Lake Breeze Effects on the Urban Environment in Chicago.

Lake breezes can strongly influence the environment in coastal areas. As the most populous city along Lake Michigan, Chicago frequently experiences lake breezes, yet their impact on air quality remains poorly understood. Using one-year measurements from meteorological and the Microsoft Eclipse low-cost sensor networks, we identify 42 lake breeze events in 2022 and assess the net effects of lake breezes on the urban environment in Chicago. We find that lake breezes occurring on weekdays generally reduce ozone (O3) mixing ratios on average up to ∼6 ppb along the northeast shoreline, contrary to many other coastal regions, where sea/lake breezes drive O3 exceedances. Reduced O3 during lake breeze events likely result from a combination of processes, including enhanced NO titration in a shallow boundary layer, especially during the early stage of the event and in areas with heavy emissions, the inflow of cleaner air, and the lower temperatures that suppress the O3 formation by reducing precursor emissions and slowing-down reaction rates. In contrast, on average, lake breezes enhance fine particulate matter (PM2.5) concentrations by up to ∼3 μg m-3, due to the accumulation of local emissions within a shallower boundary layer and enhanced partitioning of semivolatile species at lower temperatures. Additionally, we identify 51 weak events during which lake breezes remained mostly within several kilometers of the lakeshore. These weak lake breeze events show broadly consistent effects on temperature and O3, albeit with smaller magnitudes. These results highlight the interactions of emissions, atmospheric chemistry, and meteorological conditions during the lake breeze events and their impact on the urban environment, underscoring the need for region-specific evaluations of sea/lake breeze effects.