The manuscript The Determination of ClNO2 via Thermal Dissociation-Tunable Infrared Laser Direct Absorption Spectroscopy by Halfacre et al. describes a novel technique to measure nitryl chloride (ClNO2), an important molecule for the chemistry of the lower atmosphere. ClNO2 has previously been measured only by chemical ionization mass spectrometry (CIMS), and the community will certainly find it valuable to have an alternative analytical technique available. The authors have thoroughly characterized and tested the new method and convincingly shown that it is reliable both in laboratory and in ambient conditions, with similar performance to the CIMS instrument.

The manuscript is within the remit of AMT, and I recommend publications, after the authors have addressed the comments below.

Main comments

Some technical clarifications are needed in Section 2.2:  the conversion to HCl is done using a flow of methane and propane, is that right? If so, in which proportions? Please amend Figure 1 to include methane, and briefly explain why two different VOC are needed. When sampling ambient air could there be interference from ambient VOCs? Specifically, VOCs with double C bond which would not form HCl. Can you comment on this point?

It is not clear what the procedure to account for ClNO2 loss in the denuder is: a 55% loss is quite high and it isn't constant with time (page 13). Are the data corrected for this loss in post-processing? If so (section 3.3 suggests as much), how reliable/stable this correction is? With regard to the denuder, is its efficiency in removing HCl constant with time?

Can you clarify the use of the HCl permeation source in the ambient setup? Lines 163-164 are  not very clear. Why not leave the permeation source in front of the furnace (after the denuder). Also, briefly explain how PFBS mitigates HCl surface losses.

In section 3.2. The authors attribute the discrepancy between the TILDAS and the CAPS to potential NO2 artefacts from the salt bed. What is the residence time in the reactor? Did you attempt to increase it or reduce it, to see if that would affect the artefact? Did you vary the composition of the salt bed and/or its water content? I agree that speculation on the exact mechanism is out of place in this paper, but if these tests were done a brief comment would be useful.

Chloramines are mentioned as potential interferences.  Are there other compounds that could possibly create interference? Like Cl2, and chlorinated-VOCs?

In section 2.3.1 and on page 11: the CAPS instrument is know to be affected by changes in humidity. Is this a factor in your setup?

Minor comments

On page 3: either use the "g" and "(aq)" notation for all equations or  use it for none of them.

On line 149: could the the particle filter in front of the inlet affect the measurements in ambient conditions?

On line 242: please add the bond dissociation energy of ClNO2 for easy comparison.

On line 273: delete "loss".

On line 274: delete "not".

On line 282: was the furnace heated for these experiments?

Figure 5: please add the Pearson's coefficients (on the figure or in the caption).

On line 343: what changes in the CIMS inlet are you referring to here?

Halfacre et al. present a novel measurement technique using thermal dissociation (TD) tunable infrared laser direct absorption spectroscopy (TILDAS) for the quantification of ClNO₂ in the troposphere. ClNO₂ is an important reservoir species for chlorine radicals and NO_x. It is primarily quantified using chemical ionization mass spectrometry which despite high time resolution and sensitivity to ClNO₂ requires calibration by an external method such as thermal dissociation cavity ring down spectroscopy. In this manuscript the authors present a new TD-based approach coupled to TILDAS, where ClNO₂ is thermally dissociated to Cl and NO₂ at 450°C, before being converted to HCl through reaction of the Cl radicals with a hydrocarbon (methane+propane). The generated HCl is then quantified by an absorption band at 2925.89645 cm^-1 by TILDAS.

Overall, the presented manuscript is well and clearly written. The authors present a novel and convincing case for the use of TD-TILDAS for the quantification of ClNO₂. As such the work presented by the authors is most certainly of interest to anyone who studies ClNO₂ both in the laboratory and field. I recommend this manuscript for publication once the following minor comments and questions are addressed:

General comments and questions:

