Science Introduction

Air quality in the eastern half of Texas has shown substantial improvement over the last twenty years in spite of a dramatic increase in the population of its major urban areas. However, within the past few years the trend of improving air quality has flattened, and several of the major urban/industrial centers in the state still do not meet either the 1 hr, or the new 8 hr NAAQS for O3. A recent sensitivity study for the Houston-Galveston area using the CAMx model suggests that dramatic reductions (65-85%) in nitrogen oxide emissions will be required to meet the standard. Similar results were obtained from a sensitivity study of the Dallas-Fort Worth area. Other major metropolitan areas within the country face a similar situation. With the new eight-hour ozone standard, and the new PM2.5 standard being implemented over the next several years, states will have to devise strategies for meeting these new requirements. A significant issue is whether sufficient scientific understanding of either the ozone or the aerosol problems exists, to develop strategies to meet either the existing or the new NAAQS, in an effective, cost-efficient way. The Texas 2000 field study will provide much needed information about the chemistry, physics, and meteorology that control the formation and accumulation of ozone and PM2.5 in the eastern half of Texas, and provide fundamental scientific knowledge that can be applied to other areas of the country with similar problems.

A significant portion of the atmospheric emissions in eastern Texas originates in the two largest urban centers, the Dallas-Fort Worth (DFW) Metroplex and the Houston-Galveston area. Each of these urban complexes extends over 100 kilometers and has a population of approximately four million people. The eastern half of Texas also has a network of large coal-fired power plants that emit substantial quantities of sulfur dioxide (SO2), nitrogen oxides (NOx), and fine particles. The upper Texas Gulf Coast is home to scores of petroleum refineries and synthetic organic chemical manufacturing plants. These industrial activities all emit volatile organic compounds (VOCs) and NOx, and some release SO2 and fine particles. The extensive vegetation and warm temperatures that are typical of this region result in large emissions of biogenic VOCs. Agricultural emissions of ammonia (NH3) are expected to be important in the formation of ammonium sulfate and ammonium nitrate particles in the region.

A number of unusual chemical and meteorological features distinguish this region from others with similar problems. These features include a mix of sources that leads to an observed VOC/NOx ambient concentration for the Houston area, which is significantly larger than the ratio derived from emissions inventories, and is also much higher than typically observed in most urban areas. There has been a suggestion that chlorine chemistry, due to the admixture of urban emissions and sea-salt aerosol from the Gulf of Mexico, or to direct industrial emissions of molecular chlorine (Cl2), or to both, may have an influence on ozone formation. This unusual precursor mix will undoubtedly influence the rate and efficiency of ozone formation relative to other urban areas and implies that a special set of strategies may be required to control ozone and PM concentrations.

figure 1

Figure 1. Ozone concentrations In Houston during the 1993 Coast Study. Upper panel shows ozone concentrations for all days. Lower panel shows concentrations on days when flow reversal occurred. Horizontal line is the 1 hr NAAQS for O3.

The unique meteorological component of the Houston O3 problem is illustrated by Figure 1 which shows the effect of land-sea breeze flow reversal on the O3 concentration. The data are from the 1993 Coastal Oxidant Study for Southeast Texas (COAST) when there was a radar wind profiler at the airport on Galveston Island. Wind measurements recorded by this profiler, were used to separate the days during the mid-July to mid-September, 1993, COAST intensive period into those with wind flow reversal and those without. Figure 1 shows that all 20 exceedances during this period occurred under conditions where there was a land/sea breeze flow reversal.

The region also affords the opportunity to study interesting and important aspects of aerosol formation and transport. Fine aerosol mass in the region is dominated by sulfate, as it is in the Southeast. However, semi-volatile organic aerosols are also important. Additionally, near the coast marine aerosols can act as nucleation centers producing internal mixtures with significantly different properties from those formed in urban and power plant plumes.

Inter- and interregional transport may have an important role in the air quality of the region. Transport along the Gulf coast from Louisiana and farther east often defines the ozone and particulate matter background in the Houston area. Transport from the Houston-Galveston, Beaumont-Port Arthur area may have a significant effect on air quality in Dallas-Fort Worth and on Northeast Texas.