Quantifying the contribution of thermally driven recirculation to a high-ozone event along the Colorado Front Range using lidar

A high-ozone (O₃) pollution episode was observed on 22 July 2014 during the concurrent “Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality” (DISCOVER-AQ) and “Front Range Air Pollution and Photochemistry Experiment” (FRAPPE) campaigns in northern Colorado. Surface O₃ monitors at three regulatory sites exceeded the Environmental Protection Agency (EPA) 2008 National Ambient Air Quality Standard (NAAQS) daily maximum 8 h average (MDA8) of 75 ppbv. To further characterize the polluted air mass and assess transport throughout the event, measurements are presented from O₃ and wind profilers, O₃-sondes, aircraft, and surface-monitoring sites. Observations indicate that thermally driven upslope flow was established throughout the Colorado Front Range during the pollution episode. As the thermally driven flow persisted throughout the day, O₃ concentrations increased and affected high-elevation Rocky Mountain sites. These observations, coupled with modeling analyses, demonstrate a westerly return flow of polluted air aloft, indicating that the mountain-plains solenoid circulation was established and impacted surface conditions within the Front Range.