Nitric acid (HNO3) is measured from an aircraft in the planetary boundary layer and free troposphere up to 7 km on 14 flights during the Texas Air Quality Study in August and September, 2000. Using a fast response chemical ionization mass spectrometer with SiF5- reagent ions, HNO3 mixing ratios are measured at a 1 Hz sample rate. HNO3 measurement using this highly selective ion chemistry is not degraded by large water vapor concentrations or interferences from other species. Rapid time response (1 s) is achieved using a heated Teflon inlet. In-flight standard addition calibrations from a HNO3 permeation source are used to determine the instrument sensitivity of 1.1 ± 0.1 Hz/pptv. Contributions to the HNO3 signal from instrument artifacts are accounted for by regularly performing in-flight instrument background checks, where HNO3 is removed from the ambient air sample by diverting the sampled air though a nylon wool filter. Measurement accuracy, which is determined from uncertainties in the standard addition calibrations and background measurements, is the sum of ±10% and ±130 pptv. Coincident in-situ measurements of other reactive nitrogen species are used to examine NOy partitioning and HNO3 formation during this month-long measurement campaign. Rapid HNO3 formation is frequently observed in pollution plumes from power plants, urban areas, and refineries. In photochemically aged power plant, urban, and refinery plumes where approximately 75% of the NOx is oxidized, HNO3 production is most favored in power plant plumes and least favored in refinery plumes.