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

Final Report — September 2004
As indicated in Table 9-7, the model run using EDAS/FNL meteorology had the most
skill at reproducing the day-to-day variability of San Antonio tracer impacts at Big Bend,
with r 2 = 0.35, compared to r 2 = 0.14 using the MM5 winds. However, the EDAS simulation
underestimated the San Antonio tracer concentration at Big Bend by 14% while the MM5
simulation overestimated the tracer concentration by 26%. Monte Carlo modeling of the
Houston tracer with either wind field simulated the measured concentrations with about equal
skill, underestimating the average concentration by about a factor of 2 and with correlation
coefficients around 0.45.
In summary, Monte Carlo model simulations with both the MM5 and EDAS/FNL
wind fields properly transported tracer to Big Bend during the measured multi-day tracer
events, and did not transport tracer to Big Bend when the measured tracer concentration was
near the background levels for multi-day periods. The Monte Carlo model with MM5 winds
appeared to be somewhat better at simulating the near field transport (as represented by the
Eagle Pass tracer), while the EDAS winds produced better simulation of more distant scale
transport, particularly of the tracer from San Antonio.
9.5 Evaluation of the Forward Mass Balance Regression (FMBR) Method Using
Perfluorocarbon Tracer Measurements
The Forward Mass Balance Regression (FMBR) technique that was described in
Section 8.3.6 combines information from the Monte Carlo modeling with a multivariate
analysis. To test the performance of the FMBR method, it was applied to the
perfluorocarbon tracer concentrations measured throughout Texas. The analysis method and
results are summarized here; details are provided in the CIRA/NPS report (Schichtel et al.,
2004), which is included in the Appendix.
Two analyses were carried out. The first analysis tested whether the technique was
able to retrieve the tracer release site locations and tracer release rates using only the
measured ambient tracer concentrations plus estimates of air mass transport by Monte Carlo
modeling from nearly 700 potential release sites distributed throughout most of the US and
Mexico. The second analysis tested the ability of FMBR to properly attribute the
contributions from tracer source regions to total tracer concentrations at Big Bend. This
approach is similar to the test used to evaluate the TrMB method in Section 9.3.
The tracer concentrations used for these analyses are those reported from the ambient
tracer measurements, which reflect the measured concentrations minus an assumed
background from sources other than the four BRAVO release locations. Some of the
resulting concentrations are negative, which reflects the uncertainty in the measurement and
background concentration. For the northeast Texas tracer there may have been a systematic
bias in the adjusted concentrations. For example, the July average i-PPCH tracer at Big Bend
was negative. All negative values were included in this analysis, however.
9.5.1 Retrieval of Tracer Release Locations and Rates
The FMBR inversion technique was first used to retrieve the tracer release locations
and release rates of the ocPDCH tracer that was continuously released from Eagle Pass and
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