Trace-gas mixing in isolated urban boundary layers: Results from the 2001 Phoenix sunrise experiment

Measurements made from surface sites, from the 50 and 140 m levels (the 16th and 39th floors) of a skyscraper, and from an instrumented aircraft are used to characterize early morning profiles of CO, NOy, and O3 within the mid-morning summertime convective atmospheric boundary layer (CABL) over Phoenix, Arizona. Although mixing was anticipated to produce uniform values of these species throughout the CABL, this was found not to be the case. Background air advected into the upper levels of the boundary layer and entrained air from above appear to be the most likely cause for the lack of well-mixed trace gases. The results show that surface measurements may provide only limited information on concentrations of trace-gas species higher in the boundary layer.