Reply to Comments By Anonymous Referee #2

The authors have provided a will written and structed paper to quantify the sizing errors from two commonly used optical particle sizers, the UHSAS and LAS. The fast sampling rates of these instruments make them a mainstay onboard research aircraft, however sizing errors from aerosol composition and refractive index have not been well quantified. The authors carefully presented the methodology used for the quantification of these errors through a serious of lab experiments. In addition, they provided real world examples from measurements made around wildfires. Very little work has been done to quantify the sizing error from these OPSs when measuring biomass aerosol. That makes the results presented in this publication scientifically significant.

The authors have provided a will written and structed paper to quantify the sizing errors from two commonly used optical particle sizers, the UHSAS and LAS. The fast sampling rates of these instruments make them a mainstay onboard research aircraft, however sizing errors from aerosol composition and refractive index have not been well quantified. The authors carefully presented the methodology used for the quantification of these errors through a serious of lab experiments. In addition, they provided real world examples from measurements made around wildfires. Very little work has been done to quantify the sizing error from these OPSs when measuring biomass aerosol. That makes the results presented in this publication scientifically significant.

Overarching Comment
What about counting efficiency? Have any corrections for the UHSAS undercounting below 100nm been applied? Looking at Figure 10 upwind leg it appears it hasn't been applied? Is this consistent across the dataset? Please improve the UHSAS instrument description section by discussing counting efficiency and if and why corrections were or were not applied.
We have not applied any size-dependent counting efficiency corrections to either the UHSAS or LAS laboratory data nor the airborne data, which is now noted in Sections 2.3 and 2.4. The reviewer points out a good example of this impact on the size distribution as shown in Figure 10, and we've added a sentence to the discussion in Section 3.2 highlighting this point. Cai et al. (2008) wasn't mentioned anywhere in the publication.

The important work of Cai et al. (2008) was cited and mentioned in the paper.
Line 260, 261,386: Spelling of thermal denuders is not consistent. Line 260 there is a space between thermal and denuder. Lines 261 and 386 there is not a space.
Done. We have modified the text per the reviewer to ensure consistency throughout the manuscript.
Line 267: Please quantify what is considered a reasonable level? What was the range of ratios for dilutions used?
Done. We now explicitly note <2x10 4 cm -3 as our rough target for the particle concentration limits, which was often driven by the TSI CPC 3010s that were also behind the dilution system. The range of dilution ratios varied from 5-20x.
Line 278: The LAS sample flow rate is discussed however no mention of what volumetric flow rate the UHSAS was maintained at Done. We have added a sentence noting the UHSAS constant volumetric flow rate of 60 cm 3 min -1 to the text.
Line 302: CAMP2EX has a lower case x at the end.
Done. We have modified the CAMP 2 Ex acronym to ensure consistency throughout the manuscript.
Lines 375-380: More information about location of this fire, the altitude the measurements were made at, and average flight speed would help provide better context to the figure and the discussion that follows in 3.2. In addition, some meteorological information would be helpful. Please provide average wind speed and direction for the flight level these measurements were made.

Done. We have added the requested information to the beginning of Section 3.2 as per the reviewer.
Line 384: What is the local time?

Done. We now note that local time is Pacific Daylight Time (UTC-7) as per the reviewer.
Line 440-446: It was briefly mentioned in the instrument description section that LAS has a standard flow rate. Please acknowledge that this was accounted for and what impacts it might have on the FIMS and LAS comparison during FIREX.
The LAS flow rate was used to convert the measured counts to concentrations, and all comparisons in this paper are made in terms of particle concentration, which places all instruments on an equal footing. Consequently, we would not expect there to be an impact on the FIMS-LAS comparison during CAMP 2 Ex or on the multi-instrument comparison during FIREX-AQ.
Controlling the LAS flow to a mass flow instead of a volumetric flow may change the particle velocity through the laser as well as the sheath:aerosol flow ratio (which may influence the shape of the particle beam). None of these potential impacts appear to affect the particle size for the laboratory and low-level plume sampling in this study; however, the effects may be more significant for higher altitude measurements. We hope to explore some of these pressure-dependence instrument performance characteristics in a follow-on paper.
Lines 825-835: Figure 5 and 6 could be better organized and annotated. a and b appear to be zoomed in views of c and d? I feel it would be more logical if the wide view is shown first as a and b and this was discussed in the captions. In addition, these are organized as columns. I recommend labeling the top of the left hand column as LAS and the right hand column as UHSAS instead of repeating it on every image.

Done. We have modified the figure as per the reviewer.
Line 850: Figure 8. Can more information be provided on the location of this fire? Perhaps a map indicating the location of the fire would accomplish this.

Done. Have added a link to the fire InciWeb page to both the figure caption and the text.
Line 855: Figure 9. Please indicate what Local time is for these measurements Done. We now note that local time is Pacific Daylight Time (UTC-7) in the caption as per the reviewer.