DETAILED DOCUMENTATION
FOR
USAF TOXIC CHEMICAL DISPERSION MODEL
AFTOX VERSION 4.1
Capt Clifton E. Dungey, HQ AWS/XTX
5 Jan 93
A. PROGRAM DESCRIPTION
1. Purpose: The USAF Toxic Chemical Dispersion Model
(AFTOX) will determine toxic chemical concentrations and give the
user the option of calculating a toxic corridor, the concentration
at a specific location, or the maximum concentration and its
location.
2. Application: AFTOX was developed for real time
analysis of non-buoyant, toxic chemical releases. Use of AFTOX is
governed by AFR 355-1. AFTOX is written so that Air Weather
Service (AWS) base weather stations can apply AFTOX to continuous
or instantaneous, gas or liquid releases from either ground or
elevated sources. It can also be applied to heated plumes from
smokestacks. AFTOX contains 130 chemicals on file and is able to
accept additional chemicals.
3. Structure: A detailed explanation of AFTOX is
contained in AFGL-TR-88-0009 and PL-TR-91-2119. AFTOX is composed
of several executable main program files (.EXE files) which are
chained together for execution purposes. The program files are
DSP1.EXE, DSP2.EXE, DSPHP.EXE, and DSP3.EXE. The file structure
and calculations performed by each file are as follows:
a. DSP1: This executable program file determines the
chemical properties and meteorological conditions. Schematic
representation is shown in Figure 1.
Figure 1. DSP1 Flow Diagram
|
|
| |
instantaneous or continuous |
| |
_______|_______ |
| CALCULATE | |
| Solar Angle |------------------|
|
______________________|__________________________
DAY NIGHT
| |
________|_________ |
| CALCULATE | |
|Solar Insolation| |
| |
snow cover? ------yes----->| |
| | |
no | |
| | |
_____|_____
|CALCULATE|
|Heat Flux|
|
_________________|__________________
| CALCULATE |
|Friction Velocity, 10-m Wind Speed|
| Monin-Obukhov Length |
__________|__________
| CALCULATE |
|Stability Parameter| ---->buoyant plume
| |
_________|_________ to DSPHP
| CALCULATE |
|Properties of Air|
|
chemical properties on file? ------->no
| |
yes ------gas ---------->|
| |
liquid |
__________|__________ |
| CALCULATE | |
|Chemical Properties| ---->EXIT to DSP2
Major computations of DSP1 are described as follows:
(1) Solar elevation angle: Determined using date,
time, latitude, and longitude.
(2) Sensible heat flux: Determined by one of two
methods, dependent on daytime or nighttime conditions.
(a) Daytime: Cloud amount, temperature,
ground moisture, and solar insolation (from solar angle, cloud
amount, and cloud type) are used.
(b) Nighttime: Cloud amount only is used.
(3) Turbulence parameters: Friction velocity, 10-
meter wind speed, and Monin-Obukhov length are interrelated and
are determined iteratively from initial estimates based on the
wind speed, heat flux, surface pressure, and surface roughness.
(4) Stability parameter: One of two methods of
computation are used.
(a) Method 1: Turbulence parameters
described in (3) above are used.
(b) Method 2: Wind speed and standard
deviation of the wind direction are used.
(5) Air properties: Density and viscosity are
computed from temperature and pressure.
(6) Chemical properties: For a liquid chemical
which is in the data file, the vapor pressure, liquid density,
and vapor density are computed from the ch
emical data, the air
temperature, and pressure.
b. DSP2: This executable program file determines the
source conditions (e.g., emission rate, duration of spill, area
of spill, and source strength). Processing through DSP2 is
dependent on type of release. The air temperature is compared to
the chemical's boiling point to determine if it is a gaseous or
liquid spill. If the chemical of interest is not on file,
default settings may be used or user may input molecular weight,
vapor pressure, and whether release is a liquid or gas to
determine source strength. Schematic representation of DSP2 is
shown in Figure 2.
