Measurements of nonmethane hydrocarbons in Phoenix, Arizona

Nonmethane hydrocarbons (NMHCs) were measured during May 1998 at several surface sites in the Phoenix area and from aboard the Battelle Gulfstream (G-1) Aircraft. Ambient air was collected in passivated stainless steel canisters and analyzed by a cryogenic preconcentration/high-resolution gas chromatographic technique with flame ionization detection. Three-hour integrated samples were collected over 24 hour periods to examine diurnal variations of the NMHC concentrations at a surface site located in a hilly region to the east of Phoenix. Distributions of the NMHCs at this site were typical of photochemically processed vehicle exhaust. Levels of the saturated, unsaturated, and aromatic hydrocarbons reached maximum values between 0100 and 0400 local time (LT). During this period the toluene/benzene ratios were approximately 2.0 and characteristic of partially reacted emissions that may have originated from the evening rush hour. Biogenic emissions of isoprene were evident during the daylight hours. Toluene/benzene ratios in samples collected before 1600 LT were usually between 1.0 and 1.5 and typical of vehicle emissions that had been photochemically oxidized. The reactivity values of the hydrocarbon mixture in ambient air at 0100-1400 LT were greater than reactivity levels at 1300-1600 LT. Isoprene contributed a significant fraction of the hydrocarbon reactivity during the daylight hours. Data from this site and the other surface sites will be used to interpret NMHC measurements made from aboard the G-1 Aircraft.