The 2001 Phoenix Sunrise experiment: vertical mixing and chemistry during the morning transition in Phoenix

A field experiment was carried out in Phoenix during June 2001 to examine the role of vertical mixing on the O3 chemistry of the boundary layer during the morning transition from stable to unstable atmospheric conditions. Surface instruments, instruments located on two floors of a 39 story building in downtown Phoenix, and an instrumented airplane were used to characterize the evolving chemistry in the lowest 650 m of the atmosphere. Remote sensing and in situ platforms were used to obtain detailed profiles of winds and temperatures during the early morning hours and for several hours after sunrise. The analysis presented in this paper focuses on vertical profiles of CO, O3, and NO/NOy measured on the building and their relationship to the morning boundary layer evolution over Phoenix. Some features of these profiles were found that are consistent with a simple conceptual picture of nighttime trapping of pollutants in a stable surface layer and a subsequent release the following morning. On some days, however, evidence of significant vertical mixing was found during the early morning hours well before the times expected for the development of convective mixing after sunrise. Possible causes include advection, street canyon turbulence, and large-scale convergence, but a satisfactory explanation for the observed evolution of the chemical profiles has not yet been found.