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

Final Report — September 2004
Figure 8-3. Locations of 17 source areas composed of 100-km grid virtual sources used to track
transport and dispersion in the Forward Mass Balance Regression method. The values and colors of
each source region are the SO 2 emission rates in 1,000 tons per year from the BRAVO and National
Emission Trend inventories. Open regions surrounding Big Bend and in central Texas were not
included in the assessment and have near-zero SO 2 emissions. Composite source regions consistent
with those of the other attribution analyses are composed of the numbered source areas as follows:
Eastern US - 1-4; Western US - 5 and 6; Texas - 7-9; Carbón - 10; and Mexico - 10-17.
The numbers of particles from the various source regions that arrive at the Big Bend
receptor site during any period of time are related to the transit probabilities for each region.
A linear least squares technique is used to identify a set of regression coefficients which
when multiplied by their respective transit probabilities for each source region for each of the
sample periods produces a “best” estimate of the measured particulate sulfate concentrations
at Big Bend. SO 2 was also attributed to these source regions in this manner.
The regression coefficients determined by this method account for the combined
effects of emission rates, plus deposition and chemical transformation during transport for
each source region over study period. These factors undoubtedly change over time, as do
transport and dispersion. However, this approach, just like TrMB, uses a single regression
coefficient value for each source region for the entire BRAVO Study period, so it cannot be
expected to attribute accurately for periods of highly unusual emissions, deposition or
chemical transformation.
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