                            USER'S MANUAL

                                 for

            USAF TOXIC CHEMICAL DISPERSION MODEL - AFTOX

                             VERSION 4.1


                      Mr Bruce Kunkel, PL/GPA
                     Capt Cliff Dungey, HQ AWS/XTX

                               4 Jan 93


    Table of Contents

1.  Basic program information
2.  File structure
  2.1  Ancillary files
  2.2  Data files
  2.3  Printer files
3.  Setting up AFTOX
4.  Running AFTOX
  4.1  General
  4.2  Printing and storing data
  4.3  Station data
  4.4  Date and time
  4.5  Type of release
  4.6  Chemical data
  4.7  Meteorological data
  4.8  Roughness length at spill site
  4.9  Source information
  4.10  Worst cast scenario
  4.11  Continuous buoyant plume release
  4.12  Concentration averaging time
  4.13  Elapsed time since start of spill
  4.14  Types of output
  4.15  Exposure limits
  4.16  Confidence limits
5.  Special considerations


1.  Basic program information

     The USAF Toxic Chemical Dispersion Model (AFTOX) is a 
dispersion model that will determine toxic chemical concentrations 
and give the user the option of calculating a toxic corridor 
(hazard distances), the concentration at a specific location, or 
the maximum concentration and its location.  AFTOX will handle 
instantaneous or continuous, gas or liquid releases (non-buoyant) 
from either ground or elevated sources.  The program contains an 
additional option for continuous, heated (buoyant) plumes from 
stacks.  AFTOX Version 4.1 is designed to run on a Zenith Z-248 
microcomputer and the Zenith Z-180 series laptop microcomputers, 
running the Microsoft MS-DOS(TM) 3 (or later) operating system.  
Version 4.1 may operate on other true IBM-PC compatibles using EGA 
or 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.

     Use of AFTOX for Air Weather Service (AWS) units is governed 
by AFR 355-1.


2.  File Structure

     The AFTOX program contains five component program files, 
three printer files, and six data files.  Of the data files, four 
are for input data and two for output.  Three ancillary files are 
used for editing the data in three of the input data files.  CHAIN 
statements are used to connect the various program files.  A list 
of the files is shown in Table 1.

     Table 1. List of files in AFTOX
____________________________________________________________
PROGRAM FILES

   AFTOX.EXE    Introduction
   DSP1.EXE     defines chemical properties and the
                meteorological conditions
   DSP2.EXE     Defines source conditions (that is, emission
                rate, spill duration, and spill area)
   DSPHP.EXE    Defines source conditions for buoyant plume
                from a stack
   DSP3.EXE     Computes 1) the hazard area, 2) the maximum
                concentration, and 3) the concentration at a
                given point and time

DATA FILES

   SD.DAT       Station data
   CH.DAT       Chemical name and data, and toxic limits
   EVAP.DAT     Chemical data for Vossler evaporation model
   AFT.DAT      File for storing all input and output data
                when printer is off
   CONCXY.DAT   File for storing x, y positions of
                concentration
                contour
   DEVICE.DAT   Contains information on computer and screen
                type

ANCILLARY FILES

   SETUP.EXE    Establishes computer and screen type
   SDFIL.EXE    Edits station data file (SD.DAT)
   CHFIL.EXE    Edits chemical data file (CH.DAT)

PRINTER FILES

   PSCMX80.COM  Configures system for EPSON-type printer
   PSCOKI.COM   Configures system for OKIDATA-type printer
   PSCMPI.COM   Configures system for old Zenith printers
____________________________________________________________

2.1 Ancillary files

        SETUP.EXE

     Before running AFTOX for the first time, SETUP.EXE should be 
run to establish the type of computer and screen.  When running 
SETUP.EXE, a screen as shown in Figure 1 will appear.  Enter the 
appropriate code for the computer and monitor types.  The 
appropriate information will be stored in DEVICE.DAT.



____________________________________________________________
                          A F T O X

          AIR FORCE TOXIC CHEMICAL DISPERSION MODEL

                 SET UP FOR GRAPHICS PROGRAM

   Enter code for type of graphics card/monitor.

   E = EGA/VGA    C = CGA     N = Other or no graphics  ? E

   Enter code for monitor type

   C = COLOR   M = MONOCHROME     ? C
____________________________________________________________
Figure 1.  Screen display for SETUP routine.

        SDFIL.EXE

   The SDFIL.EXE file is used to set up the station data file, 
SD.DAT.  SDFIL allows one to view, delete, add, and edit data in 
SD.DAT.  The file may contain one or more stations.

     To run SDFIL, type SDFIL and press <ENTER>.  The computer 
will display a screen with several options as shown in Figure 2.  
Choose the appropriate option and proceed as instructed.

____________________________________
1--display or print station data
2--set up new SD.DAT file
3--edit data
4--add station
5--delete station
6--quit
----------------------------
Choose one of the above
 ___________________________________
Figure 2. List of options in SDFIL.


