EPA

Office of Air Quality

Pla nnin g a nd S ta nd a rds

C Air

lean

United States Office of Air Quality EPA-454/R-98-020

Environmental Protection Planning and Standards December 1998

Agency Research Triangle Park, NC 27711

AIR

A Comparison of CALPUFF with ISC3
ACKNOWLEDGMENTS

Special credit and thanks are due John Irwin, NOAA for his

technical assistance and advice through all phases of the project,

from study design and meteorological data selection to analysis and

presentation of results. In the model comparisons, credit is due to

Tom Coulter, EPA for his work on the steady state analyses and to

Pete Eckhoff for the variable meteorology analyses. Thanks are

also due Dave Strimaitis and Joe Scire of Earth Tech for their

cooperation and technical assistance with the CALPUFF runs.

DISCLAIMER

This report was reviewed by the Office of Air Quality Planning and

Standards, EPA for approval for publication. Mention of trade

names or commercial products is not intended to constitute

endorsement or recommendation for use.
PREFACE

In this report a comparison is made of two different dispersion

models, CALPUFF and ISC3. CALPUFF is a Lagrangian puff

model which simulates continuous puffs of pollutants released into

the ambient flow, whereas ISC3 is a Gaussian plume model that

treats emissions from a source as a contiguous mass. CALPUFF

may be configured to treat emissions as integrated puffs or as slugs. 

ISC3 is currently recommended for routine use in assessing source

impacts involving transport distances of less than 50km. This

report is being released to establish part of the basis for review of

the consequences resulting from use of CALPUFF in routine

dispersion modeling of air pollution impacts.
i

TABLE OF CONTENTS

1. Introduction ..............................................................1

2. Technical Background ......................................................1

3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.1 Steady State (screening) Meteorological Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.1.1 Residual Analysis ...............................................4

3.1.2 Point Sources (surface and elevated) ................................6

3.1.3 Area Source ...................................................7

3.1.4 Volume source .................................................7

3.2 Variable Meteorological Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2.1 Scenarios for Sensitivity Study ....................................8

3.2.2 Preliminary Studies .............................................9

3.2.3 Sensitivity Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4. Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.1 Steady State Meteorological Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.2 Variable Meteorological Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Appendices

A. Switch settings for CALPUFF input file to emulate ISC3's “Regulatory Default” mode

B. Meteorological conditions for the steady state CALPUFF/ISC3 comparisons

C. Characteristics for sources used in the CALPUFF/ISC3 comparisons

D. Receptor array used in the CALPUFF/ISC3 comparisons

E. Puffs versus Slugs: CALPUFF’s Two Simulation Modes

F. Summary statistics from performance matrix - point sources (Z = 3000m) i

G. Summary statistics from performance matrix - area source (emissions simulated as slugs)

H. ISCST3's treatment of virtual sources

I. Summary statistics from performance matrix - volume source

J. Wind rose patterns

K. Puff and slug model concentrations

L. Additional figures illustrating results with variable meteorological conditions 
1

1. Introduction

With the initial use of models such as CALPUFF for regulatory applications, there is the

question of how the model will behave with respect to more widely used models like the

Industrial Source Complex Short Term (ISC3ST) model, hereafter ISC3. Several sensitivity and

comparison studies were designed and performed to determine how CALPUFF would behave

when set to emulate ISC3. The results of those runs were analyzed and are discussed here.

This evaluation features a systematic, phased series of implementation modes. Section 3.1

involves simple screening modes in which conditions are extremely limited and controlled. 

Section 3.2 addresses the more general mode in which meteorological conditions are allowed to

vary hourly. Section 4 provides a summary and conclusions from this investigation. References

are listed in Section 5, followed by the appendices.

2. Technical Background

CALPUFF is a Lagrangian puff model. The model is programmed to simulate continuous

puffs of pollutants being emitted from a source into the ambient wind flow. As the wind flow 

changes from hour to hour, the path each puff takes changes to the new wind flow direction. 

Puff diffusion is Gaussian and concentrations are based on the contributions of each puff as it

passes over or near a receptor point. For these tests, CALPUFF was set to emit 99 puffs per hour

(default). A sufficiently large number of puffs is necessary to adequately reproduce the plume

solution at near-field receptors.

CALPUFF was originally designed for mesoscale applications and treated emissions as

integrated puffs. As features were added to the model for handling local-scale applications, it

was realized that use of the integrated puff approach was inefficient. A more efficient approach

was developed to treat the emissions as a slug, in which the slug is stretched so as to better

characterize local source impacts. The slug can be visualized as a group of overlapping circular

puffs having very small separation distances. When run in the slug mode, the hourly averaged

pollutant mass is spread evenly throughout the slug. For a given hour, if all of the hourly slug

has not passed over a receptor, concentrations are reduced by the mass that has not passed over

the receptor (Appendix E; Section 2.1 of Reference #2). Note that when run in a slug mode,

once the slug’s lateral dispersion (y) approaches the length of the slug itself (as eventually

happens with downwind distance), CALPUFF samples the pollutant mass as a puff to improve

computational efficiency. At sufficient downwind distance, there becomes no benefit or

advantage for the slug simulation.
2

In the comparison studies described in this report, CALPUFF was run in both the puff mode

(emissions simulated as integrated puffs) and the slug mode (emissions simulated as slugs). 

When the distinction between puffs and slugs is important or significant, they will appear in

italics (i.e., slugs or puffs; see Appendix E). In the generic sense, the use of “puffs” will be used

to connote the characterization of a continuous release of a series of overlapping averaged puffs,

in which the transport and dispersion of each puff is treated independently, based on local (time

and space varying) meteorological conditions. Whereas, the use of “plume” will be used to

connote the characterization of a continuous release, in which the release and sampling times are

long compared with the travel time from source to receptor, and the meteorological conditions

are steady state over the travel time.
3

3. Results

In this comparison, CALPUFF (Version 4.0, level 960612) was compared with the latest

version of ISC3 (dated 96113). CALPUFF was run in a mode that enabled ISC3-type

meteorological data as input, and therefore winds are horizontally homogeneous for each hour. 

ISC3 was implemented in the “Regulatory Default” mode and the input file for CALPUFF was

configured so as to emulate this to the best extent possible (see Appendix A). Both surface and

elevated sources were simulated for rural environments in flat terrain, free of obstacles.

3.1 Steady State (screening) Meteorological Conditions

In this approach to the comparison, meteorological conditions were held constant (as in

SCREEN3) so as to express true model differences, i.e., without the bias of a varying (temporally

and spatially) meteorological regime. Meteorological data sets were synthesized with fixed

meteorological conditions (Pasquill-Gifford stability category, wind speed, and mixing height)

and were of duration estimated to be sufficient to advect CALPUFF's puffs to the edge of domain

(generally 24 - 48 hours). (Of course, ISC3's steady state plume reaches the edge of the domain

instantaneously.) For Pasquill-Gifford (P-G) stability category A, 5 wind speeds were used, for

B, there were 9 wind speeds, for C, 11 wind speeds, for D, 13 wind speeds, for E, 9 wind speeds,

and for F, 7 wind speeds. A matrix describing the basis for the 54 meteorological conditions

used is provided in Appendix B.

The elevated point sources were 35m, 100m and 200m, respectively. Surface releases were

simulated with a 2m point source, a 500m X 500m area source, and a typical volume source. 

Characteristics for each source type are described in Appendix C. Sources were placed at the

center of a 2 X 2 grid cell domain, with grid spacing set to 150km. While effects within the first

50km are of most interest and significance, straight-line receptors were located with decreasing

density out to 100km (Appendix D). The 62 receptors were placed along a radial aligned at

360, coincident with the bearing used for transport winds.

Unique model runs were made for each combination of source type and meteorological

condition (i.e., Pasquill-Gifford stability category, wind speed, and mixing height). Each model

was configured to output the highest hourly average concentration for SO (no deposition or 2

chemical transformation).
%Ri  (

Ri

ISC3max

) 100

%R   %Ri

62

FBi  Ri

CALPUFF ISC3

2

R ̄

 R 2

i

R ̄

 R 2

i

R ̄ %R

4

3.1.1 Residual Analysis

For each pair of model runs (CALPUFF and ISC3), a signed residual (R , - i CALPUFF

, μgm ) was computed at each of the 62 receptors. From the 62 residuals, a mean ( , ISC3

-3

μgm ), standard deviation ( , μgm ), and sum of residuals squared ( ) were computed. -3 -3

R

The statistic provides an indication (sign) of bias along the receptor radial. The statistic R

provides general indication of the variance along the receptor radial. Because many of the

absolute residuals were quite small, provides a relatively robust indicator of accord along

the receptor radial.

