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

Tables 3‐13 and Figures 3‐2 through 3‐6 display the plume fitting model performance statistics
for the various CALPUFF sensitivity tests and the 100 km arc of receptors in the GP80 field
experiment and compares them with the previous results as reported by EPA (1998a). The
fitted predicted and observed plume centerline concentrations (Cmax) and the percent
differences, expressed as a mean normalized bias (MNB), are shown in Table 3‐13 with the
MNB results reproduced in Figure 3‐2. Similar results are seen for the predicted and observed
maximum concentrations at any monitoring site along the arc (Omax) that are shown in Table
3‐13 and Figure 3‐3. The use of either the CALPUFF (CAL) or AERMOD (AER) algorithms for the
turbulence dispersion doesn’t appear the affect the maximum concentration model
performance. Most CALPUFF sensitivity simulations overestimate the observed Cmax value by
over 40%, with the 1998EPA_PG and EXP2C_PG simulations overestimating the observed Cmax
value by over a factor of 2 (> 100%). The overestimation of the observed Omax value is even
greater, exceeding 60% for most of the CALPUFF simulations. The PG dispersion produces
much higher maximum concentrations compared to CAL/AER dispersion for experiments EXP2B
and EXP2C. But the PG maximum concentrations are comparable or even a little lower than
CAL/AER for the other experiments; although in the 1998 EPA study the PG dispersion option
produced much higher maximum concentrations. The EXP1B, EXP2B and EXP3B CALPUFF
simulations do not exhibit the large overestimation bias of Cmax and Omax as seen in the other
experiments and are closest to reproducing the observed maximum concentrations on the 100
km arc, matching the observed values to within ±25%; note that the “B” series of experiments
use MM5 data (12, 36 and 12 km for EXP1, EXP2 and EXP3, respectively) but only surface and
no upper‐air meteorological observations. The CALPUFF/MMIF simulation using the 12 km
MM5 data and PG dispersion also reproduced the maximum concentrations to within ±25%.
Most of the CALPUFF sensitivity simulations underestimate the plume spread (σy) by 20% to
35% (Figure 3‐4), which is consistent with overestimating the observed maximum concentration
(i.e., insufficient dispersion leading to overestimation of the maximum concentrations). The
exceptions to this are again the “B” series of CALPUFF/CALMET experiments and
MMIF12KM_PG. Another exception to this is the EPA1998_PG simulation which agrees with
the observed plume spread amount quite well; the explanation for this is unclear and seems
inconsistent with the fact that 1998BASE_PG overestimated the observed Cmax/Omax values.
The 1998EPA_PG results were taken from the EPA (1998a) report and could not be verified or
quality assured so we cannot explain this discrepancy.
The deviations between the observed and predicted plume centerline along the 100 km arc of
receptors in degrees is shown in Figure 3‐5. The modeled plume centerline tends to be 0 to14
degrees off from the observed plume centerline. The best performing model configuration for
the plume centerline location is the BASEA series that uses CALMET with observed surface and
upper‐air meteorological data but no MM5 data. The CALPUFF/CALMET sensitivity tests that
use surface and upper‐air (“A” series) and just surface (“B” series) meteorological observations
tend to perform best for the plume centerline location, whereas the sensitivity tests that uses
no meteorological observations (“C” series) performs the worse, with the plume centerline
tending to be 10 to 14 degrees too far west on the 100 km arc for the “C” series of
CALPUFF/CALMET sensitivity tests. The CALPUFF/MMIF runs, which also do not include any
meteorological observations, also tend to have plume centerlines that are 6 to 12 degrees too
far to the west.
Most of the CALPUFF sensitivity tests have cross wind integrated concentrations (CWIC) that
are within ±20% of the observed value along the 100 km arc (Figure 3‐6 and Table 5‐13). The
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