Reduction in surface ozone concentrations under ultra-high temperatures in central California

Central California historically experiences a substantial number of days each year with ground-level ozone concentrations that exceed state and federal air quality standards. These ozone exceedances are associated with the high summer daytime temperatures in the region, along with stagnant meteorological conditions, regional and local chemical emissions, and long range transport of ozone and ozone precursors. Studies in the eastern United States have found that surface ozone concentrations are higher when surface air temperatures are higher, with the slope of the ozone-temperature relationship either constant or increasing with higher temperatures. Analysis of observed surface ozone concentrations and air temperatures in California's Central Valley reveals that while ozone does generally increase as surface temperatures rise, ozone concentrations at ultra-high temperatures (at least 41 C) are lower than would be predicted by the trend from the lower temperature ranges. This change in the surface ozone-temperature relationship at such high temperatures could be attributed to differences in emissions, chemistry, meteorology, or a combination of these factors. The effect of variations in anthropogenic and biogenic emissions of ozone precursors at high temperatures was analyzed using a photochemical box model, which also takes into account any known changes in ozone chemistry with temperature. The influence of other meteorological factors at high temperatures, including the height of the top of the planetary boundary layer and the regional pattern of low-level wind trajectories, was also examined.