|In the revised paper, the authors have addressed some of the issues that the reviewers raised. However, I do not think that it is sufficiently improved relative to the AMTD version to be accepted for publication. Neither of my major comments is satisfactorily responded to.|
Regarding the UV field, the authors acknowledged that more lamps would greatly help and it would not be difficult to install them. Then the authors should use a design with more lamps as the base CFR design in this study. Or at a minimum, they should clearly acknowledge in the abstract and conclusions that most space in the current CFR design is photochemically dark (“not sufficient to provide uniform light distribution within the reactor” in the revised paper is misleading as a uniform light distribution in reactors is virtually impossible), propose a better lamp layout and discuss its possible improvements compared to the current design. In my comments to the AMTD paper, I already stated that the CFR in the current design is not really an effective setup producing large quantities of SOA. Even under atmospherically irrelevant conditions, PAM and PEAR can still be generally more efficient in producing SOA because their effective photochemically active volumes are larger than that of the CFR. Also, it has been shown that the condensation of gases on OA particles may be kinetically limited in OFRs, but this is not important unless the condensation sink is very low (Palm et al., 2016; Ahlberg et al., 2017; Eluri et al., 2018). A longer residence time of 25 min in the CFR cannot make a major difference in the quantity of SOA produced. Thus, the arguments that the authors used to show that the CFR has advantages in producing large quantities of SOA are not valid.
The authors acknowledged that they did not aim to use the CFR to produce atmospherically relevant SOA. Then the scientific significance of this paper is seriously limited. While I agree that the relationship between the chemical composition and physicochemical properties of SOA is not affected by its atmospheric relevance, a reactor that can only explore this relationship in a part of the composition/property space that is far from atmospherically relevance has very limited usefulness. As I said in my review of the AMTD paper, the conditions under which the CFR were run were remarkably different (orders of magnitude away) from the atmospherically relevant conditions. To my knowledge, there is not any easy solution that can efficiently produce mg of SOA that can serve as ambient SOA surrogate. If the authors can use the CFR to achieve this goal, that will be a real novelty and should be detailed in this paper. Otherwise, the authors should clearly acknowledge that the CFR is unable to produce ambient surrogate SOA when producing SOA in large quantities.
Response 1 to Referee 4: if experiments are run with ppmv of precursors, they are very likely atmospherically irrelevant. Then the only goal of these experiments is to make as much SOA as possible. It is unclear to me why “memory effects” should be considered as a problem. In other words, I am wondering why “memory effects” should be taken more seriously than atmospheric irrelevance, as to me both are types of inabilities to explore certain parts of SOA chemical composition space. Besides, I do not think that a custom-built PAM itself is expensive.
Response to my “Table 1” comment: I do not understand this response. If the authors wished to conduct low-NOx experiments, simply injecting no NO would work, since NO is not necessary for photochemical production of OH in the CFR.
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Eluri, S., Cappa, C. D., Friedman, B., Farmer, D. K. and Jathar, S. H.: Modeling the formation and composition of secondary organic aerosol from diesel exhaust using parameterized and semi-explicit chemistry and thermodynamic models, Atmos. Chem. Phys., 18(19), 13813–13838, doi:10.5194/acp-18-13813-2018, 2018.
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