The authors aim for describing a new instrument for the detection of HONO by laser-photofragmentation of HONO and subsequent detection of OH by LIF. Although the topic of the paper would have been in the scope of this journal, the way the authors structure the paper is not appropriate. The authors give an adequate clear description of the instrument and its calibration.

However, the results and discussion part is not fitting the scope of the journal. The authors mainly describe measurements and try to give a chemical explanation of the observed concentrations. This is clearly out of the scope of this journal and should be significantly shortened. Unfortunately, the authors give only little results and discussion of the performance of the instrument. Even the discussion of interferences is kept on a level of estimates from literature and the only experimental result is only mentioned to be discussed in future publications. More detailed experimental investigations in the laboratory could have been done.

A comparison of the measured HONO/OH ratio with calculations could have been valuable, if this is used to test the validity of the instrument. However, this is likely not possible, because concentrations are not only determined by the photo-stationary state of gas-phase reactions and the additional measurements may be incomplete or not of high-enough quality. Another option would have been to include a detailed comparison with the CIMS instrument, but the author decided not to do so, but to shift this to another publication.

Overall, the authors mention topics that could have been discussed to show and discuss the performance of the instruments even without further experiments, which would have been even better. Because of the lack of content, I recommend rejecting the paper. It might become suitable for the journal, if there is a broader discussion of results.

Detailed comments to the part of the experimental section:

L13: Full calibration of the sensitivity requires also the calibration of the OH detection sensitivity.

L16: Does the LOD refer to HONO and OH?

L39: The explanation of HONO accumulating during nighttime is a bit misleading, because gas-phase reaction alone would not explain the increase of HONO during the night, but only shift the photo-stationary state to HONO at dawn.

L166: Here or somewhere else the authors should mention the duration of each step in the measurement cycle.

L211/L213: There is inconsistency in the naming of the quantum yield.

L233/234: The term “effective sensitivity” is rather confusing in this context. The sensitivity of the instrument does not depend on laser power due to the normalization of the fluorescence signal to the laser power. The authors likely mean a better limit of detection that can be achieved at higher laser power, because the total fluorescence counts increase and is therefore more likely larger than the noise. Please clarify.

L233/234: The authors argue that photolytic interferences become smaller in the single-pass configuration compared to the multi-pass configuration. This would allow to operate the system at higher laser power. This is rather confusing because at higher laser power photolytic interferences will again gain in importance. Please clarify.

L242: What do the authors mean with “once a stable concentration OH and HO2 is produced”? What does need to stabilize?

L249: How was the loss of 5% determined?

L250: It would be beneficial for the reader to know the wall loss rates that are assumed and to specify the fractional loss to specific loss processes.

L252: The text sounds as if there is a significant fraction of OH left, but Fig S4 suggests that this is negligible.

L252: Can the authors exclude that reactions of NO from the calibration source leads to any back-reaction of OH to HONO after the 355nm laser pulse has been applied in the measurement cell?

Figure 4/5: Are really counts shown or normalized count rates? Why are numbers in Fig. 4 so much smaller compared to numbers in Fig. 5, if they are also derived from calibration measurements?

L267: It is not very clear for the reader, which correction is applied to S_OH. Is this needed because different losses apply, if NO is added or not? Number of corrections may help to better understand what is done.

 L271: The authors mention several possible problems with impurities of the NO added in the calibration procedure. This discussion should be extended by a quantitative estimate, if these reactions could play a role for the conditions described in this work.

L282: It would help to give a quantitative estimate about the impact of a typical additional OH concentration during midday on the limit of detection of HONO.

L283: Typical accuracies for the determination of OH concentration in a calibration source like used in this work are within the range of 10 to 20% and would significantly contribute to the overall uncertainty of the HONO calibration. Please clarify.

Development of a Laser-Photofragmentation Laser-Induced Fluorescence instrument for the detection of nitrous acid and hydroxyl radicals in the atmosphere

Bottorff et al., Atmospheric Measurement Techniques Discussions, doi.org/10.5194/amt-2021-102

The authors report the development, characterisation and initial results from a novel instrument designed to detect HONO and OH in the atmosphere. The instrument is based on the fluorescence assay by gas expansion (FAGE) technique used to detect OH radicals in the atmosphere by laser induced fluorescence, which has been developed to provide measurements of HONO by measuring the OH fragment produced following the laser photolysis of HONO.

