EPA-454/B-95-003d
ADDENDUM
USER'S GUIDE FOR THE
INDUSTRIAL SOURCE COMPLEX (ISC3) DISPERSION MODELS
VOLUME H - DESCRIPTION OF MODEL ALGORITHMS
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Emissions, Monitoring, and Analysis Division
Research Triangle Park, North Carolina 27711
June 1999
-------
ACKNOWLEDGMENTS
The Addendum to the User's Guide for the ISC3 Models has been prepared by
Roger W. Erode of Pacific Environmental Services, Inc., Research Triangle Park, North
Carolina, under subcontract to EC/R, Inc., Chapel Hill, North Carolina. This effort has
been funded by the Environmental Protection Agency under Contract No. 68D98006, with
Dennis G. Atkinson as Work Assignment Manager.
11
-------
TECHNICAL DESCRIPTION FOR THE
REVISED ISCST3 MODEL (DATED 99155)
This document provides a technical description of model algorithms for recent
enhancements of the ISCST3 model, including the most recent version dated 99155. The
algorithms described in this Addendum include the gas dry deposition algorithms based on
the draft GDISCDFT model (dated 96248), and the optimizations of the area source
algorithm. Both of these enhancements are associated with the non-regulatory default
TOXICS option introduced with version 99155 of ISCST3. A brief description of the user
instructions for these new options is presented in the accompanying Addendum to Volume I
of the ISC3 model user's guide (ISC3ADD1.WPD).
Gas Dry Deposition Algorithms
The ISCST3 dry deposition algorithm for gaseous pollutants is based on the
algorithm contained in the CALPUFF dispersion model (EPA, 1995a), and has undergone
limited review and evaluation (Moore, at al. 1995).
The deposition flux, Fd, is calculated as the product of the concentration, %d, and a
deposition velocity, vd, computed at a reference height zd:
Fd =
The concentration value, x
-------
An alternative pathway that is potentially important in sparsely vegetated areas or over water
is deposition directly to the ground/water surface. Although not involving vegetation, it is
convenient to include the ground/ water surface resistance as a component of rc.
The atmospheric resistance term (rj is given by Equations 1-81 and 1-82 in Section
1.3.2 of the ISC3 model user's guide, Volume H (EPA, 1995b).
The deposition layer resistance (r^ is parameterized in terms of the Schmidt number
(EPA, 1995a) as:
where, S0 = the Schmidt number
u = the kinematic viscosity of air ("0.15 x 10"4 m2/s),
DM = the molecular diffusivity of the pollutant (m2/s), and,
dl9 d2 = empirical parameters; dl/k=5, d2=2/3 (Hicks, 1982)
k = the von Karman constant (~0.4)
u* = surface friction velocity (m/s)
The canopy resistance (rc) is the resistance for gases in the vegetation layer,
including the ground/water surface. There are three main pathways for uptake/reaction
within the vegetation or at the surface (EPA, 1995a):
(1) Transfer through the stomatal pore and dissolution or reaction in the mesophyll cells
(plant tissue that contains chlorophyll).
(2) Reaction with or transfer through the leaf cuticle.
(3) Transfer into the ground/water surface.
These pathways are treated as three resistances in parallel.
rc =[LAI/rf + LAI/rcut + l/r I'1 (A4)
-------
where, rf = the internal foliage resistance (s/m) (Pathway 1, Transfer through
the stomatal pore and dissolution or reaction in mesophyll cells),
rcut = the cuticle resistance (s/m), (Pathway 2, Reaction with or transfer
through the leaf cuticle, a thin film covering the surface of
plants),
rg = the ground or water surface resistance (s/m), (Pathway 3,
Transfer into the ground/water surface), and,
LAI = the leaf area index (ratio of leaf surface area divided by ground
surface area). The LAI is specified as a function of wind
direction and month/season, and is included in the meteorological
input file provided by the MPRM preprocessor.
Pathway 1:
The internal foliage resistance (rf) consists of two components:
r. = r + r
s
where, rs = the resistance (s/m) to transport through the stomatal pore (see
below), and,
rra = the resistance (s/m) to dissolution or reaction of the pollutant in
the mesophyll (spongy parenchyma) cells, user input by species.
