Modeling high ozone episodes in the Houston-Galveston area: Lessons learned and remaining problems

High ozone episodes in the Houston–Galveston–Brazoria area for the Texas Air Quality Study 2000 (TexAQS 2000) and the Second Texas Air Quality Study (TexAQS-II) in 2006 are utilized to study meteorological and chemical conditions that lead to bad air quality in Houston-Gaveston Area in Texas. Modeling studies demonstrate high sensitivity of the air quality problems in the area to the synoptic and local flow conditions and the changing emission inputs from 2000 to 2006. Although there have been significant reductions in the general emissions of the highly reactive volatile organic compounds (HRVOCs) from the Houston Ship Channel area, where many petrochemical facilities are concentrated, some episodic release of the HRVOC emissions can cause extreme high ozone conditions. For normal emission condition days, the region can still experience bad air quality problems under the meteorological conditions when the weak northeasterly synoptic flows are delicately counter balanced by the southerly sea breeze flows leading to an elongated convergent zone straddling along the south of the Houston Metropolitan and Ship Channel areas in the afternoon. This presents a serious challenge to the current mesoscale meteorological modeling skills because the need to predicting the timing and location of such convergent zone accurately. First the meteorological simulations are improved with the high resolution land use land cover data. Then, a multi-stage four-dimensional data assimilation scheme (MS-FDDA) is utilized with various available observation data during the field experiment and the NCEP's EDAS/NMM reanalysis data to improve meteorological predictions further. After improving the initial and boundary conditions of the air quality model simulations, emission uncertainty issues are addressed to establish the appropriate levels of the anthropogenic NOx and VOC emissions from the mobile and point sources.