Documentation of the Evaluation of CALPUFF and Other Long ...

ETEX LRT Dispersion Model Sensitivity Tests
Sensitivity tests were conducted using the CAMx, CALPUFF and HYSPLIT models and the ETEX
field study data.
For CAMx, the effects of alternative vertical mixing coefficients (OB70, TKE, ACM2 and CMAQ),
horizontal advection solvers (PPM and Bott) and use of the subgrid‐scale Plume‐in‐Grid (PiG)
module were evaluated. The key findings from the CAMx ETEX sensitivity tests were as follows:
• The vertical mixing parameter had the biggest effect on model performance, with the
CMAQ vertical diffusion coefficients producing the best performing CAMx simulations.
• The horizontal advection solver had a much smaller effect on CAMx model performance
with the PPM algorithm performing slightly better than Bott.
• The use of no PiG module produced slightly better performance than use of the PiG
module.
• The default CAMx configuration used in the ETEX evaluation (CMAQ/PPM/No PiG) was the
best performing CAMx sensitivity test.
CALPUFF sensitivity tests were performed to examine the effects of puff splitting on the
CALPUFF model performance for the ETEX field experiment. When EPA listed CALPUFF as the
EPA‐recommended LRT dispersion model in 2003, they noted that the implementation of puff
splitting likely will extend the models applicability beyond 300 km downwind (EPA, 2003). Since
many of the ETEX monitoring sites are sited further than 300 km downwind from the release,
one potential explanation for the poor CALPUFF model performance is that it is being applied
farther downwind than the model is applicable for. Figure ES‐9 displays a time series of the
Figure of Merit in Space (FMS) performance statistic for the five LRT dispersion models.
Although CALPUFF performs reasonably well within the first 12 hours of the tracer release, its
performance quickly degrades even within 300 km of the source. Thus, CALPUFF’s poor model
performance is not due to applying the model to downwind distances beyond its applicability.
Eight CALPUFF puff splitting sensitivity tests were conducted ranging from no puff splitting to
aggressive puff splitting for all hours of the day and relaxing some of the puff splitting initiation
criteria so that even more puff splitting can occur. The CALPUFF ETEX model performance using
no puff splitting and all hour puff splitting was very similar, thus we saw no evidence to support
EPA’s 2003 statements that puff splitting may extend the downwind applicability of the model.
In fact, when some of the puff splitting initiation criteria were relaxed to allow more puff
splitting, the CALUFF performance degraded.
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