Modeling ozone formation from industrial emission events in Houston, Texas

It is now generally accepted that industrial emission events are occurring from chemical processing facilities in Houston, Texas with daily frequencies and significant temporal variability. These events have been reported to last from hours to days, and a large fraction are made up of four highly reactive volatile organic compounds (HRVOCs): ethene, propene, butenes, and 1,3-butadiene. The Texas Commission on Environmental Quality (TCEQ) has targeted industrial sources of HRVOCs by imposing annual and hourly limits and creating a market-based HRVOC emissions cap and trade (HECT) program. The HECT program uses the Maximum Incremental Reactivity (MIR) scale to calculate the magnitude needed to trade between HRVOCs. The work reported here used the TCEQ regulatory model to evaluate the HECT programʹs use of the MIR scale by simulating a series of hypothetical but observational-based industrial emission events for different VOC species. The magnitude of each release was adjusted based on the MIR scale under the assumption that this would give the same increase in ozone. The regulatory model, however, predicted that o-xylene caused the largest increase in ozone. In every simulation, ozone production was directly related to the amount of hydroxyl radicals produced from the photolysis of formaldehyde and other aldehydes. The sensitivity of ozone production to these hydroxyl radical sources appeared regardless of whether the industrial emission event plume encountered high sources of NOx. The MIR scale was developed for an average urban atmosphere and its failure in equating ozone reactivity here may be due to the extreme levels of NOx and VOC seen in event emissions in Houston.