Another robust statistic was envisioned in which the absolute residual at each receptor

was related to, say, ISC3's predicted concentration value at that receptor. Because of the

mathematical problem posed by zero values (can’t divide by zero), the statistic %R (% residual) i 

was defined in terms of the maximum concentration predicted by ISC3 for each run:

The mean % residual follows as:

As with , the statistic provides an indication (sign) of bias along the receptor radial.

Another statistic of interest was the Fractional Bias (FB):

Having by definition a distribution from -2 to +2, a value of zero indicates no bias between

and . CALPUFF ISC3
If (

CALPUFF ISC3

2 ) < 0.001μgm 3

, then FBi  0.0

0.10 FB 0.10

FB  FBi

62

R ̄ %R FB

FB

 ̄

FB

R  32 μgm 3

FB FB  ̄

R  0.3 μgm 3

FB FB

%Rmin %Rmax

R ̄

min R ̄max

5

One problem that arises with the FB statistic is when the mean paired concentration is

very close to zero: the FB statistic can be artificially inflated to a value close to ±2. Since cases

in which the mean is close to zero are of little interest in this comparison, a filter was applied:

For each run pair (i.e., CALPUFF versus ISC3), a mean fractional bias was computed as:

(62 receptors)

As with and , provides an indication (sign) of bias along the receptor radial. While a

value of zero would be ideal for , the following was established as a "goal":

Specific instances for which this goal was not met were noted.

There are some caveats to the interpretation of FB. Its behavior is closely related to its

structure. Its value is influenced not only by the absolute difference of the paired concentrations,

but by their relative magnitude as well. Thus, modest R 's related to "large" 's (e.g., from a low i

level release) yield modest FB 's (and a modest ). Such a scenario can include a fairly large i

variance ( = 56 μgm ) and mean residual (e.g., ) along the receptor radial but R

-3

still result in a fairly low (e.g., = -0.06). Conversely, modest R 's related to "small" 's i

(e.g., from an elevated release) may yield substantial FB 's. Such a scenario can include a modest i

variance ( = 0.5 μgm ) and mean residual (e.g., ) along the receptor radial but R

-3

still result in a sizeable (e.g., = 0.35). While a useful indicator of correspondence

between two quantities, the FB must be interpreted in the context of other comparison statistics.

At the conclusion of the runs, a performance matrix was created and aggregate statistics

were compiled. For basic residual analysis, the value, run (distinct combination of source type,

wind speed, P-G category, mixing height) and receptor for R and R were noted. i(min) i(max)

Likewise, across all runs, the value and run for and were noted, as were the value

and run for and . Across all runs, the value and run for R(min) R(max) and  were also

noted.
FBmin FBmax

FB

|FB| FBmax

%R

%Rmin

%Rmax

|FB| FBmax

 Certain runs may be referenced, e.g., D20H100 or B1p5H2. Under this nomenclature, the first signifies a 100m source running 1

under D stability with 20 ms winds. The second would be a 2m source running under B stability with 1.5 ms winds. -1 -1

 Each source was modeled 54 times for each of two mixing heights. In the current version of CALPUFF, it is impossible to 2

isolate impacts from more than one source per run. ISC3, however, may be configured to simulate multiple sources during a single

run and isolate impacts individually.

 For a description of integrated puff and slug formulations, see Sections 2.1.1 and 2.1.2 of the CALPUFF User’s Guide 3

(Reference #2) and Appendix E of this report.

6

Finally, for the FB statistic, the value, run and receptor for FB and FB were i(min) i(max)

noted. Across all runs, the value and run for and were noted. The values and cases

in which did not meet the "10% goal" were also noted.

3.1.2 Point Sources (surface and elevated)1

3000m mixing height

To model the four point sources, CALPUFF had to be run 216 times while ISC3 was run 54

times. As indicated in Appendix A, CALPUFF was run in the slug mode to emulate ISC3's 2

Gaussian plume simulation. The results indicated good accord (Appendix F). For all cases, 3

0.10 ( = 0.02). The maximum residual was 25.0 μgm (0.13% of the concentration -3

mean at the incident receptor), while the minimum residual was -8.0 μgm (0.03% of the -3

concentration mean at the incident receptor). Mean residuals for any run were less than one

μgm , and total range for  was 0.0 - 3.2 μgm . Overall, perhaps the most practical -3 -3

R

performance parameter was , which indicates accord well within one percent across all

release heights, meteorological conditions and receptors (the value for was -0.04% and

 was 0.13%). A qualitative inspection of residuals as they appear along the receptor array

indicated no distinct pattern of bias for any case. Across all runs, a slight negative bias

(CALPUFF relative to ISC3) is apparent for the 2m source, and the greatest variance is

associated with the 2m source, especially for P-G category A.

500m mixing height

The array of runs was redone (again, using slugs) with mixing height reduced to 500m to

assess CALPUFF's response to reflection and to evaluate whether reflection is handled

equivalently. The results were quite good. In 43 cases, the plume centerline computed by ISC3

exceeded the mixing height and set ground level concentrations to zero. CALPUFF treated the

same cases equivalently. For the remaining 173 cases, 0.10 ( = 0.02). The

comparison statistics bear a striking resemblance to those for Z = 3000m. Mean residuals for i
%R

%Rmin

%Rmax

|FB| FBmin

%R

%Rmin %Rmax

7

any run were also less than one μgm , and indicates accord to well within one percent -3

across all release heights, meteorological conditions and receptors (the value for was

0.0% and was 0.08%). A qualitative inspection of residuals as they appear along the

receptor array also indicated no distinct pattern of bias for any case. As with the 3000m Z case, i

a slight negative bias is apparent for the 2m source, and the greatest variance is associated with

the 2m source, especially for P-G category A.

3.1.3 Area Source

The area source was modeled with emissions simulated as slugs. While a significant

difference would be expected between the behavior of puffs and slugs, slugs are considered to

treat the area source more closely to the way of ISC3. This is because the "line-source"

integrator, similar to that used in ISC3 to model area sources, is only implemented when

emissions are simulated as slugs. Puffs use the effective y treatment for area sources. If there

are receptors within or very near an area source, the slug treatment is a better representation. If

receptors are farther away, the puff treatment is reasonable, and less time-consuming. Mixing

height was fixed at 3000m. These runs were done both for z(init) z(init) = 0 and for  = 2.5m

(specification of non-zero  is optional in both models). The best accord was seen for the set z(init)

in which  = 0 (Appendix G). For about one fifth of the cases, > 0.10 ( = -0.16). z(init)

The maximum residual was 561 μgm (2.2% of the concentration mean at the incident receptor), -3

while the minimum residual was -1537 μgm (33% of the concentration mean at the incident -3

receptor). Mean residuals and mean standard deviations among runs ranged over three orders of

magnitude. Analysis of the residuals and fractional biases indicate a definite trend toward

negative bias (CALPUFF relative to ISC3), and best accord for any P-G category was seen for

the higher wind speeds. Also, within any P-G category, the variance falls off with higher wind

speed. The parameter indicates accord within two percent across meteorological conditions

and receptors (the value for was -1.5% and was -0.07%) and again, the tendency

toward negative bias is indicated. A qualitative inspection of residuals as they appear along the

receptor array indicated no distinct pattern of bias for any case.

3.1.4 Volume Source

The volume source was modeled with emissions simulated as slugs. Because ISC3 does not

compute concentrations for receptors within 2.15y y of the source (it's actually 2.15 + 1m), no

residuals were analyzed for receptors closer than 200m.
|FB|

%R

%Rmin %Rmax

8

There is a fundamental feature of the way in which ISCST3 treats virtual sources (such as

the volume source in question) that is at odds with the way in which CALPUFF treats such

sources. The phenomenon is described and illustrated in Appendix H. A modified version of

ISCST3 was created to ensure conformity in the treatment of virtual sources by both models. 

Once this modification was made, the accord between CALPUFF and ISC3 was quite good

(Appendix I).