     The following is a list of data required for each station 
stored in SD.DAT.

           Station name
           1-Metric  2-English Units
           Standard Deviation of Wind Direction (Y/N)
           Standard Deviation Averaging Time (min)
           Latitude (deg)
           Longitude (deg)
           Surface Roughness (cm)
           Height of Wind Measurement (m or ft)
           Time Difference (Greenwich-local standard)
           Station Elevation (m or ft)

     Stations that are located reasonably close to each other (<50 
km) do not have to be listed separately because the latitude, 
longitude, and elevation are not highly sensitive parameters.  
However, if the stations have a different surface roughness or 
wind measurement height then they must be listed separately.

     The units refer to the meteorological and distance 
parameters.  The user will have the option of entering either 
metric or English units for the spill rate or quantity.

     As a guide for inputting the proper surface roughness, a 
table, as shown below, is automatically displayed on the screen.  
The surface roughness need not be restricted to one of the values 
shown in Figure 3, but it is recommended that the value fall 
between 0.5 and 100 cm.

____________________________________________________________
       TERRAIN DESCRIPTION       SURFACE ROUGHNESS(cm)

SNOW, NO VEGETATION, MUD FLATS, NO OBSTACLES             0.5
RUNWAY, OPEN FLAT TERRAIN, GRASS, FEW ISOLATED OBSTACLES   3
LOW CROPS, OCCASIONAL LARGE OBSTACLES                     10
HIGH CROPS, SCATTERED OBSTACLES                           25
PARKLAND, BUSHES, NUMEROUS OBSTACLES                      50
REGULAR LARGE OBSTACLE COVERAGE (SUBURB, FOREST)         100
____________________________________________________________
Figure 3.  Roughness length as a function of terrain type.

        CHFIL.EXE

     The CHFIL.EXE file is used to view, make changes, deletions 
or additions to the chemical data file, CH.DAT.  The current 
CH.DAT file contains 130 chemicals.  The chemicals are listed in 
alphabetical order.

     To run CHFIL, type CHFIL and press <ENTER>.  The computer 
will display a screen with several options as shown in Figure 4.  
Choose the appropriate option and proceed as instructed.

_______________________________________
1--display or print data
2--edit exposure limits for specific chemical
3--edit data for specific chemical
4--add chemical
5--delete chemical
6--quit
-----------------------
Choose one of the above
_______________________________________
Figure 4.  List of options in CHFIL.

     In viewing the chemical data, several options exist: 1) data 
may be displayed on the screen or sent to the printer; 2) either 
the chemical numbers and names or all the chemical data may be 
displayed or printed; and 3) chemical data for a particular 
chemical or for all the chemicals may be displayed or printed.

     The exposure limits are used only as a guide for the user.  
The user must input the exposure limits when running AFTOX, but 
may default to the STEL values.  The base bioenvironmental 
engineers can assist if you have any questions.

     When editing the chemical data under option 3, the user must 
re-enter all the data for a particular chemical, even though he 
may be changing only one value.  The old data is displayed on the 
screen to make it easy for the user to re-enter the data.  One 
word of caution--the names hydrazine, monomethylhydrazine (MMH), 
dimethylhydrazine (UDMH), and nitrogen tetroxide must be spelled 
correctly because the model must recognize the name, and not the 
number, in order for it to use the Vossler evaporation model.

  The data for each chemical consist of the following: 

  Chemical number
  Chemical name
  Time weighed average (TWA) exposure limits (ppm and mg/m3)
  Short term exposure limits (STEL) (ppm and mg/m3)
  Molecular weight
  Boiling temperature (K)
  Critical temperature (K)
  Critical pressure (atm)
  Critical volume (cm3/g-mole)
  Vapor pressure constants (3)
  Liquid density constants (2)
  Molecular diffusivity constants (2)

     If the chemical is a gas at all expected ambient 
temperatures, then only information up through the boiling 
temperature is needed.

     If the chemical is a liquid, vapor pressure information must 
be in the chemical file.  The exception is for the four chemicals, 
hydrazine, monomethylhydrazine(MMH), dimethylhydrazine(UDMH), and 
nitrogen tetroxide, that use the Vossler evaporation model (1989).  
The chemical data required for the Vossler model, including the 
vapor pressure data, are included in EVAP.DAT.  All other 
chemicals use either the Shell's SPILLS evaporation model 
(Fleischer, 1980) or the Clewell model (1983).

     The vapor pressure may be defined in one of three ways: 1) by 
the Antoine equation, in which case three constants are required, 
2) by the Frost-Kalkwarf equation in which two constants are 
required, or 3) by directly inputting the vapor pressure in atm.  
The model determines which method to use by the number of 
constants in the file.  To use the Frost-Kalkwarf equation, the 
critical temperature and critical pressure must be known.

