The manuscript by Lieberman et al. details a method to measure dissolved nitrogen dioxide (NO₂) concentrations in aqueous solutions. The method relies on the conversion of aqueous NO₂ into nitrite using S(IV) (= HSO₃^- + SO₃^2-), followed by quantitation of nitrite by the Griess method. This technical note is well-written and provides enough detail to easily reproduce the experiments. The method is useful and represents a novel approach to (near)simultaneously measuring nitrite and NO₂ concentrations arising during nitrate photolysis. The application of this method to studying nitrate photoproduct yields is notable given the importance of elucidating renoxification pathways derived from nitrate photolysis. As pointed out by the authors, previous measurements have been limited to measuring the quantum yields of the nitrite and NO₂ photolysis channels with different analytical methods.  The current method will enable the use of a single method to assess whether secondary chemistry is enhancing one channel at the expense of the other or if there is an overall enhancement of the primary nitrate quantum yield. Below are some points that came up during my reading of the manusript.

Figure 1: The content of this figure / scheme really doesn’t lend itself well to its own figure.  I feel it would be better to simply replace this with three separate equations (R1-R3) in the main text.

Line 99:  Please specifically indicate the pH (or pH range) of the solution here.

Section 2.3:  Please address the possibility that HSO₃^- can act as a scavenger of OH radical produced from nitrate photolysis.  Also, this reaction would form sulfite radical anion and I would like to know your thoughts on whether this would interfere with the chemistry. Does one have to correct the quantum yields to account for this? In addition to this, please address the possibility that H₂O₂ can oxidized the Griess reaction reagents or nitrite, converting it to nitrate via peroxynitrite. I would expect for a paper such as this for the researchers to test potential interferences with the method. This is not done beyond the limited intercomparison of nitrite and NO₂ quantum yields presented in the figures. If a more complex matrix was used (e.g., in the presence of organic molecules, dissolved organic matter, transition metals, etc.) could we expect the method to accurately quantitate NO₂ concentrations?

Line 141:  How did you determine the limit of detection?

Lines 210-218:  Practically speaking, can the authors please provide some comments on how variable these results are and how stable the system is.  That is, if I try to use this method, how important is it to follow the indicated timing here?  If one lets the reaction go longer or doesn’t “develop” the reaction solutions soon enough, does one get a different answer? Can one store the reaction solutions in the freezer for later analysis and still get comparable answers? Any insights?

Line 263:  The authors suggest there are other reactions that lead to consumption of and NO₂, quantifying it ask k_other. Can the authors please provide some insights into what they think this(ese) reaction(s) is(are)? 

This paper presents a method that converts NO₂ to NO₂^– using S(IV) so that nitrate photolysis products can be measured by the same instrument. This method can derive total nitrate photolysis quantum yield from the two channels by one analytical method, allowing determination of whether the nitrate photolysis is impacted by one channel or both channels in different environments. This method could potentially be useful to our community in better quantifying nitrate renoxification, but there are issues that need to be addressed.

Major comments:

1. This method is developed to be used for bulk nitrate solutions. However, many studies (including references cited in this paper) on enhanced nitrate photolysis were about particulate nitrates or nitrates on surfaces. The authors need to address this disconnection. How can this method be used to address particulate nitrate/surface-absorbed nitrate photolysis? With complex composition in atmospheric particles, can this method still be valid?

1. Did the authors investigate whether the added H₂O₂ would react with NO₂- and interfere with the quantum yield quantification?

Minor/technical comments

1. In equation (1)(2) and (4), the x should be a multiplication sign, not letter x.
2. Figure 3 caption: “Measured quantum yields of nitrite (yellow bars)”. I think the authors mean blue bars.