LOD”.\nLines 167-168: Please clarify “the agreement and similarity of the concentrations of pure halides” to “the similarity of the concentrations of the measured mercuric halides”.\nFigure 1, top right: It appears that there is an erroneous superscript “-“ on the label.\nFigure 1: It would be useful for the reader to state in the Figure 1 caption that these are neutral compounds being detected as NO3- adducts in the mass spectrometer.\nFigure 2E: The BrO data presented here are not discussed in the text.\nFigure S4, Top: Please clarify, in the caption, what the breaks in the data correspond to.\nFigure S6: Add the sampling location to the caption for improved clarity.\nTable S1: Add a column for the expected mass intensities, for comparison to the observed.", "reviewer_2": "Review of “Direct observations of atmospheric oxidized mercury in polar areas” by Jokinen et al.\nThe detection of oxidized mercury compounds has been an elusive goal for several decades. It is assumed that these compounds represent an important sink in the global mercury cycle, but our knowledge of them is very low. Thus unambiguous identification and quantification of oxidized compounds is a key missing link in understanding the global mercury cycle.\nJokinen et al present strong evidence that they have detected HgX2 compounds in the polar regions. The key data to support this is the Hg isotopic pattern, shown in Figures 1+2 (although figures need improvement). Based on these observations and the method development this paper will be an important contribution to the scientific/Hg literature.\nI have more questions around the claimed detection of naturally charged ions such as HgX2-NO3- . Figure 2 shows the mass pattern supporting this claim and the observations appear to be consistent with the claim. I am just unclear how this complex would form and (mostly) the source of the NO3 ion. Also, I can not see in the methods how this compound is distinguished from the NO3 ion which is used in the CIM mass spec. I do not see this discussed in either the main manuscript or SI. There should be one or two sentences explaining in the main manuscript and more details in the SI. Assuming the authors can justify the presence of these naturally charged complexes, I would like to know more about their formation. Are you assuming this is a gaseous reaction? The levels of NO3- (107 molecules/cm3) used in the equilibrium calculations (line 104) are very low, but I am unclear of the reaction or the source of the NO3. This needs to be clarified. Its also not clear from the manuscript how important is this complex. The only quantitative concentration information is shown in Figure 2 and its certainly not possible to infer which are the most important species. I recommend the authors add a brief summary of the data to a new table. This information is important as it allows other researchers to target the most important HgX2 compounds (or complexes).\nOther comments:\nLine 30: contradictory sentence “dominates in the Arctic, while HgCl2 prevails…”\nFigure 1: The detection of the isotopic pattern is critical evidence for the authors claim, yet its very hard to see in this figure. While the time series is useful, if space is tight this should go to the SI in favor of a clear view of the isotopic pattern.\nFigure 2: Similar comment as above. Its very hard to read the top sub-part that shows the isotopic pattern. Figure 2b and 2c are good. Not sure what to make of the O3 and BrO data. Seems these could go to the SI.\nFinally I would note that detection of HgX2 by MS has been reported previously, though not in ambient air (doi:10.5194/amt-9-2195-2016).\nOther comments:\nWhat are the noteworthy results?\nDetection of HgX2 in ambient air.\n- Will the work be of significance to the field and related fields? How does it compare to the established literature? If the work is not original, please provide relevant references.\nYes.\n- Does the work support the conclusions and claims, or is additional evidence needed?\nPartly. See comments above.\n- Are there any flaws in the data analysis, interpretation and conclusions? - Do these prohibit publication or require revision?\nSee comments.\n- Is the methodology sound? Does the work meet the expected standards in your field?\nYes.\n- Is there enough detail provided in the methods for the work to be reproduced?\nPartly.\nSee comments above.", "reviewer_3": "This study presents the direct, in situ measurements of individual gas-phase oxidized mercury species in the ambient atmosphere, using atmospheric pressure chemical ionization mass spectrometry. This is an important and novel contribution to the mercury community. However, I cannot agree with its main highlight, that the observations challenge existing chemistry models, based on the current results. More evidences and discussions are needed.\nSpecific comments:\n1. Line 157-158: This study just cites one reference (Shah et al., 2021) using GEO-Chem model, which cannot represent current all atmospheric mercury models. In addition, the proportions of oxidized mercury species here are global annual average, which cannot be compared with the measurements at the polar areas in specific periods.\n2. Current atmospheric mercury models have included HgBr2 mechanisms. In another cited reference (Saiz-Lopez et al., 2020), which is also written by the corresponding author of this study, HgBr2 is predicted to be a major contributor to GOM at the surface globally using the GLEMOS Model without mercury photo-reduction mechanisms. The authors can show the simulation results at the corresponding observation sites and periods. HgBr2 is expected to be more important at polar areas with abundant halogens, especially during “bromine explosion”.\n3. In the above reference (Saiz-Lopez et al., 2020), HOHgBr is predicted to be a main contributor to GOM, especially when including mercury photo-reduction mechanisms. In Shah et al. (2021), HOHgBr and Hg(OH)2 are predicted to be important contributors to GOM. However, the CI-APi-TOF method does not detect HOHgBr and Hg(OH)2, which cannot prove the larger contributions of HgBr2 than HOHgBr or Hg(OH)2.\n4. In Line 146-154, the authors try to convince us that the measured species by CI-APi-TOF represent most GOM, by comparing with Tekran measurements. When we go to the same periods, e.g. April 2020, the CI-APi-TOF measurements during MOSAiC are just nearly a half of the Tekran measurements at the Canadian Alert station. In addition, the Tekran measurements of oxidized Hg are known to be strongly biased low, and the extent of that bias can vary by almost an order of magnitude, depending on the atmospheric conditions and compositions of oxidized Hg. Considering these potential underestimations, the CI-APi-TOF measured HgBr2 may just contribute less than 20-30% of GOM.\nReferences:\nShah, V.; Jacob, D. J.; Thackray, C. P.; Wang, X.; Sunderland, E. M.; Dibble, T. S.; Saiz-Lopez, A.; Cernusak, I.; Kello, V.; Castro, P. J.; Wu, R.; Wang, C., Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury. Environmental Science & Technology 2021, 55, (21), 14445-14456.\nSaiz-Lopez, A.; Travnikov, O.; Sonke, J. E.; Thackray, C. P.; Jacob, D. J.; Carmona-Garcia, J.; Frances-Monerris, A.; Roca-Sanjuan, D.; Acuna, A. U.; Davalos, J. Z.; Cuevas, C. A.; Jiskra, M.; Wang, F.; Bieser, J.; Plane, J. M. C.; Francisco, J. S., Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere. Proceedings of the National Academy of Sciences of the United States of America 2020, 117, (49), 30949-30956."}, "author_response": "Reply: We thank the reviewer for the constructive comments on our manuscript. We\ngreatly appreciate the positive evaluation of our observational data and the novelty of the\nmethod. We also acknowledge the concerns raised regarding the model comparison and\nthe treatment of data below the limit of detection. We will carefully address these points\nbelow and revise the manuscript accordingly to ensure accurate discussion and\ninterpretation.