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

Final Report — September 2004
underestimated the observed concentration by more than 15%. This bias is at least partially
due to the regression analysis, which forced the intercept through zero and did not account
for errors in the source attribution results.
Table 9-23. Comparison of sulfate simulation performance of the synthesized REMSAD and CMAQ
models at Big Bend over the period 9 July through 28 October 1999.
Synthesized
REMSAD
Synthesized
CMAQ
r 2 0.85 0.89
Bias (%) -18 -16
RMS Error (%) 42 36
When the daily sulfate concentrations resulting from the synthesis approach are
compared with measured values, the simulated concentrations are not exactly correct, of
course. Of the predictions by the two models, the synthesized CMAQ results are likely more
trustworthy. The original CMAQ results compared better to the observed sulfate data and the
synthesized CMAQ results were closer to the original model estimates than those of
REMSAD. Therefore, the CMAQ source attributions required smaller bias corrections, and
relied less on the regression analysis to fit the observed data.
As will be shown in Section 11.3, both synthesized models were quite consistent, on
average, in their attributions of sulfate to different source regions throughout the United
States and Mexico. This good agreement between the two synthesized model results is an
improvement over the original model results. Therefore, the synthesized inversion appears to
account for different biases in the two models. It is not known, however, whether these
biases are due to underestimations in emissions or systematic errors in the modeled transport,
chemistry or removal processes.
Comparing the two synthesized model attribution results on a day-by-day basis
revealed some differences. Texas, Eastern U.S. and Western U.S. attribution results were
quite similar, with correlation coefficients of 0.88 and above. However, the source
attribution results for Mexico and the boundary conditions had correlation coefficients less
than 0.7. Mexico is the source region for which the CMAQ simulations used higher emission
rates than the REMSAD ones, an increase that was supported by more recent Mexican
emission inventories.
Although there are differences between the two models’ source attributions to Mexico
and the boundary conditions, but they are not large enough to result in different attribution
conclusions. Therefore, in Chapter 11 only the daily synthesized CMAQ results are
discussed in detail. The CMAQ results were chosen for several reasons:
• They conform better to the more current northern Mexico emission estimates,
• The original CMAQ results compared better to the observed sulfate data,
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