& EPA
United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park. NC 27711
EPA-454/R-94-018
July 1994
Air
COMPARISON OF ISC2 DRY DEPOSITION
ESTIMATES BASED ON CURRENT AND
PROPOSED DEPOSITION ALGORITHMS
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COMPARISON OF ISC2 DRY DEPOSITION ESTIMATES BASED ON
CURRENT AND PROPOSED DEPOSITION ALGORITHMS
Office of Air Quality Planning and Standards
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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DISCLAIMER
This report has been reviewed by the U.S. Environmental
Protection Agency (EPA) and has been approved for publication as
an EPA document. Any mention of trade names or commercial
products does not constitute endorsement or recommendation for
use.
11
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PREFACE
The ability to accurately estimate deposition of particulate
matter is of special concern in assessing environmental impacts
from a variety of sources including Superfund sites, municipal
waste incinerators, and surface coal mines. The limitations of
the Industrial Source Complex (ISC2) model (version dated 92273)
for use in such assessments are discussed elsewhere and are noted
in the following. The deposition algorithm currently employed in
ISC2 was developed for applications to large particles dominated
by gravitational settling (i.e., particles greater than about 20
fw. diameter) . The current algorithm was not intended for use
with small particles and, in fact, includes an assumption that
particles less than 5.7 fj.m in diameter are totally reflected at
the surface and, thus, experience no deposition. This is in
conflict with recent observations.
In light of these limitations, a new deposition algorithm
which will handle the full range of significant particle sizes
(0.1 to 100 /zm) is being considered for use in ISC2. A
description of the new algorithm including comparisons with
alternative methods for estimating deposition is provided in an
April 1994 EPA report "Development and Testing of A Dry
Deposition Algorithm (Revised)", EPA-454/R-94-015.
The new deposition algorithm has been tested within the
framework of the ISC2 model and comparisons of deposition
estimates using the old (current) and new deposition algorithms
have been made for a range of source types and particulate
emission scenarios. Similar comparisons have been made of
particulate concentration estimates as affected by the old and
new deposition algorithms. This report documents these analyses.
The Environmental Protection Agency must conduct a formal
public review before the Agency can recommend the new deposition
algorithm for use in regulatory analyses. Such a review will
enable EPA to assess the potential consequences of replacing the
old deposition algorithm in ISC2 with the new algorithm. This
report is being released in accordance with this review process.
The report is one of several reports on the ISC2 models that must
be considered before any formal changes can be adopted.
111
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TABLE OF CONTENTS
Page
1.0 Introduction 1
2.0 Methodology 2
2.1 Modeling Scenarios 2
2.1.1 Particle Size 2
2.1.2 Meteorological Data 3
2.1.3 Release Height 4
2.1.4 Receptor Location 4
2.2 Diagnostic Comparisons 5
2.3 Operational Comparisons 5
3.0 Results 6
3.1 Diagnostic Comparisons 6
3.2 Operational Comparisons 8
3.2.1 Deposition 8
3.2.2 Concentration 10
4.0 Summary and Conclusions 12
4.1 Deposition 12
4.2 Concentration 12
5.0 References 13
Appendix A A-1
Appendix B B-l
IV
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LIST OF TABLES
Table 1
Table 2
Table 3
Table 4
Table 5
Table 6
Table 7
Table 8
Particle diameter, gravitational settling
velocity, and reflection coefficient used in
modeling deposition.
Source parameters employed in modeling . . .
Highest 1-hour ISCST deposition flux estimates
(g/m2) based on the old and new algorithms
for a 35-m release.
Meteorological Conditions used in sensitivity ,
analyses.
Comparison of ISCST deposition flux estimates ,
based on the old and new algorithms for
selected averaging times, release heights,
and particle diameters as indicated.
Comparison of ISC2 annual deposition flux . .
estimates based on the old and new algorithms
for release heights and particle diameters as
indicated.
Comparison of ISCST concentration estimates as
affected by the old and new deposition
algorithms for selected averaging times,
release heights, and particle diameters as
indicated.
Comparison of ISC2 annual concentration . . .
estimates as affected by the old and new
deposition algorithms for release heights and
particle diameters as indicated.
4
7
14
15
16
17
v
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LIST OF FIGURES
Figure 1. Comparison of highest 1-hour ISCST deposition
estimates based on the old and new
deposition algorithms for a 35-m release
for 50 /zm diameter particles.
Figure 2. Comparison of highest 1-hour ISCST deposition
estimates based on the old and new
deposition algorithms for a 35-m release
for particle diameters as indicated.
Figure 3. Comparison of ISCST deposition estimates based
on the old and new deposition algorithms
for selected extreme meteorological
conditions and particle diameters as
indicated.
Figure 4. Fractional difference of highest 1-hour ISCST
deposition estimates based on the old and
new algorithms for particle diameters and
release heights as indicated.
Figure 5. Fractional difference of highest 3-hour ISCST
deposition estimates based on the old and
new algorithms for particle diameters and
release heights as indicated.
Figure 6. Fractional difference of highest 24-hour ISCST
deposition estimates based on the old and
new algorithms for particle diameters and
release heights as indicated.
Figure 7. Fractional difference of highest annual ISCST
deposition estimates based on the old and
new algorithms for particle diameters and
release heights as indicated.
Figure 8. Fractional difference of highest 1-hour ISCST
concentration estimates considering
effects of the old and new deposition
algorithms for particle diameters and
release heights as indicated.
19
. 20
21
22
. 23
. 24
. 25
VI
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LIST OF FIGURES (continued)
Figure 9. Fractional difference of highest 3-hour ISCST . 26
concentration estimates considering
effects of the old and new deposition
algorithms for particle diameters and
release heights as indicated.
Figure 10 Fractional difference of highest 24-hour ISCST . 27
concentration estimates considering
effects of the old and new deposition
algorithms for particle diameters and
release heights as indicated.
Figure 11 Fractional difference of highest annual ISCST . 28
concentration estimates considering
effects of the old and new deposition
algorithms for particle diameters and
release heights as indicated.
VI1
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Comparison of ISC2 Dry Deposition Estimates Based on
Current and Proposed Deposition Algorithms
1. INTRODUCTION
The Industrial Source Complex dispersion model [ISC2 (USEPA,
1992)] is a widely recommended refined model for use in
assessments of air quality impacts from particulate sources and
incorporates many features essential to the modeling of such
sources. These include area and volume source algorithms,
building wake algorithms, and a deposition algorithm based on
Dumbauld et al. (1976) and Overcamp (1976). The latter was
included in recently completed evaluations (USEPA, 1994)
comparing the performance of several dry deposition algorithms
and several plume depletion algorithms. As a result of these
evaluations, EPA is considering replacing the current deposition
algorithm in ISC2 (hereafter referred to as the "old" deposition
algorithm) with the deposition algorithm now employed in the Acid
Deposition and Oxidant Model [ADOM (Pleim et al., 1984)]. This
algorithm will be referred to hereafter as the "new" deposition
algorithm. In addition, EPA also proposes to incorporate a plume
depletion algorithm in ISC2. The proposed plume depletion
algorithm, a modified version of the source depletion technique
(Horst, 1977), was selected following the evaluations in USEPA,
1994.
