Understanding Haze in Big Bend National Park 

Big Bend Regional Aerosol and Visibility 

Observational (BRAVO) Study 

Big Bend National Park on a day with no haze 9-15-04

 

BACKGROUND 

The scenic beauty of Big Bend National Park is often spoiled by

haze that obscures its many vistas. In addition, Big Bend is one of 

the few national parks where haze has been increasing since the 

late 1980s. In 1999, the National Park Service and the U.S. 

Environmental Protection Agency carried out the Big Bend Regional 

Aerosol and Visibility Observational (BRAVO) study. Other 

participating agencies were the Texas Commission on 

Environmental Quality and the Electric Power Research Institute. 

The primary objective of BRAVO was to determine what causes the 

haze at Big Bend. The BRAVO study involved a four-month 

intensive monitoring period from July through October 1999, 

followed by a data analysis and modeling effort. 

Haze is caused by scattering and absorption of light (light extinction) 

by suspended particles in the air. Particles can stay suspended in 

the atmosphere for many days and be transported for hundreds of 

miles. The composition of the particles varies depending on their 

human and natural sources. Sulfate particles are the single largest 

contributor to haze at Big Bend NP, accounting for about half of the 

haze on the average and on the haziest days. Sulfate particles form 

in the atmosphere from chemical reactions of sulfur dioxide gas. 

Sources of sulfur dioxide include coal-fired power plants, metal 

smelters, refineries, other industrial processes and volcanoes. Dust 

from the suspension of soil particles and carbonaceous material

from forest fires also affect haze at Big Bend. The haze in Big Bend 

NP peaks in the spring months (April–June), when both sulfate and 

carbonaceous particles are the dominant contributors, and the late 

summer and early fall months (August–October) when sulfate 

particles alone dominate. The least hazy conditions occur during 

the winter months.

Typical Haziest BRAVO day 

Typical Least Hazy BRAVO day 

Sulfates contribute approximately

50% of the light extinction at Big 

Bend National Park 
Figure 1: Estimated contributions by particulate sulfate source regions to Big Bend particulate haze levels for the 20%

haziest days and the 20% least hazy days of the BRAVO study period. 

 

20% Haziest BRAVO Days

Other

Mexico

7%

Carbόn

9%

Texas

11%

Eastern

 US 22%

Other

2%

Western

US 4%

NonSulfate

Haze

45%

Sulfate Haze

55%

Average Particulate Haze = 56 1/Mm

20% Least Hazy BRAVO Days

Carbόn

8%

Other

Mexico

11%

Texas

4%

Other

5%

Western

 US 7%

Eastern

 US 5%

NonSulfate

Haze

60%

Average Particulate Haze = 10 1/Mm

Sulfate Haze

40%

 

BRAVO Analyses 

After the field measurement period, laboratory analyses of the air samples 

were completed. These analyses yielded the concentrations and chemical 

composition of the atmospheric particles and concentrations of unique 

tracer compounds that were released to assess transport. A number of 

data analysis methods were used to ascertain what source regions were 

contributing to the sulfate haze at Big Bend National Park. Some methods 

involved examining the relationships between different measured chemical 

components and unique tracer compounds, while others were based on 

numerical models of the meteorology, pollutant transport, and chemical 

reactions of the atmosphere. One aspect of the BRAVO study was the 

innovative way numerical atmospheric modeling was reconciled with the 

measured chemical compounds to give a more accurate assessment of the 

contributors to sulfate haze. Figure 1 summarizes the contributions on the 

haziest and least hazy days during the study. 

BRAVO Findings 

• On average, during the 

study period more than half 

of the sulfate at Big Bend 

National Park came from 

the U.S., in particular from 

the eastern U.S. and Texas. 

• On average, Mexican 

sources contributed just 

over a third of the sulfate. 

• The Carbon I and II power

plants in Mexico contributed 

about one-fifth of the total 

sulfate measured at Big 

Bend National Park. 

• Eastern US and eastern 

Texas sources were the 

largest contributors to peak 

particulate sulfate episodes 

during BRAVO.

• Airflow from eastern Texas 

and the eastern U.S. is 

most frequent during late 

summer and early fall 

months, when sulfate 

contributes most to haze.

• Mexico and the western 

U.S. were the largest 

contributors on the least 

hazy BRAVO days.

• The least hazy days were

frequently associated with 

transport from the west.

Next Steps for Improving Visibility at Big Bend National Park 

In general, sulfur dioxide emission reductions should help reverse the trend 

for worsening visibility in the Big Bend region. This is particularly true of 

sources in Texas and the eastern U.S, given the significance of the 

contribution from those regions. Sulfur dioxide emission reductions from 

the western U.S and Northern Mexico would help maintain and improve the 

least hazy days. 

There are current and pending federal regulations that should reduce sulfur 

dioxide emissions throughout the United States and make significant 

progress toward improving visibility in Big Bend National Park. 

Although sources in Mexico are also shown to contribute to visibility 

impairment at Big Bend National Park, the U.S. government has no 

jurisdiction there. There have been partnerships between agencies in 

Mexico and the U.S. to address transport of pollution. Such a partnership 

might be of use in the Big Bend Region.

For More Information on the BRAVO study go to: www2.nature.nps.gov/air/studies/bravo/index.htm