General comments

This work details on how HOM can function as an indicator for determining the sensitivity of O₃ formation. The authors have clearly communicated their approach, results, and the potential limitations of the study. In general, the manuscript is well-written and of good scientific quality. Therefore, I would recommend this manuscript for publication with the minor re-works and additions outlined below.

Specific comments

How the photolysis rate of NO₂ was determined? Maybe a brief discussion (used expressions) can be included in the manuscript.

It was not clear from the discussion (Section 2.3) and Table A2, whether wall losses have been accounted for or not. A discussion on these losses could be interesting to see the nature of the effect on indicating ratios.

There can be a discussion on why IR1 holds a better potential than IR2 for indicating O₃ formation sensitivity.

It is specified that this chamber study can estimate indicating ratios in determining O₃ formation sensitivity, both qualitatively as well as quantitatively. I suggest adding a table that shows the estimated and measured values of O₃ concentration as well as the indicating ratios, which will make it easier for the readers to refer to the values.

Technical comments

Line 205: Despite showing a faster decay compared to HOM_ON, O≤8, non-nitrate HOM monomers with fewer than 9 oxygen atoms (HOM_Mono, O≤8) also showed overall slow decays (Fig. 3). – This sentence needs to be rewritten.

After Table A1, the line ‘The figures are shown....’ should be deleted or completed.

The authors conducted chamber experiments to evaluate whether HOM can function as a real-time indicator of the O3 production regime. They classified different types of HOM, and showed that the O3 sensitivity can be estimated from the composition of HOM (nitrate-containing and non-nitrate compounds) in a single VOC system. 

Overall, this manuscript is well written, and the results are useful to researchers working on the O3 formation mechanism. However, there are several concerns as below.

General comments: 

(1) The authors used the ratios of non-nitrate HOM and nitrate-containing HOM as the indicating ratios. The reason for using HOM (not oxygenated VOC) is not clear to me. In general, HOM compounds have lower volatility than most OVOC compounds, and thus, they have more uncertainty factors, such as higher aerosol formation (for both ambient air and chamber experiments) and wall deposition (for chamber experiments). In this study, I guess the authors measured only gaseous HOM by CIMS, and thus, the production yield of HOM cannot be fully measured. The volatilities of HOM compounds varied widely, and thus, the fractions of HOM in a particulate phase or deposited on walls are different among compounds. The application of the OVOC/organic nitrate ratio as the indicating ratio seems more straightforward than the HOM ratio proposed in this manuscript. The reason for using HOM should be clearly stated.

(In a similar context, I do not understand why compounds with O<=8 are not included in this analysis. Compounds with O>8 have lower volatility, and thus, aerosol formation or wall deposition would be expected to interfere with the estimation of the HOM production yields).

(2) Is there the aerosol formation in this experimental system? If large fractions of the produced HOM are in a particulate phase, then, the interpretation of the derived indicating ratio becomes complicated. The information on aerosol formation in this experimental system is useful to readers.

(3) The estimated threshold values of the indicating ratios are the key findings of this study, whereas the derivation of the thresholds is not explained in the main text. The derivation of these threshold values (shown in Lines 307-309) should be clearly explained. 

Technical comments: 

Figure 5: “Measured” should be removed from the labels of the x and y axes, because the results of EKMA are also shown in this figure.