This report documents analyses to assess the potential
consequences (i.e., differences in estimates of deposition and
design concentrations) of replacing the old deposition algorithm
in ISC2 with the new algorithm.
The old deposition algorithm in ISC2 simulates deposition as
the movement of particles toward the surface by the combined
processes of atmospheric turbulence and gravitational settling.
The method was developed for applications to large particles
dominated by gravitational settling; these are typically
particles greater than 20 /zm diameter. At the surface, a portion
of the plume determined by a user-specified reflection
coefficient1 is reflected from the surface; the remainder is
deposited, resulting in a reduction in concentration within the
plume. It should be noted that for small particles, the ISC2
reflection coefficient was derived based on extrapolation of the
gravitational settling-dominated data for larger particles
(Dumbauld et al., 1976). The extrapolation led to the assumption
that particles less than 5.7 /im in diameter are totally reflected
1 The empirical reflection coefficients recommended for use
with ISC2 are based on experiments involving aircraft releases of
two spray carriers (Duphar and No. 2 fuel oil) in which most of
the particles (more than 99 percent) exceeded 20 /xm diameter.
Particles of this large size are nearly completely controlled by
gravitational settling.
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at the surface and thus experience no deposition. This is
contrary to recent observations which indicate significant
deposition velocities for these small particles.
The new deposition algorithm is intended to simulate
processes important over the entire range of particle sizes. For
example, the proposed algorithm includes effects due to inertial
impaction and Brownian motion which control the deposition of
small particles. These effects, which are dependent on
meteorological and surface conditions, are not easily
parameterized within the framework of a reflection coefficient
algorithm such as used in the current ISC2.
The new deposition algorithm has been tested within the
framework of the ISC2 models [ISCST2 (Short-Term) and ISCLT2
(Long-Term)] and comparisons of deposition estimates using the
old and new deposition algorithms have been made for a range of
source types and particulate emission scenarios. Similar
comparisons have been made of particulate concentration estimates
as affected by the old and new deposition algorithms.
Documentation of these comparisons is provided in the following.
2. METHODOLOGY
2.1 Modeling Scenarios
2.1.1 Particle Size
In the development and testing of dry deposition algorithms
it was noted that the important physical processes affecting dry
deposition could be segregated by particle size (USEPA, 1994).
These processes include gravitational settling, which is dominant
for large particles (greater than 20 pan diameter), inertial
impaction, which is dominant in the size range from 1.0 to 20 /im,
and Brownian motion, which is important for small particles (less
than about 0.1 jzm) . In the comparisons to be presented, seven
particle size categories were employed to represent the important
range of particle sizes. These were: 0.1, 1.0, 10, 20, 50, 80,
and 100 /urn diameter. The characteristic settling velocities
(assuming spherical particles with a density of 1 g/cm3) and
reflection coefficients assigned to these particles for use in
modeling based on the old deposition algorithm are given in Table
1. For use in modeling, each particle size category was treated
as a separate source and assigned a mass fraction of 1.0.
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Table 1 Particle diameter, gravitational settling velocity, and
reflection coefficient used in modeling deposition
Particle
Diameter
(p.m)
0.10
1.00
10.0
20.0
50.0
80.0
100.
Settling
Velocity
(m/s)
0.00000
0.00003
0.003
0.012
0.074
0.190
0.300
Reflection
Coefficient
1.00
1.00
0.87
0.76
0.55
0.27
0.00
2.1.2 Meteorological Data
2.1.2.1 ISCST2
The meteorological data employed in the ISCST2 evaluations
consisted of one year (1984) of hourly surface data and
concurrent twice-daily mixing heights for the National Weather
Service (NWS) station at Oklahoma City, Oklahoma. These data
were processed using the EPA recommended Meteorological Processor
for Regulatory Models [MPRM (USEPA, 1990)]. In addition to the
standard set of meteorological variables required by Gaussian
dispersion models (i.e., wind direction, wind speed, stability,
temperature, and mixing height), the proposed new deposition
algorithm also requires estimates of the Monin-Obukhov length and
the surface friction velocity. These additional variables were
calculated using software adapted from the meteorological
preprocessor for the Hybrid Plume Dispersion Model (Hanna and
Chang, 1991). To facilitate subsequent analyses, all hourly wind
directions were set to a fixed value of 270 degrees (flow vector
of 90 degrees).
2.1.2.2 ISCLT2
The same meteorological data (Oklahoma City hourly surface
data for 1984) were also processed for use in ISCLT2. This
processing was accomplished using the PCSTAR software which
generates a joint frequency distribution of wind direction and
wind speed by stability class. The Monin-Obukhov length and
friction velocity required in the deposition algorithm are
generated internally within ISCLT2. The algorithm for computing
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the friction velocity is based on Wang and Chen (1980) and is the
same as was used for ISCST. The algorithm for computing the
Monin-Obukhov length is based on Colder (1972).
2.1.3 Release Height
Both deposition and concentration are dependent on release
height. The modeling performed for these evaluations, included a
surface release, and three elevated releases, 35-m, 100-m, and
200-m. The source parameters associated with these four releases
are given in Table 2.
Table 2 Source parameters employed in modeling
Stack
Height
(m)
0.1
35
100
200
Stack
Diameter
(m)
1.0
2.4
4.6
5.6
Exit
Velocity
(m/s)
0.1
11.7
18.8
26.5
Exit
Temp.
(K)
293
432
416
425
Buoyancy
Flux
(m4/s3)
53
288
633
Emission
Rate
(g/s)
1000
1000
1000
1000
2.1.4 Receptor Location
All receptors were assumed to be located^ in flat terrain and
were assigned an elevation equal to the base elevation of the
source. Receptor locations for use in ISCST and ISCLT are
described in the following.
2.1.4.1 ISCST2
Receptors for use in ISCST2, were placed at intervals along
the X axis beginning at 0.1 km from the source and extending to
50 km. A total of 35 receptors were employed in the modeling.
This arrangement of receptors in combination with a fixed 90
degree flow vector facilitates the identification of the highest
1-hour results which, in this analysis, are independent of wind
direction. The results obtained for longer averaging times
(i.e., 3-hour, 24-hour, and annual) which are dependent on wind
direction, will be conservative (higher) compared to results
where the wind direction is allowed to vary.
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2.1.4.2 ISCLT2
Screening analyses employing coarse polar grids were
conducted to determine the distance to the maximum deposition
using ISCLT2. Additional screening analyses were conducted using
a refined polar grid employing 26 rings extending to 10 km. The
screening showed that the highest deposition consistently
occurred at receptors located along the 360 degree radial.
Consequently, all subsequent analyses using ISCLT2 were conducted
with receptors located on the 360 degree radial. These receptors
were located at 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0,
1.2, 1.4, 1.6, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, 6, 7, 8, 9, and
10 km.
2.2 Diagnostic Comparisons
In conducting the ISC2 analyses, it became apparent that a
limited sensitivity study was needed to illustrate the
sensitivity of the new and old algorithms to particle size,
surface characteristics, meteorological conditions, release
height, and source/receptor distance. Consequently, sensitivity
analyses were constructed using 1-hour deposition and
concentration estimates from ISCST2. These sensitivity analyses
complement and extend the analyses in USEPA (1994).
