The paper summarizes average carbon oxidation states of SOA formed from photooxidation of single and mixture VOCs (o-cresol, a-pinene, and isoprene) in the presence of NOx. SOA were sampled online by HR-ToF-AMS and FiGAERO-I-CIMS and offline by HPLC-MS to determine the average oxidation state of carbon. Results from different experiments and instruments were then compared to investigate how the chemistry of SOA formation is influenced by the presence of another precursor. There are two other papers from the group which apparently have highlighted already what the common and unique SOA products are in these systems. I therefore don't know what the value of this paper is in addition to the more robust identified products in their previous work. As the authors highlight, HR-ToF-AMS cannot provide an accurate estimate of the average OSc since it cannot determine the N-content of aerosols well. FIGAERO and HPLC-MS are each sensitive to certain classes of compounds and so don't provide a complete picture of the oxidation products either. That said, I think there is value in comparing results from one instrument across the single/mixture systems to say something about the differences in chemistry. I therefore believe the manuscript can eventually be published, but it needs major revisions in my opinion. As is, there are simple conclusions that are hidden in the paper and lost in the mere numeric comparisons presented in Sections 3 and 4. Please see below my comments: 

General comments:

Experimental section: The experimental section early on should mention that NOx was adjusted to keep the VOC/NOx ratio the same. What’s the predicted average OH in the expts?

Sections 3.1-3.3 read as sentences comparing average OSc estimates from one condition/instrument to the next and is not that valuable as written. For example, I didn’t get a sense of what to think about the different estimates or sometimes opposing trends in the estimates from the positive vs. negative ion modes of HPLC-MS. I think for this section to be useful, the data need to be better synthesized and a summary presented in a way different than just comparing pairs of average OSc values.

Section 4: I was hoping this section contains a more synthesized look at the data, but it’s a summarized set of comparisons again (without actually mentioning all the OSc values) and the discussion is more on limitations of the instruments (or preferential detection of certain compounds) rather than actual chemistry

Since there is evidence for nitrogen-containing ions to be formed in these systems and OS of nitrogen has a significant impact on the estimated average OSc, I don’t see the value in comparing OSc when OS of nitrogen is ignored, so in my mind, the data from HR-ToF-AMS should not be included in the paper and any other reference to average OSc disregarding OS of nitrogen should be removed.

Specific comments:

P3, L63: add some reference for recent research on multi-VOC systems

P3, L29-71: consider breaking this sentence to multiple or rephrase it for ease of readability

L 264: C in DeCarlo should be capitalized

L 265 Reference to Sueper doesn’t need the first name initial

L294: how long was sonication done?

L299: evaporation to dryness was carried out at a much warmer temperature than room. What’s the potential impact in driving off some of the more volatile SOA components?

L499: change significantly to significant

L639: remove either “in” or “for”

L770: delete “which”

L833: what do you mean by “at a higher magnitude”?

L928: delete “be”

L962 and 949: there are two contradictory statements on the average OSc of the mixture in these two statements

This manuscript describes results from environmental chamber experiments in which VOCs (single VOCs, and binary and ternary mixtures) were reacted with OH to form secondary organic aerosol (SOA). In particular, this paper focuses on the oxidation state of carbon (OSc) of the SOA formed, measured with three different instruments: a HR-ToF-AMS, FIGAERO-CIMS (both online methods) and UHPLC-HRMS (an offline method). A chief complication in comparing OSc from these three instruments is that they all measure it imperfectly – the HR-AMS cannot quantify nitrogen (N) accurately, and the other two methods are selective, i.e. do not detect all organic compounds. This paper presents a thorough discussion of oxidation state measured by the different instruments during the different experiments, but for me as a reader I had difficulty following the main scientific takeaways from these measurements, especially considering the caveats of the oxidation state measurements. A thus suggest major revisions of this manuscript.

General comments:

I find this paper difficult to follow in part due to the amount of quantitative information described in the results section. Perhaps some of these results could be presented in tabular format so that the text can focus more on trends and discussion (rather than numbers). I also suggest that the authors highlight the main results / scientific insights from the study in the revised version of the manuscript.

Lines 195-203: It is unclear to me how the chamber background and experimental background were subtracted from the data, and how this may impact reported measurements of oxidation state from the three different instruments. Perhaps a more detailed explanation in the SI would help, with graphs showing background, experimental data, and corrected data.

Specific comments

Line 52-53: The last sentence of the abstract is unclear, especially the phrase “to enable... atmosphere”. Please revise.

Line 316-317: “Thus, produced product ion spectrum, to inform the compound’s structural characterisation and isomer identification” Please revise the sentence – unclear.

Line 792-793: “experiments, suggesting that the dominant control by α-pinene oxidation products (Fig.2a).” Revise this sentence (remove “that”?)

Line 795-797: “However, the magnitude of average OSc in mixture system is slightly lower than in the single α-pinene experiment, the influence of isoprene oxidation products on the average carbon oxidation state of total SOA.” Please revise this sentence (especially the second phrase) – unclear.

Line 797-799: “Isoprene is known to form C4 and C5 compounds with high volatility (e.g. methacrolein (C4) and C5-hydroxycarbonyls) on OH oxidation, with potential to suppress the particulate mass form from α-pinene oxidation in the mixed system.” Please revise, unclear.

There are other grammar issues throughout the manuscript, e.g. missing periods.