Title: Reaction kinetics and multi-sulfur products formation of sulfur-containing volatile organic compounds with OH radicals

Author(s): Xun Rong, Lei Yao, Chuang Li, Cong An, Gan Yang, Jiali Shen, Runlong Cai, Douglas Worsnop, and Lin Wang

MS No.: egusphere-2026-1423

MS type: Research article

Summary:

This manuscript describes a set of experiments exploring the OH oxidation of various sulfur compounds with ranging functionality. The researchers present OH rate constants for a handful of compounds as well as exploring the potential for these sulfur species to form low volatility multi-sulfur compounds. Lastly, the work explores the emission of sulfur compounds from an algae-rich freshwater lake. The authors identify the potential of sulfur compounds to form low volatility multi-sulfur compounds under OH oxidation pointing to a potentially important source of SOA. The work presented investigates an important area of research with findings useful to the community. In the work’s current state, I personally believe the experiment design and parameters are not sufficiently outlined in the main text or SI. More descriptions and experimental information need to be presented for better replication of work and understanding. Conclusions about the multi-sulfur products provide an important finding but need further exploration of the experimental parameters and their comparison to the oxidative environment in which the explored sulfur compounds are emitted. Lastly, the methodology for OH rate constants have some large assumptions that need further exploration. I cannot recommend this publication in its current form. I would request significant revisions before publication.

General comments:

Multiple rate constants are presented in the work, but I have some questions about the OH value calculation and its usage throughout the work. The OH concentration within the chamber is approximated using the decay of DMS. While this is an appropriate application, I don’t believe that the calculated OH can be utilized for all other sulfur compounds. The steady-state or experimental OH will depend on multiple factors (in your experiment) including the O3, RH and OH reactivity. While the OH production could be relatively consistent given similar O3 and RH, the order of magnitude difference in OH reactivity of DMS compared to some faster sulfur compounds (DMDS) will lead to a different apparent OH concentration. To account for this, most laboratory experiments investigating rate constants include a reference species with a well known OH rate constant (e.g. propane, 1,2,4-trimethylbenzene). For this reason, I would encourage more discussion on the uncertainty of the rate constants. If experiments cannot be redone, I would encourage more investigation into the box modeling utilized on DMS to better approximate observations from the chamber.

I believe more discussion is needed with respect to the potential H-migration and its role in the formation and/or competition with the RO2+RO2 reactions. As per Figure S7, the HO2 and RO2 in the chamber are modeled to be very low (<50 ppt). This would allow for very long RO2 lifetimes and thus increased potential for isomerization reactions. Using 0.1 s^-1 for the DMS RO2 isomerization, 90% of this RO2 should isomerize and form HPMTF. I encourage building upon the recent work that has extensively explored the autoxidation of DMS and DMDS. This literature can help inform and constrain the RO2, then explore other sulfur compounds.  

Overall, there seems to be a lack of citations or validation for some of the comments made throughout the work. Comprehensive citations referencing mechanisms, techniques and analysis used and previous sulfur work is missing. Additionally, work on sulfur oxidation and emissions have been studied over the past 4 decades and none of that original work seems to be acknowledged in this work.

Technical comments:

Line 54: Ronald P. Kiene performed a lot of good work exploring DMSP and its transformation in the marine system. This and subsequent work should be acknowledged.

Line 77: OH is not the only oxidant of DMS and sulfur compounds, but it is the dominant during the day.

Line 80: Rate constants are temperature dependent when presented should have the temperature at which it is calculated. There are multiple instances throughout the piece where a temperature of the rate constant should be present.

Line 108: A description of the mass spectrometers needs to also be in the methods and should have more description of operation.

Line 167: There is a tilda (~) as well as error constraints on the OH concentration. This is confusing as tilda points to approximate, but you give tight constraints. This is contradictory. I also strongly encourage the removal of tildas throughout the piece especially when values were calculated and closely measured.

Line 174: Rate constants need the temperatures at which they were calculated.

Line 182: What is the “current value” of the DMDS rate constant? Please add citations

Figure 1: Please add error bars to the data points as I assume the 1 min data points are 1 min averages

Table 2: The JPL Publication 19-5 indicates that some of the sulfur compounds reported have temperature dependent rate constants, I would recommend calculating them at 291K to better compare your derived value

Line 203: Acyclic sulfur compounds and well as cyclic should and can undergo OH-addition reactions, which are enhanced under cooler temperatures. This is mostly omitted for discussions about H abstraction. I encourage more discussion here.

Line 245-262: A significant portion of this section discusses the RO2-RO2 combinations and the potential to for “multi-sulfur” species. How does the eventual yields of these compounds compare when these compounds are oxidized in the atmosphere? I would expect CH3O2 and CH3C(O)OO would be dominant in the atmosphere. I encourage discussion of how the findings here will translate to the remote atmosphere. Additionally, I encourage discussion about carbonyls and hydroxyls that can for from RO2 + RO2 reactions

Line 339: Molecular formulas (e.g CH2S, C2H6S) are given mixing ratios with little discussion of how and what absolute sensitivity was used to calculate. Without knowledge of the compound how was the sensitivity to that compound applied? I would encourage adding uncertainty or clarification on how this was calculated as DMS and EtSH are isobaric but can have different calibration factors.

Line 340: PTR-MS have been shown to produce fragmentation and artifacts generated within the drift /ionization region. During your single component calibrations was there any evidence of fragmentation and if so where fragments accounted for in the concentration calculations as well as the mass defect plots (i.e. Figure 4a)?

Line 346: What was the filter size and information? The formation of sulfur compounds is also dependent on the aquatic biota. Bacteria can break down and convert DMSP and other dissolved sulfur. Post filtration how can you discern the effect of algae vs that of additional biological processing independent of algae?

 Figure 4a: Could you describe what this figure is telling us? I understand that it gives a general display of the complex mixture, but its showing more CHO that CHS. Also, can you compare that to a post-OH experiment to highlight the formation of more sulfur compounds?

Line 409: Please add citations for the declining SO2

Line 411: I would also include passive SO2 and H2S, both of which will form H2SO4 and would also become very important under this lowering Anthropogenic SO2 world.

General Comments

In this manuscript the authors describe an experimental study of the kinetics of the reactions of a selection of sulfur-containing volatile organic compounds with OH radicals and the products formed in the absence of NOx. Experiments were conducted in a small Teflon chamber, and the decay of the reactant was monitored with a PTR-MS and the products with a nitrate-CIMS. Rate constants measured using relative rate kinetics and product yields were estimated using sulfuric acid as a calibration standard. Samples of algae were also collected from a freshwater lake, and their emissions of sulfur-containing VOCs were compared with compounds identified here.  

The experiments and data analysis were well done, and the interpretation of the results, including the proposed reaction mechanisms, are reasonable. The authors make good use of the literature, and the manuscript is clearly written. The study provides useful new results on the kinetics and products of atmospheric reactions of an important class of understudied compounds. I recommend publication in ACP and have only a few minor comments.

Specific Comments

1. The product yields are very low, with totals typically less than about 10%? Where does all the mass go? This is a tiny reactor, so I assume much is lost to the chamber walls. Have the authors estimated gas-wall partitioning of products? Do they know how much aerosol is formed?
2. Are emissions of sulfur-containing VOCs sufficient that these low product yields can matter for the atmosphere?
3. What areas in China are these NOx-free conditions appropriate for?

Technical Comments

1. The quantities reported here are reaction rate constants or rate coefficients, not reaction rates, which is the term used throughout the text.