Ort et al. uses airborne observations of O3 and CO from 12 aircraft campaigns to investigate the tropospheric zonal distribution of those two species and their ratio (with an emphasis on the northern hemisphere). The authors identify high O3-CO ratios in the subtropics (23° - 40° latitude) that extend deep into the tropospheric column. In addition to stratosphere-troposphere exchange processes (STE), the authors hypothesize that these high ratios in the subtropics are also due in part to lightning NOx (LNOx) emissions in the upper tropical troposphere that produce more O3 and OH (thus depleting CO) and is subsequently transported to the subtropics via the Hadley circulation. A standard run of a global 3D atmospheric chemistry model (EMAC) is able to reproduce the pattern of high O3-CO ratios in the subtropics. Moreover, the authors perform a sensitivity run using the EMAC model by turning off LNOx emissions and show that the O3-CO ratio is reduced by upwards of 40% in the northern subtropics. A model investigation on the seasonality of the O3-CO ratio is also included. Overall, in addition to STE, the authors show that the chemical composition of the tropospheric subtropics is influenced by LNOx emissions in the tropics.

This manuscript would be of interest to the readership of ACP, and I recommend publication after attention to the following comments:

- Lines 125-126: "statistically relevant data can be found in the northern hemisphere": Could the authors define what they mean by "statistically relevant data"? Is there a way to show on Figs 2 and 3 which grid boxes are "statistically relevant" (e.g., using thatching of some sort)?

- Lines 126-127: "all sets were averaged to 60 s before they were combined": Since individual data sets have different uncertainties (Table 2), do some data sets carry more weight than others when combining them or are they all treated equally? 

- Line 128: "sufficient number of data points for each grid box": The authors should add a figure in SI similar to Fig 1 that shows the number of data points used to calculate the value in each grid box. For example, the ~200 ppbv CO in Fig 1(a) seems out of place compared to surrounding grid boxes, and I'm curious if this is due to limited observations biasing that particular grid box. Moreover, what is the standard deviation of the data points in each grid box?

- Line 142: How does overestimated O3 in the model account for reduced NOx from lightning?

- Line 261: "but more pronounced in the northern subtropics": Southern subtropics in the model look just as pronounced as well. Is there a way the authors can quantify the difference instead of giving a qualitative statement? Admittedly, the southern subtropics appears less pronounced in the observational data (Fig 3).

- Line 275: "smaller CO and higher O3 mixing ratios in the subtropics compared to the northern extra tropics and tropics": Could this statement be conditioned since it's not necessarily true at all altitudes.

- Lines 282-283: "the decreasing gradients towards the tropics and mid- to high latitudes are reproduced by the model": Generally agree with the authors but it seems like the gradients are mainly reproduced for 0-6 km and not so much for altitudes higher than 6 km in Fig 6(c) (particularly for the higher latitudes). Why is this the case in the model?

- Line 293: "leading to higher OH concentrations in the convective outflow. This potentially shortens the lifetime of CO and other hydrocarbons": It would be helpful if the authors added a brief discussion on how much LNOx emissions is impacting the atmospheric oxidation capacity of the northern subtropical troposphere. Perhaps show the difference in OH that results from excluding LNOx in Fig 7? Also, can the authors calculate the change in lifetime for CO and a few other hydrocarbons after excluding LNOx emissions?

- Line 357: "the local high of the O3-CO ratio in the subtropics is still present, albeit weaker": Could the authors make a figure similar to Fig 5 but for the EMAC model run excluding LNOx and place it in the SI? This would help to show the local high of the O3-CO ratio in the subtropics is still present albeit weaker in a quick way. Moreover, it would be helpful when readers later see seasonal differences in Fig 11.

- Line 400: "the O3-CO ratio is still shifted to higher values by excluding LNOx": Please double-check statement. It seems like ratio is shifted to lower values when excluding LNOx in Fig 11.

- Lines 423-426: What other factors are at play for O3 since even though O3 depletion is strongest during the summer, the O3 mixing ratio from May-Aug remains relatively high compared to the rest of the year.

