|The paper has improved since the last round of revisions, but still requires further revision before it is acceptable for publication in ACP. The first part of the manuscript (excluding the abstract) that describes the data is better than the latter part that infers the dependence of ozone on NOx and VOCs. Since the authors do not present a photochemical model, the paper needs to be very clear that it cannot make recommendations about NOx and VOC control that are specific to Guangzhou, although it can make comparisons to other locations. Statements about the specific NOx and VOC regime in the latter part of the paper need to be revised with this comparison in mind, but without stating that it has determined a control strategy for Guangzhou that is appropriate.|
Page 3, line 6: Particulate matter rather than particulate matters (no “s”), and pollutant rather than pollutants (again, no “s”).
Page 3, line 17: “smog” rather than “somg”
Page 3, line 21-25: The reference here is now improved, but the statement about the 8:1 ratio is not as general as the sentence indicates. Suggest the following revision. “Although specific actual situations differ, numerical simulations in other polluted regions, such as Los Angeles in the 1980s, have shown ozone formation to be NOx limited when the VOC/NOx ratio is greater than 8:1 for VOC expressed as the mixing ratio of total carbon (Seinfeld 1989).”
Page 3, line 25-27: There is a great deal of literature examining observational relationships between VOC and NOx in O3 formation. See for example, Trainer, M., et al: Correlation Of Ozone With NOy In Photochemically Aged Air, Journal Of Geophysical Research-Atmospheres, 98, 2917-2925, 1993, which has been widely cited. Suggest simply omitting the sentence beginning with “However, little attention …”, since it is not needed to justify the work in this manuscript.
Page 4, line 7: If this is the first use of the term “MIR”, give the definition here. Also, since the authors do not actually use a photochemical model to calculate MIR but look it up from work on other places, replace “MIR” with “estimated MIR” here. Also see further comments on the use of MIR below.
Page 5, line 1: Suggest that “single measurement cell” is a better description than “single glass measuring tank”
Page 5, line 5: Replace “respectively” with “alternately”
Page 5, line 9: Add an appropriate reference to the conversion of higher oxides of nitrogen in molybdenum converters, such as: Dunlea, E. J., et al..: Evaluation of nitrogen dioxide chemiluminescence monitors in a polluted urban environment, Atmos. Chem. Phys., 7, 2691-2704, 10.5194/acp-7-2691-2007, 2007.
Page 5, line 36: Remove the phrase “you can see that”. Sentence should read simply “The correlation coefficient ranged …”
Page 5, line 40: A sentence is needed here to state that the instrument has not been intercompared with other VOC measurements, and that such an intercomparision is an important priority for future work.
Page 6, line 2: Remove the word “characteristic”, replace “relatively” with “readily” and remove “which displays the seasonal pattern” to read “The seasonal variation of ozone is readily apparent, being lower in the spring ….”
Page 6, line 11: Replace “showed” with “shown”
Page 6, line 19: Remove “in this”
Page 6, line 20: Replace “of that” with “the”, and change “may include” to “probably includes”
Page 6, line 26: Remove the word “variation”. Also, begin this sentence with an appropriate caveat about transport effects. “In the absence of transport effects, a negative derivative in ozone …”
Page 6, line 26: Low wind speed does not mean that transport is necessarily unimportant. The statement is not quantitative in any case, since “very small” is not defined, and the authors probably don’t have a means to make a quantitative statement here. Rather, the statement should read something to this effect.
“Although wind speeds are low at this site (1.4 m s 1 in different seasons, see Table 1), the effect of horizontal transport on daily variation in ozone is difficult to estimate and cannot be ruled out. Here, we interpret all variation as due to chemical production and loss, understanding that such interpretation is necessarily an upper limit to the effect of chemistry due to the likelihood of transport also influencing daily ozone variation.”
Page 6, line 32: remove “of variation”
Page 6, line 33: There is no measurement of OH radicals as the statement implies. The authors probably mean that sunlight begins to decrease at this time. Sentence should be clearer. “At 15:00, sunlight and associated photochemistry begin to decrease ….”
Page 6, line 37: remove “variation” at beginning of line. Replace “high OH radicals” with “increased photochemistry”
Page 6, bottom: First two sentences in section 3.2 are redundant. Omit the first one.
Page 6, line 15: Is it accurate to say “developed by Carter in model scenarios of Los Angeles in the 1980s” ? The MIR calculations must have some reference point that should be specified. If they are developed for a different city or time period, then give that reference point, but the reference must be stated.
Page 7, line 33: Remove the phrase “of the atmosphere at the sampling point.” End of this sentence should read “… alkanes and aromatics are shown to have the highest mixing ratio.”
Page 7, line 39. The point about alkenes and aromatics is important, but is consistent with findings from other locations. There should be a sentence and a reference to this effect. A good reference would be: Russell, A., Milford, J., Bergin, M. S., McBride, S., McNair, L., Yang, Y., Stockwell, W. R., and Croes, B.: Urban Ozone Control and Atmospheric Reactivity of Organic Gases, Science, 269, 491-495, 1995.
Page 8, line 11: The sentence is not consistent with itself. If the MIR varies with location, why should this demonstrate that it is accurate for Guangzhou? A better statement would be that the MIR values were not calculated for Guangzhou, so they may not be accurate.
Page 8, line 14: Too many significant figures. It is not possible that the ozone formation potential has been determined to four significant digits. Quote as either “approximately 30%” or “approximately 32%”.
Page 8, line 28: replace “were” with “are”
Page 8, line 28-32: the definition of the ozone isopleth is not clear. Please revise this sentence to state this more clearly. It is simply a two dimensional plot of the variation of peak ozone with VOC and NOx. Also, the Dodge 1977 reference is a difficult one to find and should be omitted.
Section 3.3, Figure 7, and associated discussion: The figure is difficult to interpret. There is no description of how the underlying VOC and NOx data are used to arrive at the contour lines on the figure, and as the authors state, there are other variables (mainly transport) beyond the VOC and NOx concentrations. There are 2 options.
1) Simplest: Simply omit this analysis and move on to the discussion of the remaining figures.
2) Average all the data in Areas 1, 2 and 3 to produce peak ozone vs. NOx for different ranges of VOCs and three single lines rather than as a contour plot. Any such plot will need to have the caveats in the text that NOx is an upper limit to actual NOx due to measurement artifacts on the molybdenum converter. If the authors choose this option, the paper will necessarily require an additional stage of review, further delaying publication. If the authors choose option (1), review may be shorter.
Page 9, line 20: New paragraph corresponding to the description of figure 8.
Page 9, line 33: “Fair” rather than “fare”. The sentence should reference the source of the number used for VOC to NOx ratio. “Simulations in Los Angeles from the 1980’s indicated a transition from VOC limited to NOx limited at a ratio of approximately 8:1 (Seinfeld, 1989). Although we have not performed specific modeling for Guangzhou, we use this approximate relationship to divide VOC and NOx limited regimes for Guangzhou.”
Page 9-10, discussion following the sentence modified in the last comment: The discussion on here must avoid explicitly stating that NOx control would be more effective than VOC control, since Guangzhou is not Los Angeles, and there is no photochemical modeling analysis in this paper that is specific to Guangzhou. Rephrase all conclusions to state more simply that Guangzhou falls in the NOx limited regime according to the simple comparison to Los Angeles, but that no definite conclusion can be reached about NOx control compared to VOC control in the absence of a photochemical model together with more accurate measurements of NOx.
Page 10, line 25: Replace “You could be seen in Figure 11” with “Figure 11 shows”