For all cases = 0.0. The maximum residual was 0.15 μgm (0.1% of the concentration -3

mean at the incident receptor), while the minimum residual was -0.92 μgm (0.4% of the -3

concentration mean at the incident receptor). Mean residuals for any run ranged from -0.2 μgm-3

to 0.0 μgm , and total range for  was 0.0 - 0.22 μgm . The parameter indicates accord -3 -3

R

well within a tenth of one percent across all meteorological conditions and receptors (the value

for was -0.07% and was 0.01%). A slight tendency for negative bias was apparent

for the stable P-G categories. As seen for the area source, for any of the stable P-G categories,

variance falls off with higher wind speed. A qualitative inspection of residuals as they appear

along the receptor array indicated slightly more bias for receptors in the near field of the source.

3.2 Variable Meteorological Conditions

3.2.1 Scenarios for Sensitivity Study

For the sensitivity study comparing CALPUFF and ISC3, meteorological conditions

were allowed to vary hourly. The first test scenario was devised to see what effects variable

meteorology would have on hourly averaged concentrations. One annual period of hourly

averaged meteorological data was selected from each of three climatically different regions of the

United States. The concentrations between CALPUFF (emissions simulated as a continuous

series of puffs) and ISC3 (emission release simulated as a continuous plume) were compared in

time and space. The comparisons were examined to try to find the underlying cause of

significant differences. The second scenario was a rerun of the first case with some

modifications. The averaging times were extended to 3-, 24-hour and annual periods. Maximum

concentrations were compared for individual receptor rings at 15 downwind distances. The suite

of four point sources described in Appendix C was used in these comparisons.

The meteorological data consist of hourly values of wind speed and direction, ambient

temperature, stability class, and mixing heights. The three sites selected were: 1991 Boise,

Idaho; 1990 Medford, Oregon; and 1964 Pittsburgh, Pennsylvania.
9

The Boise data set was selected because of the very directional nature of it winds (see

1991 Boise wind rose, Appendix J). Over 33% of the winds have a northwesterly component

and over 33% of the winds have a southeasterly component with the majority of those winds

having speeds greater than 2 ms . With such persistence in wind direction, the puffs simulated -1

by CALPUFF would be expected to be transported to the most distant receptors.

The Medford data set was selected because of the high number of calm wind situations

(see 1990 Medford wind rose, Appendix J). In 1990, Medford Oregon recorded a value of 22.5%

of calm winds. This compares to the average of 6.5% for the other two sets of data. Since

CALPUFF processes calm winds and ISC3 “zeros” concentrations during calm wind events,

there is good reason to expect differences to be seen in the simulated patterns of surface

concentration values estimated by the two models.

The 1964 Pittsburgh data set was selected because it has been used as a standard test set

for a number of years and because of its fairly well distributed wind directions and wind speeds

(see 1964 Pittsburgh wind rose, Appendix J). Although there is a bias in the wind direction

toward the southwest, this set was included because many data sets show a similar bias for a

particular wind direction and also have a low number of calm winds.

The receptor placement consisted of 15 rings of 36 receptors each for a total of 540

receptors. The rings were spaced at distances of 0.5, 1, 2, 3, 5, 10, 15, 20, 30, 50, 100, 150, 200,

250, and 300km from the source. On each ring, the receptors were spaced every 10 starting at

360.

3.2.2 Preliminary Studies

Three preliminary studies were done prior to the sensitivity study. In the first preliminary

study, CALPUFF and ISC3 were run to create a plot of concentration curves under steady state

conditions for centerline and laterally placed receptors. If there were differences in the way

dispersion coefficients were calculated between the two models, that would become apparent in

plots of concentration distributions. In the second preliminary study, the puff and slug models

were run for a two-hour segment where a large wind shift occurred in the second hour. The

purpose was to compare concentration footprints from puff and slug mode results. This study

highlights the different manner in which puffs and slugs are treated in CALPUFF. In the third

preliminary study, a detailed examination was made of the concentration output from CALPUFF

(puff mode) and ISC3 using the Boise meteorological data to help understand the large

differences in concentrations between these two models over a multi-hour period involving calm

winds and a wind shift.
10

First Study

The first study was done to see whether CALPUFF would calculate  and  values y z

differently than ISC3. The same standard input file was created for ISC3 and CALPUFF with the

idea being that any difference in the way the sigmas were calculated would be evident in the

concentration results. In this comparison, CALPUFF was run in both the puff and slug modes. 

The models were run for a 2m point source and the basic switch settings for CALPUFF set per

Appendix A. The meteorological data were kept constant except for P-G stability category. For

each run, the stability category was changed until all six stability categories, A through F, were

used for all three models (i.e., ISC3, CALPUFF puff model, and CALPUFF slug model). A

preliminary group of receptors was created with the centerline along the 360 axis and the

receptors spaced every 1 for 44.

The resulting concentrations were compared on a receptor-by-receptor basis. When the

same input data were used, all three models produced concentrations that were within a few

fractions of a percent of one another (Figure 1). Figure 1 displays six sets of three curves, one

set of curves for each stability category. Each curve in each set overlaps the other curves in that

set. The only common difference in each set is that both CALPUFF curves are truncated. This

can be seen by the extrapolation of the ISC3 thin dashed line after the thick dashed line and thin

solid lines of the two CALPUFF curves. This disparity results from CALPUFF concentration

values set to zero for receptors that are more than 3 from the centerline (Appendix E), whereas y

ISC3 sets concentration values to zero for receptors that are more than 11.75y from the

centerline. However, lateral plume spread in ISC3 is limited to 50 either side of the centerline

and may be further decreased by vertical mixing conditions.

Second Study

In the second study, CALPUFF’s treatment of emissions, puffs versus slugs, was evaluated

(Appendix E), using synthesized meteorological data. There is a general difference in the extent

of the hourly CALPUFF concentration “footprints” using the puff and slug models (Appendix K). 

Concentrations produced by the puff model produce a concentration field similar to a

concentration field produced by ISC3 but are restricted to the trajectory algorithms in CALPUFF. 

The extent of each CALPUFF downwind concentration field is limited by the average wind

speed occurring over a particular hour. The extent of the downwind concentration field in ISC3

is limited only by the farthest downwind receptor. The extent of the downwind concentration

field when the slug model is used is the same as that for the puff model. However, when the

wind direction changes from one hour and to the next, the directional orientation of the slug is

maintained while the slug is advected downwind (Figure 2). During Hour 1, the wind was from 
0 10 20 30 40

Degree Deviation of Receptor from Centerline Direction

1E-1

1E+0

1E+1

1E+2

1E+3

1E+4

1E+5

C once ntration s a t 0.5 km (u g/m 3)

C oncentration Curves

ISC3 Model

CALPUFF Puff M odel

CALPUFF Slug Model

A

B

C

D E F

-1 0 1 2 3 4 5 6 7 8 9 10

Distance (km)

-1

0

1

2

3

4

5

6

7

8

9

1 0

D istance (km )

Source Hour 1 Slug and Puff Concentration Footprints

H ou r 1 W in d D ire ctio n

H o u r 2 P u ff C o n c e n tra tio n F o o tp rin t

fro m H o u r 1 E m is s io n s

Hour 2 Slug Concentration Footprint

from Hour 1 and 2 Emissions

H o u r 2 P u ff F o o tp rin t

fro m H o u r 2 E m is s io n s

H o u r 2 W in d D ire c tio n

-40 -30 -20 -10 0 10 20 30 40

Distance (km)

-30

-20

-10

0

1 0

2 0

D is ta n c e (k m )

Centerline at Hour 62

CALPUFF Em issions

from Hours 52-56 Source

ISC3 Em issions

from Hour 62 Only

CALPUFF Em issions

from Hours 61-62

CALPUFF Em issions

from Hours 57-60

-40 -20 0 20 40

Centerline Distance Downwind from Source (km)

1E-1

1E+0

1E+1

1E+2

1E+3

1E+4

Concentrations (ug/m3)

Group Centerline Concentrations For Hour 62

Concentration Contributions

CALPUFF, Hours 52-56

CALPUFF, Hours 57-60

CALPUFF, Hours 61-62

CALPUFF Total

ISC3, Hour 62 only

Hours 61-62

Hours 57-60

CALPUFF Total

ISC3

Hours 52-56

CALPUFF Total

ISC3

Hours 61-62

Hours 61-62 Hours 57-60

Hours 52-56

11

Figure 1. Plot of ISC3 and CALPUFF slug Figure 2. Plot of CALPUFF slug and puff

and puff model concentrations at a distance of one-hour concentration footprints during a

0.5 km from the source for all stability 2-hour, 70 degree wind shift. Note the broad

classes. The curves for CALPUFF are Hour 2 slug area which was advected from

truncated because CALPUFF and ISC3 use the area of the Hour 1 slug footprint. This

different y/ criteria for deciding whether to area was coupled with the Hour 2 emissions. y

compute a concentration for a given receptor.

