Review of Yates et al., 2023

In their manuscript, the authors present results from a large number of scientific flights within the AJAX program. In a first part, the program itself is motivated and examples of scientific objectives for some of the AJAX flights are given, together with operational aspects of the program. Then, individual instruments are briefly introduced and generated data sets are characterized. Finally, examples for the usefulness of AJAX data are given: Long-term trend observations (and comparisons to ground based measurements), observations of pollutants in the boundary layer, and satellite validation.

The manuscript is well written, figures are of high quality and the analyses the authors did are sound. The paper is well within the scope of ESSD and it is in a very good shape. I only have a few minor points, which could be addressed by the authors before publication.

Minor/specific points:

- line 37: It might be helpful to the reader to give some references for the mentioned "traditional" field campaigns. For my taste, it would not be necessary to resolve the acronyms in this context, but some idea where to find context information would be nice. 

- line 43: Please also give metrical units (meters or kilometers) for the altitudes, even though elevations in "feet" are usual in aviation.

- line 81: suggestion: (Figure 1) -> (Figure 1, middle panel)

- Figure 1: the right panel with the time series of the flights is very informative, but not introduced in the main text. Please refer to it somewhere, e.g. in the beginning of section 2.

- line 99: I suggest to remove the specific FAR number in the text and just refer to regulations and give the reference as it is given now.

- line 125: Maybe not everyone is familiar with the location of Aliso Canyon, so maybe a small hint, where it is located would be helpful.

- Figure 4: For both panels, the label of the z-axis is displayed in two lines, which overlaps with the ticks of the axis. Sometimes it is impossible to make the plotting program do the labels in a beautiful way, but if possible, I would suggest to have the label in only one line.

- line 141: I think the common abbreviation of kilometers per hour is "km/h" (without "r")

- Sections 3.1 and 3.2: I miss the measurement principle of these two kind of instruments. In my opinion, it is not necessary to explain the instrument in detail in this manuscript, since references are given, but I would at least like to read the name of the technique instead of just knowing that it is a "sensor".

- line 220: mb -> hPa

- Sections 3.3 and 3.4: For the O3 and GHG instruments, a detailed procedure of data screening has been reported, which is not presented for the HCHO and MMS instruments. Is there no data screening for these instruments? 

- Figure 7: The legend of this figure has very small fonts and is hard to read. In addition, the light green color is hard to see in the plots. Further, I suggest to mention the colors in the figure caption, e.g.: "Average CO2 (top), CH4 (middle), O3 (bottom) over the entirety of each individual AJAX flight (green), the average of data collected below 2 km from each individual flight (BL, cyan), and above 2 km (FT, dark blue). Monthly mean values from a surface reference site  are shown for comparison (grey): ..."

- Section 4.1: I think it would be good to add references for the ground based measurements, the AJAX data are compared to.

- line 247: Just out of curiosity: Are the AJAX data points matching better to the THD curve those flights, which have been above the pacific ocean (as shown in the map in Fig. 1)?

- line 292: This one-sentence-paragraph feels a bit odd here and should be extended a little. It is also strange that Fig. 10b is introduced before Fig. 10a. However, I am not really sure, if Fig. 10b is really necessary in this work at all (but it would be okay if the authors want to keep it).

- Figure 10a: I am not sure, if I understand the meaning of the "n = ..." statements in this panel.

See attached pdf file for my comments.