Comments to the Author:

The paper describes a commercial TILDAS instrument for measuring hydrogen chloride in ambient air and demonstrate the ability of sampling methodology to minimize inlet artefacts. Due to the “sticky” behavior of HCl gas, quantitative sampling remains a challenge for current approaches. To improve instrument response to changes in HCl gas concentration, a custom-fabricated quartz virtual impactor is used to replace particle filters to avoid excess surface-mediated interactions with filters, and the heating and PFBS coating methods are employed to improve transmission. Its performance validates that the sampling method is effective for reducing HCl “sticky” behavior. Overall, the paper is well written, with detailed characterization in the lab as well as reliable performance in the field sampling. I recommend this paper for publication in AMT after the following minor revisions.

General comments:

Section 2.2.1: The technique description of the TILDAS device is not clear, and more technical details need to be added, such as measurement principle, structural schematic diagram, etc.

Section 2.2.2: The custom-fabricated quartz virtual impactor can effectively remove the large particles (> 300 nm diameter) in the sampling line, which was approximately 13% of the total volumetric flow. Only gas molecules and small particles (< 300 nm diameter) can flow into the TILDAS instrument. Please explain how does the impactor work and how is the ratio of flow rate obtained?

Section 3.1: The performance of HCl TILDAS is evaluated in the lab with dry zero air as well as in the field with HCl-scrubbed sample air, and its precision and LOD are superior to the previously reported methods. More technical details need to be added to explain how does the instrument achieve better performance? Did the authors perform long-term measurements of a fixed concentration of HCl gas? This approach can better represent its real performance.

Section 3.3.2: There is an obvious offset about 0.07 ppbv (shown in Figure 8) before addition of nitric acid to the passivated sample inlet flow. Please explain the reason for the offset signal.

Section 3.4: The maximum concentration of HCl in field observation is about 0.1 ppbv shown in Figure 9(a). But the HNO₃ concentration of 4 ppbv may cause an increase of 0.08 ppbv of HCl. How to evaluate the error of atmospheric HCl concentration caused by HNO₃? And the influence of a potential leak on the measurement of HCl gas concentrations during observation needs to be clearly evaluated.

Specific comments:

Page 4, L137: The references could not be found in this manuscript.

Page 14, L382-387: The influence of humidity on the measurement bias of HCl concentrations is only reported at 60% RH. In fact, the relative humidity of atmosphere is often much higher than this value. Therefore, the authors need to give the relationship between the measurement bias and the relative humidity, so that the reader can clearly grasp it.

Page 18, L490: The data should be modified to 20 June 2021.

Halfacre et al. report the construction and evaluation of a spectrometer for quantification of HCl in the atmosphere. The instrument is thoroughly described and was evaluated in the field as part of the Integrated Research Observation System for Clean Air" (OSCA) campaign in Manchester. The instrument's figures of merit are an improvement over existing technology (Table 1). The paper should be published once my comments below have been addressed.

Title: Please remove the term "Novelty" from the title. Novelty is implied when publishing. Further, TILDAS using astigmatic Herriott cells has been around for at least a quarter century. The main novelty of this work is the extension of known technology (QCL-TILDAS) to a new molecule (HCl).

line 21/301 - "high accuracy". It would help to be more quantitative here and nuanced in the discussion of accuracy. The authors report that the instrument measured 3.6% lower than a commercial HCl cylinder, certified to contain a known concentration within ±5%. The certification applies to what the manufacturer added to the cylinder; what comes out can be an entirely different matter (subject to regulator passivation etc.). As such, a comparison to a single cylinder does not suffice to validate a new instrument's  accuracy in my opinion.

In this context, are the absorption line strengths well known (and can be used to justify accuracy)?

There is also a zero offset to be considered when discussing accuracy since the instrument reports negative mixing ratios (e.g., Figure 9) which are inaccurate by default. Consider stating a slope uncertainty and a zero offset uncertainty.

Have the authors considered calibrating or comparing against a wet chemistry technique?

line 114 - replace detection with quantification

line 128 - I was wondering about the safety of perfluorobutanesulfonic acid, which is partially discussed on lines 186-188. Consider adding a comment regarding safe handling of this compound.

lines 147 - 154. Please state the line strengths (or cross-sections) used and how those were determined (Hitran?)

line 153 "well-resolved" - please state the FWHM of these lines and add a graph showing the spectrum you are discussing here (at high and low concentration), so the readers can see for themselves.

line 501-514. Please try to be more quantitative in this paragraph - for example, rather than saying 'greatly improve' or 'higher flow inlets' or 'reduce sample air residence time', state by how much or the actual value.

Figure 6 - replace sec with s

Figure 8 - what caused the second hump at 14:10? Please add an explanation to the caption.

Figure 9 panels (a) and (c) - both the HCl and NO2 data exhibit spikes and negative concentrations, even when averaged. Please add some discussion to the text as to the meaning of this, potential causes, and remedies. Consider adding a horizontal line to show limits of detection or quantification.

Figure 10 panel (a) - change units to pptv to avoid the x10³

The manuscript describes a gas phase HCL sensor, based on tunable infrared laser direct absorption spectroscopy (TILDAS). The authors highlight the importance of HCL in the atmosphere, describing its influence. They describe the difficulties of monitoring HCL, it is “sticky” and results in such effects as long instrument response times. They describe current monitoring techniques, their strengths and weaknesses, and  why their approach will be of benefit. 

The authors first describe the TILDAS sensor design, followed by techniques to minimize sticky behavior of HCL on extraction, an “inertial inlet” and active passivation. They also describe procedures for validation, field testing and data analysis. 

The authors present results for different configurations, such as with and without passivation and humidity effects, They present field data and compare sensitivity to other published set-ups. They discuss problems with HCL particulate and nitric acid.

They present 7-8 pptv at 1 Hz  and 3𝜎 limit of detection ranging from 21-24 pptv. For longer averaging times, the highest precision obtained was 0.5 pptv and 3𝜎 limit of detection of 1.6 pptv at 2.4 minutes. These values are competitive compared to other optical techniques, which are considered more complicated to set-up. I think the manuscript should be published.

I believe the manuscript requires minor revisions and clarifications.

• The title needs to be considered. I find it misleading. TILDAS is not a novel spectroscopic approach. It is the first application of TILDAS to HCL. Should be clarified to reader or manuscript changed.
• There is little detail on optical configuration of set-up. If it is new and custom made, more information can be given here.
• There is little detail to spectral fitting. They do talk about background subtration. But, error can also come from the spectra fit. I think more detail should be given here. 
• Re Methane measurement. Do you have a LOD or senitivity for this measurement? it appears in plot, methane can fluctuate by approx 2-3 ppb in a few seconds. is this real? If yes, why not see these typeos of fluctuations with HCL
• ISORROPIA II.. a few sentences on theory are not in referred section
• The conclusion seems more like an outlook, except for first sentence. 

Individual line comments/typos;

line 107... typo "it is has"

line 138... What is exact wavelength of laser and tuning range?

line 160...  “gas phase via acid displacement” can you add a reference for this. There are references earlier, but they don;t seem to fit this.

line 401 "It is well established that HCl and particulate chloride (pCl- 401 ) exist together in dynamic equilibrium" can you add refernce here.

line 484 How is nh3 measuremed here?

Figure 10. Is the time on a) and b) the same. Not clear. y scale strange