General Comments:

This study investigates the levels of oxidized mercury at the high-elevation measurement station Storm Peak Laboratory. The use of a dual-channel system reduces the systematic biases of previous oxidized mercury measurements, and the length of the dataset (2 six-month periods) allows for analysis of drivers of oxidized mercury dynamics. The study is written in a clear fashion and the conclusions are well supported by the statistical analysis. I appreciate that the authors used multiple approaches to analyze their data, including identification and analysis of anomalous events, back-trajectory analysis, and Principle Component Analysis. Not all aspects of the oxidized mercury observations can be fully explained, but this is reasonable given that very few (if any) such datasets exist and this can act as an impetus for further observational and modelling work. I believe that this study is ready for publication and provide only minor comments and questions.

Specific Comments:

-In the schematic, shouldn’t fires and anthropogenic sources be emitting Hg(II) as well? I understand that the authors conclude that this does not affect the measurements at SPL, but perhaps should be included for completeness of the cycle.

-L67 - couldn’t the oxidation mechanisms by Br and OH also occur in the boundary layer, not only the free troposphere? It might be that we have more evidence from the free troposphere, where measurements are not impacted by local emissions, but oxidation could occur throughout the troposphere (and stratosphere - Saiz-Lopez et al., doi:10.1029/2022GL097953, 2022)

-L371 - I know this is pervasive in mercury literature, but reporting a trend in % per year implies an exponential fit to me. However, all of the cited literature studies calculate linear regression trends. I think it would therefore be preferable to describe these results in ng m-3 yr-1 or % changes over the number of years studied. Could the authors find a way to report their trend results and compare with existing literature without continuing the use of these misleading units?

-L420 - Is it possible that local photochemistry and deposition dynamics could cause the diurnal pattern of HgII?

-L488 - A useful reference here may be Fu et al., ES&T, 2021, https://doi.org/10.1021/acs.est.1c02568, which also found a similar slope as the current study (-0.44), and a similar origin in the mid-troposphere (5 km)

-L561 - Correct this, ozone is not directly formed by combustion but rather through chemistry of ozone precursors (not all of which are produced by combustion)

-L582 - It’s also possible that co-emitted halogens or aerosols in smoke plumes lead to faster oxidation or scavenging of Hg(0) than clean air, leading to elevated Hg(II) and particulate Hg.

-L602 - Would aerosol number concentration be comparable with aerosol scattering, since the scattering measurements are affected by the aerosol size distribution?

Technical Comments:

-L151- should this be 1.31± 0.9 or 0.09 ng m-3? Check Table 1 as well.

-L190 - I don’t understand what the last part of the sentence is adding (There also are 6 months of data in 2022) 

-L294 - what does data were excluded listwise mean?

-L309 - statistically significant differences for which means?

-L319 - Reference Table 1 for this section?

-L637-640 - can this sentence be clarified? As it was just mentioned that local precipitation can play a role

The study by Derry et al. applied a new speciated atmospheric Hg measurement technique to measure atmospheric Hg0 and Hg(II) at 10 min interval at SPL in spring and summer 2021 and 2022. The authors combined the speciated atmospheric Hg observations with ancillary parameters, to investigate the concentrations and origins of atmospheric Hg(II) in the free troposphere. I overall agree with the measurement technique and interpretation of the observational data. The manuscript is also organized in a good manner. Overall, I feel that this study is good at concrete observations but appears to be lack of providing novel findings as compared with previous studies, and this should be improved. I have several moderate concerns which hope be to considered before the publication in ACP.

Line 27: the temporal behavior similar to previous work is not clear to me. Please specified the temporal behavior.

Line 32-33: In addition to the deposition during transport, the slope higher than -1.0 might be also caused by the lower atmospheric THg at high-altitude and removal of Hg(II) by cloud droplets,….  

Line 118: please specify the intervals for changing the cation membranes.

Line 121-123: I suggest the author mention the other factors the might contribute the analytical uncertainty of the method, e.g., would cloud water Hg(II) be collected by the thermal converter under cloudy or rainy conditions? A 2.5 min analytical interval for Hg detection using the Tekran 2537 analyzer would generate large analytical uncertainty in Hg detection due to low Hg loading?

Line 142-156: a discussion on the negative values based on the method is excellent. The strategy that excluding these negative values, however, is not convinced to me. This has a potential to reduce the reported mean concentrations during the whole study period, although the authors claim that this did not change the statistics of the data. I suppose the negative values should be mainly related to the low Hg(II) during these periods. I therefore suggest to define these negative values to be 0.

Table 1: it is better to show that negative values were excluded in the caption.

Section 2.3.3: the intervals that calculating a new trajectory should be added.

Line 306-312: the description of the temporal variations is opposite to the statement in the abstract.

Section 3.1.2.: in addition to the power plant emissions, would other anthropogenic sources affect the atmospheric Hg? A simple to detailed discussion might be of interests.

Line 360-367: a comparison between this study and previous observations to show hemispheric Hg decline has many uncertainties, considering that these sites were impacted by different regional sources or investigated by different times. In addition, all the references supporting their hypothesis did not report decline over the past 15 years. Other references regarding the hemispheric GEM trends should be added.

416-431: the diurnal distributions of Hg(II) is in contrast with most high-altitude observations. Currently, the interpretation is not very sufficient. I would suggest the authors to add other proxies (e.g., O3, humidity, CO, SO2) in Figure 2, which would be helpful for better understand the controls. The author mainly focused on the air mass origins, and would local Hg(II) production or removals contribute the diurnal trend in Hg(II). A local wind system together with large-scale wind field would help to diagnose the potential effects?

Line 541-542: where did these oxidations occur? In the continental or oceanic free troposphere?

Line 558: the study by Fu et al. did not attribute many Hg(II) enrichment event to air masses from UT/LS, instead, to the air masses from lower and middle FT over the North Atlantic Ocean.   

Line 570-583: the explanation of the higher O3 concentrations during Hg(II) events is confuse to me. if these high O3 were related to PBL pollutions and biomass burning, would these sources also contribute to high Hg(II) events? This contradicts the major conclusion of this study that Hg(II) is mainly produced by atmospheric processes. The vertical profile in O3 might be evident in the continental atmosphere, even below upper free troposphere. The authors should also consider that O3 and Hg(II) have a similar production mechanism (e.g., photochemical process under dry air conditions) or removal processes.