2.3 Operational Comparisons
In a regulatory air quality analysis, ISC2 is used to
estimate expected extreme (highest and second highest)
concentrations for averaging times of interest, commonly, 1-hour,
3-hour, 24-hour and annual. Hence, a primary purpose of this
analysis is to assess the operational consequences of the new
algorithm by examining its effect on extreme value deposition and
concentration estimates. This is accomplished by exercising
ISCST2 and ISCLT2 in an operational mode and comparing the
deposition and concentration estimates based on the old and new
deposition algorithms. The performance measure used in the
comparisons is patterned on the 'Fractional Bias' (Cox and
Tikvart, 1990) and is defined here as the 'Fractional Difference'
(FD) :
FD = 2 (NEW - OLD) / (NEW + OLD)
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3. RESULTS
3.1 Diagnostic Comparisons
The results of the diagnostic comparison are presented first
since they are essential to the understanding of the results of
the operational comparisons. An initial pilot analysis was
conducted for the 35-m release height to determine the maximum
deposition as a function of particle size for all receptors and
all meteorological conditions (i.e., unpaired in space and time),
and to identify the meteorological conditions associated with
these maxima.
Results (old and new deposition estimates as a function of
downwind distance) for the 50 /*m diameter particle are presented
in Figure 1. The maximum deposition using the old algorithm
occurs at 0.5 km under C stability with a wind speed of 15.4 m/s.
By comparison, the maximum deposition using the new algorithm
occurs at 3 km under F stability with a wind speed of 1.5 m/s.
The same conditions (F stability and 1.5 m/s) were associated
with a secondary maximum for the old algorithm which also occurs
at 3 km downwind. The higher maximum with C stability for the
old algorithm is a result of a combination of effects: first,
with the higher wind speed (15.4 m/s versus 1.5 m/s) the plume
rise from the 35-m stack is lower (19 meters versus 124 meters)
and second, the plume disperses more rapidly in slightly unstable
conditions (C stability) as compared to strongly stable
conditions (F stability). Similar graphs for other particle
sizes evaluated (10, 20, 50, 80, and 100 /im) are given in
Appendix A.
The old ISC2 deposition algorithm employs a 'tilted-plume'
approach for characterizing the gravitational descent of the
plume as it disperses downwind. The degree of tilt is dependent
on the ambient wind speed and the gravitational settling
velocity. The latter, as shown in Table 1, is a function of
particle size, density, etc. The old algorithm characterizes
deposition at the surface through the use of an empirical
reflection/absorption coefficient which is also a function of
particle size. The effective deposition velocity, defined by the
ratio of deposition flux and concentration, is dependent on plume
tilt and the interaction of reflection terms with the underlying
surface. The end result is that the effective deposition
velocity in the old algorithm decreases with downwind distance.
The new deposition algorithm also employs a tilted plume
approach for characterizing the gravitational descent of the
plume. Deposition at the surface is computed as the product of a
deposition velocity and the ground-level air concentration.
However, in contrast to the old algorithm, the deposition
velocity in the new algorithm is a function of surface conditions
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and meteorology, but is independent of distance from the source.
These differences preclude summarization of the results of these
comparisons in simple statements.
Table 3 and Figure 2 summarize the results of the above
analyses for all particle sizes (maximum deposition estimates
unpaired in space and time) for 1-hour averaging. For the
smaller (0.1 and 1.0 ^m - the old algorithm always results in no
deposition for these particles) and larger (50, 80, and 100 /zm)
particle sizes, the new algorithm results in higher deposition
estimates than the old algorithm. However, for intermediate
particle sizes (10 and 20 /zm) the new algorithm results in lower
deposition estimates.
Table 3 Highest 1-hour average ISCST deposition flux estimates
(g/m2) based on the old and new algorithms for a 35-m release.
Particle
Diameter
(Mm)
0.1
1.0
10
20
50
80
100
Maximum Deposition
(g/n?)
OLD
-
-
1.04 (*)
1.94 (*)
3.99 {*)
20.8 (**)
69.9 (**)
NEW
0.01
0.00
0.62
0.71
4.86
32.7
75.9
(*)
(*)
(*)
(*)
(**)
(**)
(**)
Distance to Max (m)
OLD
-
-
500
500
500
1400
900
NEW
600
600
600
600
3000
1200
800
Key to meteorological conditions noted
(*) C stability at 15.4 m/s
(**) F stability at 1.54 m/s
above
Additional modeling using the meteorological conditions
identified with the modeled extremes (see Table 4) was conducted
to assess the sensitivity of the algorithms to meteorology. In
the course of these analyses, two additional meteorological cases
were included: the first, C stability at 3 m/s, was included to
assess the sensitivity of the algorithms to wind speed; the
second, E stability at 1.54 m/s, was included to aid in
understanding the concentration results for a 200-m release of
100 jum particles. These latter two sensitivity analyses are
discussed in Appendix B.
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Table 4 Meteorological conditions used in sensitivity Analyses
P-G
Stability
Class
A
C
C
E
F
Wind
Speed
(m/s)
2.57
15.4
3.00
1.54
1.54
Temperature
CIO
307
295
295
297
306
Mixing
Height
(m)
1536
808
808
1368
2756
Friction
Velocity
(m/s)
0.29
1.35
0.30
0.07
0.07
M-0 Length
(m)
-8
-839
-60
10
10
Figure 3 shows the results of the analyses of the Table 3
extremes. It should be noted that this figure is similar to
Figure 2 except, in this case, the results are paired in time
(i.e., the old and new estimates were based on the same
meteorological conditions). As shown in Figure 3, the relative
magnitude of the deposition estimates based on the new and old
algorithms is dependent on the meteorology and, as indicated by
the solid lines, the ratio (slope) appears to be a constant for
particle sizes above about 20 /zm in diameter for a given
meteorological condition. In general, deposition estimates based
on the old algorithm exceed the estimates based on the new
algorithm for the unstable conditions, while the reverse is the
case for stable conditions. The two cases for stability class C
(3 m/s and 15 m/s) show the effect of wind speed on the
deposition estimates. At the lower wind speed the estimates from
the two algorithms are similar. At the higher wind speed, the
deposition estimates based on the old algorithm exceed those
based on the new algorithm.
3.2 Operational Comparisons
3.2.1 Deposition
3.2.1.1 ISCST2
Operationally, one is often interested in maximum deposition
estimates, unpaired in space and time. These would be cumulative
estimates; i.e., summed over some period of time (e.g., annual)
and over all particle sizes. The latter would be a weighted sum
based on the mass fraction of each particle size in the source
term.
Deposition estimates were computed for four time periods (1-
hour, 3-hour, 24-hour, and annual) for each of four release
scenarios and each of seven particle size categories. The
results are given in Table 5. Note that for each old/new pair of
estimates, the corresponding Fractional Difference (FD)
comparison statistic is also given. For example, the estimates
8
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for a 35-m release of 50 /xm diameter particles over a l-hour
period are 3.99 g/m2 using the old algorithm and 4.86 g/m2 using
the new algorithm. The FD comparison statistic in this case is
computed as:
FD = 2(4.86 - 3.99)/(4.86 + 3.99) = 0.20
The resulting value, 0.20 in this example, indicates that the
estimate based on the new algorithm increased (in this case by 22
percent) relative to the estimate based on the old algorithm.
