The authors use a 0-D puff model to investigate emissions and chemistry in a biomass burning plume from the Rim Fire observed during the SEAC⁴RS campaign. The time evolution of normalized excess mixing ratios (NEMRs) is constrained by observations of O₃, NO_x, a large number of NO_y species, and a large number of VOC which help constrain RO_x chemistry. Six model cases are investigated with a particular focus on HONO, 1) using only observed species, 2) adding unobserved VOC based on lab studies, 3) adding HONO as a primary emission, 4) adding HONO via particular nitrate photolysis, 5) adding HONO via heterogeneous reaction of NO₂, and 6) a combination of 2,3, and 4. Implications for the representation of the investigated chemistry in other models are briefly discussed.

The work is scientifically sound and valuable as a thorough investigation of a case study and is generally well presented. My general comments are to better contextualize the case study, and to provide additional detail on the expanded VOC reactivity and the lack of NO_y closure and how the latter relates to the HONO additions investigated. I elaborate below.

The investigation of the case study could benefit from some contextualization and summarization. While there is extensive literature available on the SEAC⁴RS campaign and this flight, certain details should be made available to the reader in this work, e.g. how did the Rim Fire compare to other fires investigated during SEAC⁴RS? A single background period is chosen, how does this background compare with other observations? What was the fuel mix for the Rim Fire? In a similar vein a number of changes in background are inferred for a variety of species many of which are likely related, e.g. an increase in biogenic background after 2 hours, these can be challenging to keep track of; I would recommend a timeline or concise summary of such changes as a single reference rather than the current references to a variety of sections above and below.

Realizing that once constructed the Lagrangian age is the time axis used, I would also encourage caution when referring to observed changes as a function of age which are attributed to fire variability. In some instances, age is used to refer to the evolution or the fire which is a separate time axis from the aging of emissions from a given point in time.

Figure 4c shows the large fraction of OH reactivity which arises from unmeasured species particularly aromatics. What is not clear is what fractions of the secondary species arise from these additional species. While the authors make a compelling case that the amount of “missing” OH reactivity cannot be explained by measurement error, it is not clear whether there are additional bounds on this from the modeling results. Understanding that the secondary VOC also changes due to the change in OH and other oxidants, can the authors offer some estimate of what fraction of the OH reactivity from secondary species is due to the unmeasured VOC?

Figure 3 shows that the observed NO_y, which is nominally a conserved family as defined here, is reduced markedly with increased Lagrangian age. As the authors discuss it is unlikely that any of the component observations are sufficiently far off to explain the discrepancy and major unmeasured components such as HONO, HO₂NO₂, and CH₃O₂NO₂ are also unlikely to fill the gap leaving open the possibility of unknown NO_y reservoirs. The HONO sensitivity studies introduce mechanisms converting observed NO_y to unobserved NO_y (although it is quickly returned), which provides a useful reference for flux out of observed NO_y. How do these compare with the observed rate of loss?

Peng et al., 2020 observed a rapid decline in the HONO NEMR in the first two hours, while Theys et al., 2020 observed a decrease in the HONO/NO₂ ratio on a similar time scale. These ratios are introduced in Sect. 2.4.2 in this work already. HONO time evolution seems to be broadly consistent with both works, but is the trend in either ratio reproduced in this work?

For the pNO₃ photolysis case, without a process to covert NO_y back to pNO₃ and with pNO₃ constrained to observations is this an unbounded production of gas-phase NO_y? Given the dominance of PN as a fraction of observed NO_y, is this the principal reason for the reproduction or is HONO chemically particularly well suited to accomplish this? As noted above, the text seems to indicate that there is evidence for unknown NO_y reservoirs, is the failure of closure across different criteria in Sect. 3.4 indicative of that or unrelated?

Technical comments:

Line 127: Can the authors provide details on the optimized lag-correlation. What was the method, what was the cost function?

Line 130: Why a single WAS sample for background? What statistics does this provide? How does this background compare with other measurements from SEAC4RS?

Line 146: It would be helpful for reproducibility to know which algorithm was used to compute the geometric median, and to what precision.

Line 149: I assume the authors mean here when the back trajectory first intersected the fire which would be when the trajectory last intersected the fire. I suggest rewording for clarity.

Line 152: When was the wind measurement for the transit time estimate taken? At observation or at time of emission? Does the 1h transit time generally comport with the back trajectories?

Line 178: “age-dependent” here should be substituted with “time-dependent” or something similar. If I understand correctly I would reserve “age” for the evolution of the trajectories and not to refer to different times of emission to avoid confusion.

Line 204: It is written “Heterogeneous chemistry is explicitly included.” I assume there is a “not” missing as this is a paragraph on limitations of the model.

Line 222: Why report the fuel composition if it is not assessed? Can the authors provide any information relating to this?

Line 235: Can the authors provide some assessment of the bulk characteristics converting compounds using this method? e.g. total carbon, average molecular elemental composition.

