Enhanced NO2 and Aerosol Extinction Observed in the Tropospheric Column behind Lake Breeze Fronts in Toronto using MAX-DOAS.

The lake breezes that frequently occur in Southern Ontario impact the levels of pollutants experienced by the populations in urban areas. The effects of lake breeze circulations on pollution transport and processing are not well understood. Few studies have measured the circulations' impact on pollutants both at the surface and within the tropospheric column. In this study, pollutants in the tropospheric column (NO₂ VCDs and AODs) were measured using MAX-DOAS concurrent with near-surface measurements of NO_x, O₃, and PM_2.5 on lake breeze days in Toronto in September and October. The arrivals of lake breeze fronts (LBFs) were identified using co-located meteorological data as well as mesoscale analyses of data from radar, satellite and a network of meteorological stations. NO₂ VCDs exhibited short-term increases of 0.8-3.4×10¹⁶ molecules cm^-2 above the pre-LBF levels following the arrival of a LBF. These measurements are the first confirmation of the theorized presence of enhanced burden of pollution within the total column behind the front within a lake breeze circulation on multiple lake-breeze days. Rapid decreases of O₃ of up to 13 ppb at the arrival of the front were unexpected based on observed increases in O₃ in other studies but can be attributed to reduced photochemical O₃ production during late summer and fall compared to mid-summer. AODs exhibited delayed enhancements compared to NO₂ VCDs, appearing to be driven by enhanced humidity following the front, in addition to enhanced particle concentration. The measurements in this study highlight the complex 2-D structure and impact of lake breeze circulations on air quality.