For use in interpreting the FD statistic, it should be noted that
a value of -0.67 (+0.67) indicates that estimates based on the
new algorithm decreased (increased) by a factor of 2 relative to
estimates based on the old algorithm.
The FD values from Table 5 are presented graphically in
Figures 4 (1-hour), 5 (3-hour), 6 (24-hour) and 7 (annual
average). It should be noted, since the old algorithm assumes
that particles less than 5.7 /urn in diameter do not deposit, that
the FD statistic for the first two particle size categories (0.1
and 1.0 /zm) will always be 2.
For surface releases, the new algorithm results in higher
deposition estimates for all except the 10 and 100 /xm particle
size categories for the 1-hour period, for all except the 100 jum
particle size for the 3-hour period, and for all particle size
categories for the 24-hour and annual periods. In general, the
magnitude of the increase in the deposition estimate for a
surface release increases as the time period of the deposition
increases. Annual deposition estimates from a surface release
(Figure 7), for example, are increased by more than a factor of 2
using the new algorithm as compared to the old algorithm.
For elevated releases, the new algorithm results in higher
estimates for the smaller (0.1 and 1.0 ion) and larger (80 and 100
/zm) particle sizes. The increase in the deposition estimates for
80 and 100 /zm particles is generally less than a factor of two.
For intermediate particle sizes (10 and 20 /zm) , deposition
estimates using the new algorithm are decreased relative to the
corresponding estimates based on the old algorithm. The decrease
generally exceeds a factor of 2 for 100-m and 200-m releases.
Results vary for elevated releases of 50 /zm particles; for
example, deposition estimates for a 35-m release increase
(generally by less than a factor of 2), whereas estimates for
100-m and 200-m releases may increase (annual) or decrease (1-
hour and 3-hour) depending on the averaging period.
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3.2.1.2 ISCLT2
Annual deposition estimates were also computed using ISCLT2.
These results are presented in Table 6. For a surface release,
annual deposition estimates based on the new algorithm exceed
estimates based on the old algorithm for all particle sizes. The
FD statistic for the surface release ranges from 1.5 to 2.0. For
elevated releases, deposition estimates based on the new
algorithm may be higher or lower than estimates based on the old
algorithm, depending on particle size and release height.
3.2.2 Concentration
Ambient concentrations are reduced by the deposition of
particulate matter and, as such, model estimates of ambient
concentrations depend on the algorithms employed to model
deposition processes.
In the case of the old algorithm, the fractional reduction
in concentration is equal to one half of the fraction deposited
(i.e., one minus the reflection coefficient). Thus,
concentrations of 100 /zm particles, which experience no
reflection, are reduced by factor of 2. It should be noted that
the treatment of the gravational settling in the old deposition
algorithm results in a 'bouncing plume'. As such, concentration
estimates using the old deposition algorithm are invalid beyond
the point of plume touch down. The latter is a function of
particle size (settling velocity), release height, and wind
speed. Because of uncertainties resulting from the bouncing
plume, most past applications of ISC2 for estimating
concentrations of particulate matter have normally been made with
the deposition algorithm turned off.
In the case of the new algorithm, the reduction in
concentration is estimated using a modified source depletion
technique based on Horst (1977). With this technique, the source
term (emission rate) is adjusted (decreased) to simulate the
increased pollutant removal with distance. In addition to these
changes, the new algorithm incorporates necessary changes to
eliminate the bouncing plume.
3.2.2.1 ISCST2
As was done for deposition, corresponding concentration
results for l-hour, 3-hour, 24-hour, and annual averaging are
presented in Table 7. The FD values from Table 7 are presented
graphically in Figures 8 (1-hour), 9 (3-hour), 10 (24-hour) and
11 (annual average).
10
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For particles up to 20 /zm in diameter, the differences in
concentration estimates are not significant (less than 10
percent).
For surface releases of larger particles, the concentration
estimates using the new algorithm are consistently lower than the
corresponding estimates using the old algorithm. The decrease in
this case ranges between a factor of 1.5 and 2 (50 to 100 percent
decrease).
For elevated releases of larger particles, the results are
mixed. For 50 /zm particles, the concentration estimates using
the new algorithm are lower than the corresponding estimates
using the old algorithm. However, for elevated releases of the
largest particle size category (100 /zm) , the concentration
estimates using the new algorithm are increased relative to the
corresponding estimates using the old algorithm. The increase
approaches a factor of 2 for 100-m and 200-m release heights (for
short-term averaging periods). For annual averaging, the
increase approaches a factor of 1.5. This apparent contradiction
with the deposition results (deposition estimates based on the
new algorithm also exceed the estimates based on the old
algorithm for elevated releases of large particles) is a direct
result of a combination of inherent differences in the two
algorithms. These differences, which are manifest in the
modeling of elevated releases of large particles, are discussed
in Appendix B.
The above results are for ISCST2 run with the deposition
(old and new) and plume depletion (new only) options activated
(turned-on). The concentration estimates from the 'old' and
'new1 versions of ISCST2 are identical when deposition is not
turned-on.
3.2.2.2 ISCLT2
Annual concentration estimates were also computed using
ISCLT2. These results are presented in Table 8. For a surface
release, annual concentration estimates using the new algorithm
are less than the corresponding estimates using the old algorithm
for all particle sizes. The FD statistic for the surface release
ranges from zero (no change) to -0.2 (a decrease of 20 percent).
For elevated releases, with the exception of 100 izm size
particles, the differences are generally not significant. For
100 urn particles, concentration estimates using the new algorithm
are higher by 30 to 40 percent.
11
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4. Summary and Conclusions
The proposed algorithm was expected to provide deposition
estimations similar to the old algorithm for particles 20 jzm in
diameter and larger, and to provide more realistic (higher)
deposition estimates for particles less than 20 /*m in diameter.
4.1 Deposition
For surface releases, this comparison shows that the new
algorithm results in higher deposition estimates for all particle
size categories for the 24-hour and annual periods. The annual
deposition estimates are increased by more than a factor of 2
using the new method.
For elevated releases, the new algorithm results in higher
estimates for the smaller (0.1 and 1.0 /zm) and larger (80 and 100
pirn) particle sizes. The increase in the deposition estimates for
80 and 100 /zm particles is generally less than a factor of two.
For intermediate particle sizes (10 and 20 /on) , deposition
estimates using the new algorithm are decreased relative to the
corresponding estimates based on the old algorithm. The decrease
generally exceeds a factor of 2 for 100-m and 200-m releases.
The ratio of the deposition estimates based on the new and
old algorithms is dependent on the meteorology and appears to be
a constant for particle sizes above about 20 /zm for a given
meteorological condition. In general, deposition estimates based
on the old algorithm exceed the estimates based on the new
algorithm for the unstable conditions, while the reverse is the
case for stable conditions (e.g., see Figure 3).
