Trace gas measurements in Phoenix, Arizona (1998)

From May 14 through June 12, 1998, measurements were made to characterize the chemical composition and vertical structure of the air above the Phoenix metropolitan area with the Department of Energy's G-1 research aircraft. The field program was performed in conjunction with the Arizona Department of Environmental Quality (ADEQ), Argonne National Laboratory (ANL), Brookhaven National Laboratory (BNL), and Pacific Northwest National Laboratory (PNNL). A total of 22 flights were conducted over the Phoenix region as well as ~ 50 km upwind and ~75 km downwind of the city. Vertical profiles made during these missions established typical boundary layer heights of 2000m during the morning and 2500m during the afternoon, approximately a factor of 2 greater than observed boundary layer heights in Nashville, TN during the 1995 SOS campaign. In almost half of these morning vertical profiles, significantly higher O3 mixing ratios were observed above the boundary layer than in the boundary layer. This observation may be attributable to stratospheric intrusion, trapping aloft of O3 produced during the previous day, or transport from another region. Trace gas measurements (O3, NO, NO2, NOy, CO, peroxides, formaldehyde and VOCs) were made to characterize the atmospheric photochemistry in the Phoenix area. Median concentrations of O3 were 58 + 13 ppbv upwind of the metropolitan area and 67 + 7 ppbv downwind. Average ratios of NOx to NOy were 0.23 upwind, 0.49 above the city in the morning, 0.42 above the city in the afternoon, and 0.21 downwind. These ratios show the fresh emissions in the city and indicate that some photochemical processing had occurred by the time the air moved downwind of the city, although O3 formation was much lower than observed in the northeast. Hydrogen peroxide concentrations in processed air downwind of the city were lower than upwind values, as expected in this high-NOx environment. Over the source region and downwind of the city, formaldehyde concentrations were substantially greater than upwind concentrations. Trace gas measurements made on the G-1 will be compared to measurements made at ground sites operated by the ADEQ and ANL.