Classification of Weather Patterns and Associated Trajectories of High-Ozone Episodes in the Houston–Galveston–Brazoria Area during the 2005/06 TexAQS-II

Abstract

The 850-hPa synoptic flow patterns over eastern Texas and adjacent states during the 2005/06 Second Texas Air Quality Studies (TexAQS-II) period were classified into six groups using a two-stage clustering method. This study identifies synoptic weather patterns that are conducive to creating high-ozone events and reveals potential emission source regions leading to ozone exceedances through backward trajectory analysis. Clusters with dominant southerly synoptic flows and higher average wind speeds (C1 and C2 clusters) were related to low mean daytime ozone levels. Easterly (C3) and northerly (C5) clusters showed a higher probability to exceed the 8-h ozone standard than the other clusters. The dry and sunny postfrontal days with weak northerly or easterly weather patterns were often associated with ozone exceedances in the Houston?Galveston?Brazoria (HGB) area during September 2006. The majority of backward trajectories on ozone episode days were from the east, which brought polluted air from the Houston Ship Channel resulting in high ozone in the southwestern part of the metropolitan area. The shifting trajectories of the C3 and C5 weather patterns were associated with surface wind turning from weak northerly to southeasterly/southerly because of the development of bay/sea breeze. Pollutants were shifting inside the metropolitan area and high ozone built up in the afternoon or early evening. A small portion of trajectories of C3 and C5 that caused high ozone passed over a short distance from the south of Houston, picking up emissions from the industries along the coast.