See enclosed file

The manuscript by Chai et al. reports on ground-based measurements of isotopic ratios (¹⁵N/¹⁴N) and (¹⁸O/¹⁶O) and concentrations of NO_x and HONO derived from fresh and aged wildfire smoke plumes. Measurements were conducted from several locations in the Western U.S. during the WE-CAN and FIREX-AQ field campaigns using state-of-the art measurement techniques.  Furthermore, the data is presented and assessed thoroughly to the full extent that the data allows.  This is a significant contribution for the following reasons: It reports for the first time the isotopic ratios of HONO in wildfire plumes and the isotopic evidence is used to evaluate the relative importance of various HONO formation/loss pathways (homo- and heterogeneous) that have until now only been studied in the laboratory or invoked with considerable speculation. Thus, I feel this work contributes significantly because it provides in situ insights into which HONO formation and loss processes are important in wildfire smoke plumes. In addition, the authors present a simple but elegant box model for assessing the importance of these pathways, that can be useful in future studies aimed at studying atmospheric processes involving reactive nitrogen. The paper is not without its weaknesses.  Most significantly, many of the parameters needed to model (e.g., the enrichment factors) are not well constrained.  However, the authors use well-reasoned assumptions and qualify their estimates by clearly discussing the limitations in the extensive appendices to the manuscript.  Overall, I feel this is not a deal-breaker since these are the best estimates that can be made using the available data (none of the enrichment factors have been evaluated in the literature). I feel this manuscript should be published in ACP after the following specific points have been addressed.

The more significant questions in my reading of the work have to do with how HONO is modeled.  If I am not mistaken, the isotopic model uses reactions R1-R4 for daytime chemistry and reactions R5-R7 to represent the nighttime chemistry controlling the HONO isotopic signature. In reality, reactions R5-R7 are also occurring during the daytime and could be important. For example, modeling studies often find that good agreement between model and measured HONO concentrations is only possible when deposition processes are included during the daytime (in addition to photolysis). Particle scavenging in smoke events will be particularly important due to the added surface area provided by particulate matter/smoke particles. For the same reason, non-photochemical sources such as R6 will occur during both the night and daytime. I feel it would be useful for the authors to justify their decision to omit reactions R5-R7 in the modeled daytime results.  I also wonder how reliable the models results are with respect to distinguishing between Reactions (R6) and (R7)?  That is, it was not clear how the parameterization of these two reactions was different and whether, due to the level of uncertainty associated with the enrichment factors and mechanisms, whether it is even possible to distinguish between them, especially since the relative contribution of R6 may be so low. Modern laboratory experiments (and theory) conducted under atmospherically relevant conditions suggest that reaction R6 is only important at very high (>100 ppbV) NO₂ concentrations when dimerization is favored. Measured NO₂ concentrations in this study were below 20 ppbV, so I would have my doubts that NO₂ levels were high enough to favor any NO₂ hydrolysis. In addition, in section B.1.2., I agree that HONO desorption involving breaking of the complex HONO...(H₂O)_n is likely important for determining KIF.  I note that the distinction between the heterogeneous NO₂ reactions (R3, R6, and R7) is the role of water.  In R3 & R7, H₂O is the medium, while in R6 H₂O is both reactant and medium, so would one not expect R6 to have a very different enrichment factor compared to R3 and R7?  

My last points have to do with readability of the manuscript and figures.  The results and discussion refer extensively to reaction equations (R1-R7) and enrichment factors that are only found in boxes within Figure 1.  The text chosen for these reactions is a small serif font placed onto a somewhat busy/distracting background; it is very difficult to read and will be even more so in final published form.  Because of their importance, I recommend simplifying Figure 1.  For example, consider turning it into a (more boring) black-white scheme that omits the graphics and provides all the relevant equations and numbers in an easy-to-read format.  I recommend checking references to equations to ensure they are referring to the correct equations.  For example, on lines 593-594, there are references to Eqs. (10)-(12); I believe this should be Eqs. (B10)-(B11).

In this manuscript the authors present the ground-based measurement results of concentrations and isotopic ratios (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) of NO_x and HONO in the wildfire smoke plumes in the Western U.S. With a simple box model, they are able to use the data to assess the relative importance of pathways of HONO formation and loss in the smoke plumes.  The research approach is innovative and is capable of providing insights into HONO formation mechanisms, although its low method sensitivity limits its applications to air masses with relatively high levels of NO_x and HONO, such as urban atmosphere and wildfire plumes.  The paper contains valuable and useful information and thus should be published.  I do have some concerns and comments below that need to be addressed before the manuscript is accepted for publication.

There were simultaneous real-time measurements of HONO, NO_x and other relevant parameters during the study, as stated in the manuscript and published in Kaspari et al. (2021).  I suggest the authors to validate the denuder sampling methods by comparing the concentrations of NO_x and HONO with those by Kaspari et al. (2021) and to address the comments by Referee #1 regarding potential interference from PAN on NO_x sampling by denuders.  It is critical to prove the methods used to be accurate and reliable before any significant conclusion can be made.

The authors reported that nitrate photolysis plays only a minor role (<5%) in HONO formation in daytime aged smoke, while heterogeneous NO₂-to-HONO conversion contributes 85-95% to total HONO production, followed by OH+NO (5-15%).  This finding is in line with what we would expect from our current understanding in HONO chemistry in the environments with moderately elevated NO_x levels.  However, it should be pointed out that HONO can be produced by different mechanisms in different NO_x concentration regimes.  Extensive field and laboratory studies in the past 30 years have shown that the HONO budgets can be well predicted and constrained by the reactions of NO and NO₂ in the high-NO_x environments.  However, other mechanisms, such as photolysis of surface nitric acid and particulate nitrate, may play an important role in the low-NO_x environments.   The real-time measurement data reported by Kaspari et al. (2021) (and also the time-series plot in Figure S3) showed very high concentrations of HONO (up to 6 ppb) and NO₂ (over 40 ppb) in bands of smoke plumes, in contract to very lower concentrations in the background air outside the plumes.   Due to the long sampling times (2-12 hours for HONO and 0.75 – 2.5 hours for NO_x) required for the concentration and isotopic measurements, the “averaged” data may not be representative of wildfire smoke plumes, especially when there were significant dilution by background air in the “aged” plume.  Cautions should be taken in interpreting the skewed averaged data.

The manuscript contains two appendixes and a supplement, and it summarizes the key reactions with isotopic fractionation information in a figure. This unusual presentation style is sometime jumpy and confusing. I suggest that some reorganizations of the manuscript should be made to smooth the flow of data presentation and discussion and to made it easier to read. 

Page 6 line 165: the minimum detection limit of 0.07 mM seems too high.  It should be 0.07 µM.

Page 14 equations (A1) and (A2): what are R and P in the equations? Is R for the rate of production/loss?  From the expression of (A2), P should be the fraction of OH-NO reaction to the total HONO production.  All the terms in equations should be defined in the text.

Page 15 equation (A4):  Since the sampling was conducted on the ground stations, ground surface should be considered in S/V; it may be important for the heterogeneous HONO production near the ground, especially during the night.

Page 16 equation (B3): Should the equation be as follows?

1/ϒ_l = 1/α + 1/Γ_b

The calculations in lines 393-494 do not make sense.

Figure 5:  How do you define the fraction of remaining HONO upon photolysis (F_rp)?  For a daytime aged plume arrived at the site from tens km away, >99% of the original HONO would be photolyzed within a few hours during the transport.  So with <1% of HONO remaining upon photolysis, >15% of R4 contribution may still be possible.