This manuscript shows evidence of a strong gradient in ozone surface concentrations along the Lake Ontario shoreline with decreasing concentrations with increasing distance from the water. This is in agreement with other research that has shown high air pollution concentrations near coastlines. However, this paper is unique in that it is able to quantify the gradient of near surface ozone concentrations with observations. The paper refers to this phenomenon as a “lake-edge removal effect,” which I have not heard before. The authors hypothesize that the observed near surface ozone gradient is due to deposition onto vegetation and chemistry. The measurements documenting and quantifying the strength of the ozone gradient are great and should be published. The analysis describing possible reasons why the ozone gradient exists needs more work before publishing. My recommendation is for this paper to be accepted after major revisions.

Major comments:

• Many lake breeze related papers were cited, but papers relating to sea and bay breezes, which are essentially the same phenomenon as lake breezes, were ignored. I suggest the authors provide more background material relating to recent studies involving gradients in air pollution near coastal areas.

• Is the “lake-edge removal effect” a common term? If not, I recommend not using it. It sounds like the lake edge is removing ozone, but I don’t think that is what is happening.

• The authors hypothesize that the observed ozone gradient is primarily due to deposition and chemistry. There are other contributing factors. I strongly suggest the authors review the following paper that discusses sharp gradients in concentrations and deposition of nitrogen species along coastlines (Loughner, C.P., M. Tzortziou, S. Shroder, and K.E. Pickering (2016), Enhanced dry deposition of nitrogen pollution near coastlines: A case study covering the Chesapeake Bay estuary and Atlantic Ocean coastline, Journal of Geophysical Research – Atmospheres, 121, 14,221-14,238.). The gradient in boundary layer height near the coastline may be a large contributing factor in the observed ozone gradient, but boundary layer height was not mentioned in the manuscript. While ozone titration might be occurring, there is no evidence provided that it is.

• I think the analysis would benefit if the analyzed gradients in observed ozone concentrations were performed separately based on wind direction (onshore vs offshore flow). When the winds are onshore, the gradient may primarily be due to the gradient in boundary layer height. When there is offshore flow, the gradient may be just due to the coastline being downwind of emissions of ozone precursors from the urban area.

• Combining this analysis with PBL height calculated from a NWP model would benefit this manuscript.

Minor comments:

• Lines 9-10: In addition to considering removing “lake-edge removal effect” here and throughout the paper (see comment above), I suggest changing “where ozone concentration decreases within the first 500 m to 1 km perpendicular to the lake” to “where ozone concentration decreases with distance from the lake within the first 500 m to 1 km” to make sure the reader understands you see this gradient near the coastline onshore and not just offshore.
• Line 27: change “airflow moving” to “airflow near the surface moving”
• Lines 30-31: end sentence after “lake” and delete the remainder of the sentence.
• Line 31: This sentence refers to a land breeze the figure shows a lake breeze.
• Line 32: change “ozone concentration” to “ozone and ozone precursor concentrations”
• Line 33: change “O3 inland” to “O3 and O3 precursors inland”
• Line 47: change “lake-breeze” to “lake-breezes”
• Line 53: change “lake breeze was” to “lake breezes were”
• Line 61: change “further” to “farther”

Line 53: Put a “The” before “Average” or make concentrations plural

Line 131: Why were measurements only made during the afternoon? Please clarify.

Line 145-146: Change “was” to “were”, data is considered plural. Check manuscript for other instances.

Line 159: For the hourly O3 and NO2 from the provincial stations, are these data only encompassing the hours of 12:00-21:00 EDT, similarly to the Aeroqual 500? It should be noted specifically how the monitoring station data is averaged

Line 265-266: The authors do not mention the shallower mixed layers, decreased photolysis rates of NO2 and overall decreased ozone production rates during winter time. These are possible contributions to the lack of a relationship between wind speed and ozone concentration. Ozone gradients may be more impacted my meteorology during summer, while during winter the regional background and local sources may contribute more. The authors should explore and describe how during winter there is less dynamic variability in ozone due to the possible reasons mentioned above.