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

• The maximum observed concentration along the arc or observed fitted centerline plume
concentration was underestimated by ‐42% to ‐72% and the plume widths overestimated
by 47% to 293%.
• The CALPUFF underestimation bias of the observed maximum concentration tends to be
improved using CALMET runs with no meteorological observations.
• The use of the PG dispersion option tends to exacerbate the plume width overestimation
bias relative to using the CAL or AER turbulence dispersion option.
• The CALPUFF predicted plume centerline tends to be offset from the observed value by 9
to 20 degrees, with the largest centerline offset (> 15 degrees) occurring when no
meteorological observations are used with either CALMET or MMIF .
• The 1998 CALPUFF runs overestimated the observed CWIC by 15% and 30% but the
current study’s BASEA configuration, which was designed to emulate the 1998 EPA study,
underestimates the observed CWIC by ‐14% and ‐38%.
The inability of most (~90%) of the current study’s CALPUFF sensitivity tests to reproduce the
1998 EPA study tracer test residence time on the 600 km receptor arc is a cause for concern.
For example, the 1998 EPA study CALPUFF simulation using the CAL dispersion option estimates
a tracer residence time on the 600 km receptor arc of 13 hours that compares favorably to
what was observed (12 hours). However, the current study CALPUFF BASEA_CAL configuration,
which was designed to emulate the 1998 EPA CALPUFF configuration, estimates a residence
time of almost half of the 1998 EPA study (7 hours). One notable difference between the 1998
EPA and the current study CALPUFF modeling for the GP80 600 km receptor arc was the use of
the slug option in the 1998 EPA study. Another notable difference was the ability of the current
version of CALPUFF to perform puff splitting, which EPA has reported likely extends the
downwind distance applicability of the CALPUFF model (EPA, 2003). Thus, a series of CALPUFF
sensitivity tests were conducted using the BASEA_CAL CALPUFF/CALMET and MMIF_12KM CAL
and PG CALPUFF/MMIF configurations that invoked the slug option and performed puff
splitting. Two types of puff splitting were analyzed, default puff splitting (DPS) that turns on the
vertical puff splitting flag once per day and all hours puff splitting (APS) that turns on the puff
splitting flag for every hour of the day. The following are the key findings from the CALPUFF
slug and puff splitting sensitivity tests for the GP80 600 km receptor arc:
• Use of puff splitting had no effect on the tracer test residence time (7 hours) in the
CALPUFF/CALMET (BASEA_CAL) configuration.
• Use of the slug option with CALPUFF/CALMET increased the tracer residence time on the
600 km receptor arc from 7 to 15 hours, suggesting that the better performance of the
1998 EPA CALPUFF simulations on the 600 km receptor arc was due to invoking the slug
option.
• On the other hand, the CALPUFF/MMIF sensitivity tests were more sensitivity to puff
splitting than CALPUFF/CALMET with the tracer residence time increasing from 6 to 8
hours using DPS and to 17 hours using APS when the CAL dispersion option was specified.
• The use of the slug option on top of APS has very different effect on the CALPUFF/MMIF
residence time along the 600 km receptor depending on which dispersion option is
utilized, with slug reducing the residence time from 17 to 15 hours using the CAL and
increasing the residence time from 11 to 20 hours using PG dispersion options.
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