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

Final Report — September 2004
Figure 9-19. Smoothed REMSAD
simulation of ground-level sulfate
concentrations at 1700 CDT on 28
September 1999. Note the small
cloud of elevated sulfate
concentrations to the east of Big
Bend.
Table 9-12 showed that intersite correlations of observed sulfate, ranging between
0.81 and 0.89, are considerably better than those for SO 2 . This is to be expected, as SO 4 has
a relatively long residence time in the atmosphere as compared to SO 2 , and the SO 4 = plume
that impacts Big Bend NP is likely to be more dispersed with weaker spatial gradients.
The final variable considered here is TS, which is the concentration of sulfur that is
contributed by both SO 2 and SO 4 = . According to Table 9-12, mean normalized biases for TS
at Big Bend range from 40 to 91%, a consequence of the overestimation of SO 2 , and mean
normalized errors are 62 to 206%. Correlation coefficients for TS are between 0.29 and 0.34,
values that are slightly higher than those for SO 2 and SO 4 . The magnitudes and timing of the
predicted TS concentrations appear to match the observations relatively well (although there
is a significant amount of missing data in July) except for October, when the amount of
sulfur predicted at Big Bend NP is significantly greater than the observed concentrations.
Also, it is interesting that the timing of the mid-October TS peak is correct, contrasted with
the premature occurrence of the simulated SO 4 peak, possibly indicating that estimated SO 4
oxidation rates are too rapid during this period.
A similar performance analysis was carried out for air quality monitors located along
the northern, eastern, southern, and western peripheries of the BRAVO monitoring network.
The monitoring sites considered include 1) the Wichita Mountains site, a Class I area in
southern Oklahoma, 2) the Big Thicket site in eastern Texas near the Louisiana border, 3) the
Laguna Atascosa site at the southern end of Texas near the Mexico border, and 4) the
Guadalupe Mountains National Park site, another Class I area in western Texas. SO 2
concentrations are overstated by REMSAD at all four sites, especially at Big Thicket, while
the SO 4 = time series shows much better agreement between the observations and predictions.
Details are provided in the CIRA/NPS report on the BRAVO Study
9-43