Figure 3. Plot of concentration footprints at

Hour 62 from three CALPUFF hourly

emission groups and one ISC3 emission Figure 4. Plot of Figure 3 CALPUFF group

group. The Hour 62 ISC3 plume centerline and total and ISC3 one-hour concentrations at

orientation is drawn through the source Hour 62. Note that the CALPUFF total is

location. Note the overlap of groups in the 5 approximately 50% greater than ISC3

to 15km downwind range. concentrations at 15km.
12

263; during Hour 2, the wind switched to 193. In Hour 1, the puff and slug model

concentration footprints are almost exactly the same. However, in the next hour, the east-west

oriented slug is advected north-northeastward. This results in a number of receptors being

impacted but at a much lower concentration of about 280 μgm . At the end of Hour 2, an area of -3

approximately 25 km has been impacted by the slug model. The puff impacts are restricted to 2

relatively narrow corridors.

Note that during Hour 2, the slug and puff models have been simulating emissions from the

source. The emissions have been transported north-northeastward (Figure 2). The

concentrations produced by both models are similar but the puff concentrations are higher in the

area of the maximum (Appendix K), due to the way in which dispersion is treated in the slug

model. Since the slug is elongated and the mass of effluent is spread evenly throughout its

volume, the newly emitted effluent close to the end of the hour has not had time to be transported

past the receptors farther out. At distances of 0.5, 1, 2, 3 and 5km along the 10 degree radial, the

Hour 2 slug concentrations are 91, 82, 64, 46, and 11 percent of the respective puff

concentrations. While the slug model may have a broader spatial impact, its average

concentrations are generally lower than those of the puff model.

Remember that receptors were placed on rings within the modeled domain and that there

were no rings between 5km and 10km. With this arrangement a truncation appeared in the puff

concentration footprint for Hour 2 beyond 5km from the source, and the actual footprint

(appearing as right side Hour 2 in Figure 2) was not detected. (This truncation was not as evident

for the Hour 2 slug footprint). To address this artifact, a finer Cartesian grid was developed for

the second preliminary study that used a spacing of 400 meters and the right side Hour 2 puff

footprint was then detected and expressed (Fig. 2). Note that the right side Hour 2 puff footprint

originates from the terminus of the Hour 1 puff. The left side Hour 2 puff footprint is the result

of Hour 2 emissions from the source. Also note the 400 meter grid resolution was not fine

enough to properly contour the left side Hour 2 puff concentration isopleths. There was no such

contouring problem evident within the other puff and slug concentration footprints. Note that the

Hour 2 slug footprint is superimposed by the Hour 1 and Hour 2 puff footprints and by the

exposed area between them.

Third Study

In the third study, a detailed examination was done on the concentration output from

CALPUFF (puff mode) and ISC3 using the Boise meteorological data to examine the cause of a

large difference in concentrations between these two models’ results. These concentrations
13

occurred 5 to 15km downwind from the source at Hour 62 and after a 4-hour period of calm

winds and then a wind reversal (Figure 3).

 

During the 10 hours preceding Hour 62, hourly emissions were released into one of three

wind regimes. First, there were 5 hours of east-southeasterly winds, followed by 4 hours of calm

winds, followed by a 180-degree wind shift for 2 hours. Emissions were advected westnorthwesterly, then stagnated but the puffs spread out evenly during this calm wind regime, and

finally all emissions were advected east-southeasterly until Hour 62.

The CALPUFF concentration field at Hour 62 consists of three groups of concentrations

based upon the prevailing wind direction at the time of emission release. One group had releases

during Hours 52 through 56. The next group had releases during the calm wind Hours 57

through 60, and the final group had releases during Hours 61 and 62. The fields were depicted to

show their respective group concentration footprints at Hour 62.

In Figure 3, note that all three groups overlap each other in the 5 to 12km range downwind. 

This is also affirmed in Figure 4, which shows the centerline concentrations oriented on the Hour

62 wind direction for each group, the total of the three groups, and the ISC3 centerline

concentrations for the receptors nearest the centerline. The centerline concentrations from the

three groups were added together to produce concentrations a factor of two greater than those

estimated by ISC3 at 15km.

Leading up to Hour 62, there were four hours of calm wind conditions. During calm winds,

CALPUFF assumes that the wind speed is zero. However, unlike ISC3 which treats the calm

hour as missing, CALPUFF increases the sigma values of each puff with respect to time. During

an hour of calm winds, the puffs have grown to the point that ground-level concentrations in this

study were calculated at 0.5 and 1.0 km from the puff centers in all directions for the first hour of

calm. After two hours, the effluent reached as far as 2km. The broadness of the Hour 52-56 and

Hour 57-60 groups is reflective of the puff spreading during the calm wind period.

 Details of this type of dispersion phenomena can be seen in Table 1. During Hour 57, the

Hour 57 puff releases penetrated a low mixing height (inversion) and continued to spread

horizontally without any concentrations contacting the ground. During the inversion rise in Hour

58, emissions were then mixed to the ground and Hour 57 emissions impacted receptors 0.5, 1

and 2km distance from the source while Hour 58 emissions were dispersed only to receptors at

0.5 and 1 km distance from the source.
14

3.2.3 Sensitivity Study

One of the major tasks of this study is to understand what types of concentrations will be

produced by CALPUFF with respect to ISC3. The results of ISC3 versus CALPUFF using the

puff and slug models were compared for three different climatological regions of the country. 

The results are displayed as a series of figures plotting the percent difference in concentrations at

various downwind distances with only ISC3 results in the denominator. Results consist of

maximum and highest of the second highest percent differences for 1-, 3-, 24-hour and annual

averages.

Table 1

CALPUFF Concentration Estimates under Calm Wind Conditions

Concentrations (μgm ) produced by: -3

Hour 57 Hour 57 Hour 58

Receptor Coordinates emissions at emissions at emissions at

X Y Hour 57 Hour 58 Hour 58

0.00 0.50 0.00 1340.44 2989.86

0.00 1.00 0.00 822.80 315.08

0.00 2.00 0.00 115.98 0.00

0.00 3.00 0.00 0.00 0.00

0.00 5.00 0.00 0.00 0.00

0.09 0.49 0.00 1340.68 2992.90

0.17 0.99 0.00 822.44 314.55

0.35 1.97 0.00 115.86 0.00

0.52 2.95 0.00 0.00 0.00

0.87 4.92 0.00 0.00 0.00

As illustrated in Figures 3 and 4, the explanation of why and how one model produces

higher concentrations than another can be complex. The effects of inversions, calm winds, wind

shifts, wind reversals, and plume and puff trajectory differences can all lead to enhanced or

reduced effluent impact. The results of these interactions are shown in Figures 5 and 6 (the

series is continued as Figures L-1 through L-7 in Appendix L).

As shown in Figure 5, the Medford plots contain the largest number of positive percentage

differences over the widest range of downwind distances. As was seen in Figures 3 and 4, the

results of calm winds and wind reversals can lead to higher than ISC3 average concentrations at
15

respective downwind distances. With the high percentage of calm winds, note also in the annual

average panel (Figure 5d) how the differences increase dramatically for the higher stacks as the

downwind distance decreases. This is caused by the ISC3 plume not reaching the ground, or not

fully dispersing to the ground, whereas CALPUFF can model effluent dispersion with wind

reversals for receptors near the stack base.

As shown in Figure 6, the overall difference pattern with respect to stack height and

downwind distances among the three sites is remarkably similar. Only the magnitude of the

differences and the downwind distance at which the values initially converge is different. The

Pittsburgh plots tend to slope downward with respect to the others but overall the patterns remain

the same with respect to stack height and downwind distance.

