|ACP-2021-531 second review. The numbered comments correspond to the authors’ replies to reviewer #1 in document “acp-2021-531-AC1-supplement” and line numbers to the authors’ revised manuscript.|
[Comment 1] This is a high quality observational dataset of HONO and related parameters that merits publication. My main concerns from the first review focused on the data interpretation side of the work, and particularly because the outcomes from the authors’ 0-D modeling analysis rely heavily on the “correct” choice of the mixing layer height. MLH is set at 50 m, and two sensitivity tests are performed with the MLH set at 35 and 100 m.
The authors’ response provides further qualitative discussion of the MLH based on their study and other literature studies. This is helpful. But it hasn’t fully answered the question. I was hoping that the authors could be *quantitative* in their response and thus they would add numerical values of the MLH found in other studies into the main text, against which the reader can judge the authors’ choice of the MLH. This would give confidence that 50 m is indeed a reasonable, *objective* choice.
MLH = 50 m brings the authors’ simple 0-D model into closest agreement with their HONO observations. But a different, more sophisticated model might produce a significantly different optimum MLH. Thus “50 m is best” is a consequence of the model, and not necessarily what is happening in the atmosphere itself. This part of the manuscript still needs further work:
* I accept the authors’ response that it isn’t possible to prove the MLH by comparing these ground-based observations with HONO measured at the mountain summit in their comparison paper.
* Were any numerical values of the MLH for HONO derived/quoted in the papers by Brown et al (2013) or Vandenboer et al (2013) or any other studies in the literature? If so, please add the MLH numbers into the main text.
* Numerical data of vertically resolved HONO measurements from Xing et al (2021) were given in the main text (and repeated in the authors’ response). Did Xing et al derive a number for the MLH? Can their data be fitted to provide a value(s) of the MLH?
* Do the sensitivity tests performed at 35 m and 100 m cover the likely range of how the MLH varies with wind speed, time of day, day vs night etc? i.e. do 35 and 100 m encompass the lower and upper limits of what is happening at their measurement site?
I’m entirely comfortable with the authors’ conclusion that ground-based HONO sources dominate at the Tai’an city measurement site, and consequently the MLH for HONO is small (of the order of 50 m). I also recognise that the exact value of the MLH is changeable with conditions at their measurement site, and the MLH will likely be different at other locations. But at the moment the manuscript asks its readers to accept 50 m as the correct choice based on the authors’ assertion. And this has consequences for the understanding of the atmospheric chemistry that follows.
Fig 4 & line 269. The addition of the J-HONO photolysis frequency to the diurnal profile in Fig 4 is informative and welcome. Likewise the more explicit reference in the text to the asymmetric shape of P_unknown. I think Line 269 should now read “Note that the profile of P_unknown *is* asymmetric around *12:00 solar noon*, indicating the unknown source is not simply photolytic…” The important point here is that the diurnal profile exhibited by P_unknown is offset with respect to the peak photolysis activity of J-HONO and/or J-NO2 at solar noon (rather than any asymmetry before/after the peak in P_unknown at 11:00).
It is good the authors have added campaign-averaged diurnal profiles of HONO, O3, NO, NO2 etc as a new figure S3 in the supporting information (also requested by referee #3). Personally I would put this new figure into the main body of the paper because its information is very important for telling the scientific story.
Continuing discussion of the sentence that began on line 269: The second half of the sentence reads “…but also includes its precursors that also have an asymmetric distribution (e.g., NO2, Figure S3)”. The text need to be much more explicit here. Panel (E) in new figure S3 certainly shows that NO2 concentrations were higher in the morning than in the afternoon – so are the authors saying the faster P_unknown HONO production rate observed before midday (and peaking at 11:00) is attributed to the greater NO2 concentrations present in the morning? This would fit with the authors’ finding that P_unknown has its biggest contribution from heterogeneous NO2 to HONO conversion at the ground (Figure 8).
I asked the authors to expand about how the conclusions of this paper translate to other locations in China and other countries? The revisions here are disappointing and I had hoped to see more discussion. The authors have added a sentence at line 523 “Model results may have uncertainties but shed light on the atmospheric chemistry in this polluted region”, which is certainly true but fails to answer the reviewer’s question.
The authors have done a lot of work on their manuscript. Overall, the revised manuscript is a great improvement because the authors have acted on the comments from 3 detailed, extensive reviews. The authors have also thoroughly proof-read their manuscript, which has removed most of the English language problems in the original manuscript. Whilst the authors thanked the reviewers in the authors’ individual responses to the 3 referees, perhaps they might also consider acknowledging the referees’ efforts in the acknowledgements section of the main manuscript?
Minor errors (These are errors I spotted whilst reviewing, so not an exhaustive list):
Caption to Fig 4, line 260. “The relative contribution of NO + OH and … *are* shown…in the pie charts”. I also suggest moving this sentence to the end of the caption, i.e. to after the sentence about the “blue shaded area” which seems to refer to the main plot (and not the blue shading in the pie charts). Or does the blue shading in the plot and the pie charts both represent the same observed-minus-model differences? I was unsure, please clarify and re-word as necessary.
Line 267. ”… were calculated *from* the measurements [plural]…”.
Line 317. “…upper limit derived from the summit measurements [plural] *in our companion paper*, see Xue et al. (2021b)…”
Line 389. “The only major exception was a period of heavy rain from 25th to 28th June…”
Line 392. “While the modified model could generally predict…, [add comma & delete “but”] it largely failed…”
Fig 10, line 470. The authors have improved the colors and now the distinctions are a lot clearer between the various OH sources in panel (A) and NO3 sources in panel (B). However the new fig 10 now uses a light scale of pastel colors. I actually preferred the style of bold colors in panels (C) and (D) of the original – it was easier to distinguish the bold colors.