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

Final Report — September 2004
For the 10-day simulations, apportionment estimates for all but the northeast Texas
source region are within the standard error of the REMSAD results. Simulations with shorter
trajectories reproduced the REMSAD apportionments in fewer source regions.
In the lower part of Table 9-10, it is interesting to note that the attribution to all of
Texas is overestimated by about the same amount that the attribution to the western U.S. is
underestimated, while the attributions for the eastern U.S. and Mexico source regions
properly reproduce the REMSAD result (i.e., they’re within one standard error). The cause
for the Texas estimating error was explored, and it was found that the FMBR technique has a
tendency to increase the attributions to closer source regions at the expense of more distant
source regions. Also, when included, boundary condition contributions tend to be incorrectly
attributed to the closer source regions. When these potential biases are taken into account,
the FMBR method can be a useful method for regional source apportionment.
9.8 Evaluation of REMSAD Simulations of Perfluorocarbon Tracers
We now turn to assessing the performance of the regional air quality models used for
source attribution in the BRAVO Study. These models were evaluated against the
perfluorocarbon concentrations measured during the tracer study and against ambient sulfur
measurements at Big Bend and throughout Texas. The tracer evaluations assessed the
performance of the regional modeling system (i.e., the air quality model and the
meteorological field) when simulating transport and diffusion. Since the perfluorocarbon
tracers are inert gases, the tracer evaluations did not assess the ability of the modeling system
to simulate chemical transformation or deposition.
This section describes the evaluation of the REMSAD system against the
perfluorocarbon measurements. A similar evaluation for the CMAQ system is described in
Section 9.10. Evaluations of the REMSAD and CMAQ-MADRID modeling systems against
ambient sulfur measurements, which also test the chemical transformation capabilities of the
models, will be presented in Sections 9.9 and 9.11, respectively.
In order to evaluate its performance at simulating the transport and diffusion of inert
emissions, the REMSAD regional air quality model was used to simulate the four
perfluorocarbon tracers that were released during the BRAVO study (as described in Section
3.2. The configuration of REMSAD for the tracer simulation was similar to that used for the
base emissions simulation described in Section 11.1, except that 1) the chemistry mechanism
was not invoked, since the tracer did not undergo chemical transformation, 2) loss via
depositional settling was not considered, since it was assumed that the tracers have very low
deposition velocities, and 3) background concentrations were set to zero. The grid scale of
the model was 36 km. Horizontal winds, temperature, and other meteorological fields were
simulated by MM5. Details of the evaluation are provided in the CIRA/NPS report
(Schichtel et al., 2004), which is included in the Appendix.
Each of the four tracer releases was treated as a point source emission. Two of the
tracers were released in the stacks of power plants, the northeast Texas tracer (PPCH) at the
Big Brown power plant and the Houston tracer (PTCH) at the Parish power plant, and thus
were lofted higher into the boundary layer due to buoyant and momentum plume rise.
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