Evidence of lower-atmospheric ozone "sloshing" in an urbanized valley

The objective of the research that is presented here was to reveal evidence of the complete diurnal process by which high concentrations of lower-atmospheric ozone are typically transported across the Phoenix (Arizona) Valley. The spatial and temporal patterns of diurnal ozone concentrations and the associated atmospheric flow patterns were examined on three spatial scales for a two-month period during summer 1998. On 11 study days marked by high ozone concentrations, ozone and atmospheric data on the micro- and mesoscales suggest the dominant ozone transport mechanism to have been a mountain-valley thermal circulation. During daytime hours an apparent anabatic wind effectively transported high concentrations of ozone from an area of high ground traffic south of the urban center northeastward toward higher terrain. On 6 of the 11 high ozone days the temporal patterns of ozone concentration and atmospheric flow indicate the nighttime return of the same ozone from the higher terrain as a result of downslope flow, or a katabatic wind. The synoptic-scale weather pattern provided a calm, clear background atmosphere. Taken together, the upslope and downslope transports of ozone produced a "sloshing" of the aerosol along the eastern side of the valley basin, affecting the eastern metropolitan population twice daily.