The values of photochemical indicators are widely used to determine the O3-NOx-VOC sensitivity with measurements, which has important policy implications. This work examined the effectiveness of four indicators such as PH2O2/PHNO3, and surface HCHO/NO2 with air quality models. It can provide decision-makers with some useful information when they use the photochemical indicators to make control strategies. The manuscript is well written and fits the scope of ACP, which is worthy of publication. However, there are a few questions as follows that need to be addressed to further improve the manuscript.  

Major comments/questions:

1. What are the influences of regional transport on the results?
2. How will NH3 and inorganic aerosols such as sulfate, nitrate, and ammonium affect the HCHO/NOy and NOy?

Some minor issues:

Page 2 line 45: “, the O3 to nitric acid ratios (O3/HNO3), and NOy)”

Figure 1. The letters are skewed.

Page 6 lines 138-140: Why are the 95^th percentile for the VOC-limited grids and the 5^th percentile for the NOx-limited grids chosen as the boundaries of the transition interval?

Page 11 lines 240-242: I suggest moving this part to the methodology section.

Page 14 lines 295-296: This seems to conflict with your argument that the emission had no significant influence on the thresholds of PH2O2/PHNO3.

The first row of Table 4 was incomplete.

Lines 450, 491, 521: The format of journal names should be consistent.

Comments to Li et al.

This paper presented the examination of different indicators for O3-NOx-VOC sensitivity based on the chemical transport model CMAQ results. Four indicators were tested, i.e. the ratio of the production rates of H2O2 and HNO3 (PH2O2/PHNO3), HCHO/NO2, HCHO/NOy, and reactive nitrogen concentrations (NOy) for the YRD region. This work determined and evaluated the threshold values of these indicators. Besides, the uncertainty caused by the method was also analyzed. Generally, the manuscript is well-written with a clear structure, and the analysis and discussion are scientifically sound. I recommend publication once the comments below have been addressed.

General comments:

1. The determination of NOx-limited and VOC-limited is changes of O3 by more than 5 ppbv if NOx and VOC emission reduction by 35% relative to the base run. This criterion is adopted from Sillman et al. (1998). However, the original analysis mainly focused on an ozone episode at the Nashville and vicinity area with relatively high O3 concentration (>80 ppbv). As indicate in Figure 2(a), the O3 concentration could match this criterion for the south part of YRD but not the North part. The relatively low O3 in the north part leads to relative low absolute change of O3 concentration when NOx or VOC emission reduce by 35%. In this case, the north part can still be attributed to NOx/VOC limited regime. It’s not clear to me how the determination of threshold for different indicators depends on this classification.
2. In section 3.4, the P_H2O2/P_HNO3 is used for an example but also suggest to add similar discussion on HCHO/NO₂ or HCHO/NOy to address the statement that “By comparing with the O₃ isopleths, it was found that HCHO/NO₂ and HCHO/NOy showed the most consistency”.
3. The overall accuracy values of NOy in some cases are higher than other photochemical indicators as shown in Fig.4, however in the section 3.3, the indicator was not recommended. Please explain the discrepancy between the result mentioned above.

Specific comments:

1. The language needs improved. For example, the tense of one paragraph should be consistent. I only list a few and suggest the authors to carefully go through the paper. Line 14: examines à examined;    Line 49: is VOC-limited à was VOC-limited      …
2. Line 48: Explain “NOz” when it appeared for the first time.
3. Line 50: If the threshold for an indicator is varying, it seems contradict to “robust”,
4. Please Briefly introduce HDDM in the section of “Methods”.
5. The introduction of OA expression in Table 4 was incomplete
6. Please elaborate the determinization of thresholds for different photochemical indicators in Fig.3.
7. Delete the extra brackets in Fig.3(a).
8. Please elaborate the approach to distinguish the O3 formation regime with shading colors as shown in Fig.5.

Reference:

Sillman, S., He, D., Pippin, M. R., Daum, P. H., Imre, D. G., Kleinman, L. I., Lee, J. H., and Weinstein-Lloyd, J.: Model correlations for ozone, reactive nitrogen, and peroxides for Nashville in comparison with measurements: Implications for O3-NO x -hydrocarbon chemistry, 103, 22629-22644, https://doi.org/10.1029/98JD00349, 1998.