Research Summary: Impact Of Lake Breezes On Air Quality In The Greater Toronto Area

^ADepartment of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, M5S 3H6, Canada

^BCloud Physics and Severe Weather Research Section, Environment Canada, 4905 Dufferin Street, Toronto, ON, M3H 5T4, Canada

The Greater Toronto Area (GTA) is home to roughly 5.5 million people and is situated on the northwestern shore of Lake Ontario. The region is often afflicted with poor air quality during summer months as a result of high levels of ozone (O₃) which is harmful to both humans and plants. O₃ is formed from a series of complex chemical reactions involving nitrogen oxides (NO_x, primarily from human activity) and volatile organic compounds (from both human and natural sources). These reactions are heavily influenced by meteorology with higher temperatures, more sunlight, and lighter winds favouring an abundance of O₃.

The presence of Lake Ontario affects summertime meteorology (and hence air quality) through a phenomenon known as a lake breeze. A lake breeze circulation results from the preferential heating of land relative to water and creates an onshore flow at ground level and recirculation of air throughout the day, as shown in Figure 1 (Sills et al., 2011). Recent studies have identified high concentrations of O₃ in air over Lake Michigan (Foley et al., 2011), Lakes Erie and St. Clair (Makar et al., 2010) and Chesapeake Bay (Loughner et al., 2014) which can be brought inland by lake breezes and recirculated throughout the day. The goals of this current study were to:

1. Determine the frequency of Lake Ontario breezes during May to September;
2. Compare meteorology and O₃ concentrations on days when a lake breeze forms versus days without a lake breeze;
3. and determine if a causal link exists between high O₃ levels at inland sites and lake breezes.

Figure 1. Cross-section of an idealized lake breeze circulation during the day. Lake breezes typically suppress cloud formation over the lake and can trap pollutants in a recirculating pattern. (Credit: Gregory Wentworth)

Methods

Data sets containing hourly measurements of O₃, NO_x and meteorological parameters (temperature, wind direction, etc…) are freely available online and were obtained from Environment Canada (http://climate.weather.gc.ca/index_e.html#access) and the Ontario Ministry of the Environment and Climate Change (http://www.airqualityontario.com/history/) for 5 sites across the GTA for the years 2010-2012. Satellite, radar, and weather station data were used together to determine the days on which a lake-breeze circulation formed (“lake breeze days”). Air quality and meteorological data were then grouped into “lake breeze days” and “non-lake breeze days” to assess the impact Lake Ontario breezes have on air quality across the GTA.

Results

Over a 3-year period it was found that Lake Ontario breezes formed on 74% of warm season (May to September) days. Figure 2 reveals that the average daytime peak O₃ concentration is between 42-49% higher on lake breeze days across all 5 monitoring sites. This proves there is a strong link between lake breeze occurrence and high O₃ levels. In order to assess whether this link is causal or merely a correlation, several case study days were identified where only some air quality sites were within the lake breeze circulation. Only sites within the circulation exhibited enhanced O₃ indicating that high levels of O₃ at these sites can be attributed to the presence of lake breeze circulations.

Figure 2. Average O3 concentration as a function of time of day for lake breeze days (dashed lines) and non-lake breeze days (solid lines). Different colours represent the five different air quality monitoring stations. (Credit: Gregory Wentworth)

Conclusions

This is the first study to examine the influence of Lake Ontario breezes on air quality using multi-year datasets. We find that these lake breezes are common summertime phenomena that can deliver and recirculate high concentrations of O₃ across the GTA which significantly degrades air quality. Future work will aim to address the mechanism(s) responsible for high O₃ over Lake Ontario and improve air quality forecasting models so they can accurately predict this phenomenon.

Full study published in Atmospheric Environment, May 2015.

Selected References

1. Foley, T., Betterton, E.A., Jack, P.E.R., Hillery, J., 2011. Lake Michigan air quality: the 1994-2003 LADCO aircraft project (LAP). Atmos. Environ., 45, 3192-3203.
2. Loughner, C.P., Tzortziou, M., Follette-Cook, M., Pickering, K.E., Goldberg, D., Satam, C., Weinheimer, A., Crawford, J.H., Knapp, D.J., Montzka, D.D., Diskin, G.S., Dickerson, R.R., 2014. Impact of bay-breeze circulations on surface air quality and boundary layer export. J. Appl. Meteorol. Clim., 53, 1697-1713.
3. Makar, P.A., Zhang, J., Gong, W., Stroud, C., Sills, D.M.L., Hayden, K.L., Brook, J.R., Levy, I., Mihele, C., Moran, M.D., Tarasick, D.W., He, H., Plummer, D., 2010. Mass tracking for chemical analysis: the causes of ozone formation in southern Ontario during BAQS-Met 2007. Atmo. Chem. Pys., 10, 11151-11173.
4. Sills, D.M.L., Brook, J.R., Levy, I., Makar, P.A., Zhang, J., Taylor, P.A., 2011. Lake breezes in the southern Great Lake region and their influence during BAQS-Met 2007. Atmos. Chem. Phys., 11, 7955-7973.

 Featured Image: View of Toronto from Lake Ontario. (Credit: Flickr User John Vetterli via Creative Commons 2.0)