A simple ozone partitioning scheme is used to separate total 8-hour ozone observed at Houston, Texas during the Second Texas Air Quality Study (TexAQS2) 2006 field intensive into two components, one representing background ozone and one representing locally produced ozone. The relative importance of each component is then examined on “exceedance days” (observing >= 85 ppb 8-hour ozone per EPA guidelines) during the field intensive and these results are compared with those observed during the ozone seasons of 1998-2005.

Both background levels and local contributions are found to be rather variable from year to year, but with an overall decline in local contributions while background levels have remained relatively steady. Controlling for key meteorological variables that strongly correlate with background and local contributions we determine the extent to which this variability and decline is attributable to changing meteorological conditions, and thus by inference how much is due to changing emissions. The TexAQS2 measurements suggest that high background ozone levels may make more stringent ozone standards increasingly difficult to attain.