Four- and Five-Carbon Dicarboxylic Acids Present in Secondary Organic Aerosol Produced from Anthropogenic and Biogenic Volatile Organic Compounds

Round 1

Reviewer 1 Report

General comments:

This manuscript conducted 21 Chamber experiments and found that four- and five-carbon dicarboxylic acids are different in the secondary organic aerosols generated by anthropogenic VOCs (toluene and naphthalene) and biogenic VOCs (α-pinene, and isoprene). It provides basic information for explaining the origin of dicarboxylic acids in the atmosphere, and is helpful for exploring the possible reaction mechanism of dicarboxylic acids formation. I recommend it to be published in the journal Atmosphere if below questions are adequately addressed.

General suggestions:

Chamber experiments：1. How to ensure the stability and repeatability of experiments when only one experiment is carried out in each run? How to determine the concentration of reactants used in each experiment? For example, run 1 and run 2 only change the concentration of toluene, why not change H₂O₂?

2. The concentrations of reactants used in Run 9 and 10 were the same. Why are the concentrations of SOA produced different? What's the reason?
3. P7, L174~175 “Succinic acid was detected in the SOA produced from isoprene under high NOx conditions (run 23) but was not detected in the SOA produced from isoprene under low NOx conditions (run 22).” However, Table 1 shows that the reactants of run 23 are NO and CH₃ONO, and run 22 is O₃. Should another set of experiments be added with the same reactants and only the concentration of the precursor be changed?
4. Table 1 shows that both the reactants and SOA concentration at run 15 (naphthalene 1.80, SOA 48) were higher than run 14 (naphthalene, 0.98, SOA, 44), But Figure 1. b f_SOA (W/W %) run 14 is significantly higher than run 15?

Results: 1. “3.3. Effect of NOx/toluene ratio on f_SOA of toluene-derived tracer” The contribution of NOx/Toluene ratio to f_SOA was discussed. As shown in Table 1, changing the concentration of CH₃ONO can have a significant impact on the generation of SOA. Should also consider the influence of CH₃ONO?

Below are some specific questions:

1. Chamber experiments, P3, L96: please make sure "The initial concentrations of CH3ONO were set higher in runs 2, 9, 10, 18, and 19 than in the other runs to investigate higher exposure to hydroxyl radicals" is correct?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The authors studied photochemical formation of four and five-carbon dicarboxylic acids from major gaseous precursors (toluene, α-pinene, and isoprene). Although, there are many studies related to photochemical formation of secondary aerosol from these precursors, the number of studies related to molecular determination of dicarboxylic acids in the produced SOA is limited.

The paper is generally well written, however, there are several minor points that should be clarified before its publication. These points are summarised below.

Lines 47-49: The sentence needs reference.

Lines 68 -69: Information about resulting CH3ONO purity is needed.

Lines 80-81: Short info about production of precursor vapours is needed.

Line 96: Number 8 should replace number 2 or   correction of the table 1 is needed.

Line 143-145: Information about typical difference between vapor pressures calculated by the used software and measured ones or calculated by other methods is needed. 

Fig 1a): Data on pinene and isoprene are hardly readable. The right vertical axis would solve the problem.

Fig 4. : Its seems that zero or very low NOx concentration makes different reaction conditions in comparison with NOx present. Therefore, the reviewer suggests to make linear fit without the lowest NOx/toluene ratio to get more realistic linear fit on this dependence.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This paper studied dicarboxylic acids present in SOA formed from the oxidation of anthropogenic and biogenic VOCs. A large set of chamber experiments was carried out and several interesting dicarboxylic acid tracers were found. I suggest the authors to consider the following comments in order to improve the quality of manuscript.

More discussions are suggested, or it looks like a result report. The authors should try to make a logical and clear expression to the section of result and discussion. Please go deeper into your chamber experimental data.

The authors should briefly explain why they focused only on the C4-C9 dicarboxylic acids and skipped the C2 and C3 dicarboxylic acids.

Why did the authors perform only two experiments for the oxidation of a-pinene and isoprene? Why did the author use O₃ in the isoprene oxidation, whereas in it was H₂O₂ in all the other experiments? I don’t understand the strategy of the experiment design. Please describe the idea of designing the experiments in detail rather than just introduce what you did.

A description of the saturation concentration as well as how it was used to estimate the phase distribution is missing.

More better examples could be provided in line 264-266 as the authors showed C4 dicarboxylic acids mainly come from the oxidation of toluene which is more abundant in urban areas rather than in rural areas.

What does “linear dicarboxylic acids” mean in line 165?

In line 298 -299, a wrong conclusion was drawn “This result indicates that there are components other than malic acid directly generated by photoxidation of toluene, a-pinene, and isoprene”. I would get the conclusion that photoxidation of toluene, a-pinene, and isoprene are not the dominant source of malic acid in the urban areas.

The authors should not focus only on malic acid when talking about the atmospheric implications.

Language needs to be further proofed, better by peers who are native speakers.

The quality of figures in this manuscript is not good enough and especially texts therein are fuzzy. Please produce the figures in a proper way and make sure all the texts are readable. e.g. I have to zoom in quite a bit to read figure 3.

Please clarify the content of each sub panel in the caption of all the figures.

References should be provided when research works are introduced especially in the introduction section.

Please format your references correctly.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report