Ozone is a secondary airborne pollutant resulting from the photochemical reactions of volatile organic compounds (VOCs) with oxides of nitrogen (NOx). Elevated concentrations of tropospheric ozone have adverse effects on public health, agriculture, and the environment. To formulate effective regulatory strategies for attainment of the National Ambient Air Quality Standard (NAAQS), detailed knowledge of regional and local ozone buildup processes is necessary.

The Houston, TX region is classified as being in moderate non-attainment of the NAAQS for 8-hr ozone. Adequate understanding of ozone formation processes for the Houston region remains lacking, however, impeding the development of optimal control strategies. The complexity and uniqueness of the ozone problem for Houston is due to both highly variable meteorology, owing to its proximity to the Gulf Coast, and the intense emissions of precursors from the petrochemical industry. The VOC emissions from the Houston Ship Channel allow for increased precursor reactivity, generally favoring increased rates of local ozone formation. Its proximity to both the Gulf of Mexico and Galveston Bay results in Houston experiencing a variety of seabreeze circulations. The seabreeze flows, which occur with varied timing, are superimposed on the larger, synoptic flow patterns influencing the entire Gulf Coast. These seabreeze flows tend to introduce a recirculating effect in the Houston area, generally favoring elevated ozone levels.

As with most urban regions, ozone levels around Houston can vary widely between different days; NAAQS exceedances typically occur under favorable (and often recurring) emissions and/or meteorological scenarios. Intense, unplanned industrial emission events are known to trigger elevated Houston ozone levels; the timing (hour of day) for such releases greatly impacts whether or not an exceedance occurs. For days lacking such accidental VOC releases, however, meteorological processes largely determine the achieved ozone levels.

In this study, we use cluster analysis and sequence analysis for the identification and characterization of a number of distinct, recurring, mesoscale flow scenarios conducive to ozone exceedances. Cluster analysis is an unsupervised form of statistics which identifies recurring patterns among a set of observations. Sequence analysis is an approach to data reduction based on the concept of a sequence similarity index. These statistical methods are applied to hourly surface wind field measurements to model the evolution in time as each day progresses through a series of mesoscale flow regimes. Beceause of its quantitative nature, this statistical approach is useful for the investigation of meteorological trends over an extended observation period. After assigning the days among a number of meteorological regimes, air quality measurements are overlaid to determine the response of Houston area ozone levels and infer mechanisms affecting transport and dispersion.

Our study focuses on the Houston area extended ozone season of 1 April through 31 October of the year 2004, for a total of 214 consecutive days. Measurements are available from two separate networks of ground level monitoring stations operated by the Texas Commission for Environmental Quality (TCEQ). All data are reported at an hourly rate, though missing measurements are a common problem associated with analysis of environmental measurements over such an extended observation period. The first network, the Continuous Air Monitoring Stations (CAMS), monitors ozone and NOx concentration levels. A second meteorological monitoring network records hourly, ground-level wind speed, wind direction, and temperature.

Cluster analysis is first applied to hourly wind measurements to determine a number of surface flow patterns; each hour of the study period is assigned to 1 of 10 identified surface flow patterns. Then a quantitative sequencing technique is applied to generate groups of days sharing similar 24 hr sequences (0000-2300 CST) of hourly flow patterns. Separate sequence analyses are performed for the exceedance and the non-exceedance days constituting the 2004 ozone season. The connection between the identified meteorological groups and air quality measurements are considered to infer physical processes affecting ozone production and buildup mechanisms. Spatially distributed, routine air quality measurements (ozone and NOx levels) are overlaid on the meteorological groups to infer transport and dispersion mechanisms affecting ozone levels in the Houston region. Additionally, weather maps at the 500-hPa pressure level are used to determine the effects of synoptic conditions on Houston flow regimes.

Our study indicates that recurring meteorological scenarios have a strong influence on regional ozone levels. Exceedances tend to occur under anti-cyclonic conditions in which strong seabreezeactivity contributes to the recirculation of pollutants in the Houston area. Resulting transport and dispersion patterns under such conditions are a superposition of the prevailing synoptic motions and shifting flows associated with the seabreeze. These different meteorological scenarios result in distinct source-receptor relationships that are associated with spatially localized ozone buildup in distinct portions of the study domain. Non-exceedance days exhibit a wider variety of meteorological conditions than the exceedance days. Days with strong synoptic influences and lacking strong sea breeze development tend to have lower ozone levels. These scenarios evidence that recirculation is involved in triggering many ozone exceedances in the Houston region.