For soluble compounds (HF, SO2, CLj, NH3), set to zero; for less
soluble compounds (NO2), it could be > 0)
Stomatal opening/closing is a response to the plant's competing needs for uptake of
CO2 and prevention of water loss from the leaves. Stomatal action imposes a strong diurnal
cycle on the stomatal resistance, and has an important role in determining deposition rates
for soluble gaseous pollutants such as SO2. Stomatal resistance (rs) is given by (EPA,
1995a):
rs = P3/ (bDM)
where, ps = a stomatal constant corresponding to the characteristics of leaf
physiology (- 2.3 x 10"8 m2),
b = the width of the stomatal opening (m), and,
DM = the molecular diffusivity of the pollutant (m2/s).
-------
The width of the stomatal opening (b) is a function of the radiation intensity,
moisture availability, and temperature. In ISC3, the state of vegetation is specified as one
of three states: (A) active and unstressed, (B) active and stressed, or (C) inactive. Irrigated
vegetation can be assumed to be in an active and unstressed state. The variation in stomatal
opening width during period (A) when vegetation is active and unstressed (Pleim et al. ,
1984) is:
b = bmax(RI/Rmax) + bmin (A7 )
where, bmax = the maximum width (m) of the stomatal opening (~ 2.5 x 10~6 m)
(Padroetal., 1991),
b,,^ = the minimum width (m) of the stomatal opening (~ 0.1 x 10~6 m),
R, = the incoming solar radiation (W/m2) received at the ground, and
is included in the meteorological input file for the model by the
MPRM preprocessor, and,
R^x = the incoming solar radiation (W/m2) at which full opening of the
stomata occur; assume constant and equal to 600.
During periods of moisture stress, the need to prevent moisture loss becomes critical,
and the stomata close. Thus for period (B), active vegetation under moisture stress
conditions, assume that b = bmin. When vegetation is inactive (e.g., during the seasonal dry
period), the internal foliage resistance becomes very large, essentially cutting off Pathway 1.
Assuming the vegetation is in state (A), active and unstressed, ambient temperature
provides an additional bound on the value of rs. During cold periods (T< 10 °C), metabolic
activity slows, and b is set by the code to bmin. During hot weather conditions (T> ~35°C)
the stomata are fully open (b=bmax) to allow evaporative cooling of the plant.
Pathway 2:
The resistance due to reaction with or transfer through the leaf cuticle (rcut) is given
by (EPA, 1995a):
rcut =efARrcutref (A8)
where, Aref = the reference reactivity parameter of SO2 (~ 8.0),
AR = the reactivity parameter for the depositing gas, (NO2 = 8, O3 = 15,
HNO3 = 18, PAN=4), and,
rcut(ref) = the empirically determined reference cuticle resistance (s/m) of
SO2, set equal to 3000 s/m (Padro et al., 1991).
-------
Pathway 3:
The third resistance pathway for rc is transfer into the ground/water surface (rg). In
sparsely vegetated areas, deposition directly to the surface may be an important pathway.
(A9)
where, rg(ref) = the reference resistance of SO2 over ground (~ 1000 s/m) (Padro
etal.,1991).
Over water, deposition of soluble pollutants can be quite rapid. The liquid phase resistance
of the depositing pollutant over water is a function of its solubility and reactivity
characteristics, and is given by (Slinn et al., 1978):
rg = H/(a. d3 uj (A10)
where, H = the Henry's law constant, which is the ratio of gas to liquid phase
concentration of the pollutant, (H ~ 4 x 10~2 (SO2), 4 x 10'7
(HA), 8 x 10'8 (HNO3), 2 x 10° (O3), 3.5 x 10° (NO2), 1 x 10'2
(PAN), and 4 x 10'6 (HCHO)),
a* = a solubility enhancement factor due to the aqueous phase
dissociation of the pollutant (a, ~ 103 for SO2, ~ 1 for CO2 10
for O3), and
d3 = a constant (~ 4.8 x 10"4).
If sufficient data are not available to compute the canopy resistance term, rc, from
Equation A4, then an option for user-specified gas dry deposition velocity is provided.
Selection of this option will by-pass the algorithm for computing deposition velocities for
gaseous pollutants, and results from the ISCST3 model based on a user-specified deposition
velocity should be used with extra caution.
Optimizations for Area Sources
When the non-regulatory default TOXICS option is specified, the ISCST3 model
optimizes the area source algorithm to improve model runtimes. These optimizations are
briefly described below.
