This paper uses tower and balloon observations and a 1-D chemical transport model to study ozone over a forest near an region producing oil sand. The manuscript is generally well-written and robust, but a few changes would further strengthen the paper.

The introduction needs some restructuring. The information is rich and almost complete, but poorly connected, thus does not provide the motivation of the study well enough. A paragraph explicitly talking about “Why ozone concentration and deposition over AOSR” is highly recommended.

L 38 – 40: NO does not ALWAYS dominate the in-canopy chemical sink of ozone (e.g. Wolfe et al. (2011) propose BVOC to be the dominant chemical sink in a warm pine forest)

L 149: Need citation for the observed “shelf shape”

L 114: The maximum height of profile measurement was 300m and most of the paper discuss about near-surface turbulent mixing and sinks. Would 1000 vertical layers be an overkill and potentially introducing unnecessary error from vertical transport? Please explain and discuss.

L 115, 156 – 157: This approach looks weird, or underexplained at best. In the model, what height does z = 0 correspond to? Assuming z = 0 refers to soil surface, this is not most of the deposition occurs (since leaf surface is mostly the major sink over healthy forest), nor what typical big-leaf model (displacement height) takes. The choice of which layer to put the “big-leaf” foreseeably affect the modelled in-canopy ozone profile. A sensitivity run to explore how the choice of level where the big-leaf is placed, or at least argument for why choosing to put the big-leaf at soil surface is needed.

L 230: Does GEM-MACH have enough resolution to resolve these regional details?

L 310: When the wind speed is higher, there should also be more vertical turbulent mixing generated by horizontal wind shear. This factor should also be considered and discussed in comparing the ozone gradients.

L 393: Does the model explicitly consider the strong diurnal variation of ozone deposition velocity? If not, explain how this might affect your result.

L 408: How long did snow cover last? Since snow and the compounding low temperature during early season can also significantly reduce ozone dry deposition. This might be a worth-discussing point.

L 464: Direct ozone (and to a lesser extent NO₂) flux measurement would also help tremendously to constrain the deposition velocity/flux.

Ozone chemistry and transport were studied in a boreal forest in the vicinity of oil sands mining in Alberta.  Observations rely on gradient measurements within a forest canopy and a few selected tethered balloon profiles.  A 1-D canopy model was run with input data from the observations, yielding a comparison between the observation data and the model output.  I have no doubt that this project was a challenging effort.

While the idea to study the influence of emission from the oil sands operations is certainly warranted, the manuscript falls short in producing significant findings that advance the understanding of the chemistry and environmental impacts, or modeling improvements.

What is the primary objective of the study?  Investigation of air pollution from oil sands mining?  Determining ozone deposition within a boreal forest?  Evaluation and advancement of photochemical modeling?  The paper provides bits and pieces to these topics, but really only scratches the surface.   I don’t see a well targeted experimental approach and comprehensive outcomes of this study.  

There are some fundamental flaws in the experimental approach for determining vertical ozone gradients within the canopy.  2B ozone monitors were used.  These instruments are favorable in applications where space and electrical power are limited.  However, their analytical performance (precision, accuracy, signal stability (i.e. lack of a drift in the instrument response), sensitivity to water vapor interference, response to monitor temperature changes, etc.) is poorer than for standard ozone monitors.  The differences that the authors report in their data for vertical ozone gradients and for ozone transported from different sectors fall well within the measurement uncertainty for these monitors.  An additional source of analytical error will likely result from the steady turning on and off of the monitors.  The manuscript does not appreciate these challenges and does not present an evaluation of the impact of this operation mode on the instrument performance, stability of its calibration, and gradient data quality.  The manuscript contains no information on how instrument response (accuracy) and stability were tracked over time and how gradients were determined to be statistically significant (see for instance [Bocquet et al., 2011]).  Statistical analysis are purely data binning and averaging.  Because of these reasons most of the interpretation is not warranted and premature.

The treatise of ozone deposition is lacking understanding of important processes (e.g. deposition to surfaces, humidity effects) and omits important recent findings and literature (e.g. [Clifton et al., 2020]).

Discussion of the data is often speculative or depends to a large degree on comparison with other studies, rather than deriving new insight and conclusions from this data set.

Understanding and presentation of ozone chemistry is at times superficial.

The manuscript presents and builds on observations of nitric oxides and sulfur dioxide but lacks a description of the experiment and data quality procedures.

The writing/wording is at times inaccurate.

I strongly recommend that the modeling work be evaluated by an expert in forest canopy chemistry and modeling.

An annotated pdf copy with some suggested corrections and more detailed comments will be attached to these summary comments.

Bocquet, F., D. Helmig, B. A. Van Dam, and C. W. Fairall (2011), Evaluation of the flux gradient technique for measurement of ozone surface fluxes over snowpack at Summit, Greenland, Atmospheric Measurement Techniques, 4, 2305-2321, doi:10.5194/amt-4-2305-2011.

Clifton, O. E., A. M. Fiore, W. J. Massman, C. B. Baublitz, M. Coyle, L. Emberson, S. Fares, D. K. Farmer, P. Gentine, G. Gerosa, A. B. Guenther, D. Helmig, D. L. Lombardozzi, J. W. Munger, E. G. Patton, S. E. Pusede, D. B. Schwede, S. J. Silva, M. Sorgel, A. L. Steiner, and A. P. K. Tai (2020), Dry Deposition of Ozone Over Land: Processes, Measurement, and Modeling, Reviews of Geophysics, 58, doi:10.1029/2019rg000670.