Photochemical Model Representation of Ozone and Precursors During the 2017 Lake Michigan Ozone Study (LMOS)

Several areas in the Lake Michigan region are violating the human health-based ozone (O₃) National Ambient Air Quality Standard. Land-water meteorology driven build-up of precursor pollutants (NO_X and VOC) from mobile and stationary sources undergo photochemical O₃ production and result in seasonal high O₃ episodes during the spring and summertime. Routine and specialized surface measurements coupled with airborne and remotely sensed measurements from the 2017 Lake Michigan Ozone Study (LMOS) provide an opportunity to evaluate photochemical grid model representation of these processes. The Community Multiscale Air Quality Model (CMAQ) was applied at a 4-km grid resolution for the Lake Michigan region and compared against routine and field study measurements to determine how well the modeling system captures emissions, meteorology, and chemical production during O₃ episodes. The model captured day to day and diurnal variability in observed O₃ along Lake Michigan but often missed peak O₃. Surface level NO₂ was generally well characterized, but the model seemed to be missing a significant source of VOC, likely both regional and local (not necessarily the same source sector) based on model sensitivities. On June 2, 2017, the model captured the timing of the lake to land breeze but underestimated near-surface wind speed which coincided with peak O₃. The underestimation of surface O₃ north of Chicago at Zion, Illinois, on this afternoon may be related to over-water vertical mixing, the modeled lake breeze being too weak to transport ozone and precursors back onshore, regional and aloft underprediction of O₃, or some combination of each of these factors. Despite underpredictions of peak O₃ at some monitors on certain days, the modeling system is generally useful for developing control scenarios for this region. Model-predicted O₃ sensitivity to precursors matched other assessments and suggests both NO_X and VOC anthropogenic emissions are important to reduce O₃ in the Chicago area.