\nAbstract, Lines 24-26: The authors state that “previous measurements have been unable\nto identify specific gas-phase oxidized mercury species at ambient levels.” Interestingly,\nthe authors cite Deeds et al. (2015, Analytical Chem), which reports ambient\nconcentrations of HgCl and HgBr in urban air in Montreal. While I agree that the data\n2 2\npresented in Deeds et al. is extremely limited (since the paper goal was methodological,\nnot an ambient study), the authors statement is inaccurate as written and should be\nrevised to take into account this prior work.\nReply: We have revised the abstract to acknowledge as the first study to demonstrate\ndetection of HgCl₂ and HgBr₂ in ambient air. To clarify the novelty of our work, we now\nstate that our study provides the first online, in situ molecular measurements of speciated\noxidized mercury in remote polar regions. The revised sentence in the abstract reads:\n“Previous measurements have only rarely identified specific oxidized mercury species at\nambient levels, but no online, in situ molecular measurements have been reported for\nremote environments.”\nIn addition, we added “speciation” in the title to better reflect the nature of the study.\nLines 128 & 133: By definition, an analyte cannot be detected by a method below its LOD.\nTherefore, it is not accurate to state “…ClHgI which is seen slightly under its LOD” because,\nby definition, the instrument did not measure (“see”) ClHgI if below the LOD. Nor is it\naccurate to state “In reality, even lower concentrations can be detected than is expected\nby the LODs, as seen with ClHgI.” Overall, it is inaccurate to state that ClHgI was observed\nat Aboa.\nReply: We thank the reviewer for this accurate observation. We agree that our previous\ndefinition of ClHgI was not precise, because the earlier LOD estimate for ClHgI was not\nbased on an optimal baseline definition. In the revised version, we recalculated the ClHgI\nLOD (all LOD for Aboa campaign) using a 24-hour clean period at the end of the Aboa\ncampaign, during which no ClHgI peaks were present. This provided a more robust\nestimate of instrumental noise. Using this method, the updated LOD for ClHgI is 25 pg\nm⁻³. The measured ClHgI concentrations at Aboa exceeded this revised LOD, and the\ndetection is therefore valid.\nWe have corrected the text accordingly and removed the previous statements implying\nobservations below the LOD (L128-130). “All compounds show values above the limit of\ndetection (LOD), which is estimated between 25 – 37 pg Hg m-3 for 60-minute time\nresolution (Table S2 and Methods for LOD determination).” We also added a figure in the\nsupplementary material to depict the peak free spectra from the period when LOD was\ndefined (Fig S8, figure attached below in the other LOD related comment).\nLine 91: Similar to the above comment, the authors state here that HgBr2 was detected in\nJune in the Arctic and refer to Fig S1. However, Figure S1 and its caption show that the\nsignal during this period was entirely below LOD. Therefore, HgBr2 was NOT detected in\nJune.\nReply: This has been corrected as it is clear that HgBr₂ was not detected in June. We now\nstate explicitly that all signals were below LOD during this period.\n“In the central Arctic, HgBr2 was the only detected neutral species in April (Fig. 2) and by\nJune the signal was below LOD (Fig. S1).”\nRelated to the above comments, the authors state, on Lines 129-130, that “…the LOD is\ndetermined using three times the standard deviation of the measured baseline during the\nwhole campaign; this baseline changed with environmental conditions such as\ntemperature inside the measurement cabin.” Given this fluctuation, why don’t the authors\ncalculate time-varying LODs to get the most out of their data (and to test their hypothesis\nthat the ClHgI signal is real (above comment))? Please also clarify in the methods what is\nmeant by “baseline”; is this an N2 background measurement?\nReply: We thank the reviewer for this important question. During both campaigns, we\nroutinely performed zero-air measurements using a HEPA filter, because the primary goal\nof the deployment was to quantify condensable vapours (such as sulfuric acid, iodic acid,\nHOMs etc). These compounds are efficiently removed by the large surface area of the\nHEPA filter. Unfortunately, mercury halides are not removed by HEPA filtration but instead,\nthe filter material releases additional organic material that is detected at the mercuric\nhalide m/z. For this reason, the HEPA-filtered zero-air measurements were not suitable for\ndetermining the instrumental baseline or the LOD of mercury halides.\nConsequently, our definition of “baseline” in this context does not refer to an external N₂\nor zero-air background, but rather to the instrumental noise level in the stick signal at the\nexact m/z of each mercury halide (added Fig. S8 for reference). To obtain a reliable\nestimate of this noise, we used two complementary approaches:\nAboa: The LODs from Aboa have been recalculated using the method described below:\nWe identified a 24-hour period at the end of the campaign during which no mercury-\nhalide peaks were present at any of the relevant m/z values. The LOD was calculated by\nadding three times the standard deviation of the unit mass resolution signal (so called\nsignal integration region, SIR, 0.4 Th width) on the average recorded and baseline\ndeducted spectrum (Fig. S8) during this clean interval. This provided a stable and physically\nmeaningful baseline (LOD), independent of short-term variability in temperature or\ncounting statistics. The new LOD have been updated on the table S2 and in Figure 1 of the\nmain manuscript text.\nMOSAiC: At the MOSAiC site, the mercury-halide m/z values always contained overlapping\nsignals from organic compounds (i.e., double peaks at the same nominal m/z). Therefore,\na continuous clean segment did not exist from which a baseline could be extracted. In this\ncase, we estimated the noise level from the inter-peak signal (local minima between two\nconsecutive peaks in the spectrum, Fig. S8), resulting in an m/z-specific background\nrepresentative of the instrumental noise.\nImportantly, this inter-peak “noise” signal corresponds to the noise integration region\n(NIR, 0.4 Th width) defined within tofTools. The NIR does not represent a pure zero-level\nmeasurement but rather the lowest non-peak ion counts in the immediate vicinity of the\nanalyte peak. As is standard in TOF mass-spectrometer processing, this noise region is\nunavoidably influenced by neighbouring peaks because the extraction window (Δm/z)\nused for both SIR and NIR integrates over a finite mass range. This means the NIR includes:\ncontributions from unresolved neighbouring ions, the detector’s electronic noise, and\nstatistical (Poisson) noise from the ion counting process.\nAlthough the NIR is therefore not a “true baseline” in the sense of a filtered zero-air\nmeasurement, it is the correct and internally consistent estimate of noise within the\ntofTools framework, because:\n- LOD is defined relative to the same integration window used for the signal (SIR).\n- Using the NIR ensures that the noise estimate reflects the exact mass-spectrometric\nconditions under which the analyte is quantified.\nThis method is widely used for compounds whose m/z windows contain nearby or\noverlapping signals, including many organic species measured with CI-APi-TOF.\nThe inter-peak NIR values provides the most accurate possible estimate of the noise level\nfor mercury-halide m/z values at MOSAiC. This is also the method we used to calculate\nAboa LODs in the original version of the manuscript.\nWe have revised the Methods section to clarify the definition of “baseline” and the\nprocedure used to determine LODs and added Fig. S8 to define SIR and NIR during times\nwhen no mercuric halide peaks were detected. The text in methods now reads as following:\n“Detection limit for mercuric halides:\nThe sensitivity of the CI-APi-TOF instrument is relatively high with low detection limits,\nonly obscured by the baseline signal due to the constant feed of charger ion-containing\ngas into the system, and electronic noise. In our processing workflow (tofTools),\nbackground correction is always applied before further analysis. The limit of detection\n(LOD) of the instruments used at Aboa (60 min time resolution, observations in January\n2015) and the MOSAiC (180 min time resolution, observations in April 2020) campaign are\ngiven in Table S2.