General comment

The authors present an interesting piece of work, regarding the partitioning of HOM (highly oxygenated organic molecules) and formation and fate of HOM in the particulate phase. With their setup they tried to compare the loss of HOM from the gas phase (what they somewhat unfortunately call condensed phase) with those they could detect in the particulate phase. For both phase they utilized NO3-CIMS, equipped with a new VIA inlet for measurement of the particulate phase (Vocus Inlet for Aerosols), though. The VIA inlet essentially uses thermal evaporation to make the SOA components available to the mass spectrometer. Within their Teflon flow chamber the authors addressed a range of different chemical regimes with the purpose to achieve a range of products distributions and to verify particulate observations in response to the change of the actual chemical regime. Unfortunately, it is not clear (yet) how far the observations in particulate late phase are methodologically biased. Here the authors suffer from the usual problems of thermos evaporation and the quantitative transfer in into the mass spectrometer (and other instruments). Insofar it is not quite clear to me in how far we look at interesting and new results in SOA formation or at interesting artefacts of baking and loss of SOA components. I find also the modelling efforts a bit simple, but ok, they are not in center of the manuscript.

Nevertheless, I find that the results of the study are very interesting, and what is more important: the courageous approach is inspiring. I see to initiate discussions and new approaches also as part of good science. Of course, the authors did efforts to characterize the limits of their methods and very positively they discuss the limits quite openly and self-critical. They speculate but never do improper claims. Moreover, the paper is well written and well structured. The material is presented clearly and in a suited manor. The figures are dense however after some looking at them keep the important information together in one place.

Having all limitations in mind, I would still say this is a quite excellent piece of work. Therefore, I suggest to publishing the manuscript as it is in ACP.

The authors may consider my suggestions for slight improvements.

Specific comments

I have questions regarding the shift from RO2 regime to HO2 regime:

Line 391/397: If you shift from RO2 to HO2 regime, wouldn't you expect that the different set of termination products in the gas-phase – more hydroperoxides -  should affect also the particulate chemistry?

Line 400: The shift to more C10H16 compounds alone does not mean that autoxidation is hold by HO2. Were the C10H16 compounds on average less oxidized than the C10H14 compounds they replaced?

Typos etc.

Line 43: I suggest to replacing “largest” by “most abundant” or so.

Line 141ff: It is not clear if you use LTOF or HTOF MS. Are you connecting to an Eisele Inlet?

Figure 2c: Were the raw signals normalized to reagent ions or total ion count. If not, why not?

Figure 2d: One could sacrifice the same scale as for the SMPS data and enlarge the data in right hand panel. Finally, you refer to details in this panel in the text.

Line 192: I think that the logic of this sentence is somewhat odd. Or did you mean more volatile components decrease already at the “lowest” temperatures?

Line 214: I don’t understand what you want to say here.

Line 450: There is probably not much NO left in the NO3 case, therefore no termination with NO. “All” NOX should be NO2, or?

Line 454: “Here”, and “in this work” is redundant.

Line 473: Is Fig S7D correct here?

Line 523: detected

Line 536: “are” compared ?

Line 539: more in between O8 and O9 !

Line 621: much “shorter” ?

Figure S8: On which axis read the pink triangles?

Table S2 and S3: The readability of these tables could be possibly improved by comparing only the same CxHy in one line, allowing for gaps where other methods did not find this class of compounds.

Table S3 header: listed

Table S4: Not easy to read. It could help if you convert the references to symbols, which you list under the table and remove the line breaks in the columns with number ranges.

Understanding the molecular level of aerosol formation has been a long-standing challenge, especially concerning the detailed chemical composition of organic aerosols. Zhao et al. used a new VIA-nitrate-CIMS technique to measure the detailed chemical composition of SOA produced by ozonolysis of α-pinene and provided some interesting new insights. They found the detected HOM molecules in the aerosol phase are significatnly different from the “condensed” phase, especially in the dimer range, and indicated that the aerosol phase reactions influenced the aging processes. The manuscript is overall well organized. And more importantly, this is an encouraging attempt and can be an important step in understanding organic molecules in the aerosol phase. I recommend it can be published after a minor revision.

1. The main findings from this work are, to some extent, similar of Pospisilova et al., 2020. Both studies suggest that aerosol phase reaction plays important role in the aging processes, and the decay of C20 and C19 HOM dimers was likely the source of C17 dimers in the aerosol phase. I suggest that the authors can discuss more on the difference from Pospisilova et al., 2020.
2. My biggest concern is still how much we can trust the VIA measurement. Is it possible the heating in the inlet speed up the decay of condensed HOM dimers? The authors suggested some possible reactions that should be responsible for the particle phase processes, e.g., the Baeyer-Villiger reactions. Does the heating process have the potential to influence the Baeyer-Villiger reactions?
3. Please provide the NO concentration for all the NOx runs
4. Although I understand this is no longer possible in this work, it would be interesting to compare the three technologies, EESI, VIA, and FIGAERO in an experiment together.

Pospisilova, V., Lopez-Hilfiker, F. D., Bell, D. M., El Haddad, I., Mohr, C., Huang, W., Heikkinen, L., Xiao, M., Dommen, J., Prevot, A. S. H., Baltensperger, U., and Slowik, J. G.: On the fate of oxygenated organic molecules in atmospheric aerosol particles, Science Advances, 6, eaax8922, doi:10.1126/sciadv.aax8922, 2020.