Figure 2. DSP2 Flow Diagram
DSP1
|
Source strength known?----no---->|
| |
| AFR 355-1,
| AWSSUP 1
| |
| pause for
yes<--------------information
|
__________________|_____________
| |
GAS LIQUID
_________|_________ ______|________
| | | |
continuous instantaneous continuous instantaneous
| | | |
| ______|___ | |
_____|_____ | Area | _____|_____ |
|CALCULATE| _____|_____ |CALCULATE|
| Amount | |CALCULATE| | Amount | |
| Spilled | | Source | | Spilled | |
| | Strength| | |
| <--------> |
| | _____|____ | |
| | |CALCULATE| |
| | | AREA | | |
| | | | |
| | |
| | | |
| | Evaporation rate= |
| | spill rate |
| | | |
| | Evaporation time= |
| | spill time |
| | | |
|
| Change area?-yes->| |
| | | | |
| | no |
| | | | ______|______
| | | |->| CALCULATE |
| | | |Evaporation|
| | | | Rate |
| | | ______|______
| | | | CALCULATE |
| | | |Evaporation|
| | | | Time |
| | | |
|---------------->|------>|-->|<------------------|
|
EXIT to DSP3
(1) Continuous gas release: Uses the emission
rate and the total time of the spill to compute the total amount
spilled. The source strength is the evaporation rate.
(2) Instantaneous gas release: Uses the amount
spilled and air density to determine the initial volume of the
spill.
(3) Continuous liquid release: The evaporation
rate into the atmosphere is the source strength. If the area of
the spill is known, it is used to determine the evaporation rate.
If the area is unknown, an area is calculated assuming a pool
depth of 1/4 inch. For example, 1 cubic foot of liquid will cover
a 48 square foot area if allowed to spread out to a depth of 1/4
inch over a flat surface. Then the emission rate is set equal to
the evaporation rate based on the calculated area. If this
calculated area appears unreasonably large as compared with the
observed spill, the user may input a smaller area which will give
a new evaporation rate. An alternate method exist for chemicals
without full data information. Evaporation rate is determined
using spill area, pool temperature, chemical molecular weight, and
vapor pressure. If variables are unknown, the model assumes the
worst case and the evaporation rate is set equal to the emission
rate.
(4) Instantaneous liquid release: Uses the amount
spilled, area covered, chemical, and air properties to compute the
evaporation rate. The source strength is set equal to the
evaporation rate. The amount spilled and the evaporation rate
determine the total evaporation time. The alternate method listed
in (3) above is used if chemical data is not available.
c. DSPHP: This executable program file determines the
source conditions for a buoyant plume from a stack (e.g., emission
rate, duration of spill, height of spill). Source strength is set
equal to the emission rate. As shown in Figure 3, atmospheric
conditions determine processing through the module.
Figure 3. DSPHP Flow Diagra
m
DSP1
|
|
|
effluent still being emitted?----no--->|
| |
yes
| |
|
Stack height above inversion?---yes--->|
| |
no |
| END
|
|
_______|_______
| CALCULATE |
|Buoyancy Flux|
_______|_______
| CALCULATE |
| Equilibrium |
| Distance |
_______|_______
| CALCULATE |
| Equilibrium |
| Height |
|
____________|___________
| CALCULATE |
|Effective Plume Height|
|
effective height greater
than inversion height? ---no-->|
| |
yes |
__________|___________ |
|Set Effective Height| |
|to Inversion Height | |
| |
EXIT TO DSP3 <-----------|
(1) If the stack height is above the inversion,
then the program terminates since surface input meteorological
conditions most likely do not apply above the inversion.
(2) Buoyancy flux: It is determined using the air
temperature, gas stack temperature, volume flow rate, and
gravitational acceleration.
(3) Unstable or neutral conditions: The buoyancy
flux is used to compute the distance downstream where the
equilibrium height is reached. This distance, the buoyancy flux,
and wind speed determine the equilibrium height.
(4) Stable conditions: The buoyancy flux, wind
speed, and potential temperature lapse rate (based on the degree
of stability) are used to compute the equilibrium he
ight.
Downwind distance where equilibrium height occurs is not needed.
(5) Effective plume height: This is equal to the
sum of the equilibrium height and stack height above ground.
