Measurement report: Observations of ground-level ozone concentration gradients perpendicular to the Lake Ontario shoreline

Abstract

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-breeze (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 affect 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−1 in summer to −8.1 ± 5.1 ppb km−1 in winter. The slope remained consistent in Toronto at −15.4 ± 6.7 ppb km−1 in summer and −16.7 ± 7.3 ppb km−1 in winter. The year-round observation of an ozone gradient and lake-edge removal effect suggests that there is an inherent “baseline” ozone concentration gradient at the lake edge, caused by the dilution effect of the rapid increase of the boundary layer there. This gradient is seen to be enhanced in the summer and dependent on local effects such as lake-breeze circulation and increased deposition to vegetation.