Impacts of Background Ozone Production on Houston and Dallas, Texas, Air Quality During the Second Texas Air Quality Study Field Mission

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A major objective of the 2006 Second Texas Air Quality Study (TexAQS II) focused on understanding the effects of regional processes on Houston and Dallas ozone nonattainment areas. Here we quantify the contributions of background (continental scale) ozone production on Houston and Dallas air quality during TexAQS II using ensemble Lagrangian trajectories to identify remote source regions that impact Houston and Dallas background ozone distributions. Global-scale chemical analyses, constrained with composition measurements from instruments on the NASA Aura satellite, are used to provide estimates of background composition along ensemble back trajectories. Lagrangian averaged O-3 net photochemical production (production minus loss, P-L) rates along the back trajectories are used as a metric to classify back trajectories. Results show that the majority (6 out of 9 or 66%) of the periods of high ozone in Houston were associated with periods of enhanced background ozone production. Slightly less than 50% (7 out of 15) of the days with high ozone in the Dallas Metropolitan Statistical Area (MSA) show enhanced background ozone production. Source apportionment studies show that 5-day Lagrangian averaged O-3 P-L in excess of 15 ppbv/d can occur during continental-scale transport to Houston owing to NOy enhancements from emissions within the Southern Great Lakes as well as recirculation of the Houston emissions. Dallas background O-3 P-L is associated with NOy enhancements from emissions within Chicago and Houston.