impact of surface wind patterns on surface ozone in Houston, Texas as revealed by cluster analysis

Results from the Texas Air Quality Study 2000 (TexAQS 2000) indicate that the air quality chemistry in Houston is in many ways unique when compared to other American cities. This is, in large part, because of the numerous petrochemical plants located in and around Houston.

In addition to the enhanced petrochemical emissions, Houston’s proximity to the coastline of Galveston Bay and the Gulf of Mexico complicates the air quality situation. The mesoscale circulations driven by the land-water contrast play a role in the transport of ozone and ozone precursors. The following conceptual model is based on case study experience. During morning hours with offshore flow, ozone and its precursors will be transported from Houston toward the bay and gulf. During the afternoon hours when the skies are relatively clear and the winds are weak, a bay breeze will form at the Galveston Bay coastline, creating a flow reversal that transports ozone and its precursors from the bay back to the city. On the larger scale, the gulf breeze, which forms at the Gulf of Mexico coastline, provides large-scale transport into Houston.

Cluster analysis of surface winds from 22 stations in the Houston area during TexAQS 2000 (July and August 2000) quantitatively supports this conceptual model. The analysis was performed on hourly-averaged winds, while simultaneously keeping track of the maximum ozone in the network for each hour. The results, which consisted of assigning each hour to one of 16 clusters, were analyzed to determine which clusters were mostly likely and least likely to occur coincident with the maximum ozone of the day and which sequences of clusters led to the maximum ozone of the day. Additionally, a frequency-of-occurrence analysis showed which clusters were most likely to occur in the summer and when, indicating which clusters were tied to the diurnal cycle.

The results were conclusive, with clusters representing the gulf breeze most likely to occur coincident with the maximum ozone of the day. The most frequently occurring cluster has been dubbed ‘the stagnant cluster,’ with weak and variable winds throughout the network. The stagnant cluster mainly occurred at night, but its occurrence during the day coincided with the worst ozone days. Cluster analysis revealed that the sequence most likely to lead to high ozone events (ozone concentrations > 120 ppbv) in Houston was morning offshore flow, at least one hour of stagnant winds, and then onshore flow.