\nFor the present study, the limit of detection (LOD) was determined separately for the Aboa\nand MOSAiC datasets because the baseline characteristics and the presence of interfering\npeaks significantly differed between the sites (Fig. S8).\nAboa:\nAt Aboa, a 24-hour period at the end of the campaign, between 26 and 27 Jan 2015,\nshowed no detectable peaks at the m/z values of any mercury-halide species. The LOD\nwas calculated by adding three times the standard deviation of the unit mass resolution\nsignal (so called signal integration region, SIR, 0.4 Th width) on the average baseline\ndeducted signal during this clean interval. This provided a stable and physically meaningful\nbaseline (LOD), independent of short-term variability in temperature or counting statistics.\nMOSAiC:\nAt the MOSAiC site, the mercury-halide m/z values always contained contributions from\noverlapping organic ions (double or partially resolved peaks, Fig. S8). Consequently, a\nclean baseline interval comparable to Aboa did not exist. Instead, we estimated the noise\nfrom the inter-peak signal, i.e. the local minima between transient organic peaks. This\nregion corresponds to the Noise Integration Region (NIR) in tofTools.\nBecause the NIR represents the lowest ion counts within the same Δm/z extraction window\nused for quantifying the analyte (the Signal Integration Region, SIR), it provides the most\naccurate and internally consistent estimate of the noise when neighboring peaks are\npresent. The NIR can include low-level contributions from unresolved ions, which results\nin slightly higher LODs compared to a clean SIR baseline; however, this approach is\nstandard for TOF mass spectrometers and necessary for masses where true zero-signal\nperiods do not exist. LODs for MOSAiC were computed as 3σ of this NIR signal for each\nm/z.”\nAdded supplementary Fig. S8:\nFigure S8: LOD definition. Example spectra from a periods when we did not detect mercuric\nhalides A) from Aboa (29 December 2014, 21:00) illustrating the Signal Integration Region\n(SIR, blue) and the Noise Integration Region (NIR, red), both 0.4 Th wide. During the Aboa\ncampaign, a 24-hour clean period existed in which no mercury-halide peaks were present\nat the relevant m/z values. This allowed the LOD to be determined directly from the\nbaseline (SIR) during a clean interval. B) Corresponding spectrum from MOSAiC (1 June\n2020, 00:00) showing that the mercury-halide m/z values consistently contained\nunidentified organic peaks even when mercuric halide signal is not detected (green arrow).\nBecause no clean baseline interval exists for MOSAiC, the noise is estimated from the\nlowest inter-peak signal (NIR), which corresponds to the baseline treatment in tofTools.\nThe NIR represents the minimum ion counts within the same extraction window used to\nquantify the analyte and includes low-level contributions from neighbouring masses,\nresulting in a slightly higher (more conservative) LOD estimate. In MOSAiC, the SIR width\nwas 0.6 Th and NIR 0.3 Th\nJustification for current model Hg oxidation mechanisms being inaccurate and the\n“discrepancy between modelled and detected species” is a general comparison to global\nmodeling results on Lines 157-158. However, it is not clear whether these global modeling\nresults of oxidized mercury speciation correspond to springtime in the Polar regions, since\nbromine chemistry, for example, is most active in the Polar regions and in springtime. If\nthey simply correspond to a global annual average, then this is not an appropriate\ncomparison. Reactive halogen chemistry varies significantly spatially and temporally across\nthe globe, with the lifetimes of halogen atoms being incredibly short. Therefore, one would\nnot expect an equal distribution of speciated oxidized Hg compounds spatially and\ntemporally across the globe. The authors allude to potential heterogeneity on Lines 189-\n191 where they call for further measurements and reanalysis of existing datasets to\n“illuminate GOM speciation trends across seaons and environments.” This discussion is\ncritical to the interpretation of the results herein and conclusions (e.g. “missing reaction channels”, Line 170). This discussion and comparison needs to be strengthened with a\nPolar springtime specific comparison in order to properly evaluate the model to\nmeasurement comparison.\nIn response to both Reviewers #1 and 3 comments on this matter, we have strengthened\nthe discussion by including chemical modelling specific to the polar springtime period.\nThe following text is now added into the methods section:\n“Photochemical and kinetics model details:\nSimulations in a (photo)chemical box model with the latest mercury chemical\nscheme3,12,13,52 (Table S4). The model allows all the HgI,II species to vary freely throughout\nthe simulation, with their coupled continuity equations (ordinary differential equations)\nsolved by using a Rosenbrock integrator. The simultaneous rate equations are converted\ninto the differential equations using the KPP 2.2.3 kinetic preprocessor53. The photolysis\nrates were calculated using the absorption cross-sections, quantum yields and the actinic\nflux vertical profile, computed using the Tropospheric Ultraviolet and Visible (TUV)\nradiation model54. The actinic fluxes during May at 79° N, 0° E, were used as representative\nof springtime condition in polar regions. The model is constrained with 1.5 ng m-3 of Hg0,\n17 ppbv ozone, 90 ppbv CO, 1.95 ppmv CH4, 8 pptv NO2 and 45 pptv DMS. Two scenarios\nfor high bromine (BrO peaking at 20 pptv) and low bromine (BrO peaking at 1 pptv) were\nrun to represent bromine explosion and normal air mass events.”\nFig. S6 shows the oxidized Hg speciation predicted by the box model. Under low bromine\nconditions, the model predicts that HOHgBr (58.7%) would dominate, followed by HgCl2\n(41%). Other oxidized Hg species are predicted to constitute less than 0.3% of total GOM.\nThis is due to Hg0 oxidation by Br forming HgBr, which rapidly reacts with O3 to form\nOHgBr, ultimately forming HOHgBr upon reaction with methane (4, 47). HgCl2 forms from\nHg0 oxidation by OH or Cl and subsequent reactions of intermediate species with HCl\n(Table S4), and its accumulation in the atmosphere is facilitated by its photostability\ncompared to other GOM species (48). During periods with higher atmospheric bromine,\nlike Arctic springtime bromine explosion events (28), the model predicts that HOHgBr\n(96.6%) would dominate the reaction products, with HgCl2 (3.3%) and other GOM forms\n(0.1%) contributing minor amounts. Thus, the photochemical box model suggests that\nHOHgBr and HgCl2 are the primary GOM species in the atmosphere for Arctic springtime\nconditions, while other HgII species contribute negligibly. However, the CI-APi-TOF\nobservations reveal that a broader range of HgII species contribute to the total observed\nGOM mass concentration.\nThe text now reads (L158 onwards):\n“The close agreement across methods indicates that HgBr constitutes a major fraction of\n2\nspring GOM in the polar regions, contradicting current atmospheric models and our\nunderstanding of Hg redox chemistry. GEOS-Chem, a Hg chemical-transport model,\npredicts that oxidized mercury is dominated globally by HgCl (49%), followed by\n2\nparticulate Hg (22%), Hg(OH)2 (19%), and HOHgBr (9%)3, reflecting oxidation via the Br,\nOH, and Cl pathways, which all involve second-step oxidation by O 3,4,15. The WACCM\n3\nchemistry-climate model also reports HgCl2 to be the dominant atmospheric oxidized\nmercury species, with a minor contribution (<0.1%) of HgBr2 globally42. Photochemical\nmodel simulations of the polar springtime period predict that HgCl2 and HOHgBr are the\nmajor components of GOM, with other HgII species contributing minor amounts (< 0.3%)\n(Fig. S6).”