# Understanding Haze in Big Bend National Park Big Bend Regional Aerosol and Visibility Observational (BRAVO) Study

Big Bend National Park on a da y with no haze 9-15-0 4



### BAC KGROUND The scenic b eauty of Big Bend Nation al Park is often spoiled b y haze that obscures its many vistas . In addition, Big Bend is one of the few national parks where haz e has been increa sing since the late 1980s. In 1999, the National Par k Service and the U.S. Environme n tal Protection Agency carri ed out the Big Bend Regi onal Aerosol and Visibility Observational (BRAVO) study. Other participating agencies we re the Texas Commission on Environme n tal Quality an d the Electr ic Pow e r R e search Institute

. The primary ob jective of BRAVO wa s to determine what causes the haze at Big Bend. The BRAVO study involved a fo ur-month intensive monitoring peri o d from July throu gh October 19 99, followed by a data analysi s and modeling effort. Haze is caused by scattering and abso r pt ion of light (light extinction) by suspende d particles in the air. Particles can sta y suspended in the atmosphere for many days and be transported for hundr eds of miles. The composition of the particle s varies depending o n their human an d n a tural source s. Sulfat e pa rticles are th e single larg est contributor to haze at Big Bend NP , accounting for about half of the haze on the aver age a nd on the haziest days. Sulfate particles form in the atmospher e from chemical r eactions of sulfu r dioxide g a s. Sources of sulfur dioxid e incl ude coal-fired powe r p l ants, metal smelters, refineries, other industr ial pro c esses and volcanoes. Dust from the suspension of soil parti cles a nd carbon aceous materia l from forest fires also affe ct haze at Big Bend. The haze in Big Bend NP peaks in the spring months (April–June), when both sulfate and carbonace o u s particles are the dom ina n t contributors, and the late summer and early fall months (August–October) w hen sulfate particles alone dominate. The l east h a zy conditions occur durin g the winter months. 

### T y pical Haz i est BRAV O da y

T y pical Lea st Haz y BRAVO da y

Sulfates contribute approximately 50 % of the l i ght extinction at Big Bend Natio nal ParkFigure 1 : Es ti ma te d c ontributio ns by pa rtic ula t e s u lfa t e s ourc e re gions to Big Be nd pa rtic ula t e ha ze le v e ls for the 2 0 % ha zie s t day s and t h e 2 0 % lea s t ha zy day s of the BR A V O s t udy pe riod.



### 2 0 % Ha z i e s t BRA V O Da y s

Ot he r M exi c o 7%

Ca rb ό n 9% T exas 11 % Ea s t e r n US 22%

Ot he r 2% We s t e r n US 4%



### N onSu l f a t e

Ha z e 45 % Su l f a t e H a z e 55%

A v er ag e P a r t i c u l at e H a z e = 5 6 1 / M m 

### 2 0 % L e a s t Ha z y BRA V O Da y s

Ca rb ό n 8% Ot he r M exi c o

11 % T exas 4%

Ot h e r 5% We s t e r n US 7% Ea s t e r n US 5%



### No n Su l f a t e

Ha z e 60%

A v er ag e P a r t i c u l at e H a z e = 1 0 1 / M m 

### Su l f a t e H a z e 40 %



### BRAVO Anal y s es After the field measurem ent period, la borator y an alyses of the a i r samples were comple ted. These a nalyses yi eld ed the concentrati ons and chemical composition of the atmospheric par ticles and concentrations of unique tracer compounds that were released t o assess tra n sport. A nu mber of data anal ysis methods were used to a scertain what source regio n s were contributing to the sulfat e haze at Big Bend Nation al Park. Some methods involved exa m ining the re lationships between differ ent measure d chemical components and uniq ue tr acer compou nds, while others were b a s ed on numerical models of the meteorolog y, pollutant transport, and chemical reactions of the atmosphe re. One aspect of the BRAVO study was the innovative w a y numerical atmospheric modeling w a s reconciled with the measured ch emical comp ounds 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 aver age, during the

study period more than h a lf of the sulfate at Big Bend National Par k came from the U.S., in particular from the eastern U.S. and Texas.

• On aver age, Mexican

sources contributed just over a third o f the sulfate.

• The Carb on I and II power

plants in Me xico contributed about one -fifth of the total sulfate measured at Big Bend Nation al Park.

• Eastern US and eastern

Tex a s sources w e re the largest contributors to pea k particulate sulfate episodes during BRAV O.

• Airflow from eastern Tex a s

and the eastern U.S. is most frequen t 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

freque ntly associated with transport from the west. 

### Next Steps for Improving Visib ilit y at Big Bend National Park In general, sulfur dioxid e emission r eductions should help re vers e 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. Sulf ur dioxide emission reductions from the western U.S and Nort hern Mexico would hel p maintain and improve the least hazy days.

There ar e current and p e nding fede ral r egulations that s hould re duce sulfur dioxid e emissions throughout the Un ited States and make significant progress toward improving visibilit y in Big Bend National Park. Although sources in Mexico are also shown to contribute to visibility impairment at Big Bend National Pa rk, the U.S. government has no jurisdiction there. There h a ve be en pa r t nerships between a genci e s in Mexico and the U.S. to ad dress transpor t of pollution. Such a p a rtnership might be of use in the Big Bend Regi on

.



### For More In formation on the BRAV O stud y go to: www2.n ature.nps.gov/air/studies/bravo/index.htm