the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Oct 2023
Measurement Report: Observations of Ground-Level Ozone Concentration Gradients Perpendicular to the Lake Ontario Shoreline
Yao Yan Huang
D. James Donaldson
Abstract. Ground-level ozone (O3) is a secondary air pollutant that has harmful effects on human and ecosystem health. Close to larger bodies of water, the well-known sea- (or lake)-breeze phenomenon plays a role in regulating ground level ozone levels. An observed lake-edge removal effect, where ozone concentration decreases within the first 500 m to 1 km perpendicular to the lake, is thought to be related to the lake-breeze circulation as well as several dilution and removal pathways. A field campaign was conducted in summer 2022 and winter 2023 in two locations on the north shore of Lake Ontario: the urban centre of Toronto, and suburban Oshawa, some 50 km east, to assess how the local environment and season effects the lake-edge removal effect. Ozone, wind speed, and wind direction were measured on 6–7 different days in each season and city along transects perpendicular to Lake Ontario’s shoreline. A consistent negative linear relationship between ozone concentration and distance from shore over the first 500 m (i.e. a lake-edge removal effect) was observed in both cities and both seasons. The ozone gradient changed in Oshawa from -23.5 ± 8.5 (1 standard deviation) ppb/km in summer to -8.1 ± 5.1 ppb/km in winter. The slope remained consistent in Toronto at -15.4 ± 6.7 ppb/km in summer and -16.7 ± 7.3 ppb/km in winter. The year-round observation of an ozone gradient and lake-edge removal effect suggests that factors other than lake-breeze circulation, such as vegetation and titration by NO, have an influence on ozone levels at the lake-land boundary.
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Yao Yan Huang and D. James Donaldson
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1751', Anonymous Referee #2, 27 Oct 2023
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”
Citation: https://doi.org/10.5194/egusphere-2023-1751-RC1 -
AC1: 'Reply on RC1', James Donaldson, 04 Dec 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1751/egusphere-2023-1751-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2023-1751', Anonymous Referee #1, 27 Oct 2023
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.
Citation: https://doi.org/10.5194/egusphere-2023-1751-RC2 -
AC2: 'Reply on RC2', James Donaldson, 04 Dec 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1751/egusphere-2023-1751-AC2-supplement.pdf
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AC2: 'Reply on RC2', James Donaldson, 04 Dec 2023
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AC3: 'Comment on egusphere-2023-1751', James Donaldson, 04 Dec 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1751/egusphere-2023-1751-AC3-supplement.pdf
Yao Yan Huang and D. James Donaldson
Data sets
Data and supplemental information for "Measurement Report: Observations of Ground-Level Ozone Concentration Gradients Perpendicular to the Lake Ontario Shoreline" D. Donaldson https://doi.org/10.5683/SP3/KETM5Z
Yao Yan Huang and D. James Donaldson
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