1. The authors explore potential interferences from a number of Cl containing species including: Cl₂, HOCl, ClONO₂ and chloroamines. To do this the authors model the TD of these species within their inlet furnace using the Kineticus model. This evaluation is important given the interest in using this technique for ambient ClNO₂ measurements, where multiple Cl containing species could dissociate under the operation conditions, contributing to Cl radical production (and HCl) and an overestimation of ClNO₂ mixing ratios. However, the authors do not mention potential interference from ClNO. ClNO has been reported in laboratory studies to form from the reaction of HCl with surfaces that have been exposed to NO2 (e.g., Raff et al., 2009) or reaction with HONO (Wingen et al., 2000), or through reaction of NO₂ with particulate chloride (Weis and Ewing, 1999), and hence could be of interest for ambient measurements. Although the reviewer is not aware of any ClNO measurements for the troposphere, it could in theory provide an additional interference for the quantification of ClNO₂, since it can dissociate to form Cl + NO (~160 kJ mol^-1).
2. The authors briefly discuss the potential for chloroamines to act as a significant interference. The reviewer agrees with the authors that this is important to highlight. In order to further emphasize this, it would be helpful to expand upon the potential magnitude of this interference slightly. Specifically, where the greatest interferences from chloroamines might occur (and give an example of mixing ratios). For example, Wang et al., 2023. This would be useful for anyone considering adopting this technique for field measurements.
3. Modelling was conducted using Kineticus to capture the TD profile of ClNO₂, formation of HCl and explore potential interferences through generation of Cl radicals from other chlorine containing species. I believe it would be useful for the reader to include a few additional details on the model (and initiation conditions and assumptions) for those who are not familiar with Kineticus.  For example, it would be useful to provide context for why in Fig. 2b the number density of ClNO₂ does not increase as the TD temperature decreases, whereas the OH and Cl continue to change during this period. In the current text, this is not clear.
4. The authors show some ‘proof of concept’ ambient measurements of ClNO₂ which were conducted on Jan 13^th, 2025. According to the x-axis of Fig. 6 these measurements lasted for ~12 hours. The authors also state on lines 335-337 that during this period the throughput of ClNO₂ in the base-coated denuder (designed to remove HCl and other acids) varied from 55%-33%. Do the authors have a sense of how this trend would hold out over longer sampling periods, such as days or weeks? How would this impact corrections applied to ClNO₂ concentrations? Additionally, at what point might the authors expect breakthrough of acidic gases such as HCl, which may interfere with the ClNO₂ measurement. These would be important to consider when using this technique for longer field deployment.

Specific comments:

Line 58- the authors refer to the instrument as a “N₂O₅-cavity ring down spectrometer” and reference Thaler et al. It is perhaps more appropriate to just say “TD-CRDS via quantification of NO₂ by absorption at 405 nm”. The “N₂O₅” instrument in question also used a laser at 662 nm for quantification of NO₃ (from the TD of N₂O₅), which would not be used for ClNO₂.

Line 87- are the costs an order of magnitude lower? Would be useful for reader to give an approximate idea of how much cheaper this method is. 

Section 2.2- Where is the temperature of the quartz furnace measured?

Fig1 – it is not currently clear where there the additional 5 m of tubing for ambient measurements was added based on the schematic in Fig. 1c. Please consider adding a few measurements for key areas on the schematic.

References:

Raff, J. D., et al. (2009). "Chlorine activation indoors and outdoors via surface-mediated reactions of nitrogen oxides with hydrogen chloride." Proceedings of the National Academy of Sciences 106(33): 13647-13654.

Wingen, L. M., et al. (2000). "A Unique Method for Laboratory Quantification of Gaseous Nitrous Acid (HONO) Using the Reaction HONO + HCl → ClNO + H₂O." The Journal of Physical Chemistry A 104(2): 329-335.                     

Weis, D. D. and G. E. Ewing (1999). "The Reaction of Nitrogen Dioxide with Sea Salt Aerosol." The Journal of Physical Chemistry A 103(25): 4865-4873.

Wang, C., et al. (2023). "Chloramines as an important photochemical source of chlorine atoms in the urban atmosphere." Proceedings of the National Academy of Sciences 120(30): e2220889120.

This manuscript describes a new method to quantify gas phase ClNO2 under ambient sampling conditions. The method couples a previously characterized approach for HCl measurement (TILDAS) with a thermal desorption approach that efficiently generates HCl from ClNO2. Potential biases and interferences are discussed, and good comparisons are made using existing techniques in lab and ambient conditions. The main advantages of the method appear to be lower cost and less demanding calibration needs than CIMS, the currently used technique. The main drawbacks are that HCl must first be removed from the sample air (necessitating a denuder and a correction for additional ClNO2 loss), and apparent interferents in the form of chloramines (or some other compound that is co-emitted). The authors acknowledge that further work is required to determine whether these interferents can be disentangled in a practical way for ambient measurements.

Overall the manuscript reads quite well and is organized in a logical way. I have only a few minor suggestions, noted below, but otherwise I find this work ready for final acceptance.

Figure 1. The readability of this could be improved- hard to read the small text, especially the gray color. Is this a vector image format? If not, that would be an improvement.

Figure 2b. It’s pretty hard to differentiate the HCl and Cl colors on this figure. Consider adjusting at least one of those.

Line 246. It would be helpful to state known or estimated mixing ratios of chloramines in any contexts presented in existing literature here.

Line 369. Is there any other way besides temperature scans that this could be achieved?