4.2 Concentration
For a surface release of large particles (50, 80, and
100/zm) , estimated concentrations using the new algorithm are
decreased relative to the corresponding estimates using the new
algorithm. The decrease ranges between a factor of 1.5 and 2 (50
to 100 percent decrease). For small and intermediate particles
(0.1, 1, 10, and 20 /zm) , the differences are not significant
(less then 10 percent).
For elevated releases of larger particles the results are
mixed. However, in all cases, the differences for these larger
particles do not exceed a factor of 2. As was the case for
surface releases, the differences for small and intermediate size
particles are not significant (less than 10 percent).
12
-------
5. REFERENCES
Cox, W.M. and J.A. Tikvart, 1990: A Statistical Procedure for
Determining the Best Performing Air Quality Simulation
Model. Atmos. Environ., 24, 2387-2395.
Dumbauld, R.K., J.E. Rafferty and H.E. Cramer, 1976:
Dispersion-deposition from aerial spray releases.
Proceedings of the Third Symposium on Atmospheric
Turbulence, Diffusion, and Air Quality, October 19-22,
Raleigh, NC.
Colder, D., 1972: Relations among stability parameters in the
surface layer. Boundary Layer Met. 3, 46-58.
Hanna, S.R. and J.C. Chang, 1991: Modification of the Hybrid
Plume Dispersion Model (HPDM) for urban conditions and its
evaluation using the Indianapolis data set. Vol. I. User's
guide for HPDM-Urban. Sigma Research Corp., Concord, MA.
Overcamp, T.J., 1976: A general Gaussian diffusion-deposition
model for elevated point sources. J. Appl. Meteor., 15,
1167-1171.
Horst, T.W., 1977: A surface depletion model for deposition from
a Gaussian plume. Atmos. Environ., 11, 41-46.
Pleim, J., A. Venkatram and R. Yamartino, 1984: ADOM/TADAP model
development program. Volume 4. The dry deposition module.
Ontario Ministry of the Environment, Rexdale, Ontario,
Canada
U.S. EPA, 1990: Meteorological Processor for Regulatory Models
(MPRM 1.2) User's Guide. EPA-600/3- 88 - 043 (Revised). U.S.
Environmental Protection Agency, Research Triangle Park,
North Carolina, 27711.
U.S. EPA, 1992: User's Guide for the Industrial Source (ISC2)
Dispersion Model. Volume 1 - User Instructions. EPA-450/4-
92-008a. U.S. Environmental Protection Agency, Research
Triangle Park, North Carolina 27711.
13
-------
U.S. EPA, 1994: Development and Testing of Dry Deposition
Algorithms. EPA-454/R-94-015. U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina
27711.
Wang, I.T. and P.C. Chen, 1980: Estimations of heat and momentum
fluxes near the ground. Proc. 2nd Joint Conf. on
Applications of Air Poll. Meteorol., AMS, 764-769.
13a
-------
TABLE 5
Comparison of ISCST deposition flux estimates based on the old and new algorithms
for selected averaging times, release heights, and particle diameters as indicated.
Fractional Difference: FD = 2 (NEW - OLD) / (NEW + OLD)
1-HOUR EXTREMES (g/m'2)
Diameter
(microns)
10
20
50
80
100
OLD
352
418
1096
1906
1296
Surface
NEW
230
727
2299
1987
1156
FO
Sfc.
-0.4
0.5
0.7
0.0
-0.1
35-meter
OLD NEW
1.04 0.62
1.94 0.71
3.99 4.86
20.83 32.67
69.86 75.88
FD
35-m
-0.5
-0.9
0.2
0.4
0.1
OLD
0.43
0.79
1.48
2.39
7.75
100-meter
NEW
0.06
0.07
0.52
3.63
14.05
FD
100-m
-1.5
-1.7
-1.0
0.4
0.6
OLD
0.27
0.49
0.93
1.51
2.48
200-meter
NEW
0.01
0.02
0.15
1.43
4.41
FD
200-m
•1.9
-1.8
•1.4
•0.1
0.6
3-HOUR EXTREMES (8/nT2)
Diameter
(microns)
10
20
50
80
100
OLD
357
647
3125
5338
3388
Surface
NEW
549
1799
6464
5533
2913
FD
Sfc.
0.4
0.9
0.7
0.0
-0.2
35-meter
OLD NEW
2.19 1.40
4.10 1.64
8.55 14.20
59.99 92.35
198.90 211.25
FD
35-m
-0.4
-0.9
0.5
0.4
0.1
OLD
0.59
1.10
2.21
6.51
21.99
100-meter
NEW
0.14
0.16
1.46
10.49
38.80
FD
100-m
-1.2
-1.5
-0.4
0.5
0.6
OLD
0.34
0.63
1.23
2.17
5.52
200-meter
NEW
0.03
0.04
0.34
4.18
10.94
FD
200-m
-1.7
•1.8
-1.1
0.6
0.7
24-HOUR EXTREMES (g/m'2)
Diameter
(microns)
10
20
50
80
100
OLD
990
2267
11659
14414
12256
Surface
NEW
2626
5957
27498
19622
14292
FD
Sfc.
0.9
0.9
0.8
0.3
0.2
35-meter
OLD NEW
6.24 6.07
12.00 7.99
29.50 41.95
172.50 254.91
563.50 583.65
FD
35-m
0.0
-0.4
0.3
0.4
0.0
OLD
0.95
1.77
3.44
17.15
56.73
100-meter
NEW
0.42
0.61
4.07
25.74
78.85
FD
100-m
-0.8
-1.0
0.2
0.4
0.3
OLD
0.43
0.76
1.45
3.80
13.44
200-meter
NEW
0.08
0.13
0.89
8.18
21.68
FD
200-m
-1.4
-1.4
-0.5
0.7
0.5
ANNUAL EXTREMES (g/m'2)
Diameter
(microns)
10
20
50
80
100
OLD
111539
243405
955851
1258514
905441
Surface
NEW
503090
1318515
4041833
3773714
2679361
FD
Sfc.
1.3
14
1.2
1.0
1.0
35 mater
OLD NEW
855.00 530.00
1677.00 1107.00
4838.00 6697.00
26225.00 36422.00
81102.00 83232.00
FD
35-m
-0.5
-0.4
0.3
0.3
0.0
OLD
62.00
120.00
343.00
2625.00
8589.00
100-meter
NEW
40.40
92.50
695.00
3928.00
9344.00
FD
100-m
-0.4
-0.3
0.7
0.4
0.1
OLD
19.00
35.00
80.00
583.00
1990.00
200-metor
NEW
8.60
20.50
159.00
989.00
2548.00
FD
200-m
•0.8
-0.5
0.7
0.5
0.2
14
-------
TABLE 6
Comparison of ISCLT annual deposition flux estimates based on the old and new
algorithms for release heights and particle diameters as indicated.
