Assessment of a regulatory model's performance relative to large spatial heterogeneity in observed ozone in Houston, Texas

Application of geostatistical approaches to predict the spatio-temporal distribution of summer ozone in Houston, Texas

Mitigation of adverse effects of air pollution requires understanding underlying exposures, such as ambient ozone concentrations. Geostatistical approaches were employed to analyze temporal trends and estimate spatial patterns of summertime ozone concentrations for Houston, Texas, based on hourly ozone observations obtained from the Texas Commission on Environmental Quality. We systematically assess the accuracy of several spatial interpolation methods, comparing inverse distance weighting, simple kriging, ordinary kriging, and universal kriging methods utilizing the hourly ozone observations and meteorological measurements from monitoring sites. Model uncertainty was assessed by leave-one-out cross-validation. Kriging methods performed better, showing greater consistency in the generated surfaces, fewer interpolation errors, and lower biases. Universal kriging did not significantly improve the interpolation results compared to ordinary kriging, and thus ordinary kriging was determined to be the optimal method, striking a balance between accuracy and simplicity. The resulting spatial patterns indicate that the more industrialized areas east and northeast of Houston exhibit the highest summertime ozone concentrations. Estimated daily maximum 8 h ozone concentration fields generated will be used to inform research on population health risks from exposure to surface ozone in Houston.

Houston's rapid ozone increases: preconditions and geographic origins

Many of Houston's highest 8-h ozone (O₃) peaks are characterised by increases in concentrations of at least 40 ppb in 1 h, or 60 ppb in 2 h. These rapid increases are called non-typical O₃ changes (NTOCs). In 2004, the Texas Commission on Environmental Quality (TCEQ) developed a novel emissions control strategy aimed at eliminating NTOCs. The strategy limited routine and short-term emissions of ethene, propene, 1,3-butadiene and butene isomers, collectively called highly reactive volatile organic compounds (HRVOCs), which are released from petrochemical facilities. HRVOCs have been associated with NTOCs through field campaigns and modelling studies. This study analysed wind measurements and O₃, formaldehyde (HCHO) and sulfur dioxide (SO₂) concentrations from 2000 to 2011 at 25 ground monitors in Houston. NTOCs almost always occurred when monitors were downwind of petrochemical facilities. Rapid O₃ increases were associated with low wind speeds; 75 % of NTOCs occurred when the 3-h average wind speed preceding the event was less than 6.5 km h^-1. Statistically significant differences in HCHO concentrations were seen between days with and without NTOCs. Early afternoon HCHO concentrations were greater on NTOC days. In the morning before an observed NTOC event, however, there were no significant differences in HCHO concentrations between days with and without NTOCs. Hourly SO₂ concentrations also increased rapidly, exhibiting behaviour similar to NTOCs. Oftentimes, the SO₂ increases preceded a NTOC. These findings show that, despite the apparent success of targeted HRVOC emission controls, further restrictions may be needed to eliminate the remaining O₃ events.