This is the review for manuscript entitled “Atmospheric oxidation of 1,3-butadiene: influence of acidity and relative humidity on SOA composition and air toxic compounds” by Jaoui et al.

This study investigates the effect of RH on the chemical composition of both gas and particle phases formed from the photooxidation of 1,3-butadiene (13BD) under acidic and non-acidic conditions. The authors identified a variety of products formed through the reaction. They find SOA mass and the most SOA products under acidic and lower RH conditions. With an increase in RH, secondary organic carbon decreased under both acidic and non-acidic conditions. Authors states that the photochemical reactivity of 13BD in our systems decreased with increasing RH and was faster under non-acidic than acidic conditions. Overall, the results from this study present potential to improve current understanding of atmospherically relevant aerosol particles. The manuscript is clearly written, and I enjoyed reading it. I mostly suggest minor revisions and clarifications, though a major revision might be needed address an alternative interpretation of the influence of acidity and RH.

General Comments:

The manuscript discusses the impacts of RH and acidity affects oxidation reactions and products. The changes in RH will modify the acidity of aerosols. It remains unclear how the authors determine the relative influence of those factors. Authors need to clarify their approach in differentiating and quantifying the impact of RH and acidity on SOA formation.

The study explores a range of RH from 11-60%. At 11% RH, ammonium sulfate particles are likely in the solid phase, which may involve different chemical mechanisms compare to those in the aqueous phase. Authors should consider how the phase states affect aerosol chemistry and the products observed.

This study did not include the toxicity of compounds formed from SOA, author may modify the title of the paper to be more relevant.  

Specific Comments：

Introduction

1. The literature citations need to be updated. Several recent studies indicating how acidity influences SOA formation should be included:

• Decreases in Epoxide-Driven Secondary Organic Aerosol Production under Highly Acidic Conditions: The Importance of Acid–Base Equilibria Madeline E. Cooke, N. Cazimir Armstrong, Alison M. Fankhauser, Yuzhi Chen, Ziying Lei, Yue Zhang, Isabel R. Ledsky, Barbara J. Turpin, Zhenfa Zhang, Avram Gold, V. Faye McNeill, Jason D. Surratt, and Andrew P. Ault Environmental Science & Technology 2024 58 (24), 10675-10684 DOI: 10.1021/acs.est.3c10851
• Initial pH Governs Secondary Organic Aerosol Phase State and Morphology after Uptake of Isoprene Epoxydiols (IEPOX) Ziying Lei, Yuzhi Chen, Yue Zhang, Madeline E. Cooke, Isabel R. Ledsky, N. Cazimir Armstrong, Nicole E. Olson, Zhenfa Zhang, Avram Gold, Jason D. Surratt, and Andrew P. Ault Environmental Science & Technology 2022 56 (15), 10596-10607 DOI: 10.1021/acs.est.2c01579Method

Method

1. The manuscript should address the wall loss of particles within the smog chamber. what is the residence time of particles in the smog chamber?
2. The manuscript does not mention the concentration of ammonium sulfate and sulfuric acid solutions under acidic and non-acidic conditions.
3. It is unclear whether each experiment started at the lowest RH, approximately 11%. If so, the ammonium sulfate seed particles would be below the efflorescence point and remain in the solid phase across the RH range, influencing SOA formation differently compared to reactions in the liquid phase. Can the authors confirm and discuss this?
4. Changes in RH also affect aerosol acidity. How do the authors assess the impact of pH changes along with RH adjustments? Furthermore, aerosol size significantly affects aerosol pH. Were the inorganic seed particles size-selected before being introduced into the smog chamber? An explanation on how size effects on aerosol acidity were ruled out would be beneficial.
5. How many experiments were conducted under the various conditions, and how repeatable are the results?
6. The RH at the ambient sampling sites is consistently higher than that in the chamber conditions. Why were higher RH experiments not performed in this study?

Discussion

1. Can the authors explain why the photochemical conversion of 13BD decreases with an increase in RH? The conversion efficiency of 13BD is higher under non-acidic conditions compared to acidic conditions. As the RH increases and the acidity decreases, this should logically result in an increase in 13BD conversion efficiency. Could the authors discuss this observation?
2. It is interesting to observe the concentration of the majority of products decreased with increasing RH, can author explain why?
3. Authors should consider add E-AIM result for non-acidic particles at different RH conditions for comparison and further understand the effect of acidity on SOA formation.
4. Page 24, Line 22, Need to add unit after 45.1.
5. Authors found that the higher concentration of the majority of reaction products under acidic condition, however it is not clear that this result is due to the effect of acidity, RH, or the phase state of seed particles.