     The liquid density is required for all chemicals in the 
liquid state, including those using the Vossler evaporation model.  
The liquid density is defined in one of two ways: 1) by the 
Guggenheim equation, in which case two constants are required, or 
2) by directly inputting the liquid density in g/cm3.  As with the 
vapor pressure, the model determines which method of defining the 
liquid density by the number of constants.  The Guggenheim 
equation requires both the critical temperature and critical 
volume.  If no density is entered, the model will assume a density 
of 1 g/cm3.

     The molecular diffusivity constants are the effective 
diameter of the molecule, (A), and the energy of molecular 
interaction, (J).  The diffusivity is required for the Shell 
evaporation model.  If either of these two constants is not 
available, the model defaults to the Clewell evaporation model.

2.2  Data files

     There are six data files, three of which have been discussed 
in Section 2.1.  The remaining three are EVAP.DAT, CONCXY.DAT, and 
AFT.DAT.  The latter will be described in Section 4.2.

        EVAP.DAT

     EVAP.DAT is the chemical data file for the Vossler 
evaporation model.  The chemicals listed in this file are 
hydrazine, monomethylhydrazine (MMH), dimethylhydrazine (UDMH), 
and nitrogen tetroxide.  The chemical data include the following:

       Molecular weight
       Boiling temperature
       Freezing temperature
       Molecular diffusion volume
       Constants and applicable temperature ranges for:
          Vapor viscosity
          Vapor heat capacity
          Vapor thermal conductivity
          Heat of vaporization
          Saturation vapor pressure
          Liquid thermal conductivity

     Data may be changed using an edit program.  However, if a new 
chemical is added to the file, the code in DSP2.BAS must be 
changed so that the new chemical will be recognized.

        CONCXY.DAT

     CONCXY.DAT is an output data file containing information on 
the X, Y coordinates of up to three requested concentrations.  
Data stored in the file include the concentration of interest, 
either mg/m3 or ppm, the time since release for an instantaneous 
or finite continuous release, and the contour half width for 
various distances downwind in either m or ft.  The half width has 
20 m resolution.  The actual half width will be within 20 m but 
always less than the calculated.

2.3  Printer files

     Three print files accompany the AFTOX program.  They are used 
to configure your computer to your printer so you can make a 
hardcopy of the graphics output from AFTOX.  Most of the newer 
computers can use the GRAPHICS.COM file in your DOS directory, but 
if this doesn't work, try one of these:
  PSCMX80.COM  for printers that emulate EPSON printers
  PSCOKI.COM   for printers that emulate OKIDATA printers
  PSCMPI.COM   for older Zenith printers (most are obsolete).

     All you have to do is place the proper command somewhere in 
your AUTOEXEC.BAT file and the configuration is done automatically 
everytime you turn on your computer.

3.  Setting up AFTOX

     AFTOX may be run directly from the diskette, but if your 
computer has a hard disk it is recommended that the program be 
transferred to the hard disk.  Create a directory on your hard 
disk where you would like the program to reside (e.g., C:>MD 
AFTOX4).  Then copy the files from the diskette to the hard disk 
(e.g., A:>COPY *.* C:>AFTOX4).

     To tailor the program to your particular computer, run 
SETUP.EXE.  Specify whether your machine is VGA/EGA, CGA, or no 
graphics.  If your machine has Hercules graphics, you will have to 
choose no graphics.  Also, specify whether your monitor is color 
or monochrome.  Once SETUP has been run, there will be no need to 
run it again unless you transfer the program to a different type 
of computer.

     Information on your location must be stored in the station 
data file, SD.DAT, by running SDFIL.EXE.  Once you input the 
station data into SD.DAT, there will be no need to run SDFIL.EXE 
again unless you change your location.

     You are now ready to run AFTOX.

4.  Running AFTOX

4.1  General

     AFTOX is a very user friendly program.  Simply proceed 
through the program, answering the questions as you go along.  
Default values are frequently given <in brackets>.  If you wish to 
go with the default value, simply press <ENTER>.  The program has 
the unique feature of being able to back up by entering <999> 
<ENTER>, thus eliminating the need to start over if you 
accidentally entered the wrong data.  The exception is that you 
can not back up into the previous file (e.g., you can not back up 
from DSP2 into DSP1).

4.2  Printing and storing data

     Type AFTOX at the DOS prompt and press <ENTER>.  This will 
start the program execution.  If you are using a printer, and wish 
the plume plot to be sent to the printer, you must load the 
appropriate PSC utility program before you start AFTOX.  If you 
haven't, the program will remind you, and you will have to start 
over.  If the printer is off, then the input and output data are 
stored in AFT.DAT for viewing or printing out at a later time.  
Each time AFTOX is run, the data in AFT.DAT is erased.  If you 
wish to save the data in AFT.DAT, the file can be saved by 
renaming the file (e.g., C:>AFTOX4:ren aft.dat spill#1.dat).

4.3  Station data

     If there is more than one station in the station data file 
(SD.DAT), you will be asked to enter the appropriate station.  The 
data stored in SD.DAT has been discussed in Section 2.1.

