1Air Quality Research Division, Environment Canada, Toronto, ON, Canada
2Department of Chemistry, University of Toronto, Toronto, ON, Canada
3Cloud Physics and Severe Weather Research Section, Environment Canada, Toronto, ON, Canada
4Meteorological Service of Canada Operations-Ontario, Environment Canada, Toronto, ON, Canada
5Centre for Atmospheric Chemistry, York University, North York, ON, Canada
Abstract. High time-resolved aircraft data, concurrent surface measurements and air quality model simulations were explored to diagnose the processes influencing aerosol chemistry under the influence of lake-breeze circulations in a polluted region of southwestern Ontario, Canada. The analysis was based upon horizontal aircraft transects at multiple altitudes across an entire lake-breeze circulation. Air mass boundaries due to lake-breeze fronts were identified in the aircraft meteorological and chemical data, which were consistent with the frontal locations determined from surface analyses. Observations and modelling support the interpretation of a lake-breeze circulation where pollutants were lofted at a lake-breeze front, transported in the synoptic flow, caught in a downdraft over the lake, and then confined by onshore flow. The detailed analysis led to the development of conceptual models that summarize the complex 3-D circulation patterns and their interaction with the synoptic flow. The identified air mass boundaries, the interpretation of the lake-breeze circulation, and best estimates for air parcel circulation times in the lake-breeze circulation (1.2 to 3.0 h) enabled formation rates of oxygenated organic aerosol (OOA/ΔCO) and SO42− to be determined. The formation rate for OOA, relative to excess CO, was found to be 2.5–6.2 μg m−3 ppmv−1 h−1 and the SO42− formation rate was 1.8–4.6% h−1. The formation rates are enhanced relative to regional background rates implying that lake-breeze circulations are an important dynamic in the formation of SO42− and secondary organic aerosol. The presence of cumulus clouds associated with the lake-breeze fronts suggests that these enhancements could be due to cloud processes. Additionally, the effective confinement of pollutants along the shoreline may have limited pollutant dilution leading to elevated oxidant concentrations.