Eight-hour ozone exceedances in eastern Texas

A conceptual model is developed for exceedances of the new 8-hour federal ozone standard for Houston and other parts of eastern Texas. Ozone levels on any given day are assumed to consist of background ozone levels (the 8-hour maximum ozone that would occur in the absence of local anthropogenic sources of precursors) and local contributions to ozone. Background levels are estimated as the lowest 8-hour maximum ozone level from a set of sensors that, in ideal circumstances, are on the periphery of a given metropolitan area and are not unduly influenced by local sources of NOx. The local contribution, as measured by a monitoring site, is the difference between the maximum 8-hour ozone measured at that site and the metropolitan area's background ozone.

Background ozone has a strong seasonal dependence. Background ozone maxima occur in mid-spring and late summer. A principal component analysis of regional wind patterns shows that background ozone levels are highly correlated with the leading principal component, which in its ozone-favoring phase involves transport from the central and eastern United States and in its ozone-lowering phase involves transport from the Tropics. The average value of this principal component reaches an extremum in September, nearly coinciding with the time of highest background ozone. The ozone minimum in early summer, which is most pronounced at southern stations, is associated with a period of low wind pattern variability. High background-ozone wind patterns also occur occasionally in the spring, but the maximum at that time largely reflects a midlatitude ozone maximum associated with wintertime precursors and long ozone lifetimes.

The local contribution to ozone is much more closely related to local weather, including temperature, wind, and precipitation. In Houston, while calm winds are desirable, high local contributions are favored whenever the background winds are light enough to support wind reversals by the sea breeze effect. At this latitude, the diurnal land-sea contrast triggers a broad inertial oscillation and a rotation of the low-level wind vectors. The background wind direction governs the timing of periods of stagnation and recirculation within the day.