Secondary Organic Aerosol Formation from Isoprene: Selected Research, Historic Account and State of the Art
Round 1
Reviewer 1 Report
This paper gives a review of the types of isoprene oxidation products that contribute to secondary organic aerosol (SOA) formation, discussing how they are analyzed and formed, and the recent history of their detection and quantification in laboratory and field studies. There is also a section on source apportionment studies of PM from isoprene, though that section is relatively brief and not that well connected to the bulk of the paper, which mostly discusses analytical methods. The emphasis of analytical methods is understandable given the well-established expertise of the authors, and this discussion should be useful to people working in this field. There is also a brief discussion of formation mechanisms, generally making the point that the mechanisms were not well established when many of these compounds were first detected, but implying that they are better understood now.
This review paper is relatively short given the large number of recent studies of SOA from isoprene, but reference is made to other reviews that give more details. But this review is well written, interesting, and relatively easy to read, and provides a useful summary of what is detected and how they are analyzed. Therefore, I think this is eventually suitable for publication. However, I think the discussions of formation mechanisms could use improvement, and think a table listing the compounds detected and giving summary information about them would greatly improve the paper. My comments and suggestions are discussed below.
In considering my comments, the authors and editor should be aware that my primary expertise is atmospheric chemical mechanisms, not analytical methods, field studies, or source apportionment. Hopefully the other reviewers can provide input in these areas. However, it looks to me like the analytical discussion is reasonably comprehensive and potentially useful to other analytical chemists, and I didn't see anything unreasonable in the discussion of source apportionment, though it seemed rather brief.
The abstract and conclusion contains the statement "SOA formation from isoprene is acid-dependent and requires sulfuric acid", but in Figures 1-3 it shows mechanisms where SOA compounds such as tetrols are formed with no sulfate involved. I can see how their formation may be acid-dependent if the hydrolysis reactions are catalyzed by acid, but there are other sources of acid in the environment. For example, Surratt et al (ref 19) observed these compounds in chamber experiments without sulfuric acid. If the authors believe that sulfate is in fact necessary, they should say more about why they believe this. Otherwise, maybe the statement should be modified to say that acid catalyzed hydrolysis is necessary, and sulfuric acid enhances SOA by forming additional nonvolatile products. (Or does the presence of sulfuric acid mainly mean that different products are formed, which may be even less volatile than the ones they replace?)
Figures 1-3 are important in that they provide an explanation of how we believe the major types of products are formed. The actual mechanisms are much more complex because of reactions at different positions and formation of different isomers, but representative examples showing the different types of reactions, such as employed in these figures, could be sufficient for this review. However, it would be useful to include some of the intermediate steps in cases where the mechanism may not be obvious. For example, they should explicitly show the cyclization of beta-hydroperoxy alkoxy radicals forming OH and epoxides (e.g. IEPOX's and MAE), which is a type of reaction that wasn't expected when the isoprene SOA products were first detected.
Also, I don't understand some of the gas-phase reactions shown on Figure 2. What are the intermediate steps between isoprene and the 1,2-dihydroxy product shown on the left side of the figure? The only mechanism I could think of would involve a disproportion reaction of the peroxy radical formed in the OH + O2 reactions with another peroxy radical, which occur but are generally unimportant in the atmosphere, and are totally negligible in the presence of NOx. In addition, they show what seems to be a particle phase hydrolysis reaction (forming a hydroxy nitrate from a dinitrate) in the gas-phase portion of the figure. Is there evidence that hydrolysis of dinitrates occur in the gas-phase under atmospheric conditions?
Reference is made to lactones in Section 3.1. If mechanisms for their formation have been proposed, perhaps a figure showing how should be included (or added to an existing figure).
The discussion of analytical methods refers to a number of compounds but does not give their structures, requiring the reader to figure them out, and how they might be formed, from their names. I think this review paper would greatly benefit by including a table listing all the isoprene SOA compounds (including sulfates) that are discussed in the paper, giving their structures, a brief indication of how they are formed if known (i.e., figure number or code), molecular weights (relevant to discussions of MS data), an indication of how they are analyzed and if they can be quantified, and information about their detection. This would help put together the information, especially Section 3.1, where many different compounds of types not shown on Figures 1-3 are discussed.
I am not sure how well the section on source apportionment fits in with the rest of the paper, though it does give examples of how the data discussed elsewhere in the review can be used. I wonder if it might be better if it were expanded and made a separate review. Although this may be due to my limited expertise in source apportionment, it seems to me that they should make it clearer what the difference is between the "organic marked based approach" and "positive matrix factorization". Don't they use the same type of data and differ mainly in the mathematics of how the data are analyzed? On the other hand, this section is useful in providing extensive references where more information is available.
The conclusions section is missing a summary of the remaining scientific questions and data needs where research is needed or currently underway. Are the main issues improving the analytical methods and quantification, or is there also needed research into how some of the observed compounds are formed, and whether they could be formed from VOCs other than isoprene.
My recommendation is publish after modification and re-review.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
Review of “Secondary Organic Aerosol Formation from Isoprene: Selected Research, Historic Account and State of the Art” by Claeys and Mauenhaut
The authors review selected research papers published in the last two decades on secondary organic aerosol (SOA) markers formed from isoprene. In 2004, the authors’ group reported that organic compounds, formed from the degradation of isoprene in the atmosphere, are present in organic aerosol collected in Amazonian rainforest. Their pioneering research triggered an enormous number of researches related to SOA from isoprene; and the atmospheric oxidation of isoprene is now widely accepted as an important source of SOA at the global scale. This review discusses knowledge elucidated for isoprene-derived SOA markers by early researchers, knowledge of isoprene-derived organosulfates, recent results of aged isoprene-derived markers and results of source apportionment studies using isoprene-derived markers. Three previous review papers related to this review are available; Carlton et al review knowledge obtained for isoprene derived SOA by early researches; Hallquist et al review comprehensive knowledge of SOA from various VOCs; and Nozière et al focus on analytical techniques of biogenic SOA markers. In contrast, this review provides a unique point of view compared to previous review papers because it focuses on formation and fate of isoprene-derived markers and covers overall research history in the last two decades. Note that their selection of papers from an enormous number of related papers is valuable for discussion of isoprene-derived markers. This review must be useful for field observation, laboratory and modeling researchers in the field of atmospheric chemistry. The manuscript suits to the scope of review articles of this journal. The reviewer strongly recommends the publication of the review in this journal as a researcher contributing to this research topic.
The reviewer provides only minor comment: In second paragraph of section 3.1, the naming of 2-sulfoxy-2-hydroxy-2-methypropanoic acid may be incorrect because 2-position secondary carbon has sulfoxy, hydroxy, and methyl functional groups. Because the authors explain that the compound does not have a terminal CH2OH group, it must be 2-hydroxy-2-methyl-3-sulfoxypropanoic acid. The naming of 2-sulfoxy-3-hydroxybutan-2-one may also be incorrect because the authors explain that it has terminal carbon. If the compound has terminal carbon, the name of the compound must be 4-hydroxy-3-sulfoxybutan-2-one.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
This revised manuscript sufficiently addresses the concerns I had in my previous review, and I now consider it acceptable for publication.
