|The revision of Zhang et al. provided a number of clarifications. Several key technical details now look clearer in the paper. However, in my opinion, it has not adequately addressed a few major issues raised by Reviewer 2:|
- Responses to Comment 2-1 and 2-6: first I do not understand the response to Comment 2-6. Did the authors mean that because the fume was only a little warmer than indoor air, it had already cooled down and condensable gases condensed onto particles? If so, it is unclear to me what role pipe heating really played and the phrase “prevent freshly warmed gas from condensing on the pipe wall” does not seem to be appropriate; otherwise, I am not convinced that pipe heating was sufficient to prevent wall losses/tubing delay of S/IVOCs in the pipe. Obviously, pipe heating could help substantially. It is however unclear whether this measure can reduce wall/tubing effects in the pipe to minor/negligible level unless the authors perform a more quantitative analysis e.g. based on Pagonis et al. (2017).
- Response to Comment 2-9: the observation that oleic acid ozonolysis made little SOA in Go:PAM does not imply that this reaction cannot produce SOA significantly in the atmosphere. Ozonolysis of oleic acid breaks its long chain and produces higher-volatility products than its oxidation by OH does. It does not surprise me that the ozonolysis products did not condense onto particles during the very short residence time of Go:PAM. However, in the atmosphere, the O3:OH ratio is even higher than in Go:PAM and the products of e.g. oleic acid ozonolysis, particularly organic peroxy radicals, have enough time to undergo several generations of autoxidation to become of sufficiently low volatility and condense. Therefore, the (lack of) contribution of ozonolysis to SOA formation in Go:PAM should be seriously discussed.
In addition, I have 2 major concerns about this paper:
- Liao et al. (2021) did OFR experiments of SOA formation from on-road vehicle emissions in Beijing. They kept their experimental conditions high-NOx and an earlier and relatively low peak of SOA formation (as a function of photochemical age), compared to literature studies of this kind, which were all conducted under low-NOx conditions in OFRs. The peak SOA age in Liao et al. (2021) was only ~1 d, significantly lower than in this study (~2 d). High-NOx pathways in organic peroxy radical chemistry allow fragmentation of organics to occur at lower degree of oxidation than low-NOx pathways. As a result, the compositions of SOA formed through these 2 types of pathways may be significantly different. I thus have doubts whether the SOA formed in Go:PAM in this study is sufficiently representative of SOA formed in urban areas, where NOx is usually high. If the SOA was not representative, the analysis in the triangle plot and the VK diagram, PMF etc. in the paper did not bring much insight. I believe that the authors should justify that the SOA in their experiments were representative of SOA in urban atmospheres.
- In a number of places in the paper, the discussions about SOA formation from cooking emissions seemed to be based on heterogeneous oxidation of POA. While I understand that this pathway may play some role, it is slow enough to be minor at photochemical ages of 1-3 days compared to gas-phase oxidation of cooking emissions, a large fraction of which is S/IVOC with C=C bonds. The authors should discuss how cooking SOA is formed from S/IVOC oxidation in the gas phase.
- Response to Comment 1-9: I agree that Manchester is not a megacity. But this would be better determined by the total population of its metro area (~3 M) than by that of the city proper (~0.5 M).
- Response to Comment 2-2: to me, the authors seemed to misunderstand the Reviewer’s point here. The Reviewer likely meant that low humidity would lead to highly viscous particles, which usually undergo slower heterogeneous oxidation than less viscous particles.
- A lot of text in Sections 2.3.2 and S2 looks too similar. Some of it may be cut to reduce redundancy.
- Line 98: a dash is needed between “Beijing” and “Chengde”.
- Ling 196: “Hongkong” -> “Hong Kong”.
Liao, K., Chen, Q., Liu, Y., Li, Y. J., Lambe, A. T., Zhu, T., Huang, R.-J., Zheng, Y., Cheng, X., Miao, R., Huang, G., Khuzestani, R. B., and Jia, T.: Secondary Organic Aerosol Formation of Fleet Vehicle Emissions in China: Potential Seasonality of Spatial Distributions, Environ. Sci. Technol., 55, 7276–7286, 2021.
Pagonis, D., Krechmer, J. E., de Gouw, J., Jimenez, J. L., and Ziemann, P. J.: Effects of gas–wall partitioning in Teflon tubing and instrumentation on time-resolved measurements of gas-phase organic compounds, Atmos. Meas. Tech., 10, 4687–4696, 2017.