Model assumes gas is released at the effective height for
dispersion calculations. If the effective height is above the
inversion height, it is set equal to the inversion height. This
is a conservative approach for calculating ground concentrations.
d. DSP3: This executable program file computes the
toxic corridor (determined by concentration), concentration at a
given location and time, or maximum concentration and location,
and outputs the results. The type of output determines the
processing through the file, as shown in the flow diagram of
Figure 4.
Figure 4. DSP3 Flow Diagram
DSP2 or DSPHP
|
|
_______________________|________________________
| | |
OPTION 1 OPTION 2 OPTION 3
(toxic corridor) (point concentration) max concentration)
| | |
of Interest> | |
|
| | |
|----->|--------X=X+DX------->|<--------X=X+DX---------|<-|
| _______|_______ |
| | CALCULATE | |
| | Dispersion | |
| |--Y=Y+DY--------->|Coefficients | no
| | | and | |
| | |Concentration| |
| | |<-----------| | |----------------> maximum?
| no | | |
| | equals contour | |
| |<-concentration? | yes
| | | |
| yes | |
| | | |
| plot concentration | |
| | | |
|<--no--plot complete? | |
| | |
yes | |
| | |
OUTPUT OUTPUT OUTPUT
contour plot concentration
concentration
max distance for | and distance
each contour | |
| | |
CHANGE? CHANGE? CHANGE?
met conditions spill duration spill duration
source conditions location height
height option option
contours | |
spill duration | |
concentration averaging time | |
option | |
scale | |
time | |
| | |
|--------------->-<--------------------------|
|
EXIT
(1) Chemical concentration computations are
identical for each output option. Dispersion coefficients are
determined using the stability parameter and the surface roughness
length. Concentration is then determined using the dispersion
coefficients, elapsed time of the spill, wind speed, source
strength, source height, and inversion height. Locations are
determined using an X-Y coordinate system with the origin at the
source site and oriented so the X-axis is in the downwind
direction and the Y-axis is perpendicular in the crosswind
direction.
(2) Option 1: This contours user-specified
concentration (up to 3 contours) and plots the toxic corridor for
the lowest concentration requested. At each distance along the X
axis, the model computes the concentration and checks to see if it
exceeds the user specified concentration. If so, concentrations
are computed for increasing values of Y until the position of the
user specified concentration is located. This location is plotted
and the procedure is repeated at the next incremental value of X.
When the plot is complete, the maximum distance for the
concentration of interest is displayed. The entire process is
then repeated for the next specified concentration.
(3) Option 2: This outputs the concentration at a
user specified location (defined by the height, downwind and
crosswind distances).
(4) Option 3: This outputs the maximum
concentration and its distance from the source for a user
specified height. Due to model assumptions that the maximum
concentration will be along the X-axis, concentrations are
calculated for increasing values of X until the maximum is
located.
(5) At the completion of each option, the user may
change the following input data:
(a) Option 1: time, meteorological
conditions, sour
ce conditions, scale, concentration averaging
time, elapsed time since the start of the spill, contours, height
of interest, and output option.
(b) Option 2: elapsed time, location, and
option.
(c) Option 3: elapsed time, height, and
option.
If the scale on option 1 is changed, the plume is replotted.
For all other input changes, AFTOX recycles to the point where the
data is input while holding all other data constant and
reaccomplishes all computations which require the changed data.
4. Files Used: The files used and their functions are
listed in Tables 1. Multiple files are used due to storage
limitations of the GW-BASIC(tm) compiler. CHAIN statements are
used to link the files.
Table 1. Files and functions for AFTOX Version 4.1
-------------------------------------------------------------
Program Files:
AFTOX.EXE 3 Nov 92 Introduction
DSP1.EXE 2 Nov 92 Defines chemical properties and
meteorological conditions
DSP2.EXE 2 Nov 92 Defines source conditions
DSPHP.EXE 27 Oct 92 Defines buoyant plume source
conditions
DSP3.EXE 2 Nov 92 Computes hazard area, concentra-
tions, and outputs results
Data Files:
SD.DAT Station data
CH.DAT Chemical data
AFT.DAT Stores all input and output data
CONCXY.DAY File for storing x and y positions
of concentration contour
DEVICE.DAT Contains information on computer
and screen type.
