The Influence of Aged Smoke on Urban Ozone: Aircraft Measurements in Chicago During the 2023 Canadian Wildfires

As large wildfires become more frequent and severe in North America due to heightened fuel aridity and fire weather conditions driven by anthropogenic climate change, wildfire smoke has an increasingly important influence on air quality. Transported smoke can impact urban boundary layer ozone (O₃) directly, via the entrainment of O₃ formed in smoke, or indirectly, through the influence of pyrogenic volatile organic compounds (VOCs) and aerosols on local O₃ production. Widespread smoke impacts from the record-breaking 2023 Canadian wildfire season coincided with the July–August 2023 Airborne Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) aircraft-based field campaign, which provided extensive in situ observations of urban chemistry in North American megacities. Vertically resolved trace gas and aerosol measurements from the August 2 AEROMMA research flight in Chicago indicated the entrainment of diffuse smoke from an aged, lofted wildfire plume into the urban boundary layer and were used to constrain a zero-dimensional photochemical model and a radiative transfer model. We perturbed the models across a range of smoke densities and urban nitrogen oxide emissions and found net pyrogenic O₃ enhancements of 3–17 ppbv, with the largest contribution from transported smoke O₃ (65%–84%), followed by locally produced photochemical O₃ from smoke VOCs (18%–46%), and finally smoke aerosol shading (−12%–0%). Although these results are specific to the observed August 2 smoke plume and Chicago urban chemistry, the modeling framework presented could be applied to other smoke-impacted locations to disentangle the various effects of wildfire smoke on urban O₃.