Impact of the on-road and mobile sources on the benzene and toluene emissions and concentrations in the Houston-Galveston area

In the Houston-Galveston area, mobile sources are responsible for more than 50% of the air toxic emissions, according to the National Emissions Inventory from 1999. We performed simulations with the air toxic version of EPA's Community Multiscale Air Quality (CMAQ) system, using the SAPRC99 chemical mechanism. We focused on two aromatic toxic species: toluene and benzene. We analyzed the CMAQ simulations for three cases with different emissions inputs: 1) all the emission source categories; 2) all the source categories except the on-road sources; 3) all the source categories except the mobile sources (on-road and non-road sources). By comparing the three cases we evaluated the impact of the on-road and mobile sources on benzene and toluene emissions and their impact on average atmospheric concentrations. We performed simulations for the Aug. 22 – Aug. 31, 2000 modeling episode at the 4-km resolution (83x65 cells), which covers the southeastern part of Texas, a part of Louisiana and the Gulf of Mexico. When on-road sources are not considered, the emissions of the above species drop by 15-40%. When mobile sources are eliminated, the benzene and toluene emissions decrease by 40-80%. In the absence of on-road sources, concentrations drop by 15-30%, while in the absence of mobile sources they drop by 35-50%. To illustrate the accuracy of the CMAQ simulations, we also compared the simulation results when all sources are considered with three observational datasets: the surface Clinton site data, the surface La Porte site data and the Baylor aircraft data.