The DOE G-1 aircraft made flights on 14 days during the TexAQS 2000 study. On seven of those days, the aircraft encountered highly localized plumes exhibiting O₃ concentrations in excess of 150 ppb; on some days, peak O₃ concentrations were in excess of 200 ppb. These ozone plumes were rapidly formed with an efficiency (O₃ per NO_x molecule consumed) much higher (7 –20) than observed in other urban areas (3-4), and were frequently associated with high concentrations (> 20 ppb) of secondary hydrocarbon species such as formaldehyde. Back trajectory analysis showed that the plumes were invariably associated with emissions from one or more of the large industrial complexes clustered about the Houston Ship Channel and Galveston Bay. Very high hydrocarbon reactivities were found in the vicinity of these facilities during morning flights. These hydrocarbon reactivities, in combination with local NO_x emissions, were large enough to support instantaneous O₃ production rates as high as 200 ppb/h. It is hypothesized that the combination of nitrogen oxides and hydrocarbon emissions emanating from this complex of industries provided a potent mixture of chemicals that caused the rapid formation of very high concentrations of ozone which, depending on the prevailing meteorology, could cause exceedance of the NAAQS ozone standard anywhere in the Houston metropolitan area.