Line 252: How is the pNO₃^- at the start of the puff constrained? If I understand correctly this is nominally the fire, which observations are taken to correspond to this?

Line 266: The linear relation is valid for the period of the observations, but J_NO2 should not be linear with SZA as a general rule especially near twilight. The value of J_NO2 at sunrise is negative using the equation. Is this relation extrapolated back in the puff model to the time of emission, if so what is the nominal SZA at emission?

Line 268: Should the values <10^-6 not be substituted by 1×10^-6 based on eq. 4? When multiplying the rate by 1000, is this lower limit similarly scaled?

Line 344: CH₃CHO is not subscripted here.

Line 395: I would move Text S2 to main text above to support this statement regarding oVOC production.

Line 402-03: I do not understand what the sentence “After 12 h, 32% of M1-simulated OH reactivity is comprised of nearly 2200 species, mostly oxygenated VOC.” Is seeking to communicate.

Line 496: This estimate is also substantially smaller than that in Theys et al., 2020 do the same reasons apply?

Line 507: Avoid Ar for Aromatic reserve for argon

Wolfe et al use observations and a photochemical model to examine the complex photochemistry in one 2013 fire.    The results show that unmeasured VOCs and OVOCs have a huge impact on the photochemistry, which is an important and believable conclusion.  However there are several significant uncertainties.  The major uncertainty that is discussed is the sources of HONO, which ends up as a downwind NOx source.   A major uncertainty that is less discussed is the fate and measurements of NOy species.    In particular Figure 3 shows a substantial decline in NOy/CO, implying a loss of NOy other than dilution.   Where does this NOy go?   Is it lost to deposition?   Deposition seems unlikely in this timescale and given particle size distribution.    Or is it transformed to other species that are not being measured?   How would this impact your conclusions?   Finally I suggest a bit more on the key measured species.  I see in the SI the list of measurements, but I recommend that a bit more be added to the main manuscript to clarify key points (like how NOy was obtained, whether HONO was measured or not…)

On the presentation, most aspects are done fairly well with the exception of Figure 5.   I found this figure very difficult to interpret.  I think the authors are trying to cram too much into one figure and the result is a figure that is very difficult (impossible) to interpret.  

Detailed comments:

Line 65: Grammar.

Line 170:  Baylon 2018 has a good discussion of the UV impacts on JNO2 and JO3 (https://doi.org/10.1002/2017JD027341)

Line 179:  Why would MCE decline with age?

Line 180:  Assumption of constant NOy EFs seems important.   Evidence?

Line 204:  What het chemistry?   Does this include HOx loss on aerosols?   Contradicts line 254.

Line 205:   Measurement accuracy…?   Don’t understand this sentence?   I think variability is more important…

Line 227: Change to “reduced by factors of 2.3 and 10…”

Line 239:   This discussion on HONO emissions/confusing is confusing.  Wasn’t HONO measured, so why do you need these scaling factors?

Line 244:  Don’t understand P-HNO3 photolysis scaling…

Line 296: Change to “VOC-to-CO emission ratio increase…”

Line 313: Fig. S12 is discussed before Fig. S11.

Fig 3 and discussion…. Does NOy include p-NO3?   Why does NOy/CO decline?   Where does NOy go?    This seems like a sig uncertainty in the results.   How does this impact the results?

Line 376:   modeled NOy-obs.    So what does M0 tell us?    It seems the bulk of NOy loss is in the first hour and is likely due to chemistry… So doesn't this imply that the NOy is probably going into unmeasured species?

Lines 391–392: Why does the base simulation over-predict the NO/NO₂ ratio during the first few hours?

Line 452: The figure for ozone is Fig. 2k, rather than Fig. 2l.

Lines 457–458: The sensitivity simulations to heterogeneous reaction of NO₂ are in Fig. S20, not Fig. S19.

Lines 517–522: Since Table S1 indicates that HCHO and NO₂ measurements were made during the SEAC⁴RS campaign, it might be useful to examine the trend in the HCHO/NO₂ ratio to see if that is consistent with your conclusions regarding the O₃ production regime based on L_N/Q.

Line 614: Grammar.

SI:

Text S2: Fig. S12 – not Fig. S13 – shows the age progression of other oVOCs.

Text S3: Fig. S13 – not Fig. S14 – shows the results for the other speciated PNs.  In addition, Fig. S14 – not Fig. S15 – shows the results for ΣPN and ΣAN.

Fig. S9: The figure caption does not state what the green lines show.

Fig. S10 caption: In the first sentence, NMB should be enclosed in parentheses.  In the second sentence, the reference should be cited as Gustafson and Yu (2012). 

Fig. S16: In the legend, the blue and red solid lines denote the NO_x NEMRs for simulations M0 and M1, respectively, rather than S0 and S1.

Figs. S18–S20: The figure captions say Figure 18, 19, and 20, instead of S18, S19, and S20.

Fig. S19: It appears that the green lines in panels (j) and (k) are not entirely visible.  If this is true, then the y-axis limit needs to be increased accordingly.