A series of low altitude sampling flights were made over the Greater Phoenix area between June 7 and July 3, 2001 to study the vertical transport and reactivity of ozone and ozone-precursors shortly after sunrise. These measurements were made as part of a campaign to study vertical mixing and to quantify the effect of mixing on chemistry within an urban boundary layer. Observations were made of NO, NO2 and NOy, VOCs, O3, CO, SO2, temperature, moisture and pressure. Airborne sampling started shortly after sunrise and continued through mid-morning, with repeated measurements made at three altitudes over Phoenix. A spiral to 10,000 ft MSL was regularly made at the start of each flight. We report here on first analysis of these measurements.

A typical flight began with sampling in the residual boundary layer of the preceding afternoon, showing a large vertical gradient in both chemical and meteorological species. With the development of the convective boundary layer, these gradients disappeared, and a more uniform value was found at all altitudes in the more slowly reacting species. Ozone levels were typically observed to be greatest aloft during the early morning hours, with values typically twice those found near the surfaces. NOy was initially inversely related to ozone, as would be expected from the O3 + NO reaction, suggesting the upward mixing of NOx rich air with the downward transport of NOx-poor, ozone rich air. The timing of the development of the convective boundary layer, as measured by the weakening of chemical stratification, appeared to be related to the intensity of the residual nocturnal stable layer. Distinct geographic regions of high NOy were repeatedly observed during the flights and appear to be related to the local boundary layer circulations. Preliminary observations from an onboard PCASP system suggest a relatively high number of large aerosols relative to fine aerosols compared to similar observations made in other major urban areas.