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

Final Report — September 2004
uncertainties in the numerical representation of various physicochemical processes in
Eulerian models (e.g., plume chemistry, cloud effects), finer resolutions may not always
yield better results in situations where transport plays a dominant role in the distribution of
the simulated species of interest.
The BRAVO study provided an opportunity to evaluate the transport components of
Eulerian models independently of chemical processes. For the San Antonio tracer, CMAQ
was able to explain 40% of the variance at K-Bar. For the other tracer releases, the CMAQ
model was only capable of explaining 5 to 13% of the variance (as estimated by the
coefficient of determination, r 2 ) there. It appears that, for inert gases at least, the CMAQ
modeling system is restricted in its ability to represent transport and diffusion from a single
point source to specific receptors (a conclusion that applies also to REMSAD).
9.11 Evaluation of CMAQ Simulations of SO 2 , Sulfate, and Other Aerosol
Components
For particulate matter simulations, the MADRID aerosol module was incorporated
into the CMAQ model, as described in Section 8.4.3. The performance of the resulting
model, called CMAQ-MADRID here, is discussed below. (More detail is provided in the
EPRI report by Pun et al. (2004), which is included in the Appendix.)
9.11.1 Development of Base Case
For the CMAQ-MADRID modeling, the boundary conditions, emissions inputs, and
model configuration that were used for the source attribution estimates (to be described in
Section 11.2) were established after evaluating the model’s performance in preliminary and
sensitivity simulations. We summarize these initial simulations and their results here and
describe the performance of the resulting modeling system. (The initial simulations and
performance are described in detail in the EPRI report (Pun et al., 2004).)
The preliminary simulation used the BRAVO emissions inventory that was described
in Chapter 4. Boundary conditions were derived from the REMSAD modeling results at the
boundary of the final CMAQ domain. The performance of this simulation was appraised by
comparing 24-hour predictions of several species concentrations with observations
throughout the BRAVO network.
Model performance for this preliminary simulation was not satisfactory for SO 2 and
sulfate over the eastern half of the CMAQ domain. To improve model performance, two
potential causes for underestimates of sulfur concentrations were explored – possible
understatement of Mexican SO 2 emissions and possible incorrect boundary conditions
resulting from overstatement of SO 2 and sulfate along the boundaries of the CMAQ domain.
Possible understatement of Mexican SO 2 emissions was suggested by systematic
underestimation of sulfate at K-Bar during periods in July when there was a predominance of
southerly winds. This hypothesis was reinforced by a September 2002 presentation by the
Undersecretary of the Mexican Ministry of Energy, titled “Mexico and USA Power Plant
Emissions in Perspective.” that presented total SO 2 emissions for three Mexican states
bordering the United States, which constitute a large portion of the CMAQ modeling domain.
9-51