Sailboat Measurements of Trace Gases in Urban Environments: In Toronto, Canada, Ground-Level NO ₂ Is Consistently Elevated over Lake Ontario versus at the Shoreline, but O ₃ Is Not

Ozone (O 3 ) and nitrogen dioxide (NO 2 ) are important urban air pollutants which are closely coupled via chemical reactions: in the atmosphere, photolysis of NO 2 forms NO and ozone, while the reaction of NO with O 3 reforms NO 2 . The local meteorology near urban centers located on coasts (e.g., sea- or lake-breezes) often gives rise to the advection of polluted air from over the land to over the water during nighttime, and the readvection of the air back ashore during the daytime. Sea- and lake-breeze effects have led to higher-than-normal ozone concentrations over coastal urban areas; concentrations of both O 3 and NO 2 have been reported to be higher over the water than inland. Most of these findings have come from remote sensing column measurements, using satellites and Pandora (DOAS), combined with land-based measurements from air quality stations. Here we present results from a 3-year campaign of measurements of “ground level” O 3 and NO 2 concentrations made over 17 cruises on Lake Ontario and at its shoreline near Toronto. Using portable sensors mounted on a sailboat, we found that while there is no clear or consistent difference in ozone concentrations over water vs at the shoreline, NO 2 was higher over the water for 12 of the 17 cruises and was the same over water and at the shoreline for the remaining 5. We discuss possible reasons for this observation.