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

Final Report — September 2004
main exception is a period of a few days in October when REMSAD sulfur concentrations
from all source areas are under-predicted by TrMB. This indicates that there was something
non-average about the chemistry and/or deposition on those days. In the case of October 4
(the point with the highest sulfate attribution), the deviation from the line indicates that the
sulfate attribution to Texas on this day could be underestimated by TrMB by nearly a factor
of 5, even though on average, the mean attribution to Texas was within one percentage point
of being correct. In the REMSAD model results, which are “truth” for this test, both October
4 and 5 have higher than average ozone concentrations over southeast Texas and an unusual
period of no precipitation over Texas, which could cause the relationship between sulfate at
Big Bend and transport from areas within Texas to be different from average. In fact,
REMSAD assigns large attributions to Texas during that period.
In summary, the TrMB method, using either the Monte Carlo or ATAD trajectory
model run with the MM5 wind fields, can reproduce the average REMSAD sulfate
attributions for the four large source areas to within ten percentage points. Examination of
the relationship between sulfate attributions and endpoints indicates, though, that caution
should be exercised when using TrMB to estimate source attributions on individual days.
HYSPLIT can reproduce the attributions to Mexico and the western U.S., but it overestimates
the attribution to Texas and underestimates that of the eastern U.S. This may be due to
trajectory heights that are too low on a single highly influential day, thus giving too much
influence to Texas and not enough to the eastern U.S.
9.7 Evaluation of the Forward Mass Balance Regression (FMBR) Method Using
REMSAD-Modeled Sulfate Concentrations
In Section 9.5, the FMBR source attribution technique was evaluated using inert
tracer concentrations. Sulfate source attribution has the complicating factor that the emitted
sulfur undergoes transformation and removal processes during transport from the source to
the receptor. To evaluate the FMBR method for a reactive species, it was applied using the
REMSAD-predicted 24-hour sulfate concentrations at Big Bend. As in the TrMB evaluation
described in the section above, the REMSAD results, provided an artificial reality in which
both the concentrations and the contributions from 10 large source regions and the model
boundary conditions were known. Therefore, the FMBR source attribution results could be
compared against “known” source attributions.
The main points concerning the evaluation of FMBR using the REMSAD sulfate
concentrations are presented below. Details can be found in the CIRA/NPS report (Schichtel
et al., 2004), which is included in the Appendix.
For this evaluation, air mass transport from 10 source regions was estimated using the
CAPITA Monte Carlo Model driven by the MM5 wind fields (which are the same wind
fields used for the REMSAD simulation). Nine of these source regions, listed in Table 9-10,
are combinations of the 17 regions shown in Figure 8-3. Monte Carlo transport of
concentrations from the REMSAD domain boundaries was not simulated, so the REMSADcalculated
contribution from the boundary conditions was added as an additional source
contribution for the FMBR analyses.
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