- Lines 436-444: "However, storm tracks... stronger isentropic STE": This section needs clarification. Seems like the main message is that the STE influence on O3 has a minor effect in summer over the northern subtropics (Lines 445-446), so what is the main message you're trying to convey in Lines 436-444?

- Lines 458-462: Please clarify the main takeaway from this discussion on convection. Lines 458-459 imply that it is not a major factor given Fig 12(a), but then Lines 459-462 suggest otherwise.

- Line 472: "consistently influences the O3 mixing ratio": Please restate the magnitude of that influence on the O3 mixing ratio in the northern subtropics from tropical LNOx.

Technical Corrections:

- Line 88: Remove apostrophe in aircraft's

- Line 100: Remove the word "are"

- Line 136: Correct typo: miscirculationaneous

- Figure 2 caption: Should "white lines" be changed to "black lines"?

- Line 207: Fix typo in sentence: "Here, ppbv values a the low hundreds could already be observed."

- Line 234: "pols" should be "poles"

- Line 387: "spacial" should be "spatial"

- Line 393: "Below, all monthly medians are shown..." Please mention the figure that this sentence is referring to.

- Line 409: "lighting" should be "lightning"

- Line 417: Remove comma after effects

- Line 418: "depend on other mechanism" should be "depend on another mechanism"

- Line 503: "investigated" should be "investigations"

- Throughout manuscript: Fix end quotes (e.g., Line 157: "REF" and "without LNOx")

The manuscript by Linda Ort and other co-authors provides a study combined in situ aircraft observations of O₃ and CO from 12 global research campaigns conducted between 2012 and 2024, covering different seasons and latitudes from the boundary layer up to the lower stratosphere, with simulations from the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model. Two simulations were performed: a reference run including all standard emissions, and a sensitivity run with lightning NOₓ emissions turned off. The authors demonstrated that observations and model both show a pronounced enhancement of the O₃-CO ratio in the northern subtropics, extending nearly to the boundary layer. They conclude that tropospheric photochemistry via tropical lightning NOₓ in the upper tropical troposphere drives high O₃-CO ratios in the subtropics, which is traditionally attributed to stratosphere-troposphere exchange (STE) near subtropical jets.

The scientific significance of this manuscript is absolutely given, as it is important to clear understanding the effects of transport and photochemistry via Hadley circulation in the upper troposphere of the tropics to the subtropics on variations of O₃ and CO. This dataset is very valuable, especially given the scarcity of such measurements with high vertical resolution. I have several concerns regarding the integration of observations and simulations. Below, I outline the main issues and offer suggestions for improvement.

Major comments:

1) Multiple instruments measured high-resolution CO (e.g., UMAQS, QCLS, TRISTAR, ATTILA) and O₃ (e.g., FAIRO UV absorption, chemiluminescence) and O₃ during these campaigns. Most of them implemented in one month spanning 12 years. The authors averaged all observations to 60 s and gridded at 1 km vertical × 1.875° latitudinal resolution to evaluate model performance. Given the relatively rapid mixing timescales within and between tropospheric hemispheres (and between the troposphere and stratosphere), simple averages of measurements in different campaigns likely biases the seasonal signal. I recommend comparing the model against co‑located, time‑matched observations to assess its ability to reproduce monthly and seasonal variability.

2) The current analysis lacks a direct comparison of observed and simulated high‑resolution vertical profiles. Such a comparison would be invaluable for diagnosing the seasonal behavior of STE on a monthly timescale and for validating the vertical transport processes in the model.

3) the authors used simulations from ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, why not ECHAM6? Given that ECHAM6 is available, with higher spatial resolution and improved representation of seasonal and intra‑seasonal transport, I suggest justifying the choice of ECHAM5 or, if feasible, repeating key simulations with ECHAM6 to determine whether the results are robust to model version.

4) I suggest the authors focusing on understanding the monthly variations and mechanism of enhancement of O₃-CO at tropospheric subtropical latitudes, because that is very important for accurate projections.

5) L234: Is “pols” a typo? Should be “poles”?