Transport and Diffusion of Ozone in the Nocturnal and Morning Planetary Boundary Layer of the Phoenix Valley

The evolution of ozone (O3) in the nocturnal and morning-transitional planetary boundary layer (PBL) of the Phoenix valley was measured as a part of the ‘Phoenix Sunrise Experiment 2001’ of the U.S. Department of Energy conducted in June 2001. The goal of the field program was to study the transport, distribution and storage of ozone and its precursors in the urban boundary layer over a diurnal cycle. The ground level O3 as well as mean meteorological variables and turbulence were measured over the entire period, and vertical profiling (using a tethered balloon) was made during the morning transition period. Approximately half of the observational days showed the usual diurnal cycle of high O3 during the day and low O3 at night, with nitrogen oxides (NOx = NO2 + NO) showing an out of phase relationship with O3. The rest of the days were signified by an anomalous increase of O3 in the late evening (∼ 2200 LST), concomitant with a sudden drop of temperature, an enhancement of wind speed and Reynolds stresses, a positive heat flux and a change of wind direction. NOx measurements indicated the simultaneous arrival of an ‘aged’ air mass, which was corroborated by the wind predictions of a mesoscale numerical model. In all, the results indicate that the recirculation of O3 rich air masses is responsible for the said high-O3 events. Such air masses are produced during the transport of O3 precursors by the upslope flow toward mountainous suburbs during the day, and they return back to the city at night via downslope winds (i.e. mountain breeze). The corresponding flow patterns, and hence the high-O3 events, are determined by background meteorological conditions. The vertical profiling of O3 and flow variables during the morning transition points to a myriad of transport, mixing and chemical processes that determine the fate of tropospheric O3. How well such processes are incorporated and resolved in predictive O3 models should determine the accuracy of their predictions.