As illustrated in Figures L - 1 through L - 7 (Appendix L), sometimes a pattern or trend can

be seen by comparing subsequent or related figures only to find an exception in another figure. 

All of this may be the result of complex interactions that are likely to occur in any of the

climatological regimes. For instance, Medford, Oregon has a high percentage of calm winds. If

these calm wind events are coupled with a wind reversal, the same situation illustrated in Figure

3 for Boise, Idaho can occur. The patterns in the percentage differences may reflect a general

pattern found at that site but the pattern can be overlaid by a situation often found at another site.
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent D ifference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent D ifference

Stack H eig ht K ey

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent D ifference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent D ifference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

16

Figure 5a. Results for 1-hour averages using Figure 5b. Results for 3-hour averages using

1990 Medford data. 1990 Medford data.

Figure 5c. Results for 24-hour averages using Figure 5d. Results for annual averages using

1990 Medford data. 1990 Medford data.

Figure 5. Percent differences (ISC3 vs. CALPUFF slug model) as a function of downwind

distance for the highest 2nd high concentrations; 1-, 3-, 24-, and annual averages. 

Data are for Medford, Oregon. Note: .
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent D ifference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent D ifference

Stack H eig ht K ey

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent D ifference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

17

Figure 6a. Results for annual averages using Figure 6b. Results for annual averages using

1991 Boise data. 1990 Medford data (repeat of Fig. 5d).

Figure 6c. Results for annual averages using

1964 Pittsburgh data.

Figure 6. Percent differences (ISC3 vs. CALPUFF slug model) as a function of downwind

distance for annual averages, all three sites. Note: .
18

4. Summary and Conclusions

4.1 Steady State Meteorological Conditions

CALPUFF and ISCST3 were run with identical meteorological data sets to compare their

estimates. The meteorological data sets were synthesized to represent a variety of wind speeds in

each of the six P-G stability categories (54 cases in all). ISC3 was run in the “regulatory default”

mode and CALPUFF runs were configured to emulate this mode, which included the simulation

of emission releases as slugs (versus integrated puffs). Receptors were located along a straight

line (“due north”) at successively distant intervals. Sources included three elevated point sources

(35m, 100m and 200m). Surface releases included a 2m point source and a rectangular area

source 500m on a side. A typical volume source was also examined. For point sources, model

runs were done for two regimes, one in which the mixing height (Z ) was set to 3000m, and the i

other for Z = 500m. The latter regime was explored to inspect CALPUFF’s treatment of i

reflection. For each source type, a comparison matrix was created to assess comparison across

all meteorological conditions in terms of a variety of robust statistical indicators.

For all point sources (with Z = 3000m), the results indicated good accord between the two i

dispersion models. For all meteorological conditions, the mean fractional bias across the

receptor radial was well below 10%. Maximum residuals at any receptor were on the order of

0.1% of the concentration mean at the incident receptor. While a qualitative inspection of

residuals as they appear along the receptor array indicated no distinct pattern of bias, a slight

negative bias (CALPUFF relative to ISC3) is apparent for the 2m source, while the reverse is true

for the elevated sources. For the low mixing height regime (Z = 500m), the comparison results i

were strikingly similar, suggesting that both CALPUFF and ISC3 treated reflection identically.

The area source was simulated with mixing height set to 3000m. One set of runs was done

with initial  set to 0, while the other was set to 2.5m. The best accord was seen for the former z

case, but for about one fifth of the cases, the mean fractional bias was greater than 10%. The

maximum absolute residual was 33% of the concentration mean at the incident receptor. There

was an apparent trend toward negative bias (CALPUFF relative to ISC3), but there was

substantial variance as well. Mean residuals and mean standard deviations ranged over three

orders of magnitude. Accord improved (and variance diminished) with higher wind speeds,

which is expected as the slugs are stretched from the point of origin. 

With a test version of ISC3 in which the virtual source treatment was “corrected”, the

models showed close agreement in their treatment of the volume source. Maximum absolute

residual was well below one percent of respective concentration means at incident receptors. For

all cases, mean fractional bias was zero. A very slight tendency for negative bias was seen for

the stable stability categories, and (as for the area source) variance diminished with higher wind
19

speed. A qualitative inspection of residuals as they appear along the receptor array indicated

slightly more bias for receptors in the near field of the source.

4.2 Variable Meteorological Conditions

To examine differences in model estimates when variable meteorological data are used,

several studies were done. Actual full-year data sets from three climatologically different sites

were used. The sites chosen were Boise, Idaho (1991), Medford, Oregon (1990) and Pittsburgh,

Pennsylvania (1964). Using a synthesized meteorological data set, a preliminary set of studies

was done to examine (1) differences in the way both models treat lateral ’s (CALPUFF was run

using both puffs and slugs), and (2) puff versus slug differences within CALPUFF alone. 

Another study was done using the Boise data to examine the occurrence and location of

concentration maxima estimated by ISC3 and the CALPUFF puff model. Then for all three sites,

extensive sensitivity studies were done in which estimates by ISC3 were compared to CALPUFF

(puff and slug models). In general, 36 - 45 receptors were placed on each of 15 concentric rings

at successively more distant intervals.

In general, the differences between CALPUFF and ISC3 concentration results are caused by

how emissions are transported and dispersed. CALPUFF limits downwind transport in based on

the wind speed while there is no such limitation in ISC3 (it is a plume model). Under calm wind

conditions, CALPUFF continues to disperse each puff while the ISC3 model is arbitrarily set to

not determine concentrations when the wind speed is less than 1 ms . CALPUFF is capable of -1

tracking the puff emitted before, during and after wind shifts and reversals while ISC3 is only

concerned with the current hour transport of its plume(s). CALPUFF continues to disperse each

puff even when they are above an inversion layer while ISC3 determines its plume is above the

inversion layer and cannot be advected to the ground (e.g,. concentrations = 0.0). When the

inversion rises above the old puffs, they are dispersed to the ground creating impacts for any

nearby receptors.

When all these and other meteorological conditions are recorded on an hourly basis and

form a complete year of meteorological data, the effects on concentrations vary between the

models and from region to region. The meteorologically induced variations in concentrations do

not appear to be so much a regional phenomena, but the variations are related to how the hourly

meteorological conditions occur preceding and during a given averaging period. It is possible to

have 4 or 5 hours of winds in one general direction followed by 4 hours of calm winds, and then

followed by several hours of reversed wind flow. This can occur in any one of the regions. 

However, the potential frequency of this occurrence may be higher for one region than another. 

Since calm winds have a causal relationship leading to higher concentrations, then a site such as

Medford with a relatively greater incidence of calms (i.e., 22% calm hours versus the other

regions having around 6%) will have higher concentrations associated with CALPUFF.
20

4.3 Conclusion

Even though ISC3 and CALPUFF can be made to produce the same concentrations in a

steady state environment, a variable state environment can produce higher-than-ISC3 groundlevel concentrations with CALPUFF. Climatological characteristics of a region appear to be a

factor, but the accumulation of hour by hour meteorological conditions on the transport of

CALPUFF puffs is the key to understanding the differences that are produced by these two

models. This should come as no surprise as the meteorological assumptions used in formulating

the downwind transport of the ISC3 and CALPUFF effluents and the dispersion from the

respective plumes and puffs are different. This is also compounded by the different treatment of

dispersion during calm wind conditions. 

This complex interaction of transport, vertical mixing, and dispersion have an effect on

concentrations with respect to downwind distances in CALPUFF. Occasionally, the

accumulation of mass released over several hours will be transported in such a manner that the

combined effect is to produce sharp localized maxima in simulated concentration values. The

occurrence of such events is not predictable. It seems to occur with greater frequency at

Medford. Calm winds play a part in these events. These maxima seem to occur at most

locations in the receptor network, at all downwind distances. When they occur, they seem to

affect in particular the results for the shorter averaging periods.

