This paper reported a newly developed IBBCEAS system to measurement NO3, NO2 and H2O near 662 nm, in which the detection of NO3 with high accuracy is the key target. The non-linear absorption of water vapor near 662 nm is a large interference and lead to the retrieval of weak NO3 absorption challenging. Several studies have been trying to address this issue. In this study, the design of the optical system was adopted from a well-established instrument for measuring the glyoxal and nitrous acid (Min et al., 2016), and added a purging flow on high-reflection mirror surfaces. I believe the novelty of work is making effort to retrieve the ambient absorption of water vapor by establish H2O absorption cross section by the instrument measurement in advance. Overall, this topic is within in the scope of AMT, and this manuscript is well written with a comprehensive characterization in the lab as well as a good performance in the field test. The authors did a good job, I would like to recommend this paper to be published subject to a minor revision.

General comments:

1. Line 187-195, I am very confused how did you established the H2O absorption cross section. Are you measured the water absorption at a certain RH or a series of RH level at room temperature? We know that the temperature and pressure in the detecting tube would influence the water absorption cross section, how to deal with these variations in ambient conditions? Given the importance of this issue, I suggest the authors provide more details about it in the revised version.
2. How about the influence of the mirror reflectivity change in the water cross section? If the R decreased to 0.99999 for example, the previous measured water cross section still working?
3. How about the temperature range in the field campaign in Arctic regions, is it possible lead to a bias in retrieving H2O absorption?
4. Line 320, how the transmission efficiency determined in the field campaign, especially the loss in the sampling tube, is it scaled by the residence time in this part?

Technical corrections:

5. Line 240, the unit of aerosol loading should be μg rather than μg cm-3? Please clarify it.
6. Line 146-147, typo error.
7. The dot size in figure 7(b, d, f) is too small

General Comments:

This paper details the development of a new BBCEAS system for simultaneous measurements of the trace gases NO3, NO2, and H2O. Unlike previously developed absorption-based sensors for NO3, this study emphasizes the utility of retrieving the water vapor signal, which has strong absorption features in the detected spectral region around 662 nm, instead of correcting for water vapor as an interference in the NO3 signal. The instrument demonstrates superior precision and comparable accuracy as compared to existing BBCEAS NO3 measurements. Overall, this paper presents a thorough characterization and evaluation of the instrument performance and its field operation. It is well within the scope of AMT, and I recommend publication subject to the minor revisions detailed below.

Specific Comments:

1. I agree with RC1 that the description of measuring the H2O absorption spectrum in the original text is unclear. I believe the authors have sufficiently addressed this concern in their response, as well as any concerns relating to temperature control of the instrument.
2. L207 states a “Fourth-order polynomial was applied to account for the optical drift and/or unaccounted extinctions such as absorption by ambient ozone.” Was there any basis for selecting this functional form? The retrieval demonstrates that the polynomial fit is a quiet a large component of the overall signal. Please elaborate or clarify why this is the case.
3. How reproducible are the NO3 transmission results to the field environment? It seems this has been clarified in the author’s response to RC1, but I’m curious if this would have to be characterized in each new environment.
4. The description of the NO3 dilutions in the linearity test are somewhat unclear. Where is the drift in the NO3 concentration evidenced in Figure 7? Or have the data in Fig 7a,b already been corrected for the linear drift? Please be explicit as to what the red and black dots indicate in these figures. It is not stated in the text or in the figure caption.
5. L321: The wording is unclear. Was the total transmission efficiency reduced by 65% of the lab-based value? Or reduced to a total transmission efficiency of 65%?

Technical Corrections:

1. It would be helpful to see all the detection limits in Table 1 for the same integration time if possible (for ease of comparison).