**ALOHA**

**Areal Locations of Hazardous Atmospheres (ALOHA Ver. 5.2)**

(The following description was drawn from ALOHA ver. 5.2 Help Software.)

ALOHA was developed by the U.S. Environmental Protection Agency and the
National Oceanic and Atmospheric Administration to simulate airborne
releases of hazardous chemicals. The National Safety Council distributes
ALOHA and provides technical support. ALOHA is accepted for off-site
consequence analyses required for risk management planning regulations
in California, Delaware, New Jersey and Nevada.

### 

ALOHA uses the Gaussian model to predict how gases that are about as
buoyant as air will disperse in the atmosphere. Such "neutrally-buoyant"
gases have about the same density as air. According to this model, wind
and atmospheric turbulence are the forces that move the molecules of a
released gas through the air so that as an escaped cloud is blown
downwind, "turbulent mixing" causes it to spread out in the crosswind
and upward directions. According to the Gaussian model, any crosswind
slice of a moving pollutant cloud looks like a bell-shaped curve, high
in the center and lower on the sides. The following are features of
ALOHA.

-   Used for heavy gas (based on DEGADIS) and neutrally buoyant
    (Gaussian) gas modeling;

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-   Estimates source strength;

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-   Used in releases from tanks, puddles, and pipes;

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-   Calculates indoor air infiltration;

-   Contains an extensive chemical library, which is user expandable;

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-   Estimates gas cloud area and concentration over time in varying
    environmental conditions;

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-   Uses weather data that can be entered by the user or directly from a
    meteorological station (list of "ALOHA-ready" portable
    meteorological measurement stations);

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-   Plots toxic cloud "footprint" onto area maps;

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-   Has easy-to-use graphic interface and display;

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-   Includes mapping program, called MARPLOT (tm), using digitized
    mapping data or other mapping images; also enables customized
    overlays showing area facilities and vulnerable populations;

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-   Available for Windows or Macintosh platforms.

####### Model Outputs

ALOHA plots a "footprint," which represents the area within which the
ground-level concentration of a pollutant gas is predicted to exceed
your level of concern (LOC) at some time after a release begins. ALOHA
footprints can be automatically scaled and displayed on a grid or scaled
to a user-selected scale in ALOHA's Footprint window.

On ALOHA's footprint plot, the shaded area represents the footprint
itself. Dashed lines along both sides of the footprint represent
uncertainty in the wind direction. The wind rarely blows constantly from
any one direction. As it shifts direction, it blows a pollutant cloud in
a new direction. The "uncertainty lines" around the footprint enclose
the region within which, about 19 out of 20 times, the gas cloud is
expected to remain. The lower the wind speed, the more the wind changes
direction, so as wind speed decreases, the uncertainty lines become
farther apart. They form a circle when wind speed is very low. A curved,
dashed line leads from the end of one uncertainty line, across the tip
of the footprint, to the end of the other uncertainty line. This line
represents the farthest downwind extent of the footprint, if the wind
were to shift to rotate the footprint towards either uncertainty line.

######## 

######## Limitations

> 1\. ALOHA cannot be more accurate than the information you give it to
> work with. But even when you provide the best input values possible,
> ALOHA, like any model, can be unreliable in certain situations, and it
> cannot model some types of releases at all.

2\. ALOHA's results can be unreliable when the following conditions
exist:

