Major comments

Key factor controlling aerosol size distribution in biomass burning plumes is interesting and important topic in current air quality and climate change studies. This work analyzed the impact of plume concentration on coagulation and OA condensation/evaporation based on FIREX-AQ aircraft measurements. They found the median particle diameter increases faster in smoke of a higher initial OA concentration than smoke of a lower initial OA concentration. Their box model simulation suggested that coagulation explains the majority of the diameter growth and OA evaporation/condensation having a relatively minor impact on the diameter growth. Although the authors provided systemic analysis with FIREX-AQ measurements, the results found by them are not novelty. Similar results have been reported by previous studies. The authors need to highlight their new findings in this work.

For the writing style of this manuscript, too much Figures were put in supporting material with unnecessary. It broke the logical flow of the paper.

There are some scientific questions need to be addressed.

1. Most of the authors’ analysis is based on smoke age. However, the definition of smoke age in this work is too simple and seems not to reflect the impact of turbulence and wind shifting. However, the uncertainties of smoke age could impact significantly on the authors’ analysis, discussions, and conclusions.

2. One of the major conclusions is the authors found OA evaporation/condensation having a relatively minor impact on the diameter growth. However, this conclusion is based on box model simulation. As shown in the simulations presented in the paper, OA condensation in young age plume and OA evaporation in old age plume seem not to be reflected by the box model.

3. The authors used linear regression to fit the dependence of median diameter with smoke age. Median diameter cannot increase without limitation. I do not think linear regression is suitable to be used here.

Minor comments

Line29: μg m-3. -3 needs to be superscript. Please check the unit in the paper.

Line200: Change x to multiple sign.

Line239-240: How do the authors think about the impact of wind direction/shifting on their method to calculate the smoke age? Wind direction may not always follow the straight-line between the fire and aircraft position.

Page8Equation2: It is unclear what do inplume and background refer to. Does inplume refer to the value sampled at aircraft position in plume? Where are the background values sampled?

Line304: More details about the assumptions, settings, and initial conditions to run the coagulation model are needed. Based on current information, it is hard to reproduce the results presented in this work.

Line339-341: I agree with the authors’ discussion that the diameter growth rates should be slowed with the smoke age. However, according to Fig.2, we did not see such slowdown. The authors need to explain this disagreement.

Line455: Why does median diameter drop after 5h in Fig.7b? Why is median diameter low during 0.5-1.5h and then rapid increased after 1.5h in Fig.7c?

Line475-477: Need to explain the reason why the change from non-size-dependent evaporation to size-dependent evaporation in model is small.

Line478: Can it be caused by the treatment of dilution in the box model?

Line489-490: Why does the impact of changing emission on number concentration is larger than size distribution?

Line491-492: In Fig.7b and Fig.7g, the consideration of OA evaporation/condensation reduced the agreement of simulation and observation. More discussions on this are needed. Usually, young age plume is dominated by OA condensation, while old age plume is dominated by OA evaporation. I did not see this reflected in these simulations. Do the authors know why the simulation works this way?

Page28Fig1: Map shown here is helpless to readers who are not familiar with local geography of Idaho. Map with land use type is more useful.

This manuscript presents airborne observations of some aerosol parameters (aerosol particle median diameter, number concentration of particles > 100 nm, organic aerosol enhancement ratio (OAER, deltaOA/deltaCO) and O:C ratio) as function of atmospheric age for eight major wildfire plumes in the western US in 2019. The main finding of the manuscript is that the median diameter of particles > 100 nm increases faster in plumes that have higher OA concentration. A box model suggests that this change is mainly due to coagulation, OA evaporation and condensation having a minor role.

Biomass burning aerosol size distribution and its evolution in sub-grid scale is an important topic for climate and air quality. However, it is unclear what are the novel findings in this study and there are also some major comments need to be addressed before this manuscript can be accepted.

Major comments

The evolution of the size distribution is described through two parameters only: median diameter and number concentration. Furthermore, the size distribution is available only for particles > 100 nm in diameter - were there really no measurements below 100 nm? Even for particles > 100 nm, please show the actual size distributions. Does the initial size distribution depend on OA concentration? Does the width of the size distribution agree with the modelled time evolution? Is one mode enough to describe the size distribution or should more modes be used?

It seems that most deviations from the modelled time evolution (e.g. Fig. 7) are explained by sampling not being Lagrangian. Please try to identify the processes that cause these deviations and discuss them in more detail. Can the model be used to evaluate whether changes in dilution and photochemical age would be enough to explain the observed differences, or if the differences are due to other factors (e.g. emission changes)?

The coagulation parameterisation by Sakamoto et al. (2016) is mentioned (e.g. lines 85-92), but no comparison is made. Please evaluate the Sakamoto et al. (2016) parameterisation with your observations.

Minor comments

On line 254 “N” is used for nitrogen, which could be confusing as it is used for number concentration lines 251 and 257.

line 258 “N is the number concentration between 50 nm and 800 nm, the range of diameters used to fit the dN/dlogDp measurements.” So are you showing only the fitted N, and not the directly measured parameter? Please use the measured quantity in the figures.

line 262 “number enhancement ratio” defined for the 2^nd time.

line 293 I believe the Pasquill stability classes are defined for boundary layer, including convective mixing. Can this method be applied to mixing above boundary layer, where the measurements were done (line 174-175)?

line 322-323 “Additionally, this assumes volume-controlled growth/shrinkage, where all particle sizes grow/shrink by the same fractional amount, preserving the lognormal modal width.” Examining the initial size distribution as function of OA concentration could help validate this assumption. At least for the median diameter (Fig. 2), there seems to be a clear distinction between more diluted plumes (Dpm ~150nm) and the more concentrated plumes (Dpm ~200nm).

line 341-343 “Castle 8/12 also has a larger uncertainty range due to a constant increase in Dpm for the first five hours of aging, but then a decrease in Dpm during the final three transects, potentially due to deviation from Lagrangian sampling.” Can you use the size distributions to rule out growth of < 100 nm particles into the sizes where they can observed?

line 428 Do you mean Fig. 6a-b?

Figure S2 Please check x-axis label.