In “Atmospheric oxidation of 1,3-butadiene: influence of acidity and relative humidity on SOA composition and air toxic compounds,” Jaoui et al. analyzed the composition of SOA prepared from 1,3-butadiene in the presence of NO_x under varying acidity and humidity levels, including both the gas and particle phases. They found increased production of SOC at elevated RH and increased acidity, although some individual compounds varied from these trends. They also identified and reported several compounds from the oxidation of 13BD for the first time. The analysis presented was in-depth and informative, and worthy of publication after the following points are addressed.

Major Feedback

1. The stated goals of the paper are not the major points of discussion throughout the work. For example, the authors state the purpose of the paper is to analyze the impacts of LWC on SOA formation, but LWC is not discussed in the paper other than Page 29 line 7 where it is related to acidity. I think LWC warrants more discussion, particularly for the non-acidified seed particle experiments where some of the seed particles are likely effloresced. I would expect a large difference in chemical behavior between solid and aqueous particles.

1. Many of the identified products appear to be OH oxidation products judging by retention of the 4-carbon backbone. Do the authors have an estimate of the steady state OH concentration? Does it change with any of the parameters they are varying (perhaps with LWC)? Given the importance of OH oxidation in the atmosphere, and that it is the major atmospheric sink for isoprene, the impact of the variables on OH concentrations should be considered.

1. If possible, the section on field measurements should be strengthened. The results would be more meaningful if 13BD had also been measured at the sites, but from my understanding there were only particle measurements taken in the field. It is stated in the intro that 13BD is mostly from anthropogenic sources, and then it is stated that the site in Godow has more anthropogenic influence than the other sites. Were the 13BD products higher there than in the other sites, or is there other evidence that the concentrations of proposed 13BD oxidation products are higher in areas where higher levels of 13BD are expected to be found?

Minor Feedback

Page 3 Line 25-30 - This statement could be clarified or perhaps made more specific. What is meant by “bulk properties” versus “composition?” Several of the listed citations for only “bulk properties” show analysis of composition. I think perhaps the authors are trying to differentiate between studies that looked at only particle composition versus studies that also included the gas phase composition. There have been several further studies on the effects of RH on particle composition since 2021.

For example:

https://doi.org/10.1039/D3EA00033H

https://doi.org/10.1039/D3EA00149K

https://doi.org/10.1039/D3EA00128H

There are likely others that I did not find in my brief search, and the earlier studies in this collection of work may be better represented by a review paper if one is available. Also, the year for the Hinks reference should be 2018, rather than 1918.

Page 5 line 2 - Does all of the radiation fall between 300 and 400 nm? It could be useful to provide a graphical comparison between the lamp spectrum and the solar spectrum in the SI, unless it is provided in one of the references.

Page 5 line 6 - What was the approximate pH of the non-acidified seed aerosol? I would expect it to be between 4 and 5, so “non-acidified” would be a more accurate description of the seeds than “non-acidic” The EAIM results for the non-acidified conditions should also be included in the paper. As the dissolved organic acids will have a significant effect on the pH for the unacidified conditions, the authors could also consider adding some concentration of a representative organic acid to the calculations to get a general idea of the pH of the particles during these experiments.

Table 1 - What is the significance of the experiment set names? I found them difficult to remember on initial read through. Since they are currently separated by acidity level, could they be referred to using an abbreviation that correlates to their acidity level?

Page 10 Line 6 - Are the authors referring to wall loss here? Or is there another type of loss they anticipate?

Table 4 – It would be informative if the authors could provide a ratio of intensity in acidified to unacidified conditions (or vice versa). They would then have the option to comment on the relative production of the compound under each condition.

Figure 6 - The authors show the less acidic carboxylic acid being deprotonated for oxaloacetic acid. This structure would make the decarboxylation they show less favorable. The fragmentation pathway for this intermediate has been presented before in acidified and unacidified ammonium sulfate solutions under conditions relevant to this work: https://doi.org/10.1021/acsearthspacechem.1c00025

Page 32 lines 18 through 20 - When the authors say the compounds they identified in this work were not found in SOA from other precursors, do they mean just the ones identified in Tables 4 and 5, or just the 4 compounds listed earlier in the section? Because several of the compounds mentioned in the paper, for example formaldehyde, should be common oxidation and photolysis products.