4.4  Date and time

     The computer date and time are displayed.  If the spill is 
for a different date or time, a new date and time may be entered.  
The program converts the date to a Julian date which it uses in 
conjunction with the time, latitude and longitude to determine the 
solar elevation, and subsequently, the solar insolation and 
surface heat flux.

4.5  Type of release

     AFTOX handles five types of releases:

        Continuous gas
        Continuous liquid
        Instantaneous gas
        Instantaneous liquid
        Continuous buoyant stack

The user has a choice of a continuous, instantaneous, or buoyant 
release.  An instantaneous release is defined as occurring over a 
15 sec period.  A continuous release is any release occurring over 
a period greater than 15 sec.  For the continuous and 
instantaneous releases, the model determines whether it is a gas 
or liquid, based on whether the air temperature is above or below 
the boiling point of the chemical.

4.6  Chemical data

     A list of 130 chemicals is displayed on the screen.  This 
list is shown in Appendix A.  The user enters the appropriate 
number.  If the chemical of interest is not listed, the user may 
press <ENTER> at the end of the list of chemicals.  He will then 
be asked the name of the chemical and its molecular weight.  If 
the molecular weight is entered, the model asks for the vapor 
pressure in mm Hg.  If the vapor pressure is known, the model uses 
Clewell's formula for determining the evaporation rate.  If either 
the molecular weight or vapor pressure is not known, the model 
assumes the worst case; that is, the evaporation rate is equal to 
the spill rate.  Also, if the molecular weight is not entered, the 
concentrations must be in mg/m3 since conversion to ppm is not 
possible without knowing the molecular weight.

     For a buoyant plume release, the model bypasses the chemical 
list and data file and asks only for the molecular weight.  Again, 
if the molecular weight is not known, concentrations must be in 
mg/m3.

4.7  Meteorological data

     The meteorological data consist of the following:

   Air temperature
   Wind direction
   Wind speed
   Standard deviation of wind direction and time over
     which it is determined (optional)
   Cloud amount
   Predominant cloud category
   Ground condition - dry, wet, snow covered (daytime only)
   Inversion base height

     The air temperature is a necessary input parameter but does 
not have a large influence on the results.  If a temperature 
reading is not available, a reasonable guess would be sufficient.

     Calm winds are not allowed.  If a zero wind speed is entered, 
the model will adjust the speed to either 0.5 m/sec or 1 kt, 
depending on which units the user chooses.  The model converts the 
wind speed measurement, whose height is specified in the station 
file SD.DAT, to a 10-m height wind speed. The 10-m wind speed is 
used in all of the calculations, and therefore the user should be 
aware that the plume may move downwind at a faster rate than the 
measured wind speed would indicate.

     If the standard deviation of wind direction is normally 
available, it is so indicated in SD.DAT along with the time over 
which it is determined.  If the standard deviation is not 
available at the time that the model is being run, simply press 
<ENTER> and the model will default to using the wind speed and 
solar conditions for computing stability and the corridor width.

     The cloud amount is entered in eighths.  There are three 
cloud categories to choose from - high, middle, and low.  If there 
are two cloud layers present, the operators should use the layer 
with the larger cloud amount.  If there are two layers with equal 
cloud amount, the operator should choose the lower cloud layer.  
The cloud type is not a factor at night.

     There are three ground types to choose from - wet, dry, snow 
covered.  If in doubt as to whether the ground is wet or dry, the 
user should choose wet, which will result in a more conservative 
hazard distance.  If the air temperature is 20C (68F) or greater, 
the model assumes no snow cover.

     If the base of an inversion is below 500 m (2000 ft), the 
height of its base is entered.  Inversion heights greater than 500 
m (2000 ft) may be entered but will have no effect on the plume.  
If the base of the inversion is below 50 m (164 ft), the model 
assumes no inversion but does assume a stability parameter of 6.  
When there is an inversion, the model assumes that the pollutants 
are trapped below its base.  In the rare case that the release is 
above the inversion, the pollutants remain above the inversion.

4.8  Roughness length at spill site

     For a non-buoyant release, the surface roughness length at 
the spill site must be entered.  For a buoyant plume, it is 
assumed that the elevated plume is minimally affected by the 
surface roughness, which is set at 3 cm.  The user has the option 
to call up the table of roughness lengths (Figure 3) for different 
types of terrain.  When uncertain as to the appropriate roughness 
length for the spill site, the user should choose a lower value, 
which will produce longer hazard distances.  Input roughness 
lengths below 0.5 cm or greater than 100 cm are set at 0.5 or 100 
cm, respectively.

4.9  Source information

     The source information required varies depending on the type 
of spill - continuous, instantaneous, gas, or liquid.

    Continuous Gas Release

     For a continuous gas release, the following information is 
required:

        Height of leak above ground (m, ft)
        Emission rate through rupture (kg/min, lb/min)
        Total time of release (min)

     The emission rate is assumed constant for the total time of 
the spill.  If the duration of the release is finite then the 
total amount released is displayed.

