This paper describes absorption measurements for wildfire smoke in Oregon and Arizona using photoacoustic spectroscopy.  The smoke is aged in a oxidation flow reactor using OH or NO3 for the equivalent of ~5-20 days.  The number of measurements are very limited (2 examples of aging with OH and 2 examples of aging with NO3), but the approach hasn't been reported for field measurements.  The paper is short and well-written.  I recommend publication after the following comments are addressed.

- Line 55: Warneke 2018 is not published.  Change to "in preparation".

- Section 2.1.1 and 2.1.2: Add the source for the vegetation identifications.

- Section 2.1.1 and 2.1.2: What were the average wind speeds during each of the four measurements?

- Section 2.1 is named "Wildfire emissions sampled during OFR experiments".  It would make sense to move the final paragraph (lines 94-114) to Section 2.2 ("Instrumentation").  That paragraph introduces the Aerodyne Mobile Laboratory and fits more logically with instrumentation than with the description of the fires.

- Line 156: How accurately did the UV flux measurement at the exit flange represent the UV flux inside the OFR?  How did you evaluate that?

- Line 160: The calculated mean residence time of the oxidation flow reactor was 123 s.  What was the observed residence time?  If the OFR was abruptly filtered or overflowed with zero air, what was the observed residence time to fully flush the volume?  Add this value as well.

- Figures S3 and S4 in the SI material show that the OH and NO3 reagent concentrations were always changed monotonically.  They began at the highest concentration, and then were decreased sequentially in steps.  If the ambient aerosol properties are changing as a function of time, this approach risks confusing that change as a dependence on the OH or NO3 concentration.  It may also introduce an error if there are systematic measurement drifts.  It would be better in the future to both 1) Introduce different reagent concentrations in random order; 2) Repeat one or two reagent concentrations to examine the repeatability of the measurements.  It would be worth noting these in the paper as future approaches.

- Line 170-172: The word "steps" is awkward.  Consider changing "steps" to "measurements."  Also, it is confusing that oxidation steps are labeled as "OFR_OH_X" and "NO3PAM_O3".  The OFR and the PAM are the same thing.  It would be better to name these consistently (not with some labels as PAM and some as OFR).

- Throughout the paper, NO3 and OH equivalent exposures are given in units of "days" or "equivalent days".  Since OH is present during the day and NO3 is present at night (so that each equivalent day would take ~2 days in the ambient atmosphere), it may be clearer to give the exposures in "hours" or "equivalent hours".

- Line 230: It would be useful to specify that the AAE values calculated here are for both BC and BrC.

- Section 3.2: This section seems to introduce several results without explanation.  Specifically:

- Line 239: NO_3,EXT is not defined.  What is it and how was it measured or calculated?

- Line 240: The NO3 reactivity of catechol is introduced, but there was no description of an instrument to measure gas-phase VOCs.  How was this measured and calculated?  There is no catechol data shown in figures or tables.

- Line 243: "The difference in external reactivities manifested in different E_MAC(λ) behavior at the two sites." What does external reactivities mean and where are they shown?

- The notation CHO₁N and CHO_gt1N are unclear.  I suggest making two changes: 1) use x to indicate that molecules may contain multiple atoms of C, H, and N; 2) change gt1 to >1.  For example: C_xH_xO₁N_x and C_xH_xO_>1N_x.

General Comments:

This manuscript investigates the dynamics of biomass burning aerosol optical properties (MAC) under different oxidation schemes.  This authors pursue this investigation through use of a well-equipped mobile lab and oxidation flow reactor, which allows near in-situ sampling, aging, physical characterization, and chemical characterization of the target aerosol.  Through investigating the plumes of two fires and their aging under two oxidation schemes the authors draw conclusions about how aging will change the optical properties of the observed aerosol.    The authors conclude that nighttime (NO3.) aging generally leads to more absorbing aerosol and daytime (OH.) aging has more complex effects, which trend toward less absorbing aerosol for longer aging times. 

Overall this short paper is well written, investigates a topic that is of pressing interest to the atmospheric community, and presents a unique application of oxidation flow reactors. The accurate constraints on aerosol optical properties, and how they change as they age, are key to accurate estimations of the earth’s radiative budget and modeling of future climate systems. I recommend that this work be accepted once the minor issues I’ve elaborated on below have been addressed.

Specific Comments:

Line 12:  These statements seem to over simplify the results.  I recommend slightly restructuring the abstract to better present the detailed results the authors discuss in the manuscript.  For example, reading the abstract only, I would believe that OH oxidation was only observed to be bleaching, whereas the authors observed a much more complex relationship.  Similarly, the NO3 results were aging and fire dependent.  This context should be included here if possible.

Line 246: Can the authors comment on the differences between the BBVOCs that drove these differences in MAC changes?  The VOCUS data is used to investigate potential differences in the NO3 reaction rate, but it would be also interesting to use this data to investigate particular precursors which contribute strongly to the observed enhancements/bleaching.

Technical Comments:

Line 58: Line suggests investigations regarding the refractive index of the sampled aerosol, but the paper only includes mentions of MAC data. 

Line 112: Small font error in reference

Line 239: Need to better define NO3, EXT class of variables. I think they are, are related to, the NO3,REXT, but it is not immediately clear.