Overall trends have been noted in the percentage difference comparisons in simulated

concentration values between CALPUFF and ISC3. For taller point sources, there is a trend

toward higher concentrations being simulated by CALPUFF in comparison to ISC3. For annual

averages, the closer a receptor is to the source and the taller the stack, the greater the chance that

the CALPUFF concentration values will be higher than those simulated by ISC3. At the more

distant downwind receptor rings, the bias changes direction from CALPUFF yielding higher

concentrations, to CALPUFF yielding relatively lower concentrations and sometimes these

concentrations are lower than their respective ISC3 counterpart.
21

5. References

1. Environmental Protection Agency, 1995. User’s Guide for the Industrial Source Complex

(ISC3) Dispersion Models. Volumes I - III. EPA-454/B-95-003a-c.

2. Environmental Protection Agency, 1995. A User’s Guide for the CALPUFF Dispersion

Model. EPA-454/B-95-006.

3. Environmental Protection Agency, 1995. SCREEN3 User's Guide. EPA-454/B-95-004.
A - 1

Appendix A

Switch settings for CALPUFF input file

to emulate ISC3's "Regulatory Default" mode

For these comparisons, CALPUFF was run to emulate ISC3's "regulatory mode” (i.e.,

default). Thus, to ensure equivalence for this emulation, certain of CALPUFF's switches were

set as follows:

METFM = 2 ASCII input file used for input

MSLUG = 1 Puffs emitted as slugs

MDRY = 0 Dry deposition NOT used, unless specified otherwise

MWET = 0 Wet deposition NOT used, unless specified otherwise

MSHEAR = 0 Vertical wind shear NOT modeled

WSCALM = 0.9999 A value of 1 ms for the calm wind speed threshold causes a rounding -1

problem

AVET = 3 Averaging times for 's is 60 min;  is adjusted as (AVET/60) y

0.2

MTRANS = 0 NO transitional plume rise (i.e., final plume rise only)

MDISP = 3 PG dispersion coefficients for RURAL areas, computed using the ISC

multi-segment approximation

MGAUSS = 1 Vertical dispersion used in the near-field is Gaussian

MCHEM = 0 NO chemical treatment used 

MROUGH = 0 PG  and  NOT adjusted for roughness y z

MPARTL = 0 No partial plume penetration of elevated inversion

MCTADJ = 1 ISC-type of terrain adjustment

MTIP = 1 Stack tip downwash used

PLXO(6) Default wind speed profile power-law exponents for P-G categories A-F

PTGO(2) Default vertical  gradient (Km ) for stable P-G categories E & F -1

For all applicable sources, CALPUFF employs buoyancy induced dispersion (BID); a feature

enabled in ISC3's regulatory mode. Consistent with ISC3's regulatory default mode, missing data

processing was NOT used.
 Wind speed is at 10m and values are the same as those used in SCREEN3. For each combination of P-G stability category, 1

comparisons for point sources were made with Z = 500m and 3000m. For the area and volume source, Z = 3000m. i i

B - 1

Appendix B

Meteorological conditions for the steady state CALPUFF/ISC3 comparisons1

P-G Wind Speed (ms )-1

A 1.0 1.5 2.0 2.5 3.0

B 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

C 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 8.0 10.0

D 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 8.0 10.0 15.0 20.0

E 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

F 1.0 1.5 2.0 2.5 3.0 3.5 4.0
 In one set of comparisons,  was set to zero. 1

z(init)

 Parameter values taken from Figure 9 of SCREEN3 User's Guide (Reference 3); buoyancy flux and momentum flux = 0; rural 2

option.

C - 1

Appendix C

Characteristics for sources used in the CALPUFF/ISC3 comparisons

Point Sources

Stack height Exit velocity Temperature

(m) (ms ) (K)

X,Y location & Emission Stack

base elevation rate diameter

(m) (gs ) (m) -1

-1

2 0, 0, 0 100 10.0 0.5 300

35 0, 0, 0 100 11.7 2.4 432

100 0, 0, 0 100 18.8 4.6 416

200 0, 0, 0 100 26.5 5.6 425

Ground-level Area Source

Area (m ) rate Release 2 Length of side Initial

(m)  (m)

Emission Effective

(gs m ) Height (m) -1 -2 z

1

250,000 500 0.0004 1.0 2.5

Volume Source2

Emission Rate (gs ): 1.0 -1

Release height (m): 10

Initial y (m): 50

Initial z (m): 20
D - 1

Appendix D

Receptor array used in the steady state CALPUFF/ISC3 comparisons

Receptors are aligned along a 360 radial at these distances (m):

1 100 32 4000

2 200 33 4500

3 300 34 5000

4 400 35 5500

5 500 36 6000

6 600 37 6500

7 700 38 7000

8 800 39 7500

9 900 40 8000

10 1000 41 8500

11 1100 42 9000

12 1200 43 9500

13 1300 44 10000

14 1400 45 15000

15 1500 46 20000

16 1600 47 25000

17 1700 48 30000

18 1800 49 35000

19 1900 50 40000

20 2000 51 45000

21 2100 52 50000

22 2200 53 55000

23 2300 54 60000

24 2400 55 65000

25 2500 56 70000

26 2600 57 75000

27 2700 58 80000

28 2800 59 85000

29 2900 60 90000

30 3000 61 95000

31 3500 62 100000
E - 1

Appendix E

Puffs versus Slugs: CALPUFF’s Two Simulation Modes

CALPUFF may be operated in one of two modes for simulating emissions: puff or slug. In

the puff mode, a continuous plume is represented as a number of discrete packets of pollutant

material. Most puff models evaluate the contributions of a puff to the concentration at a receptor

by a “snapshot” approach, where each puff is “frozen” at particular time intervals, the

concentration due to the frozen puff at that time is computed, and the puff is then allowed to

move, evolving in size, strength, etc. until the next sampling step. The total concentration at a

receptor is the sum of the contributions of all nearby puffs averaged for all sampling steps within

the basic time step. A traditional drawback of the puff approach has been the need for the release

of many puffs to adequately represent a continuous plume close to the source. Another potential

problem arises if the puffs do not overlap sufficiently, causing concentrations at receptors located

in the gap between puffs at the time of the “snapshot” to be underestimated, while those at the

puff centers are overestimated. One alternative to the problems posed by the “snapshot”

approach is the use of the integrated sampling function (originally implemented in MESOPUFF

II). This technique is available in CALPUFF as the integrated puff approach, and is fully

described in Section 2.1.1 of the CALPUFF User’s Guide (Reference 2).

Another approach available in CALPUFF uses a non-circular puff (slug) elongated in the

direction of the wind to eliminate the need for frequent releases of puffs. Thus in the slug model,

the “puffs” consist of Gaussian packets of pollutant material stretched in the along wind

direction. A slug can be visualized as a group of overlapping circular puffs having very small

puff separation distances. Actually, the slug represents the continuous emission of puffs, each

containing an infinitesimal mass. The concentrations near the endpoints of the slug (both inside

and outside of the body of the slug) fall off in such a way that if adjacent slugs are present, the

plume predictions will be reproduced when the contributions of those slugs are included (and this

is with steady state conditions). As with circular puffs, each slug is free to evolve independently

in response to local effects of dispersion, chemical transformation, removal, etc. However,

unlike puffs, the endpoints of adjacent slugs are constrained to remain connected (like country

sausages). This ensures continuity of a simulated plume without the gaps associated with the

puff approach. It should be noted that all receptors lying outside of the slug’s ±3y envelope

during the entire averaging time interval are eliminated from consideration. And for those

receptors remaining, integration time limits are computed such that sampling is not performed

when the receptor is outside of the ±3 envelope. This technique is available in CALPUFF as y

the slug approach, and is fully described in Section 2.1.2 of the CALPUFF User’s Guide

(Reference 2).

When initial CALPUFF runs were made for point sources, a disparity was seen between

concentration estimates produced by CALPUFF run in the slug mode versus those produced by

ISC3. This discrepancy was unexpected and the matter was brought to the attention of Earth

Tech (CALPUFF’s developer). Earth Tech determined that the reason the slug model in Version

960612 did not reproduce the plume model (ISC3) was due to the computation algorithm for

sigmas. In the 960612 version, the receptor-specific sigma was computed by determining the
E - 2

sigma that the puff would have at the receptor, even if the puff hasn't reached the receptor yet (as

does a plume model). This gave nearly exact reproduction of plume results under steady-state

conditions.

However, under non-steady conditions and very high sigma growth rates (e.g., under P-G

category A), this extrapolation can produce puff impacts prematurely (and hence the causality

effect is compromised somewhat). Therefore, the sigmas were "clipped" at the value at the end

of the slug when the receptor is beyond the end of the slug. This arrangement did reasonably

well for causality effects, but caused some deviation from the plume results under steady state

conditions.

