Application of a Lagrangian Process Analysis tool to characterize ozone formation in Southeast Texas

Ozone formation in the Houston area is more rapid and efficient than in many other urban areas; these features are due to the interaction of urban emissions with industrial plumes which are associated with both continuous and episodic industrial emissions. To examine the chemistry of interactions of the industrial plumes with urban emissions, a Lagrangian Process Analysis tool was embedded in a gridded photochemical model. The tool successfully isolated the plume so that, within the Process Analysis volume, the dominant process affecting ozone concentrations was chemical production. The analyses showed that the chemistry of the industrial plumes is dependent on emissions encountered downwind, and the extent of radical availability in the morning. For one episode, morning stagnation over the industrial region resulted in enhanced morning radical formation due to aldehyde photolysis. The enhanced morning reactivity led to high ozone concentrations as the plume interacted with urban emissions later in the day. In contrast, during a second episode, the dominant factor influencing ozone concentrations was high volatile organic carbon (VOC)/NOx ratios late in the afternoon, as the plume advected over wooded areas. For emission events, the main perturbation of the ozone formation chemistry occurred during the first few hours of the event due to enhanced production of free radicals from aldehyde photolysis, and more efficient utilization of free radicals due to increased reactive hydrocarbon concentrations.