(BRAVO) Study: Final Report. - Desert Research Institute

Final Report — September 2004
As was the case with all sulfur episodes, the passage of weather fronts preceded this
episode. On August 29-31 a cold front oriented east-west was driven southward and even
slightly westward by the clockwise circulation around a high pressure system centered in
Manitoba. This “back-door” cold front would be conducive to transport of air from the Ohio
River Valley into Texas. (The meteorology of this episode is discussed further in Section
5.4.)
Later episodes, starting in mid-September, had flow from the southeast as the front
approached, followed by light northeasterly and easterly flow after passage. Little if any
transport from the Ohio River Valley would be expected under these conditions, but transport
from eastern Texas and nearby areas would be expected. Back-trajectory analyses and
dispersion modeling support these conclusions, as shown later in this report.
Thus some episodes at Big Bend seemed to be related to emissions from within
Texas, while others were affected by sources farther to the east or to the south. The largescale
meteorology suggests that air quality at Big Bend may be affected by sources in the
eastern and midwestern United States during some periods.
Further insight into the spatial and temporal patterns of the particulate matter
concentrations is provided by the animations that are described in Section 8.6 and are
included in the Appendix. Additional discussion of study period meteorology appears in
Section 5.4.
5.1.2 Properties of Particulate Matter at Big Bend National Park
The IMPROVE sampling system was used at the K-Bar site in Big Bend National
Park to collect samples of particulate matter with aerodynamic diameters smaller than 2.5
and 10 µm (PM 2.5 and PM 10 , respectively). Samples were analyzed to determine
concentrations of elements, ions, carbonaceous material, and gravimetric mass less than 2.5
and 10.0 µm. The filter samples were collected each day starting at 8:00 a.m. In addition,
size resolved particle measurements were made using an 8-stage Micro Orifice Uniform
Deposit Impactor (MOUDI) sampling system that covered the same size range, and
substrates were analyzed for inorganic species, including sulfate, nitrate, and ammonium
ions.
The findings from the measurements of the IMPROVE and MOUDI samplers were
combined to develop a complete description of the ambient particulate matter. The data
analyses that were carried out are described in detail in the CIRA/NPS report on the BRAVO
Study (Schichtel et al., 2004), which is included in the Appendix. We summarize here the
procedures that were followed and the key findings.
The fine PM species at most continental sites can be classified into five major types:
sulfates, nitrates, organics, light-absorbing carbon (LAC), and soil. Other fine species such
as non-soil potassium, sea salt, and other trace elements are less important from a visibility
standpoint there. Most fine sulfates are the result of oxidation of SO 2 gas to sulfate particles.
5-4