Fractional Difference: FD = 2 (NEW - OLD) / (NEW + OLD)
Surface Release
Diameter
(microns)
0.1
1
10
20
50
80
100
OLD
0
0
1199
3202
12376
21440
28738
NEW
2831
1093
67358
117317
239284
235098
194290
FD
2.0
2.0
1.9
1.9
1.8
1.7
1.5
35-Meter Release
OLD
0
0
96
187
505
1794
5483
NEW
2
1
67
103
412
2182
5136
FD
2.0
2.0
-0.4
-0.6
-0.2
0.2
-0.1
Diameter
(microns)
100-Meter Release
OLD NEW FD
200-Meter Release
OLD NEW FD
0.1
1
10
20
50
80
100
0
0
9
17
41
149
488
0
0
5
8
37
210
511
2.0
2.0
-0.6
-0.7
-0.1
0.3
0.0
0
0
3
6
13
34
114
0.0
0.0
1.0
1.8
7.5
51.6
135.0
2.0
2.0
-1.0
-1.1
-0.5
0.4
0.2
15
-------
TABLE 7
Comparison of ISCST concentration estimates based on the old and new algorithms
for selected averaging times, release heights, and particle diameters as indicated.
Fractional Difference: FD = 2 (NEW - OLD) f (NEW + OLD)
1-HOUR EXTREMES (micro-g/m'3)
Diameter
(microns) OLD
0.1 21711142
1 21712652
10 20370922
20 18579514
50 14046924
80 5646102
100 1504833
Surface
NEW
21608136
21625196
20260306
16512426
8455137
2841743
1032195
FD
Sfc.
0.00
0.00
-0.01
-0.12
•0.50
•0.66
-0.37
OLD
2836
2836
2658
3403
20355
48452
59623
35-meter
NEW
2824
2832
2839
3414
17875
47026
69928
FD
35-m
0.00
0.00
0.07
0.00
-0.13
-0.03
0.16
OLD
897
897
836
787
2208
5386
6895
100-meter
NEW
894
895
895
865
1931
5235
12947
FD
100-m
0.00
0.00
0.07
0.09
•0.13
-0.03
0.61
OLD
439
439
410
386
648
1626
2230
200-meter
NEW
437
438
438
424
563
2063
4072
FD
200-m
-0.01
0.00
0.07
0.09
-0.14
0.24
0.58
3-HOUR EXTREMES (micro-g/m'3)
Diameter
(microns) OLD
0.1 17028218
1 17029042
10 15960174
20 14728003
50 13971580
80 5475584
100 1407853
24-HOUR EXTREMES I
Diameter
(microns) OLD
0 1 6225974
1 6226221
10 5832943
20 5392277
50 6232613
80 2300707
100 836814
Surface
NEW
16957118
16972472
16089264
13625220
7921412
2639824
875861
|micro-g/m"3)
Surface
NEW
6197444
6205344
5893759
4966362
4292841
1241776
530638
FD
Sfc.
0.00
0.00
0.01
•0.08
-0.55
-0.70
•0.47
FD
Sfc.
0.00
0.00
0.01
-0.08
-0.37
-0.60
-0.45
OLD
2490
2491
2353
3278
19687
45792
57704
OLD
1777
1778
1687
1661
7885
18098
22210
35-meter
NEW
2484
2488
2491
3270
17414
44300
64895
35-meter
NEW
1770
1775
1521
1621
7277
17401
25557
FD
35-m
0.00
0.00
0.06
0.00
-0.12
-0.03
0.12
FD
35-m
0.00
0.00
•0.10
-0.02
•0.08
-0.04
0.14
OLD
486
486
454
429
2119
5122
6734
OLD
151
151
142
148
832
1832
2319
100-meter
NEW
484
485
484
472
1797
5034
11918
100-meter
NEW
149
150
150
141
705
1756
3453
FD
100-m
0.00
0.00
0.06
0.10
-0.16
-0.02
0.56
FD
100-m
-0.01
0.00
0.06
-0.05
-0.17
-0.04
0.39
OLD
186
186
174
165
525
1242
1675
OLD
56
56
51
48
182
388
509
200-meter
NEW
185
185
186
179
416
2008
3362
200-meter
NEW
55
55
55
51
142
559
950
FD
200-m
0.00
0.00
0.07
0.08
-0.23
0.47
0.67
FD
200m
-0.01
•0.01
0.08
0.06
-0.24
0.36
0.60
ANNUAL EXTREMES |micro-g/nT3)
Diameter
(microns) OLD
0 1 2259888
1 2260180
10 2138979
20 2081605
50 1971727
80 886039
100 342726
Surface
NEW
2252064
2256171
2174798
2028710
1534105
594997
274974
FD
0.00
0.00
0.02
-0.03
-0.25
-0.39
0.22
OLD
1020
1020
985
1025
2421
5207
6485
35-meter
NEW
1015
1019
956
1017
2497
5829
8699
FD
0.00
0.00
-0.03
-0.01
0.03
0.11
0.29
OLD
85
85
82
89
266
587
756
100-meter
NEW
84
84
78
88
256
627
975
FD
•0.01
0.00
-0.05
-0.01
-0.04
0.07
0.25
OLD
18
18
17
19
63
143
185
200-meter
NEW
18
18
17
19
58
157
266
FD
-0.01
0.00
-0.05
-0.02
-0.08
0.10
0.36
16
-------
TABLE 8
Comparison of ISCLT annual concentration estimates as affected by the old and new
deposition algorithms for release heights and particle diameters as indicated.
Fractional Difference: FD = 2 (NEW - OLD) f (NEW + OLD)
Diameter
(microns)
Surface Release
OLD
NEW
FD
35-Meter Release
OLD
NEW
FD
0.1
1
10
20
50
80
100
194905
194905
182408
170238
103686
45320
23616
193914
194452
177153
156715
83371
35293
19240
0.0
0.0
0.0
-0.1
-0.2
-0.2
•0.2
88
88
84
84
128
286
376
88
88
78
83
137
336
521
0.0
0.0
-0.1
0.0
0.1
0.2
0.3
100-Meter Release
200-Meter Release
Diameter
(microns)
0.1
1
10
20
50
80
100
OLD
7.0
7.0
6.6
6.6
12.0
28.9
38.3
NEW
6.9
6.9
6.1
6.5
12.4
32.2
51.7
FD
0.0
0.0
-0.1
0.0
0.0
0.1
0.3
OLD
1.9
1.9
1.7
1.7
2.7
7.1
9.4
NEW
1.8
1.9
1.6
1.7
2.4
7.8
13.6
FD
-0.1
0.0
-0.1
0.0
-0.1
0.1
0.4
17
-------
3
FIGURE 1
Comparison of highest 1-hour ISCST deposition estimates based on old and new
algorithms for a 35-meter release for 50 micron diameter particles.
