Modeling a Chemical Release
A chemical release incident has occurred or can
potentially occur. The chemical is
toxic if inhaled. Decisions must be
made quickly on a safe evacuation distance for the general public, or whether
to shelter-in-place if there is insufficient time to do an evacuation. There are many issues involved: what is happening, what chemicals are
involved, is there fire, is the chemical escaping to the air, what are the
hazards, how should the information be communicated to the public. The first responder may have a gas
dispersion model available which predicts a protective action distance based on
a downwind concentration based on some level of concern, and then order a
public evacuation distance based on that level of concern. When a decision is made on a safe evacuation
distance, there will always be critics who have the benefit of hindsight saying
(1) the model was too conservative and over predicted evacuation distances, or
(2) this situation was more dangerous and more people should have been
evacuated, or (3) authorities failed to communicate instructions to the general
public in time.
We will model two hypothetical examples of a toxic chemical
release incident, and try to understand what is going on.
Chlorine Rail Car Accident
A 90-ton capacity rail car is involved in an accident, and a
strong odor of chlorine is present. As
far as we can tell, there are no large volumes of liquid chlorine coming from
the tank, but the accident scene is obscured by yellowish-greenish gas. Responders can’t get near to see what
exactly is happening. There is no
fire. There are no other rail cars
containing flammables or other hazardous substances which might react with
chlorine causing an explosion. There
are causalities. How should this situation
be modeled to estimate a public protective action zone?
Potential Terrorist Incident
A suspicious item is discovered under the bleachers of a
stadium prior to a soccer game. The
item appears to be a quart or liter container of clear liquid attached to an
explosive device. It could be a hoax,
or the liquid could be some smelly material designed to disrupt the game. The liquid could be nitroglycerine or even a
chemical warfare agent. The stadium is
evacuated. What protective action
distance should be established under a worst-case situation that the container
might contain the toxic chemical warfare agent Sarin?
Modeling the Chlorine Rail Car Incident
In a real incident, responders rarely have all the necessary
information required to do gas dispersion modeling. Reasonable guesses must be made.
On 6 January 2005, at about 2:30AM, three 90-ton chlorine
rail cars were involved in an accident at a crossing siding in Graniteville,
SC. The resulting chlorine gas leak
killed nine people and sent over 350 people to a nearby hospital for chlorine
inhalation. About 5,400 people were
evacuated within a one-mile radius.
Initially, responders did not know how many rail car tanks were leaking
chlorine. It was established the next
day that chlorine was leaking from only one rail car tank, and that perhaps
possibly 40% of the chlorine still remained in the tank. The chlorine gas continued to escape from a
fist-sized hole in the tank. On January
9, when a temporary patch was used to plug the hole in the tank, it was
estimated that 30 tons of chlorine remained in the tank and 60 tons had
There were reports that the ALOHA gas dispersion model was
used to estimate a protection action distance for the Graniteville incident,
and that this model grossly over predicted concentrations as a function of
distance downwind. If the modelers
assumed that all of the chlorine were released at once or during a relatively
short time period, e.g. 60 minutes, the default time in ALOHA, very large
concentrations would be predicted downwind.
The chlorine release rate was much slower in the Graniteville incident
but still deadly.
The chlorine inside a 90-ton rail car would be shipped as a
liquid under its own vapor pressure.
Probably about 85% of the volume inside the tank would be liquid and the
remaining amount vapor and some nitrogen.
Assuming an ambient temperature of 50o
F, the pressure (gage)
inside the tank would be about 60 psi before the breach. If the hole is at the top of the tank,
chlorine gas will be released. The drop
in pressure inside the tank would cause the chlorine liquid to boil resulting
in more chlorine escaping. As the
chlorine boils, the tank will become chilled reducing the evaporation
rate. It will take many days to empty the
On the other hand, if there is a large hole at the bottom
of the tank, the pressure will force chlorine liquid out the hole. The tank will empty much sooner. The chlorine liquid on the ground will also
quickly evaporate. Maximum chlorine
concentrations in the air will be much greater.
The 2004 Emergency Response Guidebook lists Initial
Isolation Zone and Protective Action Distances for hazardous chemicals involved
in transportation accidents. This
guidebook is published jointly by Transport Canada, U.S. Dept. of
Transportation, and Secretariat of Transport (Mexico), and is
available at http://hazmat.dot.gov/pubs/erg/erg2004.pdf
The user need only consider four categories for each
chemical when looking up the Initial Isolation Zone and Protective Action
Distances in the Emergency Response Guidebook.