     Continuous liquid release

     For a continuous liquid release, the following information is 
required:

   Spill rate through rupture (kg/min, m3/min,
     lb/min, gal/min)
   Total time of spill (min)
   Spill area (m2, ft2) - otherwise default value is used
   Pool temperature - for those chemicals using the Clewell
        evaporation model (default = air temperature)

     The height of the leak is not entered since it is assumed 
that the liquid spills to the ground.  The default spill area is 
based on the volume spilled and the assumption of a 1-cm deep 
pool.  For an ongoing continuous spill, the volume spilled is 
based on a 10-min spill.  The default spill area is displayed but 
if the user has information on the size of the spill area, he may 
over-ride the default value.  Based on the input data and the 
chemical properties, the model computes the evaporation (emission) 
rate into the atmosphere and the total evaporation (release) time 
for a finite continuous release.

     Instantaneous gas release

     For an instantaneous gas release, the following information 
is required:

          Release height (m, ft)
          Amount released (kg, lb)

     The model assumes a cylindrical volume source in which the 
height is equal to the radius.  The initial volume of the spill is 
a function of the amount released.

     Instantaneous liquid release

     For an instantaneous liquid release, the following 
information is required:

   Amount spilled (kg, m3, lb, gal)
   Spill area (m2, ft2) -  otherwise default value is used
   Pool temperature -  for those chemicals using the Clewell
        evaporation model (default = air temperature)

     The default spill area is based on the volume spilled and the 
assumption of a 1-cm deep pool.  Based on the input data and the 
chemical properties, the model computes the emission rate into the 
atmosphere and the total time of release.

4.10  Worst case scenario

     When the initial spill alert is sounded, quite often very 
little source information is available.  In this case, AFTOX 4.1 
has the option of computing a worst case scenario.  The toxic 
corridor length calculation is based on Air Force Regulation 355-
1/AWSSUP1, Attachment 1, dated 24 September 1990.

     This regulation states that the corridor length is equal to 
the current wind speed in knots x 6,000.  This represents the 
distance in feet that the plume will travel in one hour.

     The corridor width is a function of the wind speed, or 
standard deviation of wind direction if available.  The 
computation of the width is based on the magnitude of the wind 
speed.  The exception is that their is no width adjustment for the 
duration of the spill since the duration may be unknown.

     Having computed the worst case scenario, the user can then 
proceed and enter the source information when it becomes available 
without re-entering the meteorological information.  However, if 
you need to change the name of the chemical or the source type 
(continuous, instantaneous), then you must start over.

4.11 Continuous buoyant plume release

     For a continuous buoyant plume release from a stack, the 
following information is required:

     Molecular weight (if available)
     Emission rate (kg/min or lbs/min)
     Elapsed time of emissions (min)
     Stack height (m or ft)
     Gas stack temperature (C or F)
     Volume flow rate (m3/min or ft3/min)

     With these data, the model calculates the equilibrium plume 
height and the downwind distance at which the plume reaches the 
equilibrium height.  If the equilibrium plume height is higher 
than the inversion, the plume height is adjusted down to the 
inversion height.  If the stack height is higher than the 
inversion the program terminates because the input meteorological 
conditions most likely do not apply above the inversion.  If 
molecular weight is not available, concentrations will be in 
mg/m3.

4.12  Concentration averaging time

     The user must specify the concentration averaging time.  For 
continuous releases, the default value is 15 min, which 
corresponds to the short term exposure limit (STEL) which is 
defined as a 15 minute time-weighted average exposure.  For 
releases of less than 15 min duration, the default averaging time 
is equal to the release time.  For instantaneous gas releases, the 
averaging time is 1 min.  The user can not enter averaging times 
less than 1 min.  Also, the averaging time cannot be greater than 
the release time since changes in the concentrations during the 
averaging period are not taken into account.  The averaging time 
affects the dispersion coefficients such that the longer the 
averaging time the greater the dispersion coefficients, thus 
resulting in shorter and wider plumes.  The averaging time and 
dispersion coefficients are related by the 1/5 power law.

4.13  Elapsed time since start of spill

     For instantaneous and finite continuous releases, the user 
must specify the elapsed time since the start of the spill.  For 
an ongoing continuous release, the model defaults to a 
sufficiently large elapsed time to assure a steady state condition 
(maximum hazard distance).  For an instantaneous liquid release or 
a finite continuous release, the default elapsed time is equal to 
the release time.  Except for short duration spills (a few 
minutes), the default time would normally give the greatest hazard 
distance.  For instantaneous gas releases, the default elapsed 
time is arbitrarily set at 10 min.  For small releases, the plume 
may dispersed within 10 min, in which case the user may wish to 
enter a shorter elapsed time.

4.14  Types of output

     The user can specify one of three types of output.

        1. Toxic corridor plot
        2. Concentration at specified location and time
        3. Maximum concentration at given height and time

     The one exception is when the no graphics option is chosen in 
SETUP, the corridor plot is replaced with a printout of the hazard 
distance for a given concentration(s).

