The Impact of Wind Direction on the Location of High Ozone in the Houston-Galveston-Brazoria Area

The Houston-Galveston-Brazoria (HGB) area has historically observed some of the highest ozone concentrations within the state of Texas. Prior to 2009, the Bayland Park monitor typically observed the highest eight-hour ozone design values. In 2010 and after, the Manvel Croix Park monitor observed the highest eight-hour ozone design values in the HGB area. Although the area has recently seen steep declines in eight-hour ozone concentrations, the Manvel Croix Park monitor continues to measure ozone values that are several parts per billion (ppb) higher compared to those measured at other monitors in the area. Many factors cause high ozone, but this analysis focused on the role wind direction plays in determining the location of high ozone in the HGB area, and how that location has changed over time. To investigate the differences in location of high ozone, five groups of high ozone days from 2008 through 2014 were created. The groups were based on which monitor observed eight-hour ozone concentrations greater than 75 ppb. NOAA's HYSPLIT model was used to examine the 24-hour back trajectories for each of the five groups of high ozone days. In addition to the HYSPLIT back trajectories, six-hour surface-level back trajectories were calculated with a two-dimensional model that uses Modified Shepard's method for interpolation. The HYSPLIT back trajectories combined with the surface trajectories show how wind direction is one important factor that determines the location of high ozone in the HGB area.