This study quantified and characterized nearly all organic carbon in the laboratory oxidation of biomass burning (BB) emissions through combined mass measurements (AMS, PTR, and I-CIMS). There are several unique elements of this work, including the focus on BB organic species conversion and formation. Generally, the authors provide good context for the work and appropriate references are cited. It is very valuable work. Several specific technical and technical comments are provided below. With minor revisions, the manuscript may be appropriate for publication in ACP.

Specific comments:

1. Line 101 What is the specific concentration or approximate range of fresh plum before aging, and how to define the maximum concentration of particle? The initial concentration of BB plume is very important for the subsequent aging process.

1. The actual OH concentration/exposure in the PFA during the course of each experiment needs to be given if measured.

1. There is a large amount of ozone in the chamber during the experiment. Over the course of the reaction, not only the OH radicals-driven oxidation reaction but also the ozone-driven oxidation reaction. How can the authors prove that OH oxidation dominated in the oxidation mechanism and how much uncertainty will be caused by the existence of high concentration ozone in calculating the equivalent aging time？

1. High-NO (RO2+NO) and low-NO (RO2+HO2) conditions are very important for the aging reaction and oxidation products. The author also stressed this point, but the follow-up manuscript did not pay attention to these chemical regimes, resulting in this part of the content was not well handled.

1. As discussed in the manuscript, only 134 PTR ions and a small subset of CIMS ions were calibrated directly. Can the authors give detailed information about the identified species, such as a table? Another question, I would like to know whether all 763 unique gas-phase species calculated the carbon oxidation state, volatility, and oxidative lifetime, or only identified ions. I wonder if you use different species in the calculation and if it will change the graph.

1. What is the concentration and formation rate of OA measured by AMS over the reaction? The evolution of actual OA concentration cannot be seen in Figure 1. How did the authors calculate the carbon concentration of OA? I can not find the calculation processes in the manuscript. As the author confirmed that the increased abundance of a handful of small VOCs is driven by fragmentation reactions, however, fragmentation reactions generally lead to the reduction of OA (doi.org/10.5194/acp-11-3303-2011, doi.org/10.1002/2014JD022563). Does this conflict with the growth of OA?

1. What is the reason for the decrease in carbon concentration of AMS, I-CIMS, and PTR in the initial stage in Figure 1?

1. Are the species in Table 2 only measured by PTR or were detected by both instruments？ Can you mark which instrument detected these species respectively?

1. The authors emphatically analyzed the Fire 25 and Fire 26 experiments. The two groups of experiments used the same fuel and got similar commons. However, the MCE and moisture content of these two groups of fuels are significantly different. Many previous field and laboratory studies have emphasized the aging process of high MCE (flaming) and low MCE (smoldering) and found that there are great differences, including the SOA formation, the oxidation state of OA, and gaseous oxidation products (doi:10.1029/2021JD034534), change of optical properties (10.1021/acs.est.0c07569), and the influence of aerosol emissions from wildfires driven by MCE (doi.org/10.1021/acs.est.6b01617). Will this factor probably influence your results, which may be worth some discussion and explanation?

Technical corrections:

Line 34, the comparison of mass spectra

Line 127, in the measurement

Line 136, delete the second “and”

Line 144, due to the unavailability of standards,

Line 301, each of the individual fires becomes...

Review of Nihill et al., “Evolution of Organic Carbon in the Laboratory Oxidation of Biomass Burning Emissions”:

Summary:

The authors detail the evolution of reactive organic carbon from biomass burning emissions at the FIREX FireLab campaign. Approximate carbon closure was achieved using three instruments, following a similar analysis from previous work. Generally, the gas phase reactive carbon moves to smaller carbon number and higher carbon oxidation state. The spectra of compounds in aged smoke all looked rather similar, regardless of which fuel was burned. This work represents an important step towards understanding the chemistry of aged smoke, which has air quality impacts all around the world. The paper is very well written. I recommend for publication after addressing my comments below.

Comments:

Line 104: What is the estimated OH concentration in the chamber?

Line 146: Can you give more details about how this max sensitivity of 8000 ncps/ppt was estimated? E.g., which compound(s) were calibrated with that sensitivity? That number seems way larger than expected, and way larger than the values of 300 ncps/ppt (theoretical) or 75 ncps/ppt (empirical) used in the similar analysis of Isaacman-VanWertz et al. 2018. Can you specifically say why the value you use is so much larger in this paper? Was a different ion-molecule reactor (IMR) used on the CIMS that had a much longer residence time or some other change like that?

Line 160: The I-CIMS probably measures a lot of peroxides though. Uncertainty analysis?

Line 192: The “(1  )” looks like a typo?

Line 237: In addition to fragmentation reactions, the decreasing n_c could be due to partitioning of larger n_c compounds into the particle phase after functionalization, right?

Line 245: Do you have a possible explanation for why the volatility briefly increases before decreasing gradually, shown in Fig. S4a and S5? Please add it to the text, that will help the reader to understand why you are showing this data.

Line 325: Are the mass spectra in Fig. 5a+b combined PTR and I-CIMS data? Could be good to say that. I guess the answer would also apply to all of the figures. It might be interesting to make a version of Fig. 5a+b, or better yet Fig. 3, for the SI where you differentiate which compounds were measured by which instrument. This is not necessary for drawing your conclusions in this work, so feel free to ignore this comment, but it might be an interesting bit of extra information to show how the PTR and I-CIMS measure complementary parts of the spectrum of compounds.

Line 338: Phenolics etc. can also fragment to produce C4Ox products, so maybe make this statement a little more general that C4Ox compounds are formed from C4+ precursors including furans or larger precursors?

Line 348: I think it would be useful for you to explain how you make a cumulative distribution function (CDF) a bit more. I am unfamiliar with them, and it took me a long time to understand. I guess the compounds are added starting with the highest concentrations first?

Line 366: Again, could loss of higher n_c compounds to the particle phase through functionalization and condensation contribute to this result of decreasing average n_c and higher relative importance of small VOCs? If you find that is a minor contribution, can you say so with any evidence?

Line 366 part two: Could heterogeneous oxidation of aerosols lead to evaporation of small n_c oxidation products? Do you expect any meaningful heterogeneous oxidation at your high OH concentrations?