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

Final Report — September 2004
Big Bend is remotely located relative to the major sources that contribute to the
particulate matter responsible for its haze. That is not to say that occasional visibility
impairment from nearby disturbed soil, wind-suspended dust, or smoke from local fires
does not occur, but these are not the predominant sources of haze at Big Bend. All
other things being the same, a source region’s potential to contribute to haze at Big
Bend increases for time periods when air parcels frequently pass over and spend more
time over the source region prior to transport to Big Bend.
Hourly five-day-long trajectories of air that arrives at Big Bend National Park were
calculated for a five-year period (1998 to 2002). Each trajectory gives estimates of the
locations of air parcels every hour of the five days prior to their being transported to Big
Bend. Residence time analysis uses these trajectories for selected periods of time (e.g., a
month) or selected receptor site conditions (e.g., haziest days at Big Bend) to estimate the
frequency of air parcel transport and its duration over various potential source regions prior
to arriving at Big Bend. Histograms of the five-year average and each individual year’s
monthly residence times for different discrete potential source regions in the U.S. and
Mexico are shown in Figure 12-7.
40
Mexico
40
Eastern U.S.
40
Texas
30
30
30
20
20
20
10
10
10
0
J F M A M J
J A S O N D
0
J F M A M J
J A S O N D
0
J F M A M J
J A S O N D
40
Western U.S.
40
Gulf of Mexico
40
Other
30
30
30
20
20
20
10
10
10
0
J F M A M J
J A S O N D
0
J F M A M J
J A S O N D
0
J F M A M J
J A S O N D
Figure 12-7. Monthly fractions of time that air parcels spend in a region prior to arrival at Big Bend in 1998 to
2002 are shown as solid bars, based on five-day back trajectory calculations. The short horizontal lines indicate
the values for each of the five yeasr that make up the five-year average bars. The plot labeled “Other”
represents locations beyond the five regions shown here.
12-17