The authors present a new flow reactor designed for investigating multiphase chemistry in aerosols and cloud droplets. This is an interesting and technically challenging idea, which could potentially be an important tool to gain insights into these understudied processes. Care was taken in the design of the reactor to avoid temperature gradients due to the UV lamps, which would interfere with multiphase partitioning, to minimize losses of soluble gases, and to strictly control RH. They use a Spot sampler to generate cloud-like droplets from seed aerosol for study in the reactor.  They present preliminary results for studies of the influence of aqueous processing on SOA formation in the lab and using ambient air. I believe the manuscript is publishable in AMT after some minor issues are addressed and the manuscript is revised accordingly.

- The article is too long, with 17 figures in the main text.  Surely this can be tightened up, with some material moved to SI. At the same time the most interesting figure (the second Figure S2 - there are two Figure S2s) has been relegated to the SI.  

- The abstract is also too long. It's not immediately clear what "transmission efficiencies" are upon reading the abstract without first reading the manuscript.  

- Why is K2SO4 used instead of other particle types which may be more representative of atmospheric aerosol or more typical of laboratory studies of aerosol and cloud chemistry? I think the question is answered several pages later on line 496 but should also be acknowledged when this is first mentioned.  Also note that "neutral" isn't really representative of ambient aerosol or cloudwater pH (cf. Pye et al. 2020)

- the size dependence of the particle tranmission efficiency is mentioned, and compared with other studies, but no physical explanation for decreasing transmission for smaller particles is given.  One would expect the opposite. 

- The SO2 transmission efficiency at high RH is low. Presumably studying SO2 multiphase chemistry was one of the main intended purposes of this apparatus. Can the authors comment on how this issue may impact the design of future studies?

- line 444: 'described in by (Mitroo et al)'... clean that up  

- residence time distribution: why is there a distribution at all if the gases or particles are introduced in the same location and sampled in the same location (if this is not the case it's not possible to tell from the text)? How are three measurements sufficient to construct the whole RTD curve and detect the absence of a long tail?  

- line 490 - I would either delete or rephrase this. You are presumably introducing this apparatus for the first time in this manuscript, why are you making this statement about it not being applied very often to cloud chemistry studies when that is the most novel and unique application?

- Figure 9 and Figure S2. See my comment above about the second Figure S2.  It is much more interesting than Figure 9.  Figure 9 has been sufficiently described in the text lines 500-507. It could be made panel B of a figure which focuses on the data in Figure S2, moved to SI, or eliminated.  How was the low SO2 transmission efficiency dealt with in these calculations?

- Is the relatively short residence time of APPA compared to a chamber experiment an issue when it comes to SOA studies? 

- What is the reader supposed to take away from the SOA yield studies shown in Figure 15? Were these yields consistent with expectations?

- funding acknowledgement seems to be missing.

Xu et al. present the evaluation of their “Accelerated Production and Processing of Aerosols” (APPA) OFR that is designed to enable aqueous-phase OH-initiated oxidative aging processes. The APPA OFR combines the authors’ PFA OFR with injected K₂SO4 seed particles at the inlet and controlled humidification that enables control of the particle liquid water content (the range of RH discussed in this paper is 40 - 100% RH). The authors perform characterization studies of gas and particle transmission efficiencies, residence time distributions, droplet size distributions, and radiation/oxidant profiles. To demonstrate applications of the APPA OFR, they generated sulfuric acid from SO₂/O₃ reactions, benzene OH-SOA in the presence of dry/aqueous seeds and “cloud droplets”, and aged ambient aerosol.

