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

Final Report — September 2004
concentrations in the eastern portion of the domain. 3 On the other hand, at the twelve
stations where the mean normalized bias represented a lesser portion (on average ~30%) of
the mean normalized error (33-78%), eleven are located in the vicinity of Big Bend National
Park and/or near the US-Mexico border, showing a tendency of the model to underestimate
more often in these regions compared to other regions of the domain. 4
Particularly severe mean normalized bias and error are apparent in Figure 9-25 at two
locations in southern Texas, quite distant from Big Bend. Since poor model performance in
southern Texas is not necessarily indicative of poor model performance at Big Bend, the
impact of these two locations on the statistics in Table 9-19 was explored. Since Table 9-19
is based on 37 stations, removing two stations would not be expected to have a large effect,
and this turns out to be true. Specifically, for sulfate the effect is to slightly reduce the mean
concentration to 3.91 µg/m 3 , increase r 2 to 0.49, and slightly decrease the bias and the error
metrics. The biggest relative change is a decrease in mean normalized error from 65% to
61%. The effects on PM 2.5 statistics are even smaller.
For the duration of the BRAVO study and across the entire BRAVO Network,
particulate sulfate accounted for 62% of the observed total sulfur concentrations on average,
comparable to a sulfate fraction of 61% for the predicted total. About 3/4 of the predicted
sulfate fractions lie within 20 percentage points of the observed values. Both data sets show
that high sulfate concentrations tend to be associated with high sulfate fractions.
This discussion indicates that model performance for sulfur differed in the eastern and
western parts of the modeling domain. We can explore this difference by comparing
performance at the K-Bar and Big Thicket stations. The K-Bar station at Big Bend National
Park, apart from being the focal station of the study, is representative of the region where the
model was more likely to understate concentrations of sulfur species. The Big Thicket
station in eastern Texas is representative of the eastern portion of the domain, where the
model showed high positive bias in simulated concentrations of sulfur species due to
consistent overestimates in the region. Tables 9-20 and 9–21 provide performance statistics
for these two locations.
3 The predicted signal at the eighth station, Eagle Pass in southern Texas near the Mexican border, is
affected detrimentally by inclusion of the Carbón power plants in the same model grid cell.
4 At the twelfth station, Wichita Mountains, less than 30% of the potential data set was valid, which
precluded any significant analysis.
9-56