D • OLD
D
D NEW
D
D
a
D •
ri
a a D
D
n
0.1 1 10
Downwind Distance (km)
18
-------
80
60
0>
.2 40
FIGURE 2
Comparison of highest 1-hour ISCST deposition estimates (g|m*2) based on
the old and new algorithms for a 35-meter release for particle diameters
(microns) as indicated:
Perfect Agreement
NEW - OLD
10
20
50
20
20
40
Deposition (old)
GO
80
19
-------
15
10
FIGURE 3
Comparison of ISCSTdeposition estimates (g|m"2) based on the old and new
algorithms for selected extreme meteorological conditions and particle diameters
(microns) as indicated, for a 35-meter release
Stability (wind speed)
Perfect Agreement
NEW - OLD
50/
10
15
Deposition (old)
20
-------
FIGURE 4
Fractional difference (FD) of highest 1-hour ISCST deposition estimates based on
old and new algorithms for particle sizes and release heights as indicated
FD = 2 (NEW - OLD) I (NEW + OLD)
2.0
1.0
f 0.0
1.0
2.0
0.1
10 20 50 80
Particle Diameter (microns)
Sfc. H 35-m D 100-m ^ 200-m
100
21
-------
FIGURE 5
Fractional difference (FD) of highest 3-hour ISCST deposition estimates based on
old and new algorithms for particle sizes and release heights as indicated
FD = 2 (NEW - OLD) f (NEW + OLD)
2.0
1.0
3:
- 0.0
-1.0
2.0
0.1
10 20 50
Particle Diameter (microns)
Sfc.
35-m D 100-m M 200-m
80
100
22
-------
FIGURE 6
Fractional difference (FD) of highest 24-hour ISCST deposition estimates based on
old and new algorithms for particle sizes and release heights as indicated
FD = 2 (NEW - OLD) f (NEW + OLD)
2.0
1.0
0.0
10
1.0
2.0
0.1
10 20 50
Particle Diameter (microns)
Sfc.
35-m LJ 100-m m 200-m
80
100
23
-------
FIGURE 7
Fractional difference (FD) of highest annual ISCST deposition estimates based on
old and new algorithms for particle sizes and release heights as indicated
FD = 2 (NEW-OLD) | (NEW + OLD)
2.0
1.0
0.0
-1.0
2.0
0.1
10 20 50
Particle Diameter (microns)
Sfc.
35-m D 100-1
80
100
m
200-m
24
-------
FIGURE 8
Fractional Difference (FD) of highest 1-hour ISCST concentration estimates
considering effects of old and new deposition algorithms for particle diameters and
release heights as indicated
FD = 2 (NEW - OLD) /{NEW + OLD)
1.00
0.50
0.00
•0.50
1.00
0.1
10 20 50
Particle Diameter (microns)
Sfc. H 35-m D 100-m Wh 200-m
80
100
25
-------
FIGURE 9
Fractional Difference (FD) of highest 3-hour ISCST concentration estimates
considering effects of old and new deposition algorithms for particle diameters and
release heights as indicated
FD = 2 (NEW • OLD) /(NEW + OLD)
1.00
0.50
01
o
c
09
0.00
•0.50
1.00
0.1
10 20 50
Particle Diameter (microns)
Sfc. H 35-m D 100-1
m
200-m
80
100
26
-------
FIGURE 10
Fractional Difference (FD) of highest 24-hour ISCST concentration estimates
considering effects of old and new deposition algorithms for particle diameters and
release heights as indicated
FD = 2 (NEW • OLD) /(NEW + OLD)
1.00
0.50
0.00
•0.50
1.00
0.1
10 20 50
Particle Diameter (microns)
80
100
Sfc.
35-m
100-m H 200-m
27
-------
FIGURE 11
Fractional Difference (FD) of highest annual-hour ISCST concentration estimates
considering effects of old and new deposition algorithms for particle diameters and
release heights as indicated
FD = 2 (NEW - OLD) f( NEW + OLD)
1.00
0.50
0.00
-0.50
1.00
0.1
10 20 50
Particle Diameter (microns)
80
100
Sfc.
35-m D 100-m H 200-m
28
-------
APPENDIX A
Comparison of Highest 1-hour ISCST Deposition Estimates Based on
the Old and New Algorithms for a 35-m Release
-------
APPENDIX A
LIST OF FIGURES
Figure Page
A-l Comparison of highest 1-hour ISCST deposition ... A-3
estimates based on old and new algorithms for a
35-meter release for 10 micron diameter particles.
A-2 Comparison of highest 1-hour ISCST deposition ... A-4
estimates based on old and new algorithms for a
35-meter release for 20 micron diameter particles.
A-3 Comparison of highest 1-hour ISCST deposition ... A-5
estimates based on old and new algorithms for a
35-meter release for 50 micron diameter particles.
A-4 Comparison of highest 1-hour ISCST deposition ... A-6
estimates based on old and new algorithms for a
35-meter release for 80 micron diameter particles.
A-5 Comparison of highest 1-hour ISCST deposition .... A-7
estimates based on old and new algorithms for a
35-meter release for 100 micron diameter particles.
A-2
-------
FIGURE A-1
Comparison of highest 1-hour ISCST deposition estimates based on old and new
algorithms for a 35-meter release for 10 micron diameter particles.
5 r
• OLD
c NEW
6 3
-2.
E
c u o
ey
0.1 1 10
Downwind Distance (km)
A-3
-------
FIGURE A-2
Comparison of highest 1-hour ISCST deposition estimates based on old and new
algorithms for a 35-meter release for 20 micron diameter particles.
5 r
• OLD
D NEW
-S2.
E
= 2
D " D D . ,
D DD Q ' "SB D
D D D
1 10
Downwind Distance (km)
A4
-------
FIGURE A-3
Comparison of highest 1-hour ISCST deposition estimates based on old and new
algorithms for a 35-meter release for 50 micron diameter particles.
I 3
01
D
D
• OLD
0 NEW
on n
_l I L_
0.1
1
Downwind Distance (km)
10
A-5
-------
FIGURE A -4
Comparison of highest 1-hour ISCST deposition estimates based on old and new
algorithms for a 35-meter release for 80 micron diameter particles.
100
80
£ 60 I-
09
1 I
E
a 40
20
n
• OLD
D NEW
D
0.1
1
Downwind Distance (km)
n nuo-D-g
10
A-6
-------
40
20
FIGURE A-5
Comparison of highest 1-hour ISCST deposition estimates based on old and new
algorithms for a 35-meter release for 100 micron diameter particles.
100
80
1 GO
OJ
E
• OLD
D NEW
0.1
Pan n
-D-D-O-d
1
Downwind Distance (km)
10
A-7
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APPENDIX B
Comparison of Highest 1-Hour Concentration Estimates as Affected
by the Old and New Deposition Algorithms for Elevated Releases
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APPENDIX B
LIST OF FIGURES
Figure Page
B-l Comparison of ISCST concentration estimates as ... B-5
affected by the old deposition algorithm for C
stability with a 3 m/s wind speed for a 35-m release
for particle diameters as indicated.
B-2 Comparison of ISCST concentration estimates as ... B-6
affected by the old deposition algorithm for C
stability with a 15 m/s wind speed for a 35-m release
for particle diameters as indicated.
B-3 Comparison of ISCST concentration estimates as ... B-7
affected by the new deposition algorithm for C
stability with a 3 m/s wind speed for a 35-m release
for particle diameters as indicated.
B-4 Comparison of ISCST concentration estimates as ... B-8
affected by the new deposition algorithm for C
stability with a 15 m/s wind speed for a 35-m release
for particle diameters as indicated.
