|In a few instances, some more clarification by the authors is requested in their response. Below, in cases where more information is requested, the original reviewer comment is given, followed by the authors response, which is then followed by a new comment.|
Comment #1: Lines 85 to 92, what is missing here is some idea on the relative contribution of biomass burning to overall sulfate in various regions. Eg, since this paper is about fires in the US, can references be sited or estimates made on the relative contribution of fires to total S near and long distances from the fires.
Reply to Comment: To the knowledge of the authors, there are no published observations of how total S biomass burning emissions contribute to overall sulfate for various regions.
New Comment: Eg, even though SO2 emissions are discussed, the authors claim in the first line of the Abstract that; Fires emit sufficient sulfur to affect local and regional air quality and climate. I am simply asking to assess this. Since a large fraction of the SO2 goes to sulfate aerosol, and the sulfate is measure and discussed, why can’t the sulfate (could include organo-sulfate) in plume be compared to out of plume (eg, data away from the burning regions) for data from this study. A simple statistical result could be given. Or look at in-plume versus studies that report sulfate throughout the US. Please seriously consider the comment. (eg, Hand, J. L., B. A. Schichtel, W. C. Malm, and M. L. Pitchford (2012), Particulate sulfate ion concentration and SO2 emission trends in the United States from the early 1990s through 2010, Atmos. Chem. Phys. , 12, 10353-10365.)
Comment #3: Line 174 and on. What is the SAGA instrument (from Fig S8 it is the MC), which should be specified in the main text. Also state if both instruments sample over similar particle size ranges or not, and if not, what is the possible effect? What about comparing sulfate from the SAGA MC and Filters to see if there is substantial sulfate at higher particle sizes compared to those measured by the AMS and MC.
Reply to Comment: The SAGA instrument definition has been added at line 173. The size coverage of different instruments is important in intercomparison and has been discussed in detail for these two instruments in Guo et al. (2021) for the ATom aircraft mission on the same airborne platform. Both AMS and SAGA MC/IC sample nominally submicron particles onboard the DC8 through different inlets and agree with each other in general, as shown in multiple comparisons over the years (e.g., Fisher et al. 2011). Despite some nuances between the two, (1) size selection of particles at ambient conditions (SAGA MC/IC) vs. dry (AMS); (2) the cutoff size is on aerodynamic diameter (SAGA MC/IC) vs. vacuum aerodynamic diameter (AMS; due to the aerodynamic lens inlet of the AMS being the main source of large particle loss), (the following has been added at line 175) both MC/IC and AMS sample submicron particles with very similar effective size cutoffs for the range of altitudes sampled in the FIREXAQ (van Donkelaar et al., 2008; Guo et al., 2021). Supermicron sulfate is rare throughout FIREX-AQ, such as the Aug 3rd flight. However, substantial coarse mode sulfate, especially in the 2nd pass during the Aug 7th flight, was observed and up to several times higher than the submicron MC/IC sulfate.
New Comment: This needs more clarity since very little information is given on what the SAGA MC is reporting. From the wording I assume this is a sulfate, but does the MC-IC analysis also measure S(IV)? I assume the S(IV) is from dissolved SO2, but does SAGA MC collected SO2 in biomass burning plumes affect the measured sulfate? This would be in addition to conversion of particulate organo-sulfates being converted to inorganic sulfate as discussed. It seems it is not a big issue given the good comparison shown in Fig. S8.
What about the SAGA filter measurements of sulfate – the replay above does not address the question? Why are the SAGA filters not used to assess HMS (or S(IV) in general) vs sulfate? Is the data not available? Much of this manuscript is on a model predicting HMS, HMS data may exist, or it may not, and a number of readers of this paper will know that. It is thus strange that this is never mentioned. Adding a sentence or two stating (if it is true) that the SAGA filter data cannot be used to determine HMS would provide clarity (and the reason why or why not). The same applies to the AMS, see (Dovrou,et al, Atmos. Meas. Tech. , 12, 5303-5315). This paper is cited, but it is not explicitly noted why the AMS is not used to directly quantify HMS.
Comment #6: Lines 347 to 362: Maybe the lack of correlation with MCE is that the MCE
dynamic range is small? One might try looking at BC/OA ratios, just out of curiosity.
Reply to Comment: Because the range of MCE values presented in Fig. 3 covers a similar range as used in the cited analyses, we believe this range is adequate for the determination of a lack of correlation.
New Comment: In other studies (in fact analysis of this FIREX data), it has been shown that BC/OA is in fact better than MCE for separating out some properties that depend on smoldering vs flaming, I suggest the authors do the calculation to actually test it.
Comment #10: Why not make plots of pH and LWC vs plume age for all these modeled
plumes, given that these are key variables?
Reply to Comment: We believe that the trends of these variables are clear in the data presented in the supplementary
New Comment: The trends are not clear. For example, I cannot tell from Fig S3 what causes the systematic jump in pH back and forth between two levels, is this due to in vs out of plumes (it is hard to line up with the SO2 and sulfate data. The pH does seem to increase when the aircraft climbs. Same applies to Fig S4, expect in this case the plot shows no pH when the aircraft changes altitude. Some form of plot other than a time series I think would help clarify the odd behavior in pH.