This paper reports enhancement ratios (ERs) and "inferred" emission factors (EFs) for gas- and particle species measured in the smoke plumes of 9 western U.S. wildfires (and one eastern prescribed fire) during the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. The measurements were collected aboard the NASA DC-8 with what may be the most comprehensive instrument payload ever deployed for measuring atmospheric chemistry of biomass burning emissions. ERs and EFs are based on 16 cross-wind plume transects deemed to have the least photochemical aging based on measured ratios of furan, a fats-reacting primary biomass burning product, and maleic anhydride (a slow reacting secondary product). The paper focuses on emissions of non-methane organic gases (NMOG) and reactive nitrogen-containing compounds (NOy). In addition to ERs and EFs, the paper provides emission parameterizations for total NMOG and NOy which could be used with satellite observations of CO and NO2 to provide boundary conditions for atmospheric chemistry modeling.

Until very recently, detailed chemical speciation of relatively un-aged wildfire plumes in the western U.S. were unavailable. The 2013 SEAC4RS campaign provided the first such measurements, but for only a couple of wildfires. The wildfire focused 2018 Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) provided detailed chemical speciation of emissions and ERs and EFs for multiple western wildfires.

This paper builds upon these previous studies and further expands and improves our understanding of emissions from western U.S wildfires:

1. Additional measurements of highly variable process - The relative abundance of pollutants in "fresh" wildfire smoke is highly variable as it depends on fire behavior, fuels, and environmental conditions, all of which have high natural variability. Therefore, the additional ER and EF measurements provided in this study improve our characterization of emissions for western U.S. wildfires.

2. Expanded chemical speciation - The extensive payload allows for more detailed speciation of NMOG and nitrogen-containing species than provided by previous studies.

3. Reduced measurement uncertainty - NMOG were predominantly measured using four instruments: the NOAA proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS), two whole air samplers (WAS) and the NOAA fast online gas chromatograph outfitted with a Time-of-Flight mass spectrometer (TOGA). Many of the NMOG species were measured by multiple instruments providing a means to reduce uncertainty.

4. Direct tie-in to extensive laboratory experiments - The NOAA PTR-ToF-MS was the same instrument used in FIREX 2016, the laboratory prequel to FIREX-AQ, provding a direct tie-in to the extensive lab measurements of biomass burning emissions form western U.S. fuels. This provides an improved interpretation of airborne measurements

5. The study uses a "photochemical clock" to select least processed emissions - ERs and EFs from measurements of smoke that is less photochemically aged than previous studies, as demonstrated by the relative yields of primary and secondary products.

6. Parameterization of NMOG and NOy emissions - provides a potential pathway for employing satellite based measurements of total column CO and NO2 to estimate fire emissions for atmospheric modeling.

Minor Comments

1. Table 1- Please described how the fuels were determined. Also, were the fuels extracted from area burned on day of flights?

2. The monoterpene ratio for FIREX-AQ/Firleab = 1.57. Did Firelab have better speciation of monoterpenes? If so, does this reflect differences in harvested vs. in-situ fuels?

3. FIREX-AQ EF vs. WE-CAN EF (Table S6): FIREX-AQ/WE-CAN is 0.16 for HCN and 3.33 for HNCO. Can the authors provide any thought on possible reason for these large differences?

4. Did the authors consider testing the ethyne/furan ratio based partitioning of NMOG emissions presented in Sekimoto et al. (2018) with the FIREX-AQ field data?

5. L489-491: "The dotted lines and shaded regions show FireLab parameterizations that describe how these ratios respond to changes in MCE (Roberts et al., 2020) for one fire burned during FireLab"

What fuel type in Firelab burn featured in Fig. 6? Are these curves indicative of typical Firelab burns?

6. L255-256: Fire plumes sampled closest to the emission source that showed significant chemical processing with a MA/F > 0.20 are excluded from this analysis.

How was the photochemical processing threshold of MA/F > 0.2 selected?

Given that maleic anhydride has multiple precursors in biomass smoke (other furans), that it is produced a couple reaction steps after the initial OH rxn, and that it has many production pathways, do the authors expect the relationship between MA/F ratio and photochemical activity be roughly the same across the 16 plumes? Could different emission rates/relative abundances of other furans and NOx or different photolysis environment significantly impact yields of MA?

7. L313: …uncertainties due to differences in calibration or canister effects are small.

Table S5. In the case of furan, the WAS/NOAA PTR and iWAS/NOAA PTR agree with one another but are less than half the TOGA/NOAA PTR. This would seem to suggest that furan loss in the canisters is significant.

8. L593-595: We suggest that these differences could be due to differences in the fuel, quantification methods applied for each study, as well as due to differences in photochemical loss of reactive species prior to detection.

Perhaps fire behavior and lofting of smoke (i.e. differential mix of flaming & smoldering) play a role as well.

Technical Comments

Figure 7 - Using fire names as labels makes it difficult to gauge location of actual measurement values on the plots. I suggest using markers instead.

Fig S4. Are the R2 for individual NMOG based on NMOG and CO medians of the 16 transects?

General Comments:

This manuscript presents calculated emission ratios and emission factors from the FIREX-AQ campaign, comparing them to WECAN and the FIREX-AQ Laboratory emission factors.  This work presents a detailed analysis of differences in the observed emissions between these campaigns due to instrumentation and smoke age.  While not particularly exciting the manuscript is an important contribution to understanding wildfire emission factors and ratios based on the limitations of the instrumentation and field sampling design.

Specific Comments:

Line 154:  “performed fast response in situ measurements”  seems to include the whole air samplers which isn’t accurate since those canisters are collected on the aircraft and analyzed later.  Clarifying this point and adding addition information on how many of the WAS samples met the low photochemical processing threshold for the EF and ER calculations would be useful since the number of data points per flight is limited.

Line 334: For the equation. What if there is no carbon in the compound (much of Figure 3b)? 

Figure 7: It doesn’t seem like the fire name is important to the figure (the fires aren’t specifically referenced in the text) and just make the plots more difficult to read. 

Table 3: In the text you discuss some compound show better a relationship with NO2.  Clarify in table 3 that the ERs are to CO.  And consider adding a table (in text or to the supplement) of those species that should use NO2.

Technical Corrections:

Figure 3: CO is too light and could be offset some to see the trace better.  The overall quality of the figures should be improved.

Figure 5: it is very difficult to read these figure – the resolution seems poor and the size is rather small.

Line 476: instead of “On the contrary” -  In contrast