Summary Evaluation

This is an excellent and well-prepared paper that presents a valuable intercomparison of airborne formaldehyde (HCHO) measurements from four instruments (TOGA-TOF, CAMS, PTR-ToF, and ISAF) during the FIREX-AQ campaign. The study is well designed and supported by a large dataset of co-located observations, allowing robust statistical analysis across a broad concentration range. The close agreement between TOGA-TOF, CAMS, and PTR-ToF confirms that TOGA-TOF provides what the author’s describe as reliable and accurate in situ HCHO data.

The TOGA-TOF measurements seem especially useful because it can measure both HCHO plus a large suite of precursor VOCs. As they are on the same instrument this can eliminate some of the sources of instrumental and sampling error. It would be nice if some recommendations could be made with regards to maintaining sample integrity and calibration best practise.

The authors’ use of volume-weighted averaging demonstrates careful consideration of the different temporal resolutions among the instruments, rather than simple averaging. This approach can be considered for all instruments taking discrete samples, such as a sample pre-concentration or quite commonly, the capture of whole air samples (where fill rate is not constant across the entire sampling period) and is a useful demonstration of this type of analysis.

The manuscript is clearly written, easy to follow, and provides an important contribution to airborne trace gas measurement science. The results are directly relevant to ongoing efforts to improve instrument consistency and model–measurement comparison

Specific Comments

1. Calibration methodology

Different instruments have different inlets, pumping speeds, pressures etc. Can the authors suggest what might have an effect on HCHO sampling integrity? Is there an understanding of the favourable parameters from each instrument and what would be the optimal sampling strategy for HCHO? 

The authors note that differences among instruments (TOGA-TOF, CAMS, ISAF, PTR-ToF) may stem partly from different calibration approaches and standards. What are the pros and cons of the different methods and is there a preferred technique? Is there one instrumental technique that shows the best accuracy, or are certain techniques limited by practicalities? 

It is stated that calibration differences likely explain the observed offsets, if possible, it would also be useful to indicate how much of the stated total uncertainty (≈35%) is attributed to calibration versus instrumental variability or sample integrity?

2. Sampling frequency and averaging

 The use of volume-weighted averaging is a good way to align the slower TOGA-TOF data (33 s) with the faster instruments (1 Hz). It might be worth adding one or two sentences to acknowledge that while this approach slightly limits the instrument’s ability to resolve rapid plume variability, the time resolution and inherent averaging is actually well suited for model comparison and regional-scale studies, if of course this is the conclusion.

3. Use of CO as a strictly conservative tracer of dilution. CO is useful because it is as separate measurement to all the instruments being compared, but I believe it can have secondary production in the plume from VOC oxidation and could be oxidised itself? perhaps a VOC that TOGA measures could be used for estimating plume aging, perhaps acetonitrile? This would be interesting in future analysis because by being measured by the same instrument, some systematic errors could be eliminated. 

technical corrections:

Excellent with just a few inconsistencies such as hypens in situ/ in-situ, 1 Hz /1-Hz and some spacing around parentheses.

The manuscript "Trace Organic Gas Analyzer Time-of-Flight mass spectrometer (TOGA-TOF) system for airborne observations of formaldehyde." presents measurements from a newly designed analytical system that couples a fast GC with a time-of-flight mass analyzer to perform minutes-scale measurements of formaldehyde and near about 100 volatile organic compounds alongside. The instrument was tested during the FIREX-AQ campaign aboard the NASA DC-8 aircraft flying transacts across plumes. The measurements were compared against two other specialized instruments onboard the aircraft that performed targeted measurements of formaldehyde. 

I really enjoyed reading this paper. It is very well written, structured and thorough in its comparison of the data obtained from the three instruments. The figures showcase comprehensive analyses and are easy to understand. I do not have any major comments but just a few minor ones. Following their resolution, I am happy to recommend the paper for publication:

1. Lines 192-193: The authors correct the fluctuations in sensitivity by normalizing with ambient tetrachloromethane. It would be good to add a citation for this or provide a rationale for selecting this compound from the broader suite of persistent ambient background species. Would this method work everywhere on the planet with the selected compound? 

2. I would like the authors to expand a little more on the 35% uncertainty for HCHO measurements via TOGA-TOF. They attribute this to variability in repeat calibrations. However, a commercially available standard cylinder is used, the outflow from which is dynamically diluted with clean air or N2. So, what should create a variability this high in calibrations? 

3. Looking at figure 3a, I would like authors to acknowledge somehow also the limitations of this instrument for measurements on airborne platforms. Minute-scale time-resolution for a GC-TOF setup is great but figure 3a shows that it may still be slow for capturing chemically dynamic environments often experienced on fast moving platforms. 

4. I suggest authors explain the volume-weighting in a bit more detail either in the main text or the SI. The current description somehow did not come across clearly to me as a reader. 

Other minor points: 

- Line 342: Lay out the mathematical description in equations-format for ease of understanding. 

- Line 349: "(described in Sect. 2.4). The note is already in section 2.4. 

- Lines 487-488: Do the authors mean a reduction in plume heterogeneity with aging is what suppresses the bias? "evolution" is a capture-all sort of a term but does not really tell what the authors intend to say. I acknowledge the explanation provided in subsequent lines.