Ancillary Files:
SETUP.EXE 26 Dec 90 Established computer and screen
type
SDFIL.EXE 1 Aug 92 Changes data in station data file
CHFIL.EXE 31 May 91 Changes data in chemical data
files
Supporting Files:
BRUN20G.EXE GW-BASIC(tm) Compiler Run-time
Library
https://www.doczj.com/doc/2a11245395.html, Utility for MPI printer
https://www.doczj.com/doc/2a11245395.html, Utility for MX-80 printer
https://www.doczj.com/doc/2a11245395.html, Utility for OKIDATA printer
B. GENERAL INFORMATION
1. Version:
a. This detailed documentation describes Version 4.1.
b. AFTOX Version 4.1 is designed to run on a Zenith Z-
248 microcomputer, equipped with EGA video, running the Microsoft
MS-DOS(tm) 3 operating system. It may operate on other true IBM-
PC compatibles using EGA video and MS-DOS(tm) 3.
c. AFTOX Version 4.1 also runs on the Zenith Z-180-
series laptop microcomputers, equipped with CGA video, running the
Microsoft MS-DOS(tm) 3 operating system. Version 4.1 may operate
on other true IBM-PC c
ompatibles using CGA video and MS-DOS(tm) 3,
such as the Zenith Z-150 series. Version 4.1 will not run on the
Zenith Z-100 microcomputers, which are not true IBM compatibles.
2. Language Used: Microsoft GW-BASIC(tm) programming
language. The Microsoft GW-BASIC(tm) compiler has been used to
prepare the executable program files (.EXE files) from BASIC
source code files (.BAS files), and the executable files are
provided to the user, so no BASIC compilation or interpretation is
needed to run AFTOX. The version of AFTOX described here is the
version dated Jan 1993. File creation dates of the executable
files directly related to AFTOX are given in Table 1 above.
3. Number of lines of code in Version 4.1: There are a
total of 3257 lines of source code, subdivided among the program
files as follows:
Lines of
Module BASIC Source Code Executable Code
AFTOX 115 AFTOX.EXE
DSP1 889 DSP1.EXE
DSP2 728 DSP2.EXE
DSPHP 163 DSPHP.EXE
DSP3 1449 DSP3.EXE
4. Memory and Disk Storage Requirements: Executable code
and documentation files are contained on one 5 1/4" double-sided,
double-density floppy diskette.
5. Peripheral Requirements: One 5 1/4" floppy disk drive
and a dot matrix printer if hardcopy contour plot is desired.
C. TECHNICAL BACKGROUND
1. Scientific Theory: The basic equation used by AFTOX
for dispersion is the Gaussian puff equation. The equation
assumes that the material is conserved during transport and
diffusion, and further assumes that the distribution of
concentration within the puff is Gaussian. Horizontal and
vertical dispersion parameters are contained within the Gaussian
puff equation and are used to statistically estimate the
dispersion of the plume. Determination of the stability and the
chemical source strength are based on accepted scientific
theories. Refer to AFGL-TR-88-0009 or PL-TR-91-2119 for a
detailed description of the scientific theory used by the model.
2. Equations and Algorithms: All equations, with
variables and constants, are listed and defined in AFGL-TR-88-
0009.
3. Accuracy and Limitations: AFTOX has wide
applicability. Unlike many methods developed for specific
situations, AFTOX can be used equally for all atmospheric
stability conditions and release scenarios. However, caution must
be used in interpreting the results. Although experience has
shown that Gaussian equation models are best suited for most
applications, one of their limitations is that calculated
distances for concentration values are average values. In order
to alleviate this potential problem, 90 percent confidence level
contour (i.e., the toxic corri
dor) is given. The toxic corridor
is plotted for all types of releases and represents, to the 90%
confidence level, the furthest extent of the chemical
concentration requested by the user.
