Houston High Ozone August 26, 2008

Analyses for a major air pollution event

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High ozone was measured in the Houston area on Tuesday, August 26th. The highest measured eight-hour average was 99 parts per billion (ppb) at the Pasadena Continuous Ambient Monitoring Station (CAMS) 672 and rated as Level Red, Unhealthy, on the U.S. Environmental Protection Agency (EPA) Air Quality Index (AQI) Exit the TCEQ scale. The ozone data at this site is from an experimental Fourier Transform Infrared (FTIR) instrument, but the measurements appear to be accurate and valid. The highest measured one-hour average was 119 parts per billion (ppb) for the hour from 1:00 to 2:00 p.m. Central Daylight Time (CDT), also at Pasadena CAMS 672. Level Red ozone was measured only at one site and Level Orange ozone was measured at five sites, with "Moderate" or higher ozone at 27 sites out of 38 sites reporting complete data for the day. This day was the fourth day this year with Level Red ozone measurements somewhere in the Houston area and the 19th day with Level Orange or higher ozone measurements, based on the new ozone standard and AQI. It was the seventh day with measured levels exceeding the old 8-hour ozone standard.

Skies were clear in the early morning and partly cloudy mid-day. Thundershowers developed in the area in the afternoon which helped to disperse the ozone. Winds were light from the northeast in the early morning and then shifted to the east mid-day and then to the southeast in the afternoon with the bay breeze and sea breeze. The high temperature was 95 degrees Fahrenheit (°F) at George Bush Houston Intercontinental Airport, 93°F at Hobby Airport, and 90°F at Galveston Airport.

Regional background levels of ozone coming into the Houston area were around 56 to 63 ppb as indicated by the peak eight-hour averages at Conroe CAMS 78, Crosby CAMS 553, and Galveston Airport CAMS 1034. The difference of 36 to 43 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 36 to 43 percent of the measured 99 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.

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