1996 Preliminary Regional Visibility Study

Overview

As a result of concerns over declining visual air quality at Big Bend National Park, a preliminary regional visibility study was conducted in Texas and northern Mexico in September and October 1996.

The primary objective of this study was to obtain information that would allow for the identification of possible source regions in both countries and source types responsible for visibility degradation at Big Bend National Park. The study was not intended or designed as an attribution study to quantify impacts of specific sources on Big Bend air quality. The preliminary study was intended to obtain information on pollutant gradients over a broad area of Texas and northeast Mexico to assist in the design of a future study to identify the causes of visibility impairment at Big Bend National Park. The preliminary study was conducted at 19 monitoring stations in Texas and Mexico from September 9 through October 13, 1996. The stations sampled PM_2.5 at all sites and PM₁₀ at Big Bend and Guadelupe Mountains National Parks. The PM_2.5 filters were analyzed for chemical composition.

Consensus was not reached on all issues by all of the participants and care also should be taken in interpreting the results of this preliminary study due to its limited duration and geographical coverage. Key consensus results included:

• To the northeast of Big Bend are large sources of sulfur associated with selenium, likely from coal-fired power plants at distances that can exceed 700 km. These sources sometimes cause high concentrations of fine particulate and fine particulate sulfur through much of Texas, including Big Bend National Park.
• On some occasions with southerly flow, Mexican emissions appear to be associated with significant sulfur concentrations at Big Bend National Park.
• During periods with southeasterly winds, emissions from both Mexico and the United States may contribute to PM_2.5 mass and fine particulate sulfur at Big Bend National Park. Also, because of the lack of correlation between sulfur and selenium and vanadium, sources in addition to power plants are contributing to these concentrations.
• Transport from areas to the northwest of Big Bend is associated with relatively low concentrations of fine particulate mass and fine particulate sulfur.
• Relative humidity plays a large role in visibility impairment at Big Bend.
• Fine particulate sulfur plays a large role in visibility impairment at Big Bend and most of the particulate sulfur is in the form of sulfate.

The preliminary study concluded with the following recommendations:

1. A more extensive field study will be needed to quantify the impacts from specific sources to visibility impairment at Big Bend National Park.
2. The spatial domain of the study should be expanded, particularly to the northeast, the south, and into the Gulf of Mexico.
3. The design of the extensive study should be based on the findings from the final report of the preliminary regional study. The results of the preliminary study and the extensive study to follow should be analyzed in the context of historical measurements made at Big Bend National Park.

A detailed description of the study and its results are contained in the report "Big Bend National Park Regional Visibility Preliminary Study" prepared by the Big Bend Air Quality Work Group for the USEPA, National Park Service, and the Mexican governmental agencies PROFEPA and SEMARNAP on January 7, 1999.