B-5 Comparison of ISCST concentration estimates as ... B-9
affected by the old and new deposition algorithms for
E stability with a 1.5 m/s wind speed for a 200-m
release for 100 /im diameter particles.
B-2
-------
The results presented in Appendix B reflect diagnostic
analyses of selected extreme 1-hour concentration events for
elevated releases.
Figures B-l through B-4 show results for a 35-m release for
C stability with a 3 m/s wind speed [Figure B-l (old) and Figure
B-3 (new)] and with a 15 m/s wind speed [Figure B-2 (old) and
Figure B-4 (new).
Effect of Wind Speed on Concentration
For the 3 m/s wind speed, the maximum concentration
increases, and the distance to the maximum decreases with
increasing particle size. This is the case for both the old
(Figure B-l) and new (Figure B-3) algorithms. These results
reflect the influence of gravitational settling and its affect on
plume tilt. Gravitational settling of a 100 /xm particle (0.3
m/s) is significant relative to a 3 m/s wind speed. Under these
conditions, an elevated plume of 100 /xm particles descends more
steeply and intersects the surface closer to the source than a
plume composed of smaller particles.
By comparison, gravitational settling and plume tilt are
not as significant with a 15 m/s wind speed. In this case,
concentrations with the old deposition algorithm (Figure B-2)
actually decrease with increasing particle size (note that
deposition increases with particle size). With the new algorithm
(Figure B-4), concentrations appear to increase with particle
size (at least to a downwind distance of 1 km.) - this reversal
may possibly be due to the affect on deposition of a higher
friction velocity.
Effects of Deposition on Concentration
Concentration estimates were also computed with the
deposition option turned off. These estimates are shown by the
solid curves in Figures B-l and B-3 for the 3 m/s wind speed, and
by the solid curves in Figures B-2 and B-4 for the 15 m/s wind
speed.
For both old (Figures B-l and B-2) and new (Figures B-3 and
B-4) algorithms, the no deposition curve coincides with the
concentration estimates for the 1 /xm particle size category.
This is as expected for the old algorithm since it considers
effects due to gravitational settling only and assigns a
reflection coefficient of 1.0 (no deposition) to this size
category. For the new algorithm, the 1 /xm particle size does not
have a noticeable effect on concentration estimates. For larger
particles, the maximum concentration increases with particle
size.
B-3
-------
Results for a 200-m Release of 100 /on Particles
Figure B-5 shows the estimated concentrations for a 200-m
release of 100 /im particles for the extreme 1-hour meteorological
conditions associated with this case (E stability at 1.5 m/s).
Concentration estimates as affected by the new deposition
algorithm are shown for two cases (with and without plume
depletion). It is seen that the effects of plume depletion
become significant at downwind distances beyond the location of
the maximum (about 5 km). The results show that the
concentration estimates as affected by the new deposition
algorithm are about a factor of 2 greater than the corresponding
estimates for the old algorithm.
B-4
-------
FIGURE B • 1
Comparison of ISCST concentration estimates as affected by the old deposition
algorithm for C stability with a 3 m/s wind speed for a 35-m release for particle
diameters as indicated
4000
3000 -
1 micron
50 microns
100 microns
no deposition
2000
1000
0.1
1.0
Downwind Distance (km)
10.0
B-5
-------
FIGURE B-2
Comparison of ISCST concentration estimates as affected by the old deposition
algorithm for C stability with a 15 mfs wind speed for a 35-m release for particle
diameters as indicated
4000
3000
1 micron
50 microns
100 microns
no deposition
C"J
«
E
2000
1000 u
0.1
1.0
Downwind Distance (km)
10.0
B-6
-------
FIGURE B -3
Comparison of ISCST concentration estimates as affected by the new deposition
algorithm for C stability with a 3 m/s wind speed for a 35-m release for particle
diameters as indicated
4000
3000 -
* *
1 micron
50 microns
100 microns
no deposition
2000
o
o
1000 -
0.1
1.0
Downwind Distance (km)
10.0
B-7
-------
FIGURE B-4
Comparison of ISCST concentration estimates as affected by the new deposition
algorithm for C stability with a 15 m/s wind speed for a 35-m release for particle
diameters as indicated
4000 r
3000 -
D
1 micron
50 microns
100 microns
no deposition
u
'i
7 2000
o I
e
U
1000
0.1
1.0
Downwind Distance (km)
10.0
B-8
-------
FIGURE B-5
Comparison of ISCST concentration estimates for a 200-m release of 100 micron
particles as affected by the old and new deposition algorithms.
Meteorological Conditions: Stability E at 1.5 m/s
5000
4000 -
3000 !-
u
'i
o
o
2000
1000
10
Downwind Distance (km)
100
B-9
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TECHNICAL REPORT DATA
(Please read Instructions on reverse before completing)
1 REPORT NO
EPA-454/R-94-018
3 RECIPIENT'S ACCESSION NO
5 REPORT DATE
4 TITLE AND SUBTITLE
Comparison of ISC2 Dry Deposition Estimates Based on
Current and Proposed Deposition Algorithms
July 1994
6 PERFORMING ORGANIZATION CODE
7 AUTHOR(S)
8 PERFORMING ORGANIZATION REPORT NO
Desmond T. Bailey
9 PERFORMING ORGANIZATION NAME AND ADDRESS
10 PROGRAM ELEMENT NO
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
11 CONTRACT/GRANT NO
12 SPONSORING AGENCY NAME AND ADDRESS
13 TYPE OF REPORT AND PERIOD COVERED
Director
Office of Air Quality Planning and Standards
Office of Air and Radiation
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
14 SPONSORING AGENCY CODE
EPA/200/04
15 SUPPLEMENTARY NOTEi
16 ABSTRACT
The ability to accurately estimate deposition of particulate matter is of special concern in assessing environmental
impacts from a variety of sources including Superfund sites, municipal waste incinerators, and surface coal mines. The
current deposition algorithm in ISC2 simulates deposition as the movement of particles toward the surface by the combined
processes of atmospheric turbulence and gravitational settling. The method was developed for applications to large particles
dominated by gravitational settling; these are typically particles greater than 20 /*m diameter. The current algorithm was not
intended for use with particles smaller than about 20 /tm which are often of concern in air toxics assessments. In light of this
limitation, a new deposition algorithm is being considered for use in ISC2. The proposed algorithm is intended to simulate
processes important over the entire range of significant particle sizes (0.1 to 100 /ttm). The proposed algorithm employs a
deposition velocity based on a resistance model. In this approach, deposition flux is calculated as the product of the near-
surface air concentration and the deposition velocity. The latter is computed as the inverse sum of the aerodynamic layer and
deposition layer resistances, plus gravitational settling. The new deposition algorithm has been tested within the framework of
the 1SC2 model and comparisons of deposition estimates using the old (current) and new deposition algorithms have been
made for a range of source types and particulate emission scenarios. Similar comparisons have been made of particulate
concentration estimates as affected by the old and new deposition algorithms.
KPY WORDS AND DOCIMBP/T ANALYSIS
IDBf/nplBRS/OPBN BNDBD TERMS
Air Pollution control
» (BCURITY O-A3S
Unclassified
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