The categories are (1) small spills, daytime conditions, (2) large
spills, daytime conditions, (3) small spills, nighttime conditions, and (4)
large spills, nighttime conditions. A
breach in a 90-ton railcar is a large spill.
The Emergency Response Guidebook lists the Initial Isolation distance
for large spills as 0.24 km, and the Protective Action Distance as 2.4 km
(daytime conditions), or 7.4 km (nighttime conditions).
What is a large spill and what is a small spill? For most hazardous chemicals, the Emergency
Response Guidebook considers anything greater than 55 or 60 gallons as a large
spill. A 90-ton rail car is a large
spill, but a breach in a one-ton chlorine tank would also be considered a large
spill. We will discuss the Emergency
Response Guidebook further, but first let’s consider gas dispersion modeling.
Gas dispersion models ask the user basic information such as
(1) the chemical itself, or at least information about the chemical such as
molecular weight and boiling point; (2) amount of chemical released, (3)
information about the terrain, which is usually expressed in terms of a
“surface roughness”, and (4) meteorology.
The user then specifies a location downwind, and the model predicts a
concentration at that location.
Alternatively, the user may specify a concentration representing a
“level of concern”, and the model predicts a distance downwind corresponding to
that concentration. Information
requested on meteorology could include (1) wind speed and direction, and (2)
atmospheric stability (estimated from time of day, geographical location, date,
percent cloud cover, etc.).
What model should the responder use? Some popular models in the public domain are
ALOHA and SLAB. Chlorine is a dense gas
which tends to “hug” the ground as it travels downwind until either solar
heating creates atmospheric instability or turbulence from wind eventually
disperses the gas. Therefore any model
selected should have a “dense gas” component, which both ALOHA and SLAB
have. The model in the PEAC tool also
has a dense gas component.
Let us do a model comparison for a chlorine release rate of
1 lb/second under daytime and nighttime conditions. We don’t know the release rate but need to proceed with a
The ALOHA model is available at no
cost from the U.S. Environmental Protection Agency. Version 5.3.1 of the ALOHA model can be downloaded at http://www.epa.gov/ceppo/cameo/aloha.htm
. The PEAC tool is available from AristaTek,
. SLAB is a dense gas model developed by
Lawrence Livermore National Laboratories under U.S. Dept. of Energy contract. SLAB is available from a number of sources
such as http://www.weblakes.com/lakeepa4.html
The models shown above were
compared at a surface roughness of 0.1 meters (cropland, light residential
terrain), and the wind speed was measured at a 2 meter height.
Usually, the Emergency Response
Planning Guideline Level 2 (ERPG-2) is used as the Level of Concern for public
evacuation distance. The ERPG-2 is also
used in the 2004 Emergency Response Guidebook for their Protective Action
Distance. ERPG-2 is established by the
American Industrial Hygiene Association and is defined as the maximum airborne
concentration below which it is believed that nearly all individuals could be
exposed for up to one hour without experiencing or developing irreversible or
other serious health effects or symptoms which could impair an individual’s
ability to take protective action. For
chlorine, the ERPG-2 is 3 ppm. For the
daytime condition at 10 mph wind, the corresponding distance ranges from 0.7 to
1 miles (1.1 to 1.6 km) depending upon what model is used. For nighttime conditions at 2 mph wind, the
distances corresponding to 3 ppm varied from 2 to 7 miles (3.2 to 11.3 km)
depending upon the model.
The lethal concentration of
chlorine (LC50) for a one-hour exposure based on rat studies is just under 300
Note that the model comparisons
are plotted logarithmically. The level
of concern represents the ground level, plume cloud centerline concentration
for different distances (in miles) downwind.
The models agree fairly close to each other under windy or overcast
conditions as represented by the top graph, but depart from each other under
the low-wind, clear nighttime condition.
The first graph represents a neutral, or “D” atmospheric stability;
“neutral” meaning that there is little ground cooling or solar heating
resulting in a minimal temperature gradient in the air. The second graph represents a “stable”
nighttime atmospheric condition where the cooler air tends to sink to the
ground, sometimes referred to as the “F” atmospheric stability. The toxic gas cloud is not readily dispersed
under stable, nighttime conditions, and as a consequence, the distances
matching up with Levels of Concern are much greater.
