Cluster Analysis of Surface Winds in Houston, Texas, and the Impact of Wind Patterns on Ozone

The city of Houston, Texas, is near a complex coastline and numerous petrochemical plants, the combination of which plays a large role in Houston’s air pollution events. It has long been known that the thermally driven afternoon onshore flow (sea breeze or gulf breeze) transports ozone-rich air inland. As a way of quantifying the role of the gulf breeze in Houston’s high-ozone events, cluster analysis of hourly averaged surface winds from a regional network of meteorological sensors was performed for 27 summer days of 2000, with the dates coinciding with the Texas Air Quality Study 2000 (TexAQS 2000). Hourly averaged winds were partitioned into 16 independent clusters, or wind patterns, while simultaneously keeping track of the maximum ozone in the network for each hour. Clusters emerged that represented various wind patterns, including thermally driven flows, stagnant winds, and a thunderstorm outflow. All clusters were used to assess which wind patterns were most likely to be coincident with the maximum ozone of the day. High ozone was most likely to occur with clusters representing the gulf breeze. Clusters occurring before the ozone maximum of the day were analyzed to determine which sequences of wind patterns were most likely to precede high ozone. A transition from offshore flow to onshore flow, with at least 1 h of stagnant winds in between, routinely occurred in the 6 h preceding ozone measurements reaching ≥ 120 parts per billion by volume (ppbv). On nontransition days with high ozone, ozone maxima ≥ 120 ppbv often occurred the hour after a wind direction shift of greater than about 45°.