\nAdded material to the supplementary:\nFigure S6. Comparison of CI-APi-TOF measurements with HgII modelling results. Panels a and b:\nmodelled partitioning of HgII gas-phase reaction products under high and low bromine conditions,\nrespectively. Panels c and d: HgII partitioning for the CI-APi-TOF measurements in Antarctica for\nperiods representing high and low GOM concentrations, respectively.\nTable S4. List of reactions and rate coefficients used in the (photo)chemical box model.\nNo. Reaction Rate Coefficients (a) Reference\nInitial reactions for elemental mercury\nBromine Reactions\n2\n6b HgBr + Br + M → HgBr + M 2.5 × 10-10 × (T/298)-0.57 8\n2\n7b HgBr + OH + M → HOHgBr + M 2.5 × 10-10 × (T/298)-0.57 8\n8b HgBr + Cl + M → BrHgCl + M 3.00 × 10-11 9\nk = [M] × 4.3 × 10-30 × (T/298)-5.9\n0\n2 ∞\nf = 0.6\nc\nk = [M] × 4.3 × 10-30 × (T/298)-5.9\n0\n2 ∞\nf = 0.6\nc\n11c HgBr + O → OHgBr + O 7.50 × 10-11 12\n3 2\n4 3\n2\nOHgBr + hv → 0.56 HgO + 0.56 Br + 0.44 Hg\n15d 16\n+0.44 Br + 0.44 O\n16d HgBr + hv → 0.6 HgBr + 1.4 Br + 0.4 Hg 17,18\n17d 16,17\nHg + 0.65 OH + 0.15 HgBr\nBrHgCl + hv → 0.6 HgBr + Cl +0.4 Br + 0.4\n18e 17\nHg\nBrHgONO + hv → 0.9 OHgBr + 0.1 HgBr +\n19d 16,17\n0.1 NO + 0.9 NO\n20d 17\n0.66 Br + 0.69 HO + 0.03 HgBr\n22b HgOH + Br + M → HOHgBr + M 3.00 × 10-11 9\n23b HgOH + OH + M → Hg(OH) + M 3.00 × 10-11 9\n2\n24b HgOH + Cl + M → HOHgCl + M 3.00 × 10-11 9\nk = [M] × 3.69 × 10-17 × T-4.75\n0\n2 ∞\nf = 0.6\nc\nk = [M] × 7.68 × 10-19 × T-4.25\n2 k = 1.24 × 10-4 × T-2.53\n∞\nf = 0.6\nc\n27c HgOH + O → OHgOH + O 7.50 × 10-11 12\n3 2\n4 2 3\n2\n2 2\n2 2 2\n2 2\nHg(OH) + hv → 0.35 HgOH + 0.85 OH + 0.5\n34e 2 20\nHg + 0.65 OH + 0.15 HgOH\n2\n37b HgCl + Cl + M → HgCl + M 3.00 × 10-11 9\n2\n38b HgCl + Br + M → BrHgCl + M 3.00 × 10-11 9\n39b HgCl + OH + M → HOHgCl + M 3.00 × 10-11 9\nk = [M] × 4.3 × 10-30 × (T/298)-5.9\n0\n2 ∞\nFcr = 0.60\nk = [M] × 4.3 × 10-30 × (T/298)-5.9\n0\n2 ∞\nFcr = 0.60\n42c HgCl + O → OHgCl + O 7.50 × 10-11 12\n3 2\n4 3\n2\n2\n2 2\nOHgCl + hv → 0.673 HgO + 0.327 Hg +\n48 20\n0.327 O + 1.0 Cl\n49e HgCl + hv → 0.6 HgCl + 1.4 Cl + 0.4 Hg 17\n50e 17\n0.1 NO + 0.9 NO\n51e 17\n0.66 Cl + 0.69 HO + 0.03 HgCl\n52 20\n0.906 Hg + 0.937 OH + 0.031 HgCl\n(a)Unimolecular rate coefficients are in s-1, bimolecular reaction rate coefficients are in cm3 molecule-1\ns-1; [M] is a third body, in this case an air molecule.\n(b)These reactions take place at the high-pressure limit in the atmosphere and the rate coefficient is given\nfor the effective bimolecular reactions\n(c)Assumed temperature-independent within the relevant range of atmospheric temperatures,\nconsidering the large exothermicity of the reaction and that the rate constant at room temperature is\nclose to the gas-phase kinetic limit.\n(d)Calculated relative molar yields of photolysis products for excitation energies at λ >290 nm.\n(e)Calculated relative molar yields of photolysis products as estimated from the photodissociation of\nsimilar compounds at excitation energies at λ >290 nm. In the case of HgCl , the yield associated with\n2\nthe double dissociation of the system must be considered as a lower-bound estimation. At shorter\nwavelengths, a higher production of the Cl + Hg photoproducts is expected.\nSimilar to the above comment, clarify what is meant by “incorporation of our speciation\ndata into chemical-transport models” (Line 178; similar phrasing again on Line 186). How\ncan this be done if there are “missing reactions channels” (Line 170) that haven’t been\nidentified? This also points to confusion about the authors implying that HgBr2 and HgCl2\nformation aren’t included in models.\nReply: We understand the reviewers’ point that the speciation data cannot be directly\nincorporated into chemistry-transport models and require intermediate research steps.\nOur intent was to advocate for future studies to explore additional chemical reactions that\ncan explain the new speciation measurements, which could then be incorporated into\natmospheric Hg chemistry models. We have made this point more explicit in the revised\nversion:\nL183: “Refining the Hg chemistry schemes in chemical transport models to better\nrepresent measured HgII speciation could lead to strong shifts in regional deposition\npatterns, with direct consequences for ecosystem exposure assessments.”\nL191: “Further analysis of these new speciation insights is required to identify the missing\nchemical reactions that can be incorporated into chemical transport models. The inclusion\nof these missing pathways will improve predictions of regional mercury deposition and\nhelp clarify how photolysis, heterogeneous reactions and halogen availability drive the\npolar mercury cycle.”\nThe presence of the ambient NO3- clusters in Antarctica is noted but not related to\nreaction mechanisms. Further discussion of how this fits into knowledge of reaction\npathways would be helpful. To support this, LODs need to be added to Figures S2 and S3,\nwhich present these data.\nReply: The referee is correct that the detection of ambient HgX NO - clusters have not\n2 3\nbeen linked with the formation mechanism. In the lower troposphere, ions and trace gases\ngo through collisions that result in the transfer of the negative charge to the lowest proton\naffinity species (and in the transfer of positive charge to the highest proton affinity\nspecies). Measuring ambient ions can thus provide important insight into the most\nnegative trace species and their ion – molecule clusters (lowest proton affinity species).\nNO - ions are one of the most abundant anions species detected in the ambient air (e.g.\nclustering when NO - collides with HgX2 (NO3- + HgX2 <-> HgX2NO3-). Figure S2\n3\nmeasurements are used to support our findings and to validate the observational results\nnot to be an artefact from the used CI-inlet, gas system or tubing material used.\nAdditional Comments:\nLines 34-35: It is stated “Our results show that real-time molecular measurements are able\nto…guide effective emission-control strategies.” Please revise this phrasing because this\nmanuscript does not show results evaluating emission-control strategies.\nReply: Line 34–35: Rephrased to: “Our results show that real-time molecular measurements\ncan substantially advance global mercury monitoring and provide a stronger observational\nbasis for evaluating mercury deposition in sensitive ecosystems”.\nLine 50: It is stated that Br2 and BrCl are released from “photochemical activation of sea-\nice surfaces”. However, it was shown by Pratt et al. 2013 (Nat. Geosc.) that the sea ice\nsurface is buffered and does not produce Br2 (or BrCl). Therefore, the authors should\nchange the phrasing to “photochemical activation of saline snowpacks”.\nReply: Line 50 is now rephrased as: “Polar regions are hotspots for this cycle, especially\nduring spring when photochemical activation of saline snowpack releases reactive\nbromine, triggering ozone depletion events and rapid Br radical oxidation of Hg0 to HgII\nwithin hours”\nLine 65: I recommend splitting this long paragraph here.\nReply: Noted, thank you for the suggestion.\nLine 76: “real-time, (sub-)hour detection” seems somewhat contradictory as phrased.\nPlease clarify – perhaps replace with “online detection”.\nReply: Line 76: Changed to “online detection”.\nLines 78 and 80: Add instrument references.\nReply: The reference is given on line 70, where the instrumentation is introduced the first\ntime. We added Junninen et al., AMT, 2010 as for APi-TOF citation.\nLine 110 and elsewhere: The authors alternate between referring to “Antarctica” and\n“Aboa”. For clarity, I suggest referring to Antarctica, rather than the station name, in the\ndiscussion, since the discussion refers to the “central Arctic”.\nReply: We changed the station name “Aboa” to Antarctica in the main text (or Aboa,\nAntarctica) for clarity. We opted to keep the station name visible because measurements\nhave also been carried out at other Antarctic research stations, and specifying Aboa avoids\nambiguity. For consistency, we continue to use the campaign name MOSAiC when\nreferring to the central Arctic expedition.\nLines 137-138: This HgBr2 seasonality is consistent with the seasonality of Arctic reactive\nbromine chemistry (Burd et al. 2017, JGR). Relating to this prior work would further support\nyour finding.