D. PROGRAM MAINTENANCE
1. Conventions: AFTOX is a fully interactive, user-
friendly program. Default responses to AFTOX prompts and
questions are displayed to the user in brackets. No input other
than
keyboard) is required to accept these responses. All other input
is done by typing in a response, followed by
questions asked interactively by the program.
2. Variable Names:
Variable Definition
A area of spill (m2 or ft2)
AD liquid density coefficient
ALB albedo
AMW molecular weight of stack affluent
ANGMAT wind direction adjustment for plot
AO flag for (1) print or (2) save/print
ASP aspect ratio for plot
AT concentration averaging time (min)
BD liquid density constant
BR vapor pressure constant
CC cloud category (1-high 2-middle 3-low)
CHM$ chemical name
CLOUD cloud amount in eighths
CMIN smallest concentration of interest (mg m-3)
CMTC convective mass transfer coefficient (m/s)
CONC concentration at a given point (mg m-3)
CONCEN concentration at a given point (ppm)
CONCI(I) array of concentrations to plot (mg m-3)
CONCP(I) array of concentrations to plot (ppm)
CONCMI minimum concentration (mg m-3)
CONCMX maximum concentration (mg m-3)
CONCON concentration conversion factor
CR vapor pressure constant
CSA cosine of ANGMAT
CT1 lag that allows user to back up to inversion height
input
DFT difference between Greenwich and local standard time
DIFUS diffusion coefficient (cm2/s)
DLIQ chemical liquid density (g cm-3)
DTE$ date
DVAP chemical vapor density (g cm-3)
DW ground moisture (1-dry 2-wet)
DX downwind distance increment (km)
DZ buoyant plume height above stack (m)
EOK energy of molecular interaction (J)
F buoyancy flux (m4 s-3)
FAK DX factor
G soil heat flux (W m-2)
H height of leak or effective emission height (m)
HET station elevation (m)
HF sensible heat flux (W m-2)
HIFT height of inversion base (ft)
HINV height of inversion base (m)
HS stack height (m or ft)
HTCONV convective heat transfer rate (W/min)
HYGFAC hydrazine factor used in alternate evaporation formula
ISTAT spill type
IUCON code for concentration units (1-mg m-3 2-ppm)
IUNCO flag for chemical mol. wt. (0-known 1-unknown)
IUNIT flag for units used (1-metric 2-English)
K total no. of overlapping puffs for given location
LA station latitude
LO station longitude
LX
natural log of X
MP mass of pollutant per puff (g)
MU dynamic viscosity (g/cm-s)
MW molecular weight of chemical
MWA molecular weight of dry air
N station number in SD.DAT
NN chemical number in CHEM.DAT and CH.DAT
NNNN number of contour plots
NP number of puffs/min
NS flag for new scale
NU index number for spilled chemical
NUM station number of interest
OPT option number for output
OPTN sigma theta flag (1-unknown 2-known)
PC critical pressure (atm)
PRES atmospheric pressure (mb)
PRT flag for printing contour plot (1-printout requested)
Q net radiation at surface (W m-2)
QPIPE emission rate for continuous spill (g/min)
QSPO evaporation rate for instantaneous liquid spill (g/min)
QSS source strength (g/min)
RATE the kernal to be integrated to find concentration
RCONS gas constant for dry air (erg/mol K)
RENL Reynolds number for instantaneous liquid spill
RENNL Reynolds number for laminar flow
RENNT Reynolds number for turbulent flow
RHO air density (g m-3)
SAN angle of safety zone (deg)
SCALE scale of contour plot (km or mi)
SCN Schmidt number
SHNL Sherwood number
SIG effective diameter of molecule (A)
SIGTH sigma theta (standard deviation of wind direction)
SIGZ vertical diffusion coefficient before roughness
correction
SN$ station name
SOLAL solar angle (deg)
SR solar radiation factor
STB stability parameter
STBV vertical stability parameter
SX horizontal diffusion coefficient = SY
SY horizontal diffusion coefficient
SZ vertical diffusion coefficient
T elapsed time since start of spill (min)
TA air temperature (C)
TAK air temperature (K)
TB chemical boiling point (K)
TC chemical critical temp (C)
THETA angle of tic mark (deg)
