| dri | HISTORIC U.S. AIR-QUALITY
STUDY REVEALS CAUSES OF HAZE AT NATIONAL PARK IN TEXAS |
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BRAVO FINDINGS Big Bend is remotely located relative to major source regions of particulate matter. Potential for a source region to contribute to increased particulate results when blocks of air linger over these regions for extended periods and pass over the park. Composition and concentration of particulate matter in park haze demonstrate considerable interannual variation. In addition, considerable interannual variability exists in potential combinations of source regions likely to contribute to park haze throughout the year. For most years, appearance of coarse particles and fine soil tends to be greatest between February and July. Air flow in the first months of this period comes from the west, including the southwestern United States and northwestern Mexico. Carbonaceous particles contribute most often to the haze during this period due to smoke from large seasonal fires in Mexico and Central America. This smoke has been documented as the source of some of the most intense episodes of haze and may be responsible for much of the carbonaceous particulate matter in park haze. Sulfate compounds are often the largest contributor to particulate haze throughout the year, e specially in late summer and fall. No single source or source region dominants seasonally or monthly averaged contributions of particulate during this period. However, some multi-day episodes of elevated concentrations of particulate sulfate emanate predominantly from a single-source region . T he most-intense episodes of haze occurring at the park are associated with relatively infrequent air-flow patterns that are capable of transporting substantial particulate sulfate from U.S. sources. Sources of sulfur-dioxide emissions in Texas and in states east of Texas contribute more particulate sulfate during intense haze episodes than do states west of Texas. In spring, summer, and fall, frequent air flow from the southeast results in greater contributions of particulate sulfate from sulfur-dioxide sources in northeastern Mexico than from the United States. Because Big Bend is frequently upwind from these sources, it is thought that Mexican sources contribute more particulate sulfate to haze than do U.S. sources over a yearly average. Further, the Mexican Carbón power plants—about 225 km east-southeast of Big Bend—contribute more than any other single facility to the average particulate-sulfate concentrations.
Clear visibility at Big Bend occurs most often in the winter, with air emanating from the north or west of the park flows over areas that produce relatively low emissions. In addition, climatic conditions associated with the rapid flow of marine air from the Gulf of Mexico over northeastern Mexico produce occurrences of clearest visibility. For additional information, |
Sulfates,
organic carbon, and coarse particulate matter are responsible for most
of the Big Bend haze . As shown in the pie charts, p articles composed
of light-absorbing carbon (LAC), soil, and nitrates are relatively minor
contributors to park haze. Spring and early summer bring carbonaceous,
sulfate, and coarse particulate matter that contribute substantially
to haze and create the periods of greatest park haze. In late summer
and fall, particulate sulfate compounds comprise the larger fraction
of haze. Episodes of high haze interspersed with relatively clear periods
are experienced in late summer and fall, and winter is generally the
clearest time of year. 
During
the summer, particulate matter is transported by easterly winds. Coarse
particles (primarily from soil) and fine soil are carried frequently
by winds from the southeastern United States, northwestern Mexico, Gulf
of Mexico, and Africa. 