     Toxic corridor plot

     If the user chooses not to use the default concentration, he 
may specify up to three contours in any order.  The concentrations 
can be specified in either mg/m3 or ppm.  The exception is if the 
molecular weight is not known, then the concentration is in mg/m3.  
The default concentration is the short term exposure limit (STEL) 
in ppm, if available, otherwise the time weighted average (TWA) 
exposure limit.  If neither is available then the user must enter 
one or more concentrations.  The user must also input the height 
of interest or choose the default height of 2 m (6 ft).

     AFTOX computes the Y position of the concentration of 
interest for increasing values of X.  X varies in increments of 
100 to 400 m depending how rapidly Y is varying in the X 
direction.  For slowly varying Y, X increments approach 400 m.  
The model starts the Y computation at 100 m from the source.  
However, if the centerline concentration at 100 m is less than the 
specified concentration of interest, calculations start at 10 m 
and progress outward in 10 m increments.  Plumes < 10 m in length 
will not be plotted.  The model first computes the concentration 
at the centerline and then moves outward in the Y direction in 20 
m increments when X>100 m, and 2 m increments when X<100 m.  The 
point at which the computed concentration is less than the 
concentration of interest defines the Y position.  The X, Y 
positions are stored in CONCXY.DAT.

     The model first computes the appropriate scale by calculating 
centerline concentrations for the specified time and height.  The 
concentration contours are then plotted on the screen as 
computations are taking place.  The contours are plotted in order 
of increasing concentrations no matter in what order they are 
entered.  These contours represent average distances, which means 
that 50 percent of the time the actual distance may be greater 
than shown and 50 percent of the time may be less than shown.  The 
90 percent hazard area for the lowest concentration is outlined on 
the contour plot and represents the area within which the plume is 
confined 90 percent of the time.  This 90 percent area represents 
the toxic corridor.

     The plume plot represents the position and size of a plume at 
the specified time after the start of the release.  Depending on 
the time after release, this may not be the maximum distance that 
the plume will extend downwind, especially if it is an 
instantaneous release.  The 90 percent hazard area, however, 
represents the area at the time when the plume reaches its maximum 
distance downwind.  It may or may not be at the specified time 
after release.  For more discussion on the confidence limits, the 
reader should refer to Section 3.3.15 in Kunkel (1991).

     Once the contour plot is completed, the user may proceed to 
make any of the changes listed below, obtain a printer plot, run 
another case, or terminate the program.

        1.  time and meteorological conditions
        2.  source conditions
        3.  concentration averaging time
        4.  elapsed time since start of spill
        5.  concentration contours
        6.  height of interest
        7.  scale
        8.  option
        9.  no change

     Concentration at a specified location and time

     If this option is chosen, the user must enter the downwind 
and crosswind distances, the height, and the time after start of 
the release.  The model then computes the concentration for that 
particular point and time.  The user may then change the location 
and/or time, choose another option, run another case, or terminate 
the program.

     Maximum concentration at a specified height and time

     This option will give the user the maximum concentration and 
its location for a specified height and time.  The user enters the 
height of interest and the time from start of release.  If it is a 
continuous spill that is still taking place and the specified 
height is the release height, the maximum concentration will be at 
the source.  In this case, AFTOX computes the concentration at 30 
m from the source.  If the specified height is different than the 
release height, or if the time after start of release is greater 
than the duration of the release, the maximum concentration will 
most likely occur at a distance greater than 30 m from the source.  
The location of the maximum concentration is determined within an 
accuracy of +/-5 m along the X axis, and of course would be 
located on the centerline of the plume at the specified height.  
When completed, the user may change the height and/or time, choose 
another option, run another case, or terminate the program.

4.15  Exposure concentrations

     If exposure concentrations have been established for the 
chemical of interest, AFTOX lets the user choose from two default 
concentrations.  In general, they represent a short- (15 min) and 
long-term (8 hours) exposure limit.

     In soome instances, several government organizations have 
established or recommended exposure limits for particular 
chemicals.  When available, AFTOX uses evacuation concentrations 
which are intended for the general public rather than employees in 
an occupational environment.  Evacuation concentrations are 
generally higher than occupational concentrations.  This is one 
reason AFTOX forecasts longer toxic corridors than other toxic 
dispersion programs.

     If your unit is responsible for calculating toxic corridors 
at your base, your operating instructions should include 
contacting the bioenvironmental engineers.  You know the weather 
observation and the forecast, but they know the chemicals.  They 
should be in the loop, especially when the chemical is not 
contained in the AFTOX chemical list.