\nReply: Line 147: Added citation to Burd et al. (2017).\nLine 147: It is not possible to measure 0 pg m-3. The range minimum should be noted as\n“<LOD”.\nReply: Line 149: Reported “<LOD” instead of “0 pg m⁻³.”\nLines 167-168: Please clarify “the agreement and similarity of the concentrations of pure\nhalides” to “the similarity of the concentrations of the measured mercuric halides”.\nReply: Line 169–171: Rephrased to “similarity of the measured mercuric halide\nconcentrations.”\nFigure 1, top right: It appears that there is an erroneous superscript “-“ on the label.\nReply: We thank the reviewer for their careful reading! Typo fixed.\nFigure 1: It would be useful for the reader to state in the Figure 1 caption that these are\nneutral compounds being detected as NO3- adducts in the mass spectrometer.\nReply: The figure caption now reads as: “Hourly averaged example spectra of neutral\nmercuric halides charged by nitrate ions. HgCl ·NO -, BrHgCl·NO - and HgBr2·NO - spectra\n2 3 3 3\nwere measured on 29 November 2014, and BrHgI·NO -, HgI2·NO - and ClHgI·NO - on 11\n3 3 3\nJanuary 2015 at Aboa, Antarctica.”\nFigure 2E: The BrO data presented here are not discussed in the text.\nReply: We have now removed panel D) from Fig. 2 since it is not discussed in the text.\nFigure S4, Top: Please clarify, in the caption, what the breaks in the data correspond to.\nReply: Gladly. We added the following text in the caption: “The gaps in observational data\ncorrespond to power cuts, instrument maintenance or zero sampling periods.”\nFigure S6: Add the sampling location to the caption for improved clarity.\nReply: Done: ”Example of the CI-APi-TOF capabilities with lower averaging time (15\nminutes) for a week in January 2015, Antarctica.”\nTable S1: Add a column for the expected mass intensities, for comparison to the observed.\nWe thank the reviewer for this suggestion. Table S1 already contains the calculated\n(expected) isotopic mass intensities of the observed mercuric halides, which correspond\nto the black vertical lines plotted in the spectra of Fig. 1. To avoid confusion, we have\nrevised the table caption to state this explicitly.\n“Table S1. Mass-to-charge ratios and calculated isotopic mass intensities (>1%) of the\nobserved mercuric halides. These calculated intensities correspond to the black vertical\nbars shown in the spectra of Fig. 1, for direct comparison with the observed peak patterns.”\nReviewer #2 (Remarks to the Author):\nReview of “Direct observations of atmospheric oxidized mercury in polar areas” by Jokinen\net al.\nThe detection of oxidized mercury compounds has been an elusive goal for several\ndecades. It is assumed that these compounds represent an important sink in the global\nmercury cycle, but our knowledge of them is very low. Thus unambiguous identification\nand quantification of oxidized compounds is a key missing link in understanding the global\nmercury cycle.\nJokinen et al present strong evidence that they have detected HgX2 compounds in the\npolar regions. The key data to support this is the Hg isotopic pattern, shown in Figures\n1+2 (although figures need improvement). Based on these observations and the method\ndevelopment this paper will be an important contribution to the scientific/Hg literature.\nReply: The authors thank the reviewer for the careful reading of the manuscript and the\ncomments provided. We answer all of the suggestion and questions below in details.\nI have more questions around the claimed detection of naturally charged ions such as\nHgX2-NO3- . Figure 2 shows the mass pattern supporting this claim and the observations\nappear to be consistent with the claim. I am just unclear how this complex would form and\n(mostly) the source of the NO3 ion.\nReply: In the lower troposphere, ions and trace gases go through rapid collisions that result\nin the transfer of the negative charge to the lowest proton affinity species (and in the\ntransfer of positive charge to the highest proton affinity species). Measuring ambient ions\ncan thus provide important insight into the lowest proton affinity trace species and their\nion – molecule clusters. NO - ions are one of the most abundant anions species detected\n3\nin the ambient air (e.g. Junninen et al., 2010, AMT). They form from proton transfer\nreactions between nitric acid and pre-existing ions such as hydrated oxide ions or\nhydroxide ions. The most probable formation pathway of naturally charged ions such as\nHgX -NO - is noncovalent clustering of NO - with the neutral mercuric halide HgX (NO -\n2 3 3 2 3\n+ HgX <-> HgX NO -). As an example, we may estimate the steady state concentration\n2 2 3\nof HgBr NO - in ambient air, assuming that its dominant sink is re-dissociation:\n2 3\nHgBr + NO - <-> HgBr NO -\n2 3 2 3\nKeq=[HgBr NO -]/([HgBr ][ NO -])\n2 3 2 3\nWe have carried out an ab initio calculation of the bonding energy of HgBr NO - and its\n2 3\nequilibrium constant Keq=5E-7 cm3 molecule-1 (Table 1). The maximum concentration of\nHgBr that has been measured with the CI-APi-TOF is less than 4E5 molecule cm-3. The\n2\ntotal concentration of negative ions in polar regions is typically between 500 and 2000\nions. Let’s assume [NO -] = 50 ions cm-3 (Junninen et al., 2010, AMT). Then [HgBr NO -] =\n3 2 3\nKeq x [HgBr ] x [NO -] = 5E-7 cm3 molecule-1 x 4E5 molecule cm-3 x 50 ions cm-3 = 10 ions\n2 3\ncm-3. This is well within detectability for APi-TOF instruments (<1 cm-3 for 1-hour\nintegration in this mass range).\nAlso, I can not see in the methods how this compound is distinguished from the NO3 ion\nwhich is used in the CIMS mass spec. I do not see this discussed in either the main\nmanuscript or SI. There should be one or two sentences explaining in the main manuscript\nand more details in the SI.\nReply: The NO3-CIMS working principle is to feed an artificial charger ion to ionize ambient\nneutral molecules. The APi-TOF functions without the charging unit as an individual\ninstrument (details of the two instruments are given in the SI: referred as Junninen et al.,\n2010, AMT for APi-TOF, and Jokinen et al., 2012, ACP for the CI-APi-TOF in the main text).\nThus, no distinguishing from the charger ion is necessary in this case.\nAssuming the authors can justify the presence of these naturally charged complexes, I\nwould like to know more about their formation. Are you assuming this is a gaseous\nreaction? The levels of NO3- (107 molecules/cm3) used in the equilibrium calculations (line\n104) are very low, but I am unclear of the reaction or the source of the NO3. This needs to\nbe clarified. Its also not clear from the manuscript how important is this complex. The only\nquantitative concentration information is shown in Figure 2 and its certainly not possible\nto infer which are the most important species. I recommend the authors add a brief\nsummary of the data to a new table. This information is important as it allows other\nresearchers to target the most important HgX2 compounds (or complexes).\nReply: The reviewer appears to be mixing the CI-APi-TOF and APi-TOF methods. As\nmentioned before, these two detections methods (CI-APi-TOF and APi-TOF) are\nindependent from one another. In the CI-APi-TOF system NO - is artificially generated\n3\nfrom nitric acid ([NO -]~107 molecules/cm3) and added to the inlet flow to chemically\n3\nionize neutral compounds in ambient air. Both Figure 1 and Figure 2 give quantitative\nconcentrations from the CI-APi-TOF measurements (neutral species) from two different\nsites. In the clean conditions in Antarctica, we detected several HgX2 compounds, while in\nthe Arctic, only one species (HgBr2) was detected. Figure S2 measurements are used to\nsupport our findings and to validate the results not to be an artefact from the used CI-\ninlet, gas system or tubing material used. Note that the typical NO - levels in ambient air\n3\nare much lower than in the CI-APi-TOF, in the range of tens per cm3 (e.g. Junninen et al.,\n2010, AMT), and that the, which implies extremely low concentrations of naturally charged\ncomplexes, as estimated above.