|The study proposed by Zhang et al, presents new results related to the effect of relative humidity on the formation and composition of SOA generated from the OH-initiated oxidation of xylene. While it is an interesting topic, this study cannot be published as it is, as different aspects/conclusions of the paper are not well constrained or speculative (see below). I suggest that the authors have a close look at the existing literature and provide strong(er) evidence (e.g., MS2 spectra, ...) to support their statements.|
Page 2, lines 33-34: Please provide references and/or results.
Page 3. Lines 4-15. This section is not useful/needed, especially as it is disconnected to the rest of the introduction. The authors mainly focused on particle phase processes. Differents aspects are actually missing including acidity effect (directly linked to LWC) and/or viscosity/phase effect on particle-phase reactions and can be added to the introduction.
Paragraph 3.1. This section should be revised and better organized. As it is, it is very difficult to follow. In addition, the authors should discuss the impact of reacted VOC on SOA yields when comparing different studies.
Page 5, line 20. Why did the authors stop the experiments after 4h, while the SOA mass was still increasing? Usually, SOA yields are calculated when the oxidation of the precursor is over. Was H2O2 still present in the chamber?
Page 5, lines 25-27; 30-32. I do not think it is a correct explanation. Indeed, as reported in many laboratory studies, SMPS measurements are reliable and do not exhibit such a large variability (15-20%). The authors should better constrain the evaluation of the uncertainties and provide references supporting their hypothesis.
Page 6, line 5. How did the authors conclude that the underestimation was negligible? Please provide a clear explanation.
Page 6, lines 20-28. To help the reader and underline the differences, the authors should make a figure presenting the SOA yields as a function of RH for the different aromatic systems (toluene, xylene,...). It would help picturing the effect of RH. In addition, they can use the size of the markers as the function of reacted VOC as the concentration of reacted VOC has a great impact on SOA yields.
Page 8, lines 23-25. Please provide references to support the assignments. if the band at 1605 cm-1 corresponds to liquid water, why is the absorption lower in humid conditions when the LWC is expected to be larger?
Page 8, lines 29-30. Are the ratio calculated for 1 experiment or for all the experiments?
Page 9, lines 15-16. Please provide the MS2 spectra in the SI to support the discussion and the structure assignments
Page 9, line 24. It would be very surprising to have such reactive species (e.g., 155, 187) within the particle phase. Please provide much deeper MS2 analyses to further confirm the presence of such species and add references.
Page 10, lines 1-5. The most intense peak corresponds to C10 compounds, so according to the author's definition, those compounds are oligomers. It is surprising that oligomers (dry conditions, no seed) contribute to a large fraction of SOA mass. Indeed, larger SOA mass would result in a larger aerosol surface area and lead to a greater condensation sink for SVOC and LVOC. In other words, it can be expected that larger SOA formation leads to a greater concentration of SVOC (i.e., monomer) within the particle phase. Please provide a deeper analysis of gas-particle partitioning in your experiments: what's the surface area of the aerosols under dry vs wet conditions and compare it with the surface area of the wall. Under dry conditions the aerosol would act as a main sink, while under wet conditions the chamber walls would be the main sink.
Page 10, line 15. What should be the production of glycolaldehyde to explain the formation/concentration of such high molecular weight compounds? It is confusing to not see any dimers and/or lower oligomers (nC = 3,4,5,... monomers). In other words, the authors are suggesting that from gaseous glycolaldehyde, such aldol condensation reaction will form only/mainly oligomers with 8 monomers. If such a process takes place the authors should be able to find the distribution of the different oligomers. Is it the case? The authors should look at the MS/MS spectra of the compounds from C4 to C10 to look for the presence of oligomers (nC = 3,4,5,..). The MS/MS spectra should be provided. Is there any evidence of oligomerization between a xylene-monomer and a carbonyl (e.g., glycolaldehyde)? Finally, such aldol processes are generally considered to be too slow to be observed, especially without the presence of an acidic solution (Herrmann et al., 2015).
Have the authors considered the reaction of glycolaldehyde with C7-C8 monomers?
Paragraph 3.4. This section is incorrect and should be revised according to the existing literature on HOM. The authors characterized the distribution of oxygenated species in particle phase as a function of RH. According to Bianchi et al., 2019, HOM refer to gas-phase highly oxygenated molecules formed from autoxidation. So it is speculative to claim that RH impacts HOM formation as the authors did not measure such species. In addition, it is contradicting a recent study proposed by Li et al. 2019 (doi.org/10.5194/acp-19-1555-2019) showing that RH does not impact the formation of HOM. It is possible that HOM undergo further particle-phase reactions as it has been suggested in a few studies (i.e., Bianchi et al., 2019).