4.16  Confidence limits

     The model predicts the mean hazard distance.  However, 
operationally, one would like to be at least 90 percent confident 
that the actual hazard distance will not exceed the predicted 
distance.  The model's concentration contour plot also shows the 
90 percent confidence level hazard area, or toxic corridor.  The 
evaluation study described in Kunkel (1988) concluded that the 
predicted hazard distance must be multiplied by 2.1 to be 90 
percent certain that the actual will not exceed the predicted 
distance.  Earlier versions of AFTOX showed the 90 percent hazard 
area only for continuous releases.  In AFTOX 4.1, the 90 percent 
hazard area is shown for all spills even though the 2.1 factor was 
derived from continuous release data.  It should be pointed out 
that in deriving the 90 percent hazard area, it is assumed that 
the input data is correct.  Errors in the input data, such as the 
wind speed and source strength will enlarge this area.  
Zettlemoyer (1990) examined the effect that data input 
uncertainties have on the concentration uncertainties using a 
Monte Carlo simulation technique.  He look at wind speed, emission 
rate, spill height, and the horizontal and vertical dispersion 
coefficients.  He concluded that errors in the wind speed had 
greater effect on the concentration uncertainty than the other 
parameters.  The greatest uncertainties occurred within one 
kilometer of the source.

     The method used to determine the width of the hazard area for 
the 90 percent confidence level remains similar to the method used 
by the Air Weather Service (see Kahler, et al., 1980).  If the 
measured wind is less than 1.8 m/sec, (<3.5 kt), the hazard area 
is a circle of radius equal to 2.1 times the predicted hazard 
distance.  If the standard deviation of wind direction (SD) is 
known and the wind speed is greater than 1.8 m/sec, then the toxic 
corridor width (W) is equal to:

     W = 6 SD

The measured SD is adjusted by the one-fifth power law to the time 
duration of the spill, up to a maximum of one hour.  If the width 
is calculated to be less than 30 degrees, it is set at 30 degrees.

     If SD is not known, then the following rules apply:

     1) For neutral or stable conditions, that is, a stability 
parameter of 3.5 or greater, the width is equal to 90 degrees for 
measured winds of 1.8 to 5.15 m/sec (3.5 to 10 kt).  For winds 
greater than 5.15 m/sec (10 kt), the width is equal to 45 degrees.

     2) For unstable conditions, the width is a function of the 
stability parameter (STB) as shown in the following equation.

     W = 165 - 30 STB

The width will vary from 60 degrees for neutral conditions to 150 
degrees for very unstable conditions.


5.  Special Considerations

     a.  Time input must be in local standard time (LST).  If 
local time changes to Daylight Savings Time, then time input 
should remain in LST.

     b.  Calm winds are not allowed because of the mathematics of 
the program.  If a zero wind speed is entered, the model assumes 1 
knot (0.5 m/s).

     c.  If the user is uncertain about roughness length, the user 
should choose a lower number, which gives a more conservative 
answer (i.e., larger corridors and higher concentrations).

     d.  If two or more cloud layers are present, user should use 
the layer with the largest cloud amount.  If layers are equal 
amounts, use the lowest layer.

     e.  If the user is unsure whether the ground is wet or dry, 
the user should use wet.  Choosing the wet option will give a more 
conservative answer.

     f.  If chemical molecular weight or vapor pressure is not 
known, the model sets the evaporation rate equal to spill rate.  
This is a worst case scenario.

     g.  If molecular weight is not known, concentration contours 
must be in mg m-3.

     h.  Concentration averaging time must be greater than or 
equal to 1 minute, but less than 60 minutes, and less than the 
release time.

     i.  When computing concentration at a specified location, 
downwind distance must be greater than or equal to 30 m 
(approximately 100 ft).

     j.  If stack height is above inversion height, then program 
terminates because surface input conditions most likely do not 
apply above the inversion.


REFERENCES

Clewell, H.J. (1983) A Simple Formula for Estimating Source 
Strengths from Spills of Toxic Liquids, ESL-TR-83-03.

Fleischer, M.T. (1980 SPILLS - An Evaporation/Air Dispersion Model 
for Chemical Spills on Land, Shell Development Company, PB 
83109470.

Kahler, J.P., Curry, R.G., and Kandler, R.A. (1980) Calculating 
Toxic Corridors, AWS/TR-80/003, ADA101267.

Kunkel B.A. (1988) User's Guide for the Air Force Toxic Chemical 
Dispersion Model (AFTOX), AFGL-TR-88-0009, ADA199096.

Kunkel B.A. (1991) AFTOX 4.0 - The Air Force Toxic Chemical 
Dispersion Model - A User's Guide, PL-TR-91-2119.

Vossler, T.L. (1989) Comparison of Steady State Evaporation Models 
for Toxic Chemical Spills: Development of a New Evaporation Model, 
GL-TR-89-0319, ADA221752.

Zettlemoyer, M.D. (1990)  An Attempt to Estimate Measurement 
Uncertainty in the Air Force Toxic Chemical Dispersion (AFTOX) 
Model, Master of Science Thesis, The Florida State University.