The manuscript is well presented and provides a detailed description of the instrument, its calibration, and initial results obtained in both outdoor and indoor field measurements. The description of the development and calibration of the instrument are within the scope of the journal, with the field measurements providing an indication of the capabilities of the instrument and as such are relevant to the publication.

There are, however, a number of areas in which the manuscript could be improved prior to publication which are listed below:

Line 26: ‘hydroperoxy’ is generally preferred over ‘hydroperoxyl’.

Line 37: Although likely clear to most readers, the terms in equation 2 ought to be defined for clarity.

Line 40: It may also be worth commenting that the wavelengths at which HONO photolyses to produce OH compared to other OH sources contribute to its role as a significant early morning OH source.

Line 95 onwards: There have been reports of photofragmentation-LIF technique used to measure HONO in laboratory experiments (e.g. Dyson et al., 2021 doi.org/10.5194/acp-21-5755-2021) which should be included in the discussion.

Line 102: The wavelength limit for O(¹D) production from ozone photolysis is usually given as 340 nm. Please clarify.

Line 178 (and elsewhere): ‘Criegee intermediates’ is preferred over ‘Criegee radicals’.

Lines 180-183: It is not entirely clear from the description whether this method is used in any of the HONO measurements reported to remove ambient OH. Please clarify.

Line 195: Add the physical state for HONO.

Line 196: What are typical concentrations generated by this method?

Line 199: How would production of ClNO result affect the measurement of HONO? Is it a problem for this technique?

Line 201: It’s not clear what the “long warmup times” refer to, what requires warming up with the method described?

Line 221: It might be more appropriate to refer to an effective oxygen absorption cross-section rather than state the absorption cross-section is dependent on operating conditions. The cross-section itself is a fundamental physical property, it is more the measurement of the cross-section that depends on conditions.

Line 242 onwards: Is there any other chemistry that should be considered? Model simulations are referred to in line 250 but no information is provided on the model or mechanism. Some additional details are required on this, and it may be appropriate to include Figure S4 in the main text.

Lines 258 and 271: Can the authors comment on the identities and possible concentrations of the impurities in the NO leading to the production of OH or reduction in apparent photofragmentation efficiency? Could the same species result in interferences in ambient measurements? See also comment below. Are scrubbers to remove NO₂ used in the calibrations described in this work?

Line 285: Are there any measurements to show how much (or how little) the photofragmentation efficiency varies throughout a prolonged measurement period? A table summarising detection limits, photofragmentation efficiencies, uncertainties, conditions etc. for calibration experiments and the measurements reported in sections 3.1 and 3.2 would be useful.

Line 306: What is the estimate based on?

Line 380 onwards: The manuscript would benefit from a more detailed description of potential interferences. Previous work by Liao et al. is referenced, but a more complete discussion of this work and its relevance to the current work is required. It would help to show some model calculations of species that may photolyse at 355 nm and lead to OH signals to demonstrate the impact of potential interferences. HOCl is mentioned as a potential problem during the indoor measurements, it would help if the authors could present some model calculations to indicate whether this is likely only a problem for indoor measurements, or whether ambient concentrations of HOCl could be problematic in outdoor field measurements. Similarly, are there any potential issues relating to HOBr or HOI in marine environments? Are there are other species which might significantly impact the measurement through production of OH or a reduction in the effective photofragmentation efficiency? Does absorption of the 355 nm light by other ambient species, such as formaldehyde, cause any potential interference by reducing the effective photofragmentation efficiency of HONO?

Line 412 onwards: The conclusions section reads more as a section on future work. It would help to include a summary of the operating conditions and measurement capabilities.

Figure 3: The image quality may need to be improved prior to final publication.

Figure 4: Can the authors clarify what is meant by ‘S1,3’, is this the average of S1 and S3?

Figure 5: Can the symbols be matched to those given in Figure 4?

Figure S1: Please clarify whether the laser used to photolyse HONO was a Nd:YAG (355 nm third harmonic) or Nd:YLF (351 nm third harmonic).

Figure S2: A schematic diagram may be more helpful than the figure provided.