Comments

1. While the APPA OFR appears to be capable of generating sulfuric acid from the aqueous-phase SO₂/O₃ reaction, its ability to initiate aqueous-phase OH oxidation chemistry was not conclusively demonstrated here. The authors did investigate SOA generated from gas-phase OH oxidation of benzene, followed by partitioning of OVOC/SOA into ALW/droplets. While this is a novel application that the APPA OFR seems to be well suited to, this is not a demonstration of aqSOA formation according to even the authors’ own definition: “water-soluble products of gas-phase chemistry [that] enter cloud droplets or aerosol liquid water and react in the aqueous phase with the hydroxyl radical (OH) or other oxidants” (L83-L86). In that regard, I think they should have used K₂SO₄ seed particles containing H₂O₂ and/or H₂O₂/FeSO₄ to initiate aqSOA formation (e.g. Nguyen et al., 2013; Daumit et al., 2016), then repeated the same experiments without H₂O₂/FeSO₄, and inferred the difference in aerosol loading and composition as aqSOA. It is not clear to me why this was not done here – this is what I would need to see to be convinced that the APPA OFR can be used to investigate aqSOA formation.
2. Similarly, how did the authors conclude that aqueous phase OH oxidation was responsible for the increase in ambient OA oxidation state as RH was increased from 40%-->85%->100% (Fig. 17 and related text)? Hypothetically, couldn’t this change have been driven by the higher RH (and LWC) promoting more efficient partitioning of low-volatility gas-phase oxidation products into the aerosol? For the reasons mentioned in above comment, it is not clear to me that this evolution in OA oxidation state was in fact due to aqueous phase oxidative aging in the APPA OFR.
3. Unless signification dilution flow is added downstream of the APPA OFR, its relatively low 1.5 L min^-1 sample flow capacity limits its application outside of measurements that can be made with particle counter(s) and instruments such as an AMS. What design changes would need to be made to increase this flow capacity to something in the range of 5-10 L min^-1 that is closer to other commonly used OFR techniques?
4. L25 and L230 - Quantify “low RH”
5.  L70 – Clarify which “new pathways” are being referred to here
6. L160 – Rather than “The APPA…is typically operated as a 254 nm-type OFR”, I suggest instead saying “the APPA reactor is typically operated in OFR254 mode”
7. L376 – typo (“to a to a”)
8. L446-L448 – Please indicate the relative humidity that was established in the APPA OFR when the RTD measurements were conducted, and please clarify if the lamps were on or off. Is there any humidity-dependence to the RTD?
9. L450 – Why are the gas/particle RTD’s in the APPA OFR narrower than in the PFA OFR when the two reactors are nominally the same design?
10. L464 – I did not notice any explicit discussion of temperature control in the APPA OFR in this section.
11. L492 - The experiments describing sulfuric acid formation from SO₂/O₃ were not clearly described. I assume sulfuric acid was generated from SO₂ + O₃ --> SO₃ + O₂ followed by SO₃ + H₂O --> H₂SO₄, but it would be useful to clarify this. How is the concentration of “dissolved” SO₂ controlled and measured? Is O₃ uptake onto the K₂SO₄ seed particles required to initiate this reaction?
12. L583 – Typo (the)
13. L608- Assuming that the authors are referring to the benzene/OH system here, I disagree that the “distribution of [benzene] oxidation products and their OH reaction constant(s) are generally unknown.”See, for example, Xu et al. (2020); Priestley et al. (2021).
14. L721. Please clarify the author contributions of C. Le and D. R. Cocker.
15. Some of the figures should be moved to the Supplement - in my opinion, Figures 4, 8, 9, 10, 12, 13, and 16 would be a better fit there.
16. The KinSim mechanism and case files that were used here should be uploaded with the Supplement.

References

T. B. Nguyen, M. M. Coggon, R. C. Flagan, and J. H. Seinfeld, Reactive Uptake and Photo-Fenton Oxidation of Glycolaldehyde in Aerosol Liquid Water. Environmental Science & Technology 2013 47 (9), 4307-4316. DOI: 10.1021/es400538j

Kelly E. Daumit, Anthony J. Carrasquillo, Rebecca A. Sugrue, and Jesse H. Kroll . Effects of Condensed-Phase Oxidants on Secondary Organic Aerosol Formation. The Journal of Physical Chemistry A 2016, 120 (9) , 1386-1394. https://doi.org/10.1021/acs.jpca.5b06160

Lu Xu, Kristian H. Møller, John D. Crounse, Henrik G. Kjaergaard, and Paul O. Wennberg, New Insights into the Radical Chemistry and Product Distribution in the OH-Initiated Oxidation of Benzene, Environmental Science & Technology 2020 54 (21), 13467-13477. DOI: 10.1021/acs.est.0c04780.

Priestley, M., Bannan, T. J., Le Breton, M., Worrall, S. D., Kang, S., Pullinen, I., Schmitt, S., Tillmann, R., Kleist, E., Zhao, D., Wildt, J., Garmash, O., Mehra, A., Bacak, A., Shallcross, D. E., Kiendler-Scharr, A., Hallquist, Å. M., Ehn, M., Coe, H., Percival, C. J., Hallquist, M., Mentel, T. F., and McFiggans, G.: Chemical characterisation of benzene oxidation products under high- and low-NO_x conditions using chemical ionisation mass spectrometry, Atmos. Chem. Phys., 21, 3473–3490, https://doi.org/10.5194/acp-21-3473-2021, 2021.