PNNL

Abstract

A central goal of the Phoenix 2001 Field Campaign was to study vertical mixing with the onset of convection and to quantify the effect of this mixing on chemistry within an urban boundary layer. As part of this study, a series of low altitude aircraft sampling flights were made over the Greater Phoenix area between June 16-30, 2001. The resulting observations, in conjunction with a series of surface measurements and meteorological observations, are being used to study the vertical transport and reactivity of ozone and ozone-precursors shortly after sunrise. 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 inversely related to ozone at the start of the flights, as would be expected from the O3 + NO reaction, suggesting the upward mixing of NOx rich air with the downward transport of NOx-poor, O3 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.