September 25. Wednesday.
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TOPAZ data were collected from about 0800 to 2000 CDT. Shallow fog layer early in the morning, otherwise clear skies all day. Light northerly winds thru midday, rotating to easterly and then southeasterly directions as the Bay and Gulf breezes got established. High NOx concentrations at La Porte and over Ship Channel and downtown Houston. Distinctive brown cloud visible towards the NW. Very low surface O3 (0 - 10 ppbv) early, increasing slowly at first to about 30 ppbv by 1030, then increasing rapidly to 95 ppbv by 1230. Moderate O3 (60 - 70 ppbv) aloft above 400 m AGL. After 1230 the boundary layer grew rapidly from about 500 to 1500 m AGL, which most likely caused a drop in O3 to the 70s ppbv due to down-mixing of the 60 - 70 ppbv layer aloft. After 1400 CDT, O3 increased again to about 120 ppbv and was fairly well mixed thru a boundary layer that was at least 1800 m deep. After 1800 CDT, the sea breeze brought in cleaner air in the lowest 300 m AGL (~ 75 ppbv). O3 aloft eventually also declined slightly to about 90-100 ppbv. The La Porte Sylvan Beach TCEQ monitoring site a few miles to the east recorded the highest O3 concentrations in the Houston area with a maximum 8-hour-average value of 124 ppbv. This is the highest MDA8 O3 value recorded so far this year in Texas and the second highest in the last 7 years.    
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Note: Biases due to clouds have not yet been fully corrected. Due to high O3 and aerosol extinction and, consequently, low signal SNR, there are slight positive biases near 50, 150, and 450 m AGL (where the profiles from the 4 different elevation angles are spliced together).