Review of "Technical note: Apportionment of Southeast Asian Biomass Burning and Urban Influence via In Situ Trace Gas Enhancement Ratios" by DiGangi and coauthors

This is a straightforward technical note presenting the usefulness of enhancement ratios of CH4 to CO to identify/constrain various origin source signatures in regions influenced by various air masses transported to a measurement location. This technique is reasonably sound and appears to be useful for apportioning data from different source regimes in the absence of measurements of more specific chemical tracers (i.e., VOCs or other non-organic gases like HCN that are emitted nearly exclusively from specific anthropogenic or biomass burning emissions sources.) This should be made clear in the paper.

Once this and the following comments and technical corrections are addressed, this technical note should be published in ACP.

Lines 129-130 and Figure S1: The brief description of the use of a ±5 ppb CO and CH4 hysteresis could use a little more explanation. The dashed lines for CH4, 1.85 + 0.04 = 1.89 ppm, and ± 5 ppb (± 0.005 ppb) would be 1.885 – 1.895, and for CO, 65 ppb + 55 ppb = 120 ppb, and then ± 5 ppb would be from 115-125 ppb? The dashed lines in Fig. S1 are each 5 ppb above those (1.89 – 1.90 ppm for CH4 and 120 – 130 for CO), which seem too high for the explanation given in the text.

Technical corrections:

Lines 14, 73, 78, etc.: “Seas” should be capitalized.

Lines 21, 22, 23, 36, 38, etc.: “air mass” and “air masses” should both be two words.

Line 29: I believe it should be “enables” (novel approach is singular).

Line 53: it would be better to spell out “many days to weeks”.

Lines 118, 137, and Figs. 2 and S2 captions: “vs.” should have a period.

Line 130: Supplemental Fig. S1. (Technically, “Supplemental” isn’t needed, either – the S is sufficient.) (Similarly, Fig. S2 – line 157).

Figure 2 caption: I recommend making this a proper sentence: “… colored by regime excluding Clark-influenced data using (a) a global background method, (b) a rolling slope method, and (c) a final combined method.”

Table 1: in the first column there are two CH₄s that need the 4s subscripted.

Lines 156-158: This sentence seems awkward. I recommend either add another comma, or change the comma to a semicolon and add a “was” before identified, maybe?

Line 160: remove “ Jr.” -- generally, suffixes aren’t included in in-text citations.

Line 161: Fig. S2c-d -- it is still a single figure being referenced.

Lines 182, 191, Fig. 3 caption, etc.: “back trajectory” and “back trajectories” should each be two words.

Line 191: “Figure 3 shows…”

Line 219: “Figure 4a shows…”

Line 223: remove “the” before “February-April”

Line 228: “Fig. 4b”

Line 215: The legend colors in Fig. 5 do not correspond to the colors in the pie charts in (a)-(f).

Lines 245 and 253: Maybe use “BB/urban” similar to the Fig. 5g category name instead of “biomass/urban”, to be clear that this isn’t a mixture of biogenic and urban emissions.

Line 250: “Fig. 5a”

Line 257: “Figure 6 shows…”

Line 260: delete one “urban”.

Line 270: remove “sloped” x2: “… with higher ΔCH₄/ΔCO corresponding to local emissions and lower ΔCH₄/ΔCO corresponding…” Similarly, consider using “relationship” instead of “slope” in the rest of this paragraph.

Lines 305-end: I believe the journal names should be abbreviated.

Line 347: there is a rogue “$” in the CAMP²Ex name.

Line 411: CO₂ should have a subscripted 2.

This paper presents a new method that uses the gas enhancement ratios of CH4 to CO for apportionment of different airmass sources during the CAMP2Ex campaign. The developed method is very useful and the results presented are also very interesting. Below are a few minor comments/suggestions.

Is “Technical note:” necessary in the title?

The abstract is somewhat difficult to follow, as it takes several sentences before clearly stating the main objective of the study—using CH₄-to-CO enhancement ratios to separate airmass influences. The initial portion of the abstract, while informative, may distract from the core contribution and could be shortened or moved to the introduction. I suggest streamlining the abstract to more quickly convey what was done and what was found in this study.

Can you provide a figure showing the flight tracks of CAMP2Ex?

I suggest providing more details and explanations in section 3.1 and 3.2, as the methods described are not very straightforward to follow.

Figure 3, what is “BT”?

Line 231: “the high and low ΔCH4/ΔCO urban regimes exhibited distinctive ΔNOy/ΔCO slopes”. This is a particularly interesting result. I encourage the authors to elaborate further on the potential reasons behind these differences and their implications for understanding urban emission sources or atmospheric processing.

Line 249: “The high ΔCH4/ΔCO urban regime exhibited less aerosol on average (8.8 μg/m3) than the low ΔCH4/ΔCO urban regime and had very similar composition with also ~40% each organic and sulfate aerosol mass” This is interesting. Just out of curiosity, any possible explanations?