\nOther comments:\nLine 30: contradictory sentence “dominates in the Arctic, while HgCl2 prevails…”\nReply: True, prevail is a wrong choice of words. We have changed the text accordingly:\n“that HgBr2 dominates in the Arctic, while HgCl2 is abundant…”\nFigure 1: The detection of the isotopic pattern is critical evidence for the authors claim, yet\nits very hard to see in this figure. While the time series is useful, if space is tight this should\ngo to the SI in favor of a clear view of the isotopic pattern.\nReply: We understand the comment and we have now plotted the spectra as large as\npossible without removing the timeseries part. Figure 1 is available as an individual high\nresolution file, that can be zoomed in for a better view of the isotopes.\nFigure 2: Similar comment as above. Its very hard to read the top sub-part that shows the\nisotopic pattern. Figure 2b and 2c are good. Not sure what to make of the O3 and BrO\ndata. Seems these could go to the SI.\nReply: Figure 2 is also available as an individual high resolution file, that can be zoomed in\nfor a better view of the isotopes. We have decided to remove the D) panel from Figure 2\nbut to keep panel C) because O data are important to show the correlation between O\n3 3\nand Hg0, which is discussed in the text.\nFinally I would note that detection of HgX2 by MS has been reported previously, though\nnot in ambient air (doi:10.5194/amt-9-2195-2016).\nReply: Thank you for pointing this out.\nReviewer #3 (Remarks to the Author):\nThis study presents the direct, in situ measurements of individual gas-phase oxidized\nmercury species in the ambient atmosphere, using atmospheric pressure chemical\nionization mass spectrometry. This is an important and novel contribution to the mercury\ncommunity. However, I cannot agree with its main highlight, that the observations\nchallenge existing chemistry models, based on the current results. More evidences and\ndiscussions are needed.\nReply: The authors thank the reviewer for careful reading and comments on the\nmanuscript. We hope to address all the criticism adequately below.\nSpecific comments:\n1. Line 157-158: This study just cites one reference (Shah et al., 2021) using GEO-Chem\nmodel, which cannot represent current all atmospheric mercury models. In addition, the\nproportions of oxidized mercury species here are global annual average, which cannot be\ncompared with the measurements at the polar areas in specific periods.\nReply: There are other atmospheric mercury models, but the study by Shah et al. (2021) is\nmost relevant in that: 1) the chemistry has been updated to the state-of-the-art chemistry;\n2) the simulated speciation of HgII is actually reported in the study. We have revised the\ntext to acknowledge that the cited study is from GEOS-Chem. To address this concern and\nto further demonstrate the discrepancy with existing chemical models, we have added\nphotochemical box modelling for polar springtime conditions, detailed in the Response to\nReviewer #1.\nThe text now reads (L158 onwards):\n“The close agreement across methods indicates that HgBr constitutes a major fraction of\n2\nspring GOM in the polar regions, contradicting current atmospheric models and our\nunderstanding of Hg redox chemistry. GEOS-Chem, a Hg chemical-transport model,\npredicts that oxidized mercury is dominated globally by HgCl (49%), followed by\n2\nparticulate Hg (22%), Hg(OH)2 (19%), and HOHgBr (9%)3, reflecting oxidation via the Br,\nOH, and Cl pathways, which all involve second-step oxidation by O 3,4,15. The WACCM\n3\nchemistry-climate model also reports HgCl2 to be the dominant atmospheric oxidized\nmercury species, with a minor contribution (<0.1%) of HgBr2 globally42. Photochemical\nmodel simulations of the polar springtime period predict that HgCl2 and HOHgBr are the\nmajor components of GOM, with other HgII species contributing minor amounts (< 0.3%)\n(Fig. S6).”\n2. Current atmospheric mercury models have included HgBr2 mechanisms. In another\ncited reference (Saiz-Lopez et al., 2020), which is also written by the corresponding author\nof this study, HgBr2 is predicted to be a major contributor to GOM at the surface globally\nusing the GLEMOS Model without mercury photo-reduction mechanisms. The authors can\nshow the simulation results at the corresponding observation sites and periods. HgBr2 is\nexpected to be more important at polar areas with abundant halogens, especially during\n“bromine explosion”.\nReply: The results referred to by the reviewer are from a model version not including Hg(I)\nand Hg(II) photoreduction reactions (“run 1, no-photolysis” in Saiz-Lopez et al., 2020),\nwhich thus cannot represent the current state of knowledge on atmospheric Hg chemistry.\nSeveral studies have been published establishing with quantum chemical calculations that\nHg(I) and Hg(II) species photolyze under tropospheric conditions (Saiz-Lopez et al., 2018;\n2019; 2020; Francés-Monerris et al., 2020). Therefore, the no-photolysis run from Saiz-\nLopez et al. (2020) is no longer representative of state-of-the-art atmospheric Hg\nchemistry models. The run including photolysis from Saiz-Lopez et al. (2020), while not\nincluding several later updates to the Hg chemistry scheme (Shah et al., 2021; Saiz-Lopez\net al., 2022; Gomez Martin et al., 2022; Hewa Edirappulige et al., 2023), shows that HgBr2\nis simulated to be a minor component of HgII once photolysis is considered.\n3. In the above reference (Saiz-Lopez et al., 2020), HOHgBr is predicted to be a main\ncontributor to GOM, especially when including mercury photo-reduction mechanisms. In\nShah et al. (2021), HOHgBr and Hg(OH)2 are predicted to be important contributors to\nGOM. However, the CI-APi-TOF method does not detect HOHgBr and Hg(OH)2, which\ncannot prove the larger contributions of HgBr2 than HOHgBr or Hg(OH)2.\nWe have acknowledged the point that HOHgBr and Hg(OH) are not detected by CI-APi-\nthan those two species, because we cannot comment on the concentrations of those\nspecies from the current measurements. What we do state is that HgBr2 is a major fraction\nof spring GOM in polar regions (L155), which is incoherent with the photochemical box\nmodelling that suggests that HgBr2 would be less than 0.3% of total GOM. We have now\nclarified this aspect in the revised manuscript.\n4. In Line 146-154, the authors try to convince us that the measured species by CI-APi-TOF\nrepresent most GOM, by comparing with Tekran measurements. When we go to the same\nperiods, e.g. April 2020, the CI-APi-TOF measurements during MOSAiC are just nearly a\nhalf of the Tekran measurements at the Canadian Alert station. In addition, the Tekran\nmeasurements of oxidized Hg are known to be strongly biased low, and the extent of that\nbias can vary by almost an order of magnitude, depending on the atmospheric conditions\nand compositions of oxidized Hg. Considering these potential underestimations, the CI-\nAPi-TOF measured HgBr2 may just contribute less than 20-30% of GOM.\nReply: While we understand the reviewer’s point, our ultimate interpretation differs. If we\nassume that we can compare April 2020 MOSAiC (mean HgBr concentration = 33 pg Hg\n2\nm-3) and Alert Tekran measurements (Tekran GOM concentration = 52 pg Hg m-3),\nthen\nwe get to a 63% contribution of HgBr . If we furthermore assume that Tekran\n2\nmeasurements are biased, we can apply a factor of 3–6 bias in the springtime Arctic based\non collocated measurements of GOM from Tekran and cation exchange measurements\nfrom Osterwalder et al. (2021), leading to an assumed GOM concentration of 156–312 pg\nHg m-3. In the case of this conservative back-of-the-envelope calculation, HgBr would still\n2\ncontribute 10–20% of total GOM, which is two orders of magnitude larger than the\nsimulated contribution of HgBr (< 0.3%). This highlights that existing chemical schemes\n2\nfor atmospheric Hg are inconsistent with the current speciation measurements from CI-\nAPi-TOF, even despite the detection limitations that are currently present.\nNew references:\n44. Saiz-Lopez, A. et al. Photoreduction of gaseous oxidized mercury changes global\natmospheric mercury speciation, transport and deposition. Nat. Commun. 