Photochemical model simulations of air quality for Houston–Galveston–Brazoria area and analysis of ozone–NO x –hydrocarbon sensitivity

Abstract

As part of the State Implementation Plan for the Houston–Galveston–Brazoria (HGB) area which has been declared as a non-attainment area, it was required to assess the impacts of pollutant emissions, meteorological conditions, and initial and boundary conditions on air quality. In this study, photochemical model simulations using the Comprehensive Air Quality Model with extensions (CAMx) were conducted for three ozone episodes between May and October of 2006. The CAMx simulations compared fairly well with the hourly average concentrations of ozone measured at the monitoring sites, and the index of agreement (IOA) values for all the three ozone episodes were found greater than 0.65, which proved to be good for CAMx model performance. Both simulated and observed peak ozone concentrations were observed to occur between 12:00 and 15:00 h. The CAMx simulation results showed that there were 68 days where the maximum 8-h mean ozone concentrations exceeded the NAAQS of 75 ppb out of a total of 72 days simulated in the three ozone episodes. For all three episodes, ozone concentrations were highest during the third quarter of the day. The photochemical indicator ratios of hydrogen peroxide (H2O2) to nitric acid (HNO3) ([H2O2]/[HNO3]) showed the HGB area to be a nitrogen oxide (NO x )-sensitive regime for the episodes studied. This indicated that volatile organic compounds (VOCs) are plentiful, and NO x is the limiting precursor for ozone formation in the HGB area. The plentiful VOCs in this region are possibly produced from industries and refineries in the HGB area.