> a\. Very low wind speeds---ALOHA's footprint accurately depicts a
> pollutant cloud's location only if the wind direction does not change
> from the value that you entered. Generally, wind direction is least
> predictable when wind speed is low. To show how much the cloud's
> position could change if the wind were to shift direction, under the
> particular weather conditions that you enter, ALOHA draws two dashed
> lines, one along each side of the footprint. ALOHA predicts that about
> 95 percent of the time, the wind will not shift direction enough to
> steadily blow the pollutant cloud outside of either line. The wider
> the zone between the lines, the less predictable is the wind direction
> and the more likely it is to change substantially. At the lowest wind
> speeds acceptable to ALOHA (about 2 knots, or 1 meter per second, at a
> height of 3 meters), these lines form a circle to indicate that the
> wind could blow from any direction.
>
> b\. Very stable atmospheric conditions---Under the most stable
> atmospheric conditions, there is usually very little wind and almost
> no mixing of the pollutant cloud with the surrounding air. Gas
> concentrations within the cloud can remain high far from the source.
> The cloud spreads slowly, and high gas concentrations may build up in
> valleys or depressions and remain for long periods of time, even at
> distances far from the release point. ALOHA does not account for
> buildup of high gas concentrations in low-lying areas.
>
> c\. Wind shifts and terrain steering effects---ALOHA assumes that wind
> speed and direction are constant (at any given height) throughout the
> area downwind of a chemical release. ALOHA also expects the ground
> below a dispersing cloud to be flat and free of obstacles. In reality,
> though, the wind typically shifts speed and direction as it flows up
> or down slopes, between hills and down into valleys, turning where
> terrain features turn. In urban areas, wind flowing around a large
> building forms eddies and changes direction and speed, significantly
> altering a cloud's shape and movement. Through streets bordered by
> large buildings can generate a "street canyon" wind pattern that
> constrains and funnels a dispersing cloud. ALOHA ignores these effects
> when it produces a footprint plot.
>
> d\. Concentration patchiness, particularly near the source---No one
> can predict gas concentrations at any particular instant downwind of a
> release, because they result partly from random chance. Instead, ALOHA
> shows you concentrations that represent averages for time periods of
> several minutes (it uses the laws of probability as well as
> meteorologists' knowledge of the atmosphere to do this). ALOHA
> predicts that average concentrations will be highest near the release
> point and along the centerline of any pollutant cloud, and will drop
> off smoothly and gradually in the downwind and crosswind directions.
> However, especially near the source of a release, wind eddies push a
> cloud unpredictably about, causing gas concentrations at any moment to
> be high in one location and low in another. Meanwhile, the average
> concentrations are likely to behave approximately as ALOHA predicts.
> As the cloud moves downwind from the release point, these eddies shift
> and spread the cloud, evening out concentrations within the cloud so
> that they become more similar to ALOHA's predictions.
>
> 3\. Avoid using ALOHA's Gaussian model to predict how a large *heavy
> gas* cloud will disperse. Large gas clouds that are denser than air
> ("heavy gases") are not buoyant, and disperse in a very different way.
> They are affected by gravity and other forces besides wind and
> turbulence. As they move downwind, they remain much lower to the
> ground than neutrally-buoyant clouds and flow like water. Ground-level
> concentrations within such clouds may reach much higher levels at some
> locations than the Gaussian model would predict.
>
> "Heavy gases" form vapor clouds that are heavier and denser than air.
> Heavy gases include not only gases with molecular weights heavier than
> air (the average molecular weight of air is about 29 kilograms per
> kilomole), but sometimes also gases such as anhydrous ammonia that are
> normally lighter than air, but that are stored liquefied under
> pressure. Liquefied gases typically escape from storage as a cold,
> heavy cloud containing a mixture of gas and fine aerosol droplets. A
> release of such a mixture is called a two-phase flow. The aerosols
> weigh the cloud down and make it more dense, and their evaporation
> cools the cloud.
>
> Heavy gases behave in a complicated way when they escape from storage.
> A heavy gas cloud first slumps away from the source in all directions,
> then flows downwind like water, propelled by the wind, gravitational
> slumping, and its forward momentum. As it moves downwind, air is
> stirred into the cloud, and it becomes less and less dense, eventually
> behaving like a neutrally buoyant gas. ALOHA takes more time to model
> this behavior than to predict dispersion of a neutrally buoyant gas.
>
> 4\. ALOHA doesn't account for the effects of:
>
> a\. Fires or chemical reactions---The smoke from a fire, because it
> has been heated, rises before it moves downwind. ALOHA doesn't account
> for this initial rise. It also doesn't account for the by-products of
> combustion, or for chemical reactions generally. ALOHA assumes that a
> dispersing chemical cloud does not react with the gases that make up
> the atmosphere, such as oxygen and water vapor. However, many
> chemicals react with dry or humid air, water, other chemicals, or even
> themselves. Because of these chemical reactions, the chemical that
> disperses downwind might be very different from the chemical that
> originally escaped from containment. In some cases, this difference
> may be substantial enough to make ALOHA's dispersion predictions
> inaccurate.
>
> b\. Particulates---ALOHA does not account for the processes that
> affect dispersion of particulates (including radioactive particles).
>
> c\. Solutions and mixtures---ALOHA is designed to model the release
> and dispersion of pure chemicals only; the property information in its
> chemical library is not valid for chemicals in solution or for
> mixtures of chemicals. It's difficult for any model to correctly
> predict the behavior of a solution or a mixture of chemicals because
> it's difficult to accurately predict chemical properties such as vapor
> pressure for solutions or mixtures. ALOHA's predictions are greatly
> affected by this and other chemical properties. When an incorrect
> property value is used in ALOHA, the model's release rate and
> dispersion estimates will not be valid.
>
> d\. Terrain---ALOHA expects the ground below a leaking tank or puddle
> to be flat, so that the liquid spreads out evenly in all directions.
> It does not account for pooling within depressions or the flow of
> liquid across sloping ground.

[Chemical Reactivity Considerations in ALOHA]{.underline}

To predict how a pollutant cloud will disperse in the atmosphere, ALOHA
assumes that the molecules in the cloud do not react with each other or
with the gases that make up the atmosphere, such as oxygen and water
vapor. That is, ALOHA assumes that the molecules that disperse in the
atmosphere are the same molecules that originally escaped from a
container. However, this is not always true. Some chemicals react with
dry or humid air, water, other chemicals, or even themselves. Because of
these chemical reactions, some or all of the molecules that disperse
downwind sometimes may be very different from the molecules that
originally escaped from containment. They may be heavier or lighter than
the original molecules, may have different properties and behave
differently in the atmosphere, and may be more or less toxic than the
original chemical. In some cases, these differences may be substantial
enough to make ALOHA's dispersion predictions inaccurate.