Why are the models predict
essentially the same results under “neutral” conditions but depart under the
stable, nighttime, “F” stability condition?
Models in their development require data sets for calibration. The data sets allow development of algorithms
which predict how the chemical cloud will spread and disperse as it travels
downwind. For example, the ALOHA model
uses Briggs’ dispersion algorithms (called “sigmas” ) which were developed from
sulfur dioxide releases tests in a Kansas field in the 1960’s. SLAB used some data sets for chemical
releases done at the Nevada Test Site near Mercury, Nevada. There are many data sets developed under “D”
or neutral stability conditions; the data sets can easily be done in a wind
tunnel under controlled conditions. But
data sets are few and far between under the stable, nighttime “F” stability
condition. Consequently the models
differ depending upon what data sets are used.
Aristatek, Inc., completed a
series of chemical releases at the Nevada Test Site under a variety of
meteorological conditions ranging from the neutral “D” to the stable “F”
stability. The chemical was released
and the air concentrations in the chemical cloud was measured by a complex
array of sensors placed downwind.
Various structures were in the cloud path mimicking conditions which
might occur at an industrial complex such as a refinery. Data sets taken under neutral or “D”
stability conditions were easy to characterize. The “F” nighttime stability was much more difficult, the data
collected was very much dependent upon micrometeorology. There can be a “near F” condition where
there was enough local turbulence to disperse the chemical cloud, and a “far F”
condition where the chemical cloud remained.
Also, meteorological conditions were not uniform as the cloud moved from
Therefore, it should not come as a
surprise if available models differ under the “F” stability condition. The discrepancy becomes worse as the wind
speed decreases. Available models
cannot handle the “zero” wind condition.
A small chlorine release
incident occurred in Springfield, MA during June 1988 under calm and overcast
nighttime conditions. Based on odor
reports, the chlorine cloud appeared to move in all directions from the site,
bypassing some locations, with no discernable pattern. People within about ¼ mile from the site
were evacuated. A couple of days later,
on June 19, the release was much greater with an accompanying fire. Under a daytime 8 mph wind condition, the
chemical cloud was described as several miles long and only a few city blocks
The concentrations graphed are
maximum, centerline concentrations. If
a person moves crosswind from the chemical cloud centerline, the concentrations
will usually rapidly drop off. But if
conditions are calm or a very low wind speed, especially after sunset,
conditions are often unpredictable. If
the chemical is a dense gas such as chlorine and there is no fire, the person
may be advised to seek higher ground.
What can we conclude from this?
Modeling the Potential Terrorist
Back to the suspicious package at the sports stadium
real accidents responders rarely have all the necessary information
required to run a gas dispersion model.
Reasonable guesses must be made.
One of the biggest unknowns is the release rate to the
atmosphere. Usually first
responders can’t even get close to the site to determine exactly what is
guesses must be made as to the release rate and meteorology. If the responders guess too
conservatively (e.g. all of the chemical released at once or within a
short period of time), critics may say that the model is too conservative.
available popular models generally give similar results under “neutral”
atmospheric conditions or under light or moderate wind conditions. There may be major differences under
low wind, clear, nighttime conditions.
: The package at a distance looks like an
explosive device attached to a bottle containing a liquid. The stadium is emptied. A sensitive gamma radiation counter is
available, but there is no elevation in radioactivity as the package is
approached. The possibility that
package might contain nitroglycerine was brought up. The web site, http://www.respondersafety.com/downloads/standoff.doc
lists safe outdoor evacuation distances for explosives taking into account
fragmentation from shrapnel or glass.
Judging from the size of the bottle and the package, there could be up
to 2 kg of explosives, or a safe outdoor evacuation distance of almost 1000
feet. Under protection of a vehicle,
responders might safely approach up to 100 feet. But what if the bottle contained a chemical warfare agent
Sarin? What is the safe evacuation
distance for Sarin, if the bottle explodes releasing all of its contents at
All of the above models were
executed in a passive (Gaussian) mode (as opposed to a dense gas mode). The D2PC Model is a Gaussian dispersion
model developed by the military for gas dispersion of chemical warfare agents,
especially if released from munitions.
The reference citation is C. Glenvil Whitacre et al, 1987, Personal
Computer Program for Chemical Hazard Prediction (D2PC)
, CRDEC-TR-87021, U.S. Army Armament Munitions
Chemical Command, Aberdeen Proving Ground, MD.
