During the Texas Air Quality Study (TexAQS 2000) ambient air was analyzed on-board the National Center for Atmospheric Research Electra aircraft using the Innsbruck proton transfer reaction mass spectrometer (PTR-MS). Molecular-level information of volatile organic compounds (VOC) such as acetaldehyde, isoprene, toluene, ketones and peroxyacetyl nitrate (PAN) were obtained at parts per trillion (pptv) levels with a time resolution of typically 30 seconds. This high time resolution is useful in characterizing individual contributions of distinct sources for ozone production in the greater Houston area. On several flights, wind direction and speed were such that these airborne VOC measurements allowed one to differentiate among anthropogenic petrochemical, urban, and power plant plumes. In-situ acetaldehyde measurements, taken in aircraft transects of the Houston metropolitan area, confirm the importance of propene emissions from localized point sources to the photochemical processing of NO_x and the rapid formation of ozone and PAN within short distances from the co-located NO_x and propene emission sources. Biogenic isoprene emissions contribute also to ozone formation. Fast response measurements of this compound and its photooxidation products methylvinylketone, methacrolein and hydroxyacetone revealed that biogenic emission sources are primarily located north east of Houston and contribute to ozone formation in the metropolitan Houston area only under certain metrological conditions.