Characterization of the relationship of mixing height development and collapse to surface-level ozone concentrations

The eastern Tennessee ridge-and-valley region has recently experienced tropospheric ozone levels that approach the national ambient air quality standards. The objective of this research was to characterize the atmospheric mixing height and vertical structure of ozone during morning and evening at a site within the ridge-and-valley. Historically, high ozone concentrations in eastern Tennessee occurred in summer months during hot, stagnant days. Thus, this study focused on periods when the region was under the influence of high pressure systems and elevated temperatures during July and August, 1995. Meteorological and ozone measurements were made during daytime and nighttime to provide information on the development and subsequent collapse of the mixing height during morning and evening, respectively; measurements were obtained using a tethered balloon sounding system. Morning profiles were characterized by growth of the mixing height and entrainment of upper ozone reservoirs. Evening profiles depicted reduction of the mixing height and growth of the nocturnal, stable boundary layer. Ground-level ozone concentrations decreased whereas ozone concentrations at greater heights (100-300 m) remained comparatively high. This study demonstrated that the entrainment of ozone reservoirs aloft played a significant, if not dominant, role in increased ozone concentrations at the surface. Experimental data were used to construct an alternative model for mixing heights in regions under meteorological conditions of elevated temperatures and high pressure systems.