As a result of Earth Tech’s investigation of this disparity, an experimental version of

CALPUFF was made available to EPA for the purposes of this comparison, and all analyses were

done with this version. This version compromised between the two solutions described above. 

The version only allows concentrations to be computed for receptors that are within 4 x (where

x is the horizontal puff dispersion parameter) of a puff centroid. This technique was seen to

perform much better with respect to both treating causality and reproducing plume results, and

will be incorporated in the next model to be released (Joe Scire, pers. comm., December 1997).
 ̄ ( CALPUFF ISC3

2 )

 ̄

0.10 FB 0.10

%Rmin

%Rmax

R ̄

min

R ̄

max

FB

FBmin

FBmax

 See text for explanation of ( C1H200 ), etc. a

 This value for R is 0.03% of at this receptor. = 24836 μgm ; = 24844 μgm b -3 -3 i CALPUFF ISC3

 This value for R is 0.13% of at this receptor. = 19040 μgm ; = 19015 μgm c -3 -3

i CALPUFF ISC3

 There were 216 distinct cases. The "goal" for this range is: d

 There is no unique run associated with this value. e

F - 1

Appendix F

Summary statistics from performance matrix - point sources (Z = 3000m) i

Emissions simulated as: SLUGSa

 (%) -0.04 (C1H200)

 (%) 0.13 (D1H100)

R (μgm ) -8.0 (A3H2 @ 100m) i(min)

-3 b

R (μgm ) 25.0 (F2p5H2 @ 500m) i(max)

-3 c

 (μgm )-3 -0.1 (see footnote e)

 (μgm )-3 0.4 (F2p5H2)

 (μgm ) 0.0 (see footnote e) R(min)

-3

 (μgm ) 3.2 (F2p5H2) R(max)

-3

# cases "out of range":d NONE

0.0 (see footnote e)

0.02 (D20H100)

FB -0.18 (A1H2 @ 100km) i(min)

FB 0.53 (D20H100 @ 800m) i(max)
 ̄ ( CALPUFF ISC3

2 )

 ̄

0.10 FB 0.10

%Rmin

%Rmax

R ̄

min

R ̄

max

FB

FBmin

FBmax

 There is no unique run associated with this value. a

 This value for R is 33% of at this receptor. = 3869 μgm ; = 5406 μgm b -3 -3

i CALPUFF ISC3

 This value for R is 2.2% of at this receptor. = 25719 μgm ; = 25158 μgm c -3 -3

i CALPUFF ISC3

 There are 54 distinct cases. The "goal" for this range is: d

G - 1

Appendix G

Summary statistics from performance matrix - area source (emissions simulated as slugs) 

Initial z (m): 0 2.5

 (%) -1.5 (F1AREA) -3.2 (F1AREA)

 (%) -0.07 (see footnote a) -0.66 (see footnote a)

R (μgm ) -1537 (F1AREA @ 3500m) -4212 (F1AREA @ 300m) i(min)

-3 b

R (μgm ) 561 (E1AREA @ 100m) 0.08 (A1AREA @ 85km) i(max)

-3 c

 (μgm )-3 -548 (F1AREA) -969 (F1AREA)

 (μgm )-3 -1.1 (D20AREA) -10.5 (D20AREA)

 (μgm ) 2.4 (D20AREA) 28.3 (D20AREA) R(min)

-3

 (μgm ) 510 (F1AREA) 895 (F1AREA) R(max)

-3

# cases "out of range":d 10 19

-0.16 (see footnote a) -0.19 (F1AREA)

-0.02 (see footnote a) -0.04 (see footnote a)

FB -0.39 (E1AREA @ 4000m) -0.40 (E1AREA @ 4000m) i(min)

FB 0.05 (A1AREA @ 85km) 0.05 (A1AREA @ 85km) i(max)
H - 1

Appendix H

ISCST3's Treatment of Virtual Sources 

For volume sources and point sources subject to building wake dispersion, ISC3 makes use of

a virtual source to simulate an initial plume size. That is, if a source has a finite size at the point of

release, its initial y z and  are "matched" to a point on the corresponding dispersion curve. Because

these curves prescribe the dispersion parameters as a function of distance (starting with a value of

zero at a downwind distance equal to zero), matching the curve to a source with a non-zero initial

sigma entails shifting the apparent position of the source upwind. This shift is known as the virtual

position of the source. If x denotes the distance of the virtual location of the source upwind of its v

actual location, then the value of the dispersion parameter at some distance (x) downwind of the

source should be evaluated at the modified distance (x + x ). v

ISC3 adopts this general method, but modifies its implementation in the following way. 

Because the P-G curves for z are expressed as the function ax , where the parameters a and b b

themselves depend on the distance, "the ISC model programs check to ensure that the x used to v

calculate z v at (x + x ) is calculated using coefficients a and b that correspond to the distance

category specified by the quantity (x + x )." (Vol. II of the ISC3 User's Guide (Section 1.1.5.2, v

p. 1-20) with the notation for the virtual distance changed from x to x . The term x is calculated zv v

using Equation 1-36.)

The result of this implementation is that the virtual distance becomes a function of receptor

distance downwind of the source, and in fact x is reevaluated at each receptor the plume encounters v

as it moves downwind. Thus, the computed curve of z as a function of distance is no longer the

continuous P-G curve. This error is illustrated in the following figures. ISC3 was applied to a

volume source with an initial  of 5m and 20m, respectively, and concentrations were obtained at z

receptors within 1000m, for both P-G stability classes A and F (Figs. H-1 to H-4). Using

strategically placed write statements in CALC1.FOR (one of ISCST3's files), the computed virtual

distances and the corresponding z values were written to a diagnostic file. These values were then

plotted in the figures below as open squares (virtual distances) and as solid circles ( ) in the figures z

below. Figures H-1 and H-2 are for the P-G A stability category, while Figures H-3 and H-4 are for

P-G F.

In Figure H-1, the virtual distance begins at 33.76m, and grows in steps corresponding to the

"distance ranges" (Table 1-3 of Vol. II) for this P-G curve to almost 120m. The corresponding z

values "jump" each time a new virtual distance is used. The same phenomenon can be seen in the

other figures, and the departure (ISC3 z z versus P-G  ) increases with downwind distance. Had

more receptors been placed near each of the transition points, a clear "break" in the z curve would

have been resolved.

For the purpose of the CALPUFF/ISC3 comparison, ISCST3 was re-configured so that a single

value of the virtual distance is computed as a joint function of P-G category and initial z, with due

regard for the distance ranges imposed on selecting a & b. This single value is then added to all

receptor distances, and the corresponding value for z computed. Figure H-1 indicates the resulting

contour (depicted with open triangles) and suggests the continuous P-G curve for stability class A,

for a virtual location 33.76m upwind of x = 0m.

A version of ISC3 with a corrected virtual source algorithm (dated 97363), as was used in this

comparison, was released in January 1998 and uploaded to EPA’s SCRAM web site for public use.
Stability A; Initial σz = 5m

10

100

1000

10000

100000

10 100 1000 10000

Distance Downwind (m)

Vertical Dispersion, Sz (m)

20.00

40.00

60.00

80.00

100.00

120.00

Virtual Distance, Xz (m)

Current Sz

Corrected Sz

Xz

H - 2

Figure H-1. Profile of  with distance (m); P-G A and  = 5m. z z(init)
Stability A; Initial σz = 20m

10

100

1000

10000

100000

10 100 1000 10000

Distance Downwind (m)

Vertical Dispersion, Sz (m)

140.00

160.00

180.00

200.00

220.00

240.00

Virtual Distance, Xz (m)

Current Sz

Corrected Sz

Xz

H - 3

Figure H-2. Profile of  with distance (m); P-G A and  = 20m. z z(init)
Stability F; Initial σz = 5m

0.00

10.00

20.00

30.00

40.00

10 100 1000 10000

Distance Downwind (m)

Vertical Dispersion, Sz (m)

50.00

100.00

150.00

200.00

250.00

300.00

Virtual Distance, Xz (m)

Current Sz

Corrected Sz

Xz

H - 4

Figure H-3. Profile of  with distance (m); P-G F and  = 5m. z z(init)
Stability F; Initial σz = 20m

