ALOHA's accuracy depends on the quality of 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.
Conditions that Reduce Atmospheric Mixing
When making concentration estimates, ALOHA assumes that the chemical is released into the atmosphere and immediately becomes mixed so that the concentration looks like a bell-shaped curve throughout the cloud (the highest concentration is downwind along the centerline). Even though that is not exactly what happens in a chemical release, this "Gaussian" assumption is fairly typical and provides reasonable concentration estimates in most cases.
However, ALOHA's concentration estimates can be less accurate when any condition exists that reduces mixing in the atmosphere. For example:
- Very low wind speeds. At very low wind speeds (less than 3 miles per hour) the pollutant cloud does not mix quickly with the surrounding air. The concentration of the gas in the chemical cloud may remain higher than ALOHA predicts, especially near the source.
- Very stable atmospheric conditions. Very stable atmospheric conditions (stability classes E and F) generally occur at night or in the early morning, and may be indicated by conditions such as low-lying fog. Under these atmospheric conditions, gas concentrations within a pollutant cloud can remain high far from the source.
Concentration Patchiness, Particularly Near the Source
Concentration patchiness is the term used for situations where the gas concentration cannot be described as a bell-shaped curve (as mentioned above). Concentration patchiness occurs in every dispersing cloud, particularly very near the source.
Near the source, ALOHA's concentration estimates may overestimate or underestimate concentrations, because ALOHA uses concentration averages. For the average concentration to be valid, the cloud must travel downwind to the point where enough eddies have mixed the air and the gas. This distance varies depending on the stability, wind speed, and release details. If the maximum distance to the toxic Level of Concern (LOC) concentration is less than 50 meters, ALOHA will not show the threat zone, because concentration patchiness makes the estimate unreliable near the source of the release (where patchiness is most pronounced).
ALOHA Doesn't Account for Some Effects
When using ALOHA, keep in mind that the program doesn't account for the effects of:
- Byproducts from fires, explosions, or chemical reactions. ALOHA doesn't account for the byproducts of combustion (such as smoke) or chemical reactions. The smoke from a fire, because it is has been heated, rises before it moves downwind. ALOHA doesn't account for this initial rise. ALOHA assumes that a dispersing 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 enough to make ALOHA's dispersion predictions inaccurate.
- Particulates. ALOHA does not account for the processes that affect dispersion of particulates (including radioactive particles).
- Chemical mixtures. ALOHA is designed to model the release and dispersion of pure chemicals and a few select solutions; the property information in its chemical library is not valid for mixtures of chemicals.
- Wind shifts and terrain steering effects. ALOHA assumes that wind speed and direction are constant throughout the area downwind of a chemical release. ALOHA also expects the ground below a dispersing cloud to be flat. In reality, though, the wind typically shifts speed and direction as it flows up or down slopes, between hills or down into valleys, turning where terrain features turn. In urban areas, wind flowing around large buildings forms eddies and changes direction and speed, significantly altering a cloud's shape and movement. ALOHA ignores these effects when it produces a threat zone estimate.
- Terrain. ALOHA assumes the ground is flat, which has different implications depending on the release scenario. For liquid releases, ALOHA does not account for pooling within depressions or the flow of liquid across sloping ground. ALOHA assumes that the liquid spreads out evenly in all directions, which may cause the puddle size and release rate to be overestimated when the ground is not flat. For gas releases, ALOHA does not account for changes in wind flow that can occur as the cloud is diverted by tall buildings and mountains.
- Hazardous fragments. If a chemical release involves an explosion, there will be flying debris from the container and the surrounding area. ALOHA does not model the trajectories of the hazardous fragments.
Questions: Contact us with questions, comments, or suggestions about ALOHA.