Analysis of the climatic mechanism contributing to the summertime transport of lower atmospheric ozone across metropolitan Phoenix, Arizona, USA

Abstract

High concentrations of lower atmospheric ozone can adversely affect the health of humans, plants, and animals. Over the past decade, unhealthy levels of ozone across the Phoenix, Arizona (USA) urban area have been a focus of attention for the United States Environmental Protection Agency (EPA) and local government agencies. As ozone concentrations exceeding the standard set forth by the EPA occur in a preferred location within the Phoenix metropolitan area (eastern suburb of Mesa), it has become important to gain an understanding of the mechanisms that transport ozone within Phoenix and its suburbs. The objective of the study that is presented here was to examine the climatic factors that contributed to the spatial distribution of lower atmospheric ozone across the eastern portion of the Phoenix metropolitan area in mid-to-late summer 1998. Microclimatic and synoptic-scale atmospheric contributors were linked to the mesoscale transport of ozone. Forty-three study days were stratified into 3 categories: High ozone days (exceeded the EPA standard), moderate ozone days (approached the EPA standard), and low ozone days. Eleven days of high ozone were differentiated from the days of the remaining 2 categories by an atmosphere containing less water vapor, and therefore a greater surface receipt of insolation. This is not surprising given the fact that ozone production is a photochemical process. However, the movement of the lower atmospheric ozone is also an important issue. The results of the study suggest that the drier atmosphere and resultant high insolation at the surface were associated with a proximal area of high pressure aloft. Taken together, the result was warmer ground surface and overlying air temperatures, light winds, an apparent lack of turbulent lower atmospheric mixing, and light southwesterly winds at 850 mb that advected little moisture into the area. Associated with the light synoptic flow and warm surface condition on days of high ozone was a wind regime that appears to be the product of a mesoscale thermodynamic circulation. The daytime flow on high ozone days became directed upslope toward the higher elevations of the eastern Phoenix Valley. Embedded within the flow was a plume of high ozone concentrations that extended from an urban area of high ground traffic eastward into the eastern suburb of Mesa. Conversely, moderate and low ozone days were associated with an eastward displacement of high pressure aloft, greater advection of low-level moisture from the south and southeast, smaller insolation receipt, less surface heating, and a much less organized movement of lower atmospheric ozone than on high ozone days.