> 1\. Air-Reactive Chemicals: Some of the chemicals in the ALOHA library
> are known to react readily with air. The reaction is rapid, and often
> the chemical spontaneously catches fire. The burning chemical can
> endanger anyone close to the point of release. A toxic cloud probably
> will form if an air-reactive chemical is released. If the cloud reacts
> with air, however, ALOHA cannot accurately predict the threat zone,
> since it does not account for effects of chemical reactions.
>
> 2\. Water Reactive Chemicals: Some of the chemicals in the ALOHA
> library are known to spontaneously react with water. Such reactions
> are rapid and often produce toxic byproducts and heat. Boiling and
> splattering liquid can pose a special hazard to responders if a
> chemical spills on water or wet ground, or if water is sprayed on the
> chemical. Even when the available water is limited to the humidity in
> the air, a reaction can occur that produces toxic byproducts and
> affects the dispersion of the toxic cloud. ***ALOHA cannot***
> accurately predict the threat zone if there is a chemical reaction.
>
> 3\. Self-Reactive Chemicals: The molecules of some chemicals join
> together to form larger molecules or break apart to form smaller
> molecules. These reactions can cause the molecular weight of molecules
> in a dispersing cloud to be different from that of the molecules that
> escaped from containment.

***ALOHA will allow you to model any reactive chemical.*** When you
select an air- or water-reactive chemical, ALOHA will alert you that the
chemical is reactive and will describe the type of reaction and reaction
products to expect. ***ALOHA will not alert you*** when you select a
self-reactive chemical.

**Definitions**

**Terms Used in ALOHA\***

The following definitions were taken from the ALOHA Help Software.

Level of Concern or Output Concentration: A level of concern (LOC) is a
threshold concentration of an airborne pollutant, usually the
concentration above which a hazard may exist. ALOHA plots a "footprint,"
which represents the zone where the ground-level pollutant concentration
is predicted to exceed your LOC at some time after a release begins.

**The Immediately Dangerous to Life or Health (IDLH)** level is the
default LOC in ALOHA (check the "IDLH" help topic to learn more about
this LOC). An IDLH has been established for about one-third of the
chemicals in ALOHA. You may choose to use either the IDLH, when a value
is available, as your LOC, or another threshold concentration. Besides
the IDLH, a variety of toxic thresholds have been established by several
organizations; check the references listed below to learn about some of
them. ***You can add your own default LOC for any chemical*** to ALOHA's
chemical library. ALOHA then will use your LOC by default rather than
the IDLH for the selected chemical.

**Threshold limits:** The American Conference of Governmental Industrial
Hygienists (ACGIH) publishes recommended occupational exposure limits
for hazardous chemicals. The TLV, or *threshold limit value,* is the
maximum airborne concentration of a given hazardous chemical to which
nearly all workers can be exposed during normal 8-hour workdays and
40-hour workweeks for an indefinite number of weeks without adverse
effects. ***Do not use TLV values alone*** to evaluate the relative
toxicity of different chemicals or to identify safe or hazardous
conditions during an accidental chemical release.

TLV-TWA is the maximum allowable *time weighted average* concentration
for an 8-hour day and 40-hour work week. TLV-TWA values are obtained
either from industrial experience, from experimental human and animal
studies, or from a combination of both. If a TLV-TWA level has been
established for a chemical that you select, this value will be displayed
on ALOHA's Text Summary window.

**Stability Class:** The atmosphere may be more or less turbulent at any
given time, depending on the amount of incoming solar radiation as well
as other factors. Meteorologists have defined six "atmospheric stability
classes," each representing a different degree of turbulence in the
atmosphere. When moderate to strong incoming solar radiation heats air
near the ground, causing it to rise and generating large eddies, the
atmosphere is considered "unstable," or relatively turbulent. Unstable
conditions are associated with atmospheric stability classes A and B.
When solar radiation is relatively weak, air near the surface has less
of a tendency to rise and less turbulence develops. In this case, the
atmosphere is considered "stable," or less turbulent, the wind is weak,
and the stability class would be E or F. Stability classes D and C
represent conditions of more neutral stability, or moderate turbulence.
Neutral conditions are associated with relatively strong wind speeds and
moderate solar radiation.

Stability class has a big effect on ALOHA's prediction of footprint
size. Under unstable conditions, for example, a dispersing gas will mix
rapidly with the air around it. ALOHA will display a SHORTER footprint
than it would for more stable conditions, because the pollutant will be
diluted more quickly below your level of concern (LOC).