An analysis of the vertical structure of the atmosphere and the upper-level meteorology and their impact on surface ozone levels in Houston, Texas

Abstract

Despite emission reductions, Houston continues to be designated as a nonattainment area for ozone (O 3) by the Environmental Protection Agency. Upper-level synoptic maps and information about the vertical structure of the lower troposphere obtained by in situ measurements were analyzed to characterize ozone exceedances in which peak 8-h average concentration exceeded 85 ppb during the Texas Air Quality Study-II in August-September 2006. Cluster analysis of meteorological conditions showed that the highest background surface O 3 concentrations occurred under northerly or easterly flow regimes at 850 hPa, coinciding with the advection of dry continental air. Exceedance days in September 2006 occurred almost exclusively in postfrontal environments. These frontal passages are associated with shifts in wind direction and may lead to increases in background O 3 from 30 ppbv (marine) to 60-70 ppbv (continental) throughout the lower troposphere. Several factors are identified to be important for 8-h average ozone peaks in Houston under well-developed land-sea-bay breeze conditions, including (1) the presence of easterly winds advecting industrial emissions from the Ship Channel, and (2) the presence of persistent large-scale northerly flows aloft advecting elevated continental background ozone levels that are eventually entrained into lower layers through the growth of the convective planetary boundary layer. Copyright 2008 by the American Geophysical Union.