Review of Wilkerson et al, 2021: In situ observations of stratospheric HCl using three-mirror integrated cavity output spectroscopy.

This manuscript presents a novel instrument (RIM-ICOS) for the in-situ measurement of stratospheric hydrochloric acid (HCl), based on off-axis integrated cavity output spectroscopy, and a demonstration of its performance. As anthropogenic chlorofluorocarbons continue to control the chemistry of the stratosphere, it is important that both the levels and the variability of key compounds, such as HCl, are known. This is especially important given projected climate induced changes in stratospheric temperatures and deep convective injection events. The ability of the RIM-ICOS to be deployed as part of a balloon payload provides an incredibly useful capability to measure this important compound in-situ.

The manuscript is well written and describes the instrument in the required level of detail, including important factors such as the minimization and characterisation of HCl surface adsorption. The authors also present data from the instrument from a balloon descent between 18-29 km altitude on 24^th August 2018, and compare these data with satellite HCl retrievals and correlations with other collocated measurements. The RIM-ICOS demonstrated a 30 s precision of 26 pptv, more than adequate for the target environment, and agreed with satellite retrievals within the combined uncertainty. Overall, I think that this is an excellent piece of work that warrants publication in AMT once the following minor comments have been addressed.

Minor comments:

• It would be useful for the reader if the dimensions, weight and power requirements of the RIM-ICOS was provided, as it’s deployment on a balloon suggests it could also be applicable to other platforms with payload constraints.
• The authors state that the instrument inlet and measurement cell are held at a temperature of 310 K in order to reduce temperature-related changes to the HCl spectra being fitted and also minimise surface effects from both HCl and water. One possible consequence of this heating is that sampled HCl in the particle or ice phase could repartition into the gas phase under these warmer temperatures. This potential interference has been discussed previously for both stratospheric and tropospheric HCl observations (e.g., Webster et al. 1994 & Crisp et al. 2014). The authors should discuss the potential impact of this repartitioning of HCl on their measurements.

References

Webster, C.R. May, R.D., Trimble, C.A., Chave, R.G., and Kendall, J.: Aircraft (ER-2) Laser Infrared Absorption Spectrometer (ALIAS) for In-situ Stratospheric Measurements of HCl, N2O, CH4, NO2, and HNO3, Applied Optics, 33, 454- 472, https://doi.org/10.1364/AO.33.000454, 1994.

Crisp, T. A., B. M. Lerner, E. J. Williams, P. K. Quinn, T. S. Bates, and Bertram, T. H.: Observations of gas phase hydrochloric acid in the polluted marine boundary layer, J. Geophys. Res. Atmos., 119, 6897–6915, doi:10.1002/2013JD020992. 2014.

This paper reports the development of an off-axis integrated cavity output spectrometer (ICOS) with re-injection performance. A precision of 26 pptv was achieved with 30 s integration time in the lab. The developed instrument was evaluated in the HUSCE campaign, and the results were compared with MLS data. The manuscript was well-written and well structured. I recommend for publication after considering following comments.

1) Page 1, Line 13. The abbreviation of MLS is incorrectly marked here.

2) Page 3, Line 67. The response time is related to the volume of the cell and sample flow rate. This argument needs the support of the detailed information of these parameters.

3) Section 2.2, page 5. What is the bandwidth of the detector? The mirror reflectivity or effective cavity length was determined by the ring-down measurement. What is the ring-down time of the empty cavity? By using re-injection performance, more light will enter the cavity. It is no longer appropriate to use the base length divided by 1-R to express the effective optical path, which is usually used for laser beam one-time injection into the cavity.

4) Section 4. What kind of interference does “balloon interference” mean? What is the ascent and descent speed of the balloon? Will the release of the helium affect the measurement of HCl? Is the pressure of the sample cell kept constant or the same as the ambient pressure? Will the residence time of the sample in the cavity change?

5) During the flight, will the changes in the atmospheric temperature affect the performance of the cavity?