                        Appendix A

1.  ACETALDEHYDE                67. FORMALDEHYDE(56%)
2.  ACETONE                     68. FREON 12
3.  ACETONITRILE                69. HYDRAZINE
4.  ACROLEIN                    70. HYDRAZINE(54%)
5.  ACRYLIC ACID                71. HYDRAZINE(70%)
6.  AEROZINE-50                 72. MONOMETHYLHYDRAZINE(MMH)
7.  ALLYL ALCOHOL               73. DIMETHYLHYDRAZINE(UDMH)
8.  ALLYL CHLORIDE              74. HYDROGEN CHLORIDE
9.  AMMONIA                     75. HYDROGEN CYANIDE
10. AMMONIA (29%)               76. HYDROGEN FLUORIDE
11. ANILINE                     77. HYDROGEN SULFIDE
12. BENZENE                     78. ISOAMYLENE (2-METHYL-2-
13. BENZYL CHLORIDE                   BUTENE)
14. BROMINE PENTAFLUORIDE       79. ISOBUTANE (2-
15. BROMOFORM                         METHYLPROPANE)
16. BROMOMETHANE                80. ISOBUTYRALDEHYDE
17. BUTADIENE (1 3)             81. ISOPRENE (2-METHYL-1 3-
18. BUTANE                            BUTADIENE)
19. BUTYL ALCOHOL(n)            82. ISOPROPANOL (2-PROPANOL)
20. BUTYL ALCOHOL(sec)          83. ISOPROPYL ETHER
      (BUTANOL-2)               84. JP-4
21. BUTYL ALCOHOL(-t)           85. JP-9
22. BUTYLENE                    86. JP-10
23. BUTYRALDEHYDE               87. MESITYL OXIDE
24. CARBON DISULFIDE            88. METHANE
25. CARBON MONOXIDE             89. METHANOL
26. CARBON TETRACHLORIDE        90. METHYLENE CHLORIDE
27. CHLORINE                    91. METHYL ETHYL KETONE
28. CHLORINE PENTAFLUORIDE      92. METHYL IODIDE
29. CHLORINE TRIFLUORIDE        93. METHYL ISOBUTYL KETONE
30. CHLOROBENZENE               94. METHYL METHACRYLATE
31. CHLOROETHANE                95. NITRIC ACID (PURE)
32. CHLOROFORM                  96. FUMING NITRIC ACID 
33. CHLOROMETHANE                     (IRFNA)
34. CRESOL(-o)                  97. NITROBENZENE
35. CRESOL(-m)                  98. NITROGEN DIOXIDE
36. CRESOL(-p)                  99. NITROGEN TETROXIDE
37. CUMENE (ISOPROPYL BENZENE) 100. NITROGEN TRIFLUORIDE
38. CYCLOHEXANE                101. NITROPROPANE
39. DIALLYL AMINE              102. OXYGEN DIFLUORIDE
40. DIBUTYL PHTHALATE          103. PERCHLORYL FLUORIDE
41. DICHLOROBENZENE (1 2) (-o) 104. PENTABORANE
42. DICHLOROBENZENE (1 3) (-m) 105. PERCHLORYL FLUORIDE
43. DICHLOROBENZENE (1 4) (-p) 106. PHENOL
44. DICHLORODIMETHYLSILANE     107. PHOSGENE
45. DICHLOROETHYLENE (1 2)     108. PROPANE
      (cip)                    109. PROPIONALDEHYDE
46. DICHLOROETHYLENE (1 2)     110. PROPYLENE
      (trans)                  111. PROPYLENE OXIDE
47. DICHLOROPROPANE (1 2)      112. PYRIDINE
48. DIETHANOL AMINE            113. STYRENE
49. DIMETHYLANILINE(n n)       114. SULFUR DIOXIDE
50. DIMETHYL PHTHALATE         115. TETRACHLOROETHANE 
51. DIOXANE (1 4)                     (1 1 2 2)
52. DI-t-BUTYLETHYL DIAMINE    116. TETRACHLOROETHYLENE
53. EPICHLOROHYDRIN            117. TITANIUM TETRACHLORIDE
54. ETHANOL                    118. TOLUENE
55. ETHYL ACRYLATE             119. TOLUIDINE(-o)
56. ETHYL BENZENE              120. TRICHLOROETHANE (1 1 1)
57. ETHYL CHLORIDE             121. TRICHLOROETHANE (1 1 2)
58. ETHYL HEXANOL (2)          122. TRICHLOROETHYLENE
59. ETHYLENE                   123. TRICHLOROTRIFLUOROETHANE
60. ETHYLENE DIBROMIDE         124. TRIMETHYLBENZENE (1 3 5)
61. ETHYLENE DICHLORIDE        125. VINYL ACETATE
62. ETHYLENE GLYCOL            126. VINYL CHLORIDE
63. ETHYLENE OXIDE             127. VINYLIDENE CHLORIDE
64. FLUORINE                   128. XYLENE(-o)
65. FORMALDEHYDE(PURE)         129. XYLENE(-m)
66. FORMALDEHYDE(37%)          130. XYLENE(-p)
