Houston High Ozone June 18, 2008

• Ozone Animation
• Plume Animation
• Backward Air Trajectory for Ozone Peak
• Satellite Animation
• Satellite Image True Color - Texas 2:16 pm CDT (from UW SSEC)
• Satellite Image True Color - Houston 2:16 pm CDT (from UW SSEC)
• Winds Aloft at UH La Marque
• Winds Aloft at UH La Marque Mid-Day

Description
High ozone was measured on the east and northeast side of the Houston area on Wednesday, June 18th.   The highest measured eight-hour average was 106 parts per billion (ppb) at the Houston Regional Monitoring (HRM) Site 11 east of Baytown and rated as Level Red, Unhealthy, on the U.S. Environmental Protection Agency (EPA) Air Quality Index (AQI) scale.   The highest measured one-hour average was 148 parts per billion (ppb) for the hour from 5:00 to 6:00 p.m. Central Daylight Time (CDT), also at HRM Site 11.   Level Red ozone was measured at two sites in the Houston metro area, with Level Orange ozone or higher at 13 sites, and "Moderate" or higher ozone at 26 sites out of 44 sites reporting complete data for the day.   This day was the first day with measured Level Red ozone in the Houston area and the 9th day this year with Level Orange or higher ozone measurements somewhere in the Houston area.

Skies were partly cloudy all day with thundershowers developing in the area in the late afternoon and early evening.   Winds were light from the northeast in the early morning and then shifted to the west and west-northwest mid-morning and then to the south and southeast in the afternoon with the bay breeze and sea breeze.   The high temperature was 98 degrees Fahrenheit (°F) at George Bush Houston Intercontinental Airport, 96°F at Hobby Airport, and 92°F at Galveston Airport.

There was a gradient in the regional background levels of ozone coming into the Houston area, ranging from about 28 to 39 ppb as indicated by the peak eight-hour averages at Lake Jackson CAMS 1016 and Galveston Airport C1034 on the south side of the Houston area to as much as 47 to 51 ppb on the north and west side of the Houston area, as indicated by measurements at Conroe CAMS 78, Northwest Harris County CAMS 26, and Katy Park CAMS 559.   It appears that the highest ozone formed in the air mass represented by upwind measurements on the north and west side of the area.   The difference of 55 to 59 ppb between the measured eight-hour area maximum and the approximate regional background level was likely caused by local air pollution sources in the Houston area.   The approximate local contribution was about 52 to 56 percent of the measured 106 ppb area eight-hour peak.

The Plume Animation shows the estimated plume tracks from large industrial sources of oxides of nitrogen (NOx) and volatile organic compounds (VOC), along with the plume tracks for the centers of the broad urban plumes coming from downtown Houston and other major urban centers.   The plume animation suggests that urban and industrial emissions from the Houston Ship Channel area were in the vicinity of some of the highest ozone measurements.  

The backward air trajectory indicates that the air with the highest ozone peak for the day had earlier passed across the central Ship Channel area and over Galveston Bay before reaching HRM Site 11.   The passage over Galveston Bay should have restricted vertical mixing of the air to help increase the concentration. Skies were also clear over Galveston Bay, allowing greater exposure to ozone producing ultra-violet sunlight.