The DEGADIS model is documented in an EPA publication, Spicer, T.O., and
J.A. Havens, Users Guide for the DEGADIS 2.1 Dense Gas Dispersion Model
The models do not predict the same
results. The major reason for the
differences is in how the models average concentrations when dealing with an
instantaneous release, as in an explosion.
At one end of the spectrum is the ALOHA model which treats the release
as lasting one minute. The other models
treat an instantaneous release as lasting a much shorter period, and as such,
the peak concentrations within the toxic cloud as it travels downwind are
greater. There are also differences in
the concentration averaging time, and what data sets are used to calibrate the
The 2004 Emergency Response
Guidebook lists initial isolation zones and protective action distances for
small and large spills of Sarin (when used as a weapon), under daytime and
nighttime conditions. For the purpose
of the ERG, a small spill is 2 kg and a large spill is 100 kg. “When used as a weapon” implies an
instantaneous release as in this example.
The initial isolation zone for small spills is defined by a radius of
150 meters in all directions from the incident. The protective action distance (PAD) s 1.7 km (1 mile) downwind
for daytime conditions, and 3.4 km (2.1 miles) for nighttime conditions.
What concentration of Sarin should
be used as the Level of Concern? An
ERPG-2 has not been established for Sarin.
The 2004 ERG uses 0.01 times the 1-hour LC50
for rats as
their Level of Concern, or 0.001 ppm (0.006 mg/m3
). Perhaps most suitable would be the Acute
Exposure Guideline Levels (AEGLs are published at the EPA Website, http://www.epa.gov/oppt/aegl/
). The U.S. Military with some exceptions has
adopted the AEGLs for their military exposure limits for chemical warfare
agents. These numbers used by the
military are available at: http://chppm‑www.apgea.army.mil/imo/ddb/DMD/TG/TECHGUID/Tg230.pdf
. Perhaps most appropriate for a PAD is
the Level 2 AEGL, or AEGL-2, defined as:
AEGL-2: Airborne concentration of a substance at or above which it is
predicted that the general population including “susceptible” but excluding
“hypersusceptible” individuals could experience irreversible or other serious,
long-lasting effects or impared ability to escape.
The AEGLs are exposure time dependent. AEGLs are available for various exposure
times (5 minute, 10 minute, 30 minute, 1 hour, 4 hours, and 8 hours). Considering that Sarin was modeled here as
an instantaneous release, the toxic cloud is expected to pass quickly. Shelter in place is a viable option. Therefore a 5-minute or 10-minute AEGL-2 is
appropriate. The 10-minute exposure
time as used by the military is 0.087 mg/m3
for Sarin. Referring to the graphs presented above, the
daytime downwind distance corresponding to 0.087 mg/m3
Concern ranges from 1 to 4 km depending upon what model is selected, or 2 to
over 10 km for clear, low wind, nighttime conditions.
Fortunately, Sarin will hydrolyze
with water vapor in the air. None of
the models available consider the destruction of Sarin by water vapor or
sunlight. Therefore the concentrations
far downwind from the incident should be less than that predicted by any of the
models. How much less is difficult to
say, but this should be taken into consideration for any model predictions over
a couple of miles.
Shelter in place is certainly an
option and should be considered for distances away from the stadium location.
Protective foams and bleach
solutions may be a viable action to mitigate or reduce the dispersal of the
liquid in the container if an explosion takes place.
What can we conclude from the
are many unknowns when confronting a suspicious package. Responders must consider all possible
situations and develop a plan for possible situations which may occur.
or sudden releases are more difficult to model than the constant release
rate situation. The release time,
how the material is dispersed at the source, and downwind concentration
averaging times all come into play when using a model. Default situations may be built into the
model. Therefore available models
will give different answers depending upon how the model is structured.
of Concern will be different for an instantaneous or short duration
release compared with a steady state, continuous release. If there is any wind, the toxic cloud
will travel downwind and pass quickly.
Similarly if there is solar heating of the ground the toxic cloud
will rapidly disperse.. Therefore,
the Level of Concern can be based on a short exposure time, for example,
the 10-minute AEGL-2. For steady
state conditions, or any release under clam nighttime conditions with a
clear sky, an ERPG-2 (or 1-hour AEGL-2 if ERPG-2 is unavailable) may be
more appropriate. Shelter-in-place
may be a viable option for short duration releases.