BRAVO Study Design

The BRAVO study was designed specifically to investigate the causes of haze at Big Bend National Park and to estimate haze contributions from source regions and source types. Several important bodies of knowledge weighed heavily in the design of the BRAVO air-quality and visibility study. These included data summarized monthly regarding visibility and aerosol composition at the park and information developed during the planning stage to include emission-transport patterns, spatial distribution of sulfur-dioxide emissions, and frequency of emission transport toward the park.

Sulfate particles form in the atmosphere from chemical reactions of sulfur dioxide. Coal-fired power plants, metal smelters, refineries, other industrial processes, and volcanoes are all potential sources of sulfur dioxide. Since sulfate particles were determined to be the most significant contributor to haze in the park, sources of sulfur dioxide were of particular concern.

Coarse atmospheric particles (2.5–10 microns in diameter)from suspended soil and carbonaceous material from forest fires and other sources also contribute to diminished visibility. Because particulate carbon also contributed substantially to the park haze, an aerosol-monitoring program was designed to gather additional data regarding the types of sources responsible for carbonaceous aerosol.

The BRAVO study utilized data from a 38-site network to characterize spatial and temporal aerosol patterns in the atmosphere. In addition, upper-air measurements were made to aid in evaluating and calibrating wind models as input to air-quality models. Extensive optical measurements were gathered at Big Bend to characterize the effects of relative humidity on light scattering and fine and coarse particles on both light scattering and light absorption. Because monitoring and source characterization activities were conducted only in the United States, the study design included additional monitoring and tracer releases along the U.S./Mexican border.

Generally, visibility at Big Bend has been most impaired during May to October. A four-month span from July to October was selected as the study period to maximize opportunities to monitor impacts from various potential sulfur-source regions, including northeast Mexico, eastern United States, and eastern Texas. These regions were of particular interest because the 1996 preliminary study indicated coal-fired power plants were likely to be significant contributors to park haze. CIASTA researchers also conducted source testing to characterize a variety of emission sources, compiled an emissions inventory used for air-quality modeling, and measured light-absorbing properties of the air through photoacoustic techniques.