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

6.4.4 CALPUFF Sensitivity Tests
Most CALPUFF applications have limited the distance downwind that the model is applied for to
less than 300 km from the source. However, the evaluation of CALPUFF in the ETEX study has
applied the model to much farther downwind distances. The issue of the downwind
applicability of the CALPUFF model was raised in the FLAG (2000) report and EPA’s June 26‐27,
2000 7 th Conference on Air Quality Modeling 19 that proposed to list CALPUFF as an EPA
recommended model for far‐field applications. However, when CALPUFF was designated an
EPA recommended far‐field model in a 2003 Federal Register (FR) notice, EPA noted that
“…since the 7th Modeling Conference, enhancements were made to CALPUFF that allow puffs to
be split both horizontally (to address wind direction shear) and vertically (to address spatial
variation in meteorological conditions). These enhancements likely will extend the system’s
ability to treat transport and dispersion beyond 300 km” (68 FR 18441). EPA goes on to further
state that “…Future performance comparisons for transport beyond 300 km are likely to extend
the applicability and use of the modeling system, and we intend to watch for such evaluations
very diligently. In an effort to keep the public abreast with the latest findings, EPA requests that
evaluation results of the CALPUFF modeling system be sent to us (SCRAM webmaster) in an
electronic format suitable for distribution, or that citations be provided for copyrighted material.
EPA will post this information on its website for review and assessment” (EPA, 2003).
Despite the passage of eight years since EPA’s request for CALPUFF evaluation regarding its
suitability for application beyond 300 km, no such documentation has been submitted. Thus,
the ETEX CALPUFF evaluation serves as an important source of information on the downwind
applicability of CALPUFF. In this section we present two types of performance analysis:
• Analyze the CALPUFF model performance as a function of distance from the source to
determine whether the poor performance of CALPUFF relative to the other LRT models is
related to applying the model beyond its downwind distance of applicability; and
• Perform CALPUFF puff splitting sensitivity tests to determine whether puff splitting can
increase the downwind distance applicability of CALPUFF, as suggested in the 2003 Federal
Register notice.
6.4.4.1 Time Dependent Model Performance
Figure 6‐19 displays the FMS model performance statistic for the five LRT models as a function
of time from the beginning of the tracer release in the ETEX experiment. Although the CALPUFF
model performance does degrade with time (distance), even close to the source it is performing
worse than the other LRT models. This was also seen in the spatial maps of the model
performance presented previous in Figure 6‐16 where the CALPUFF model had spatial
alignment problems compared with the observed tracer 24 hours after the tracer was released.
Thus, CALPUFF does not perform comparably to the other evaluated LRT models even within
300 km of the source.
19 http://www.epa.gov/ttn/scram/7thmodconf.htm
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