20.00

24.00

28.00

32.00

36.00

40.00

10 100 1000 10000

Distance Downwind (m)

Vertical Dispersion, Sz (m)

1550.00

1600.00

1650.00

1700.00

1750.00

1800.00

Virtual Distance, Xz (m)

Current Sz

Corrected Sz

Xz

H - 5

Figure H-4. Profile of  with distance (m); P-G F and  = 20m. z z(init)
 ̄ ( CALPUFF ISC3

2 )

 ̄

0.10 FB 0.10

%Rmin

%Rmax

R ̄

min

R ̄

max

FB

FBmin

FBmax

 There is no unique run associated with this value. a

 This value for R is 0.4% of at this receptor. = 238.6 μgm ; = 239.5 μgm b -3 -3

i CALPUFF ISC3

 This value for R is 0.1% of at this receptor. = 135.1 μgm ; = 135.0 μgm c -3 -3

i CALPUFF ISC3

 There were 54 distinct cases. The "goal" for this range is: d

I - 1

Appendix I

Summary statistics from performance matrix - volume source 

Emissions simulated as: SLUGS

 (%) -0.07 (see footnote a)

 (%) 0.01 (see footnote a)

R (μgm ) -0.92 (F1VOL @ 200m) i(min)

-3 b

R (μgm ) 0.15 (C1VOL @ 200m) i(max)

-3 c

 (μgm )-3 -0.2 (F1VOL)

 (μgm )-3 0.0 (see footnote a)

 (μgm ) 0.0 (see footnote a) R(min)

-3

 (μgm ) 0.22 (F1VOL) R(max)

-3

# cases "out of range":d NONE

0.0 (see footnote a)

0.0 (see footnote a)

FB -0.18 (A1VOL @ 100km) i(min)

FB 0.06 (A1VOL @ 85km) i(max)
J - 1

Appendix J

Wind Rose Patterns
J - 2

Appendix J, continued.
K - 1

Appendix K

CALPUFF Concentrations Estimated by Integrated Puff and Slug Model

Coordinates Hour 1 Hour 2

X Y Puff Puff Slug

0 0.5 0 0 0

0 10 0 0

0 20 0 0

0 30 0 0

0 50 0 0

0.09 0.49 0 4400.88 4011.7

0.17 0.99 0 4606.96 3785.43

0.35 1.97 0 2366.87 1525.9

0.52 2.95 0 1408.23 658.51

0.87 4.92 0 670.78 74.94

0.17 0.47 0 0 75.18

0.34 0.94 0 0 211.99

0.68 1.88 0 0 284.43

3.19 2.82 0 0 271.96

1.71 4.7 0 0 233.71

0.25 0.43 0 0 93.83

0.5 0.87 0 0 234.87

 1 1.73 0 0 284.04

1.5 2.6 0 0 266.32

2.5 4.33 0 0 229.02

0.32 0.38 0 0 108.62

0.64 0.77 0 0 248.26

1.29 1.53 0 0 282.78

1.93 2.3 0 0 262.57

3.21 3.83 0 0 226.52

0.38 0.32 0 0 118.39

0.77 0.64 0 0 254.07

1.53 1.29 0 0 261.61

2.3 1.93 0 0 261.61

3.83 3.21 0 0 226.13

0.43 0.25 0 0 122.47

0.87 0.5 0 0 255.92

1.73 1 0 0 281.55

2.6 1.5 0 0 261.75

4.33 2.5 0 0 227.49

0.47 0.17 0 0 120.31

0.94 0.34 0 0 254.23

1.88 0.68 0 0 282.08

2.82 1.03 0 0 263.61

4.7 1.71 0 0 230.65

0.49 0.09 4445.83 0 103.69

0.99 0.17 4593.7 0 234.05

1.97 0.35 2440.53 0 269.69

2.95 0.52 1413.91 0 254.14

4.92 0.87 678.28 0 226.08

0.5 0 0 0 0
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

Stack Height Key

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

L - 1

Puff vs. ISC3 Slug vs. ISC3

Figure L-1(a). Boise meteorological data. Figure L-1(b). Boise meteorological data.

Figure L-1(c). Medford meteorological data. Figure L-1(d). Medford meteorological Data.

Figure L-1(e). Pittsburgh meteorological data. Figure L-1(f). Pittsburgh meteorological data.

Figure L-1. Maximum 1-hour average concentrations by distance. Figures a, c, & e show

CALPUFF puffs, whereas figures b, d, & f show slugs.

Note: .
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

Stack H eig ht K ey

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (k m )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

L - 2

Puff vs. ISC3 Slug vs. ISC3

Figure L-2(a). Boise meteorological data. Figure L-2(b). Boise meteorological data.

Figure L-2(c). Medford Meteorological Data. Figure L-2(d). Medford meteorological Data.

Figure L-2(e). Pittsburgh meteorological data. Figure L-2(f). Pittsburgh meteorological data.

Figure L-2. Maximum 3-hour average concentrations by distance. Figures a, c, & e show

CALPUFF puffs, whereas figures b, d, & f show slugs.

Note: .
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Dow nwind (km )

-200

0

200

400

600

Percent Difference

Stack H eig ht K e y

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (k m )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

L - 3

Puff vs. ISC3 Slug vs. ISC3

Figure L-3(a). Boise meteorological data. Figure L-3(b). Boise meteorological data.

Figure L-3(c). Medford meteorological data. Figure L-3(d). Medford meteorological data.

Figure L-3(e). Pittsburgh meteorological data. Figure L-3(f). Pittsburgh meteorological data.

Figure L-3. Maximum 24-hour average concentrations by distance. Figures a, c, & e show

CALPUFF puffs, whereas figures b, d, & f show slugs.

Note: .
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

Stack H eig ht K ey

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

L - 4

Puff vs. ISC3 Slug vs. ISC3

Figure L-4(a). Boise meteorological data. Figure L-4(b). Boise meteorological data.

Figure L-4(c). Medford meteorological data. Figure L-4(d). Medford meteorological data.

Figure L-4(e). Pittsburgh meteorological data. Figure L-4(f). Pittsburgh meteorological data.

Figure L-4. Maximum annual average concentrations by distance. Figures a, c, & e show

CALPUFF puffs, whereas figures b, d, & f show slugs.

Note: .
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

Stack H eig ht K ey

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

L - 5

Puff vs. ISC3 Slug vs. ISC3

Figure L-5(a). Boise meteorological data. Figure L-5(b). Boise meteorological data.

Figure L-5(c). Medford meteorological data. Figure L-5(d). Medford meteorological data.

Figure L-5(e). Pittsburgh meteorological data. Figure L-5(f). Pittsburgh meteorological data.

Figure L-5. Highest of the second highest 1-hour average concentrations by distance. Figures

a, c, & e show CALPUFF puffs, whereas figures b, d, & f show slugs.

Note: .
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

Stack H eig ht K ey

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

L - 6

Puff vs. ISC3 Slug vs ISC3

Figure L-6(a). Boise meteorological data. Figure L-6(b). Boise meteorological data.

Figure L-6(c). Medford meteorological data. Figure L-6(d). Medford meteorological data.

Figure L-6(e). Pittsburgh meteorological data. Figure L-6(f). Pittsburgh meteorological data.

Figure L-6. Highest of the second highest 3-hour average concentrations by distance. Figures

a, c, & e show CALPUFF puffs, whereas figures b, d, & f show slugs.

Note: .
1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

Stack H eig ht K ey

 2 M eter

 35 M eter

100 M eter

200 M eter

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

1 10 100 1000

Distance Downwind (km )

-200

0

200

400

600

Percent Difference

% Difference  100 ( CALPUFF  ISC3

ISC3

)

L - 7

Puff vs. ISC3 Slug vs. ISC3

Figure L-7(a). Boise meteorological data. Figure L-7(b). Boise meteorological data.

Figure L-7(c). Medford meteorological data. Figure L-7(d). Medford meteorological data.

Figure L-7(e). Pittsburgh meteorological data. Figure L-7(f). Pittsburgh meteorological data.

Figure L-7. Highest of the second highest 24-hour average concentrations by distance. 

Figures a, c, & e show CALPUFF puffs, whereas figures b, d, & f show slugs. 

Note: .