9, 4796 (2018).\n(This was not numbered in the original manuscript)\n53. Peterson, K. A. & Puzzarini, C. Systematically convergent basis sets for transition\nmetals. II. Pseudopotential-based correlation consistent basis sets for the group 11 (Cu,\nAg, Au) and 12 (Zn, Cd, Hg) elements. Theor. Chem. Acc. 114, 283–296 (2005).\n54. Francés-Monerris, A. et al. Photodissociation Mechanisms of Major Mercury(II)\nSpecies in the Atmospheric Chemical Cycle of Mercury. Angew. Chem. Int. Ed. 59, 7605–\n7610 (2020).\n55. Damian, V., Sandu, A., Damian, M., Potra, F. & Carmichael, G. R. The kinetic\npreprocessor KPP-a software environment for solving chemical kinetics. Comput. Chem.\nEng. 26, 1567–1579 (2002).\n56. Madronich, S. & Flocke, S. The Role of Solar Radiation in Atmospheric Chemistry.\nin Environmental Photochemistry (ed. Boule, P.) 1–26 (Springer, Berlin, Heidelberg, 1999).\ndoi:10.1007/978-3-540-69044-3_1."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have significantly improved their manuscript, particularly through reanalysis and reconsideration of CIMS LODs and the addition of photochemical box modeling of the polar springtime. My comments here pertain to the newly added box modeling comparisons to the measurements and need for further quantitative assessment and discussion in the main text.\nFor the photochemical box modeling, it would seem that the Cl atom concentration is important in the calculation of HgCl2 (reactions 3, 24, & 37 in Table S4), which is important for interpretation of the model-measurement comparison that lead to the paper conclusions. I could not find any information provided for how chlorine was constrained or introduced in the model, nor whether the simulated Cl atom concentrations are in line with that expected for the polar springtime. Also, just like Br, Cl can also vary significantly in the polar springtime (Liao et al. 2014, Nat Geosc.; Custard et al. 2016, ES&T; McNamara et al. 2019, ES&T), so a sensitivity test, like for Br, seems important for Cl as well.\nThe paragraph starting on line 156 that compares model results with measurements is improved, and the new Fig S6 is useful. However, comparing percentages from models that include compounds not measured by the CIMS is misleading. It would be an improved comparison to calculate box model percentages solely for the compounds measured by the CIMS. This is to say – if you remove HgBrOH (and any additional species that the CIMS couldn’t measure it) from Fig S6, what is the calculated % of HgBr2? Adding pie charts to Fig S6 that only contain compounds that the CIMS could measure would be useful.\nIn addition, the sentence on lines 164-165 about the box model results should include the range in modeled percentages of HgCl2 and HOHgBr, as currently the reader must consult the SI for these values. These results are far more useful than the global percentages given in prior sentence, since the polar springtime is unique (which motivated this box modeling for this manuscript revision).\nFor the box model results, comparison of the measured and modeled speciated mercury concentrations would be highly valuable to evaluate on an absolute basis, rather than only by percentage.\nConsidering Fig 1, it appears that there is temporal variation in the percentages of the various mercuric halide species. This seems important to calculate and discuss when comparing the measurements to the model.\nIn the authors’ response, on pages 10 and 14, they refer to ambient NO3- being one of the most abundant ambient anions in ambient air. However, they reference an observational study in Helsinki, which should have more NOx than Antarctica. Since the authors have the full ambient ion mass spectrum from their measurements, inclusion of this mass spectrum in the SI (presumably showing NO3- as the most abundant anion), like the one in the Junninen et al. paper, would better support their observation.\nThe content in the response at the bottom of page 17 would be useful to incorporate into the manuscript.", "reviewer_2": "Second Review of “Direct observations of atmospheric oxidized mercury in polar areas” by\nJokinen et al.\nThe authors have done a good job of responding to my earlier comments and the research represents a significant step forward in our understanding of mercury observations chemistry. I have just a few small comments that should be addressed prior to final acceptance.\nLines 135-136: I am unclear what p=0.72 refers to. If this is a probability, it certainly is not a strong correlation, as the authors state. Also not clear what “Th” refers to.\nLines 153-155: Confusing sentence.\nLine180: unclear meaning “”as fast as…”\nFor Figures 1 and 2, the authors should provide a one or two sentence explanation of why the isotopic patterns are lines, whereas the m/z patterns are broader. I assume this is due to some type of instrumental broadening, but a one sentence explanation would be helpful.\nOverall, this is a nice and important piece of research.\nDan Jaffe", "reviewer_3": "My comments have been addressed. No further comments."}, "author_response": "Thank you for these constructive suggestions to further improve the manuscript. Our responses to each point are given below in blue typeface.\nFor the photochemical box modeling, it would seem that the Cl atom concentration is important in the calculation of HgCl2 (reactions 3, 24, & 37 in Table S4), which is important for interpretation of the model-measurement comparison that lead to the paper conclusions. I could not find any information provided for how chlorine was constrained or introduced in the model, nor whether the simulated Cl atom concentrations are in line with that expected for the polar springtime. Also, just like Br, Cl can also vary significantly in the polar springtime (Liao et al. 2014, Nat Geosc.; Custard et al. 2016, ES&T; McNamara et al. 2019, ES&T), so a sensitivity test, like for Br, seems important for Cl as well.\nAuthor response: While the chlorine atom concentrations are important for atmospheric chemistry, they are not involved in the rate-limiting step of Hg0 oxidation. The first step in the oxidation of Hg, primarily by Br radicals, is the rate-limiting step towards the formation of HgII, resulting in limited roles for reactions 3, 24, and 37 under polar tropospheric conditions. Here, we have considered an average Cl atom concentration of 1 × 105 molecules cm-3, which is similar to the values reported in the studies mentioned above (Liao et al., Nat. Geosci., 2014; Custard et al., ES&T, 2016). Changing this by one order of magnitude (the range reported in the above studies) does not make a large difference to the modelled HgCl , as the reaction with bromine atoms remains the rate-limiting step (refer to table S4).\nThe paragraph starting on line 156 that compares model results with measurements is improved, and the new Fig S6 is useful. However, comparing percentages from models that include compounds not measured by the CIMS is misleading. It would be an improved comparison to calculate box model percentages solely for the compounds measured by the CIMS. This is to say – if you remove HgBrOH (and any additional species that the CIMS couldn’t measure it) from Fig S6, what is the calculated % of HgBr2? Adding pie charts to Fig S6 that only contain compounds that the CIMS could measure would be useful.\nAuthor response: While we appreciate the suggestion to use percentage comparisons, we feel this gives a somewhat misleading picture of the difference between the model and observations. For example, in the high bromine scenario, HgBrOH contributes 96.6% of the total GOM. If we remove this, the total GOM would be much lower than the observations. In addition, the model predicts that HgCl would account for 3.3% of the total GOM, and the other species would contribute about 0.1%, which is much lower than the current relative contribution of the other species in the observations (62.5%). Hence, such a direct comparison of only species that are measured is not useful and may be misleading.