Houston’s rapid ozone increases: preconditions and geographic origins

Abstract

Environmental context Ozone pollution in Houston, Texas, has been a public health concern for decades. Unusually large hourly changes in observed ozone concentrations have been correlated with a greater likelihood of violating the federal air quality standard. We investigate the geographic and chemical origins of these large hourly increases, which should help regulators better control ozone violations. Abstract Many of Houston’s highest 8-h ozone (O3) peaks are characterised by increases in concentrations of at least 40ppb in 1h, or 60ppb in 2h. 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 6.5kmh–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.