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.