\nIn addition, the sentence on lines 164-165 about the box model results should include the range in modeled percentages of HgCl2 and HOHgBr, as currently the reader must consult the SI for these values. These results are far more useful than the global percentages given in prior sentence, since the polar springtime is unique (which motivated this box modeling for this manuscript revision).\nAuthor response: As suggested by the reviewer, we have added into this sentence the range in percentages of HgCl and HOHgBr as suggested by the box model analysis: “Photochemical model simulations of the polar springtime period predict that HgCl (3–41%) and HOHgBr (59–97%) are the major components of GOM, with other HgII species contributing minor amounts (<0.3%) (Fig. S6).”\nFor the box model results, comparison of the measured and modeled speciated mercury concentrations would be highly valuable to evaluate on an absolute basis, rather than only by percentage.\nConsidering Fig 1, it appears that there is temporal variation in the percentages of the various mercuric halide species. This seems important to calculate and discuss when comparing the measurements to the model.\nAuthor response: Our box model analysis has focused on two case studies for the polar springtime, with high and low bromine, already showing substantially different results to the measurements conducted in Antarctica and the central Arctic. Given that these discrepancies are already illustrated with these two cases, we think it would be superfluous to compare the temporal variability of the measured speciation with the modelled speciation, since the model predictions are not able to simulate the presence of the major measured species. For the purposes of this study which focus on the novel measurements of speciated oxidized mercury, the current comparison with box models is sufficient to show that the existing chemistry schemes are likely missing reaction channels.\nRegarding absolute values, Fig. S6 has provided the absolute values of GOM concentrations within the figure (high bromine case, GOM = 110 pg m-3; low bromine case, GOM = 20 pg m-3), which are comparable to our measured values in Antarctica (interquartile range 29–110 pg m-3) and Arctic (17–49 pg m-3).\nIn the authors’ response, on pages 10 and 14, they refer to ambient NO3- being one of the most abundant ambient anions in ambient air. However, they reference an observational study in Helsinki, which should have more NOx than Antarctica. Since the authors have the full ambient ion mass spectrum from their measurements, inclusion of this mass spectrum in the SI (presumably showing NO3- as the most abundant anion), like the one in the Junninen et al. paper, would better support their observation.\nAuthor response: Thank you for the suggestion. We added a spectrum in Figure S3 to further illustrate the dominance of nitrate peaks during non-new particle formation (NPF) event days. Bisulfate (HSO -) and its clusters dominate the spectrum during NPF days as reported in Jokinen et al., 2018.\nCaption now reads as following:\nFigure S3. Aboa, Antarctica. Top) The ambient-air (NO -) signal of naturally produced nitrate ions, measured with the APi-TOF instrument during the Aboa campaign. The naturally charged nitrate-mercuric halide clusters are likely products of the reactions between the naturally-produced NO - ions and neutral mercury halides in the ambient atmosphere. Bottom) An example spectrum from Antarctica, measured on 23 Dec 2014, during a non-new particle formation day depicting the dominance of nitrate containing peaks. Other larger peaks represent bisulfates (HSO -), methane sulfonate ions (CH SO -) and mixed clusters.\nThe content in the response at the bottom of page 17 would be useful to incorporate into the manuscript.\nAuthor response: We agree, and have now briefly addressed the previous response in the manuscript with the following:\nL165-167: “Even when accounting for uncertainties associated with past measurements and known instrumental biases, the observed HgBr₂ concentrations remain approximately two orders of magnitude larger than the simulated contribution of HgBr₂ (<0.3%; see Supplementary Information for details).”\nReviewer #2 (Remarks to the Author):\nSecond Review of “Direct observations of atmospheric oxidized mercury in polar areas” by Jokinen et al.\nThe authors have done a good job of responding to my earlier comments and the research represents a significant step forward in our understanding of mercury observations chemistry. I have just a few small comments that should be addressed prior to final acceptance.\nThank you for these comments on the revised manuscript, which are addressed below.\nLines 135-136: I am unclear what p=0.72 refers to. If this is a probability, it certainly is not a strong correlation, as the authors state.\nAuthor response: We refer to linear relationship between the measured signals (R² = 0.52; Pearson’s r = 0.72). In ambient field conditions this is considered strong or at least moderate-strong. We changed the wording to “moderate to strong” in the text and changed “rho” to r.\nAlso not clear what “Th” refers to.\nAuthor response: The Thomson (Th) is a unit used in mass spectrometry and it presents the mass-to-charge-ratio, not mass directly. It is defined by 1 Th = 1 Da/e (Dalton, atomic mass unit / e, elemental charge). As it is not a SI unit, we added an unit name at the first occurrence:\nL136: …”HgBr signal and the Br- signal (78.9189 Th, Thomson, likely from HBr30).”\nAuthor response: The reviewer does not specify what is confusing about the sentence:\nLines 153:155 in the current manuscript: …“Acknowledging that Tekran measurements can underestimate GOM, we also compared our data to reactive mercury (GOM + particulate Hg) values from cation-exchange membrane detection at Zeppelin (IQR 37–86 pg Hg m-3)41, noting that these measurements include also a contribution from particulate Hg39.)”\nWe wanted to emphasise that the comparison isn’t direct since one measurement contains only gas phase components and the other includes gas and particulate phase mercury. We would prefer keeping the sentence as it is.\nBecause Tekran measurements have been suggested to underestimate total GOM concentrations14, we also compared our data to reactive mercury (GOM + particulate Hg) values from cation exchange membrane measurements at Zeppelin (IQR 37–86 pg Hg m-3)40,41. We note, however, that the reactive mercury measurements also include a contribution from particulate Hg39, which would not be measured by CI-APi-TOF.\nLine180: unclear meaning “”as fast as…”\nAuthor response: changed to “faster than”\nFor Figures 1 and 2, the authors should provide a one or two sentence explanation of why the isotopic patterns are lines, whereas the m/z patterns are broader. I assume this is due to some type of instrumental broadening, but a one sentence explanation would be helpful.\nAuthor response: Good catch! The isotopic patterns are displayed as theoretical exact-mass stick spectra of that specific elemental composition (exact peak position), whereas the measured ambient spectra are broadened by the finite mass resolving power of the CI-APi-TOF or APi-TOF (m/Δm ≈ 4000-4500). This is now added to the methods section under both CI-APi-TOF (line 425) and APi-TOF (Line 455-457).\nThe text now reads as following:\nLines 424-425: “In both campaigns, a nitrate-based CI-APi-TOF mass spectrometer (Tofwerk, Switzerland, “HTOF”, with finite resolving power of approximately m/Δm ≈ 4000-4500) with a so-called “Eisele” type CI-inlet was employed18”\nLines 455-457: “The TOF analyser is identical with the CI-APi-TOF (Tofwerk, Switzerland, “HTOF”, with finite resolving power of approximately m/Δm ≈ 4000-4500).”\nThe actual peak fitting is not shown for better readability. You may refer to Fig. 2 in Jokinen et al., 2012, ACP in order to better visualize how peaks are fitted (integrated). Jokinen et al.:Ref 18: https://acp.copernicus.org/articles/12/4117/2012/\nOverall, this is a nice and important piece of research.\nThank you!\nDan Jaffe\nReviewer #3 (Remarks to the Author):\nMy comments have been addressed. No further comments.\nThank you!"}]

Decision letter