In the regulator}' default mode, the ISCST3 model utilizes a Romberg numerical
integration to estimate the area source impacts, as described hi Section 1.2.3 of the ISC3
model user's guide, Volume n (EPA, 1995b). While the Romberg integration performs
well relative to other approaches for receptors located within or adjacent to the area scarce,
-------
its advantages diminish as the receptor location is moved further away from the source. The
shape of the integrand becomes less complex for the latter case, approaching that of a point
source at distances of about 15 source widths downwind. Recognizing this behavior, the
TOXICS option in ISCST3 makes use of a more computationally efficient 2-point Gaussian
Quadrature routine to approximate the numerical integral for cases where the receptor
location satisfies the following condition relative to the side of the area source being
integrated:
XU - XL < 5*XL
(All)
where,
XL = the minimum distance from the side of the area source to the
receptor, and
XU = the maximum distance from the side of the area source to the
receptor.
If the receptor location does not satisfy the condition in Equation Al 1, then the
Romberg numerical integration routine is used. In addition, for receptors that are located
several source widths downwind of an area source, a point source approximation is used.
The distance used to determine if a point source approximation is applied is stability
dependent, and is determined as follows:
where,
X > FACT * WIDTH
X
(A12)
= the downwind distance from the center of the source to the
receptor,
FACT = a stability-dependent factor (see below), and
WIDTH = the crosswind width of the area source.
Values of FACT:
Stability Class
A
B
C
D
E
F
Rural
3.5
5.5
7.5
12.5
15.5
25.5
Urban
3.5
3.5
5.5
10.5
15.5
15.5
When area sources are modeled with dry depletion, the TOXICS option also allows
the user to specify the AREADPLT option, which applies a single effective dry depletion
factor to the undepleted value calculated for the area source. The effective dry depletion
-------
factor, which replaces the application of dry depletion within the area source integration, is
intended to provide potential runtime savings to the user. Since dry depletion is distance-
dependent, the effective dry depletion factor is calculated for an empirically-derived
effective distance. The effective distance is calculated as the distance from the receptor to a
point within the area source that is one-third the distance from the downwind edge to the
upwind edge. For receptors located upwind of the downwind edge, including receptors
located within the area source, the effective distance is one-third the distance from the
receptor to the upwind edge of the source.
In addition to the area source optimizations described above, when the TOXICS
option is specified, the dry depletion integration is performed using a 2-point Gaussian
Quadrature routine rather than the Romberg integration used for regulatory applications.
References
Environmental Protection Agency, 1995a. A User's Guide for the CALPUFF Dispersion
Model. EPA-454/B-95-006. U.S. Environmental Protection Agency, Research
Triangle Park, NC.
Environmental Protection Agency, 1995b. User's Guide for the Industrial Source Complex
(ISC3) Dispersion Models, Volume n - Description of Model Algorithms.
EPA-454/B-95-003b. U.S. Environmental Protection Agency, Research Triangle
Park, NC.
Hicks, B.B., 1982: Critical assessment document on acid deposition. ATDL Contrib. File
No. 81/24, Atmos. Turb. and Diff. Laboratory, Oak Ridge, TN.
Moore, G., P. Ryan, D. Schwede, and D. Strimaitis, 1995: Model performance evaluation
of gaseous dry deposition algorithms. Paper 95-TA34.02, 88th Annual Meeting &
Exhibition of the Air and Waste Management Association, San Antonio, Texas, June
18-23, 1995.
Padro, J., G.D. Hartog, and H.H. Neumann, 1991: An investigation of the ADOM dry
deposition module using summertime O3 measurements above a deciduous forest.
Atmos. Environ, 25A, 1689-1704.
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.
Slinn, W.G.N., L. Hasse, B.B. Hicks, A.W. Hogan, D. Lai, P.S. Liss, K.O. Munnich,
G.A. Sehmel and O. Vittori, 1978: Some aspects of the transfer of atmospheric
trace constituents past the air-sea interface. Atmos. Environ., 12, 2055-2087.
7
-------
Wesley, MX. and B.B, Hicks, 1977: Some factors that effect the deposition rates of sulfur
dioxide and similar gases on vegetation. /. Air Poll. Control Assoc., 27, 1110-1116.
------- |