| 05 Dec 2025
Stability and selectivity of pre-concentration methods for gaseous oxidized mercury in the air
Abstract. The determination of gaseous oxidized mercury (GOM, HgII) in the atmosphere at ultra-trace levels necessitates a preconcentration step. KCl-coated denuders and cation exchange membranes (CEMs) are widely used preconcentration methods for sampling HgII. Sampling losses during preconcentration could result in measurement biases. This study evaluated the performance of denuders and CEMs in retaining HgII, to accurately estimate biases through a precise mass balance approach using 197Hg radiotracer and generation of specific HgII species by non-thermal plasma oxidation of Hg0 in the presence of reactant gases. The results showed that recovery of HgII by thermal decomposition from freshly prepared denuders were approximately 100 %. The retention of HgII on denuders during sampling period was much poorer compared to CEMs. Freshly prepared denuders lost up to 61.5 % of HgII, while reused denuders lost up to 79.7 % after 2 hours of ambient air exposure. CEMs exhibited much lower losses over 72 hours: on average, 16.7 % for HgO, 2.7 % for HgCl2, and 3.8 % for HgBr2. The standard digestion procedure for CEMs by BrCl digestion was found to be incomplete, with a few residual amounts detected on CEMs post-digestion (5.3 % on average). Additionally, the overall mass distribution within the CEM filter pack cartridges revealed that some HgII was retained on their inner Teflon parts. These findings underscore the critical importance of addressing biases in methods adopted for HgII measurements, to improve the accuracy and reliability of atmospheric Hg data, an integral component in evaluating Hg model outputs for Minamata convention effectiveness evaluation.
General Comments:
I found the overall quality of the manuscript to be high. The measurement of gaseous oxidized mercury (GOM) compounds is an important scientific issue, and understanding the biases of current measurement techniques is a relevant scientific question within the scope of this journal. HgII is an extraordinarily difficult compound to make quantitative measurements of due to its propensity to stick to any surface it comes in contact with, but the experimenter’s careful approach allows them to achieve near 100% mass balance, giving additional confidence in their results. There are some typos in the text, but they do not significantly affect the reader’s understanding of the text. The findings are substantive, and the manuscript publication-worthy.
Specific Comments:
I find the discussion from line 283 onwards rather confusing:
Membrane-based measurements were reported to be from 1.3 to 3.7 times higher than denuder-based measurements, both in field and laboratory experiments. Gustin et al. (2019) reported that RM measurements from the RMAS 2.0 (Reno-Reactive Mercury Active Systems) were 3.7 times higher than those from the Tekran 2537/1130 system. Additionally, Tekran 1130 HgII measurements were correlated with HgII data from the CEM in the RMAS+PTFE system. These findings indicate that while both systems measure similar HgII/RM concentrations, the values differ significantly in magnitude.
I am unsure what is intended to be communicated that, “both systems measure similar HgII/RM concentrations, the values differ significantly in magnitude.” Is the intended meaning that there is a strong correlation between the values despite the significant difference in magnitude? It would be good to consider rewriting for clarity.
Line 315 – 320: The authors are entitled to their view on the potential for continued use of the KCl-denuder method, but as a reader I find the prospect of (see line 217) 33 +/- 17% or 21 +/- 7% Hg losses from this method very unappealing. Yes, with appropriate calibration anything can work, but that seems very difficult to correct appropriately.
Technical Corrections:
At various places in the document there seem to be some small typos. No specific response is needed to these notes, but I recommend the text be checked by an experienced editor. (All super/subscripts are correct in the original, but lost when pasted as plain text)
31: suspect this should be re-emission
36: is ‘the’ necessary? (the ambient air)
37: ‘and reported as GOM fraction’ is there a missing article?
38: ‘have mostly been performed using commercially available Tekran 2537/1130 unit’ missing article?
58: ‘lack proper calibration to the SI traceability,’ perhaps, ‘lack SI-traceable calibration’ or ‘lack proper calibration to SI traceable standards’
71: ‘This approach helps validate how the methods behave to different HgII species’ perhaps ‘respond’ rather than ‘behave’?
73: ‘Hg radiotracer’ perhaps ‘radiotracers’?
74: ‘utilized the 197Hg’ unnecessary article
87: ‘irradiated ampoule was carefully opened, and the solution was transferred in
separate vial for storage.’ Missing article before ‘irradiated’ and ‘separate’
101: ‘197Hg2+ ions in solution reduced by SnCl2 to 197Hg0 vapor was trapped in a primary’ Change ‘was’ to ‘were’
102: ‘This primary Au trap with known amount of 197Hg0’ missing article, ‘with a known’
113: ‘The Cl2 gas was produced in the laboratory based on the reaction’ On the subsequent line, Br2 is also produced in situ, but is not described as being, ‘produced in the laboratory’, perhaps, “Cl2 and Br2 gas were produced in the laboratory…” or remove the text, “in the laboratory”
117: ‘CEMs (Pall Corporation - Mustang MSTGS3R) cut off into standard diameter (47 mm) was used for all experiments.’ I think there are some missing words in this sentence.
119: ‘A pair of copper electrode was attached around the outer surface of the CEM filter pack, leaving behind approximately 20 mm gap between both electrodes.’ ‘Electrodes’; remove ‘behind’
121: ‘taking into consideration of the breakthrough’ ‘Of’ is unnecessary.
135: ‘for loading primary Au trap.’ Missing article.
135: ‘For *the* well-type detector, standards were prepared in *triplicate* by spiking *a/the* radiolabeled solution (*at the suitable concentration*) into MQ with 5% HNO3 (v/v) **for desired concentrations** (Not sure if needed). 8 mL of each *standard was* transferred into separate vials for measurements in the well-type HPGe detector similar to the analysis *for* all sample solutions.’ See the suggested changes.
142: ‘The quantification of losses 197HgII from the denuder’ ‘Lost’ perhaps?
145: ‘As air was pulled through denuder inlet’ Missing article
146: ‘A KCl-trap was also plugged before the Au trap to check whether’-perhaps ‘inserted’ rather than ‘plugged’
150: ‘ambient air flow for longer duration’-should be either ‘for a longer duration’ or ‘for longer durations’.
152: ‘at the flow rate of 1 L min’ ‘a flow rate’
156: ‘each CEMs were’ should this be ‘each CEM was’?
157: ‘The Filter pack’ capitalization unnecessary.
169: ‘loading efficiency between the denuders were not statistically’-change to ‘was’
170: ‘The breakthrough during loadings were averaged at 6.5%’-neither ‘were’ nor ‘at’ are necessary.
195: ‘Discussions on implication of HgII recovery’ missing article
232: ‘lost upto’ correct typo
232: ‘that have previously exposed’ previously been exposed
234: ‘upto’ correct typo
241: ‘For HgO, *the* average of total sample losses from CEM*s* was quantified at 16.7’
242: ‘downstream between CEM and Au trap,’ missing article
254: ‘Meanwhile, HgCl2 and HgBr2 are Lewis acids (Sandström et al., 1990) which are strongly bonded with *the* CEM surface, *which* thus exhibits *a* very high retention of these compounds.’
277: ‘The study highlights the limitations of both denuder’, should this be, ‘This study’? Or is this sentence referring to the study cited in the preceding sentence?