TMSP total mass spilled (g)
TNU total number of chemicals listed in CHEM.DAT and CH.DAT
TP pool temperature (C)
TPUFF total number of puffs
TRAVT travel time to X+4SX (s)
TRAVTI travel time to X-4SX (s)
TS gas stack temperature (C or F)
TTSP total time of spill (min)
TTT flag in DSP3 for changing parameter
TW(1) time weighted average exposure limit (ppm)
TW(2) time weighted average exposure limit (mg m-3)
TW(3) short term exposure limit (ppm)
TW(4) short term exposure limit (mg m-3)
U wind speed at 10 m height (m/s)
UU wind speed at height ZW (m/s)
U9 flag indicating new met conditions have been entered
VC chemical critical volume (cm3/g-mole)
VF stack volume flow rate (m3/min or ft3/min)
VISCK kinematic viscosity of air (cm3/s)
VP chemical vapor pressure (atm)
WAT wind averaging time (min)
WD wind direction (deg)
WDTHL width of spill assuming laminar flow (m)
WDTHT width of spill assuming turbulent flow (m)
WWIDTH width of spill (m)
X downwind distance (km)
XMX maximum distance for a given concentration (km)
XO initial downwind distance (0.1 km)
XS(I) X position of current concentration contour
XS1(I) X position of contour 1
XS2(I) X position of contour 2
XS3(I) X position of contour 3
XST equilibrium downwind distance for buoyant plume (m)
XY X + virtual distance
Y crosswind distance (m)
YO initial crosswind distance (m)
YS(I) Y position of current concentration contour
YS1(I) Y position of contour 1
YS2(I) Y position of contour 2
YS3(I) Y position of contour 3
YSM(I) = -YS(I)
Z receptor height (m)
ZO spill site roughness length (cm)
ZOO wind site roughness length (cm)
ZW height of wind measurement (m)
3. Guidelines to Assist Validation: AFTOX has been
through several revisions based upon technical evaluations and
user comments from field testing. HQ AWS/XTX has reviewed the
software and has found the versions described in this document
technically sound.
4. Error Handling: During revisions and field testing,
errors and possible problem areas were identified and corrected.
All problems identifiable at this level of testing have been
accounted for. It is not possible to test all possible
paths/options in programs as complicated as this one; instead, the
operationally most common paths/options are tested and asymptotic
input values are submitted. Some errors may have gotten through
our testing. If errors are detected during operational use, they
should be brought through channels to the attention of:
HQ AWS/XTX
102 W. Losey Street, Room 105
Scott AFB IL 62225-5206
(DSN 576-4598)
5. Pseudocode and File Structure: See schematic
representation in section A.3 for illustration of structure and
processing sequence.
6. Special Maintenance Considerations: none
E. OTHER INFORMATION
1. Security Requirements: none
2. Privacy Act Considerations: none
3. Copyright Information: AFTOX is written in compiled
Microsoft GW-BASIC(tm). The executable (filename.EXE) program
files result from compilations of GW-BASIC(tm) source code and
hence require the GW-BASIC(tm) Compiler Run-time Library
(BRUN20G.EXE) to execute. Under special agreement with Microsoft,
the GW-BASIC(tm) Compiler Run-time Library is distributed with
AFTOX. The resulting distribution diskette carries external
markings, and the executable code displays a message to the effect
that "Portions (C) Copyrighted Microsoft Corp (1986)." The
"portions" referred to are, of course, the GW-BASIC(tm) Run-time
Library (BRUN20G.EXE), wh
ich cannot be further distributed except
as a member of the AFTOX distribution software.
F. ADMINISTRATIVE DETAILS
1. POC: HQ AWS/XTX
102 W. Losey Street, Room 105
Scott AFB IL 62225-5206
(DSN 576-4721)
2. Configuration Manager: AWS/SCR
This program has been validated and satisfactorily performs the
function for which it was designed. It is approved for release to
all government agencies and supported contractors.
APPROVAL AUTHORITY___________________________DATE__________________