Source contributions of volatile organic compounds to ozone formation in southeast Texas

A source-oriented SAPRC-99 gas phase photochemical mechanism was incorporated into the Community Multiscale Air Quality (CMAQ) model to determine the contributions of volatile organic compounds (VOCs) to predicted net ozone (O-3) formation rates during the Texas Air Quality Study (TexAQS) from 16 August to 7 September 2000. Contributions from biogenic sources, diesel vehicles, highway gasoline vehicles, off-highway gasoline vehicles, solvent utilization, petroleum industries, other industries and wildfires were determined. Peak column-averaged O-3 formation rate due to industrial sources averaged over this episode was approximately 8.5 ppb hr(-1). Contributions of gasoline vehicles and solvent utilization were large in urban areas to the west of the industrial region with highest column-averaged formation rates of 3.7 and 1.4 ppb hr(-1), respectively. Large regional contributions of biogenic sources to O-3 formation were predicted with highest O-3 formation rate of 11.9 ppb hr(-1) in downwind rural areas. Wildfires could contribute to large O-3 formation but their influence was generally localized. Analysis of 2400 back-trajectories from areas with maximum daily 8-h O-3 greater than 90 ppb showed that industrial sources were the largest anthropogenic sources of VOC that contributed to the these high O-3 events, followed by gasoline vehicle sources. The median of relative contributions from biogenic and anthropogenic sources from this analysis was approximately 60% and 40%, respectively. Analysis of the back-trajectories where 1-h peak O-3 concentrations were greater than 120 ppb showed that the median relative contributions due to anthropogenic sources were increased to over 60%. This suggests that high O-3 events in the HGB region were driven by anthropogenic VOC emissions from industrial sources.