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

the duration the tracer resides on the 600 km arc, with values of 14 hours (1998EPA_PG) and
13 hours (1998EPA_CAL). Since the 1998 EPA CALPUFF runs estimated that the tracer arrives
after the sampling started (hour 3), then this is a true overstatement of the tracer residence
time and not an artifact of the tracer sampling starting after, or at the same time, the observed
tracer arrived at the arc. There are a couple exceptions to the initial CALPUFF simulations
performed in this study that understated the observed tracer duration on the arc by
approximately a factor of 2, which are discussed below.
The BASEA_PG scenario estimates that the tracer is on the arc for 12 hours, the same as
the observed. However, it estimates the tracer leaves three hours earlier (hour 14) than
observed (hour 11). Why the BASEA_PG tracer plume time statistics are so different from
the two companion turbulence dispersion CALPUFF sensitivity tests (BASEA_CAL and
BASEA_AER) is unclear. The same meteorological fields were used in the three BASEA
CALPUFF sensitivity tests and the only difference was in the dispersion options. This large
difference in the CALPUFF predicted tracer residence time due to use of the PG versus CAL
or AER dispersion options (12 hours versus 6‐7 hours) was not seen in any of the other
CALPUFF sensitivity experiment configurations. Although use of the PG dispersion
sometimes increases the estimated tracer residence time on the arc by one hour in some
of the CALPUFF sensitivity tests (Table 3‐14).
The EXP2C series of experiments have estimated tracer plume duration times (11‐13
hours) that is comparable to what was observed. EXP2C uses 36 km MM5 data and
CALMET was run using a 4 km grid resolution with no meteorological observations (NOOBS
= 2). When meteorological observations are added, either surface data alone (EXP2B) or
surface and upper‐air measurements (EXP2A), the tracer duration statistics degrades to
only 5 to 8 hours on the arc. It is interesting to note that all of the “C” series of
experiments (i.e., use of no meteorological observations in CALMET) exhibit better plume
residence time statistics than the experiments that used meteorological observations
(with the exception of BASEA_PG discussed previously). But only experiment EXP2C (and
BASEA_PG) using 36 km MM5 data and CALMET run with 4 km grid resolution was able to
replicate the observed tracer residence time.
Most of the initial CALPUFF sensitivity tests were unable to reproduce the observed tracer
residence time on the 600 km arc, as was done in the EPA 1998 study using earlier versions of
CALPUFF. Even the BASEA_CAL sensitivity test, which was designed to be mostly consistent
with the 1998EPA_CAL simulation, estimated tracer plume residence time that was half of what
was observed and estimated by the 1998EPA_CAL simulation. In addition to using difference
versions of the CALPUFF model (Version 4.0 versus 5.8), the BASEA_CAL simulation also did not
invoke the slug option as was used in 1998EPA_CAL (MSLUG = 1). The use of the slug option is
designed for near‐source applications and is not typically used in LRT dispersion modeling, so in
this study the initial CALPUFF sensitivity tests did not use the slug option for modeling of the
600 km arc. The effect of the slug option is investigated in additional CALPUFF sensitivity tests
discussed later in this Chapter.
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