Modeling the impacts of emission events on ozone formation in Houston, Texas

The Houston/Galveston region has a large number of industrial point sources of volatile organic compounds (VOCs) and NOx, and observational data have shown that high concentrations of ozone are formed rapidly in the plumes of these industrial facilities. Some of the highly reactive VOC emissions from these facilities can be episodic, so in this work, photochemical air quality models, capable of simulating hundreds of possible types of emission events, have been developed and used to assess the impact of event magnitude, duration, timing and composition on ozone formation and accumulation. Simulations showed that ozone formation varied from nearly zero to up to 30 ppb per 1000 lb of release and that ozone formation depended strongly on the time of day of the release. The time of day that leads to maximum increases in ozone varies from day to day, depending on meteorology and NOx availability. On days and times that were conducive to ozone formation, the maximum additional ozone formation due to emission events was linearly related to emission magnitude, but the slope of that dependence varied based on the composition of the events. Overall, only a small percentage of emission events have the potential to lead to substantial ozone formation. Approximately 1.5% of emission events were projected to produce more than 10 ppb of additional ozone, and 0.5% of emission events were projected to produce more than 70 ppb of additional ozone, compared to base case simulations with no emission events.