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

Final Report — September 2004
include HYSPLIT with EDAS/FNL input, the CAPITA Monte Carlo model with MM5,
ATAD with MM5, and ATAD with raw sounding data. In general, choice of 5, 7, or 10-day
back trajectories made little difference for this evaluation and the results were usually the
same within the standard error of the regression coefficients, irrespective of the trajectory
length. (Note that the results given here depend somewhat on the geometries of the tracer
release regions shown in Figure 9-7. Different boundaries for the regions surrounding each
tracer source may produce different perceptions of performance.)
The worst performance is by HYSPLIT with MM5 input, a combination that
dramatically overstates the Eagle Pass tracer contribution and understates that from San
Antonio. Eagle Pass is approximately 250 km from K-Bar, while San Antonio is almost
twice as distant at approximately 450 km. This may be an indication that HYSPLIT with
MM5 has too many back-trajectory endpoints close to the receptor at the expense of too few
farther away.
Other problem combinations were ATAD with EDAS/FNL input and the Monte
Carlo model with EDAS/FNL input, both of which overestimated tracer arriving from
Houston and underestimated the contribution from San Antonio. Houston is the most distant
of the three modeled release sites at approximately 750 km from K-Bar, so these latter
combinations are overestimating the most distance source area.
Results of this evaluation suggest that the best model/wind field combination for
tracer attributions, and thus for accurate back trajectory placement within south Texas, is
HYSPLIT with EDAS/FNL input. The combinations of CAPITA Monte Carlo or ATAD
with MM5 input and ATAD with raw sounding input also were able to re-create the known
tracer attributions. Because HYSPLIT with MM5 input performed poorly in both the tracer
test described in this section and the simulated sulfate tests described in Section 9.6, this
combination was not used for BRAVO sulfate source attribution. Attribution modeling using
back trajectories from either the Monte Carlo or ATAD models with EDAS/FNL input is also
suspect.
9.4 Evaluation of the CAPITA Monte Carlo Model Using Perfluorocarbon Tracer
Measurements
The CAPITA Monte Carlo particle dispersion model was tested against the
perfluorocarbon tracer measurements, in order to evaluate that model's ability to simulate
near field and synoptic scale transport and to further explore the effects of different wind
fields. The approach used for this evaluation and the results it produced are described here;
details can be found in the CIRA/NPS BRAVO report (Schichtel et al., 2004), which is
included in the Appendix.
As described in Section 8.4.1, the CAPITA Monte Carlo model is a long-range
transport model that simulates air mass transport and diffusion by tracking the movement of
multiple particles released from a source. A modeled wind field is used to advect the
particles in three-dimensional space. The vertical mixing that takes place within the
atmospheric boundary layer is simulated using a Monte Carlo technique that evenly
distributes the particles between the surface and the mixing height.
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