# Presentation: 295484

## Modeling Dispersion of Chemical Hazards, using ALOHA 

**Prepared by Dr. Erno Sajo, Associate Professor, Department of Physics and Astronomy, Louisiana State University**

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Objectives

**3.1****	****Define the nature of the hazards associated with the air dispersion of hazardous chemicals.**
**3.2****	****Discuss the interaction of air dispersion of hazardous chemicals with the natural environment.**
**3.3**	**Discuss the goals and objectives of air dispersion modeling of hazardous chemicals, specific to the intended user.**
**3.4****	****Outline the capabilities of the ALOHA model and discuss data requirements. **
**3.5****	****Review available results and discuss importance to different user groups.**
**3.6****	****Discuss the application of ALOHA air dispersion model results for both planning and emergency management operations.**

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Overview of Atmospheric Dispersion 

- The effects of atmospheric motions on suspended pollutants
- Effects of large size turbulence
- Effects of small turbulence
- Particle pollutants & gaseous pollutants
- Behavior of cold or dense gas
- Behavior of hot gas
- Behavior of buoyant gas
- Behavior of mixtures
- Behavior of particulates and aerosols
- Dispersion in urban and rural environments

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Sources of Air Pollutants

- Industrial process accidents
- Transportation accidents
- Routine industrial fugitive emissions
- Nuclear, chemical or biological terrorism
- Source geometry as a factor impacting air pollutant sources

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Consequences of Nuclear, Chemical & Biological Releases

- Concentration of the release & health impacts
- Concentration by specific locations (points)
- Doses for radioactive materials
- Distribution of observed or calculated concentrations
- Level of concern (LOC) or threshold concentration
- Comparison of concentration distribution & LOC
- Comparison of radiation dose & regulatory standards

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Types of Hazard from Atmospheric Releases

- Direct exposure (inhalation, skin or eye contact)
- Indirect exposure (radiation)
- Contaminated food or water supply
- Pathways of pollutants from food or water
- Terrestrial or aquatic pathways
- Classification of pathways

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Importance of Hazard of Airborne Pollutants

- Protection of life and human health
- Protection of property
- Vulnerability of population near chemical sites or transportation routes
- Clearer ways to describe behavior of hazards

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Meteorology and Boundary Layer

- Wind factors
- Vertical temperature structure & stability
- Surface roughness
- Meteorological instrumentation

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Source Term, Dilution & Plume Rise

- Release parameters from source
- Release parameters of the chemical
- Plume rise
- Release height for buoyant sources
- Volume & area point sources
- Building effects

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Dispersion of Neutrally Buoyant Gases 

- The plume is not rising nor sinking
- Equal probability of spreading in different directions
- Neutrally buoyant gases are described using Gaussian statistics or normal distributions

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Dispersion of Heavy Gases	

- Types of heavy gases
- Gravitational spreading
- Phases of heavy gas dispersion
- Release
- Transition
- Dispersion

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Goals of Modeling Atmospheric Dispersion

- Assess nature & extent of hazard
- Enhance preparedness
- Assess actions to be taken
- Examine evacuation routes

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Atmospheric Dispersion Models

- Types of models and data requirements
- Computer requirements
- Limitations of computational models
- Uncertainties in modeling

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Air Dispersion Models

- Models specific for type of release
- Industrial Source Complex (ISC3-ST)
- CALPUFF & SCIPUFF (buoyant gas)
- DEGADIS & SLAB (heavy gas)
- ALOHA (both buoyant & heavy gas releases)

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## ALOHA Capabilities and Data Requirements

- Meteorological data requirements
- Chemical database
- Capabilities
- Automatically determines heavy or buoyant gas
- Models emissions from many sources
- Predicts indoor & outdoor sources
- Displays concentration footprint

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## ALOHA (continued): Limitations

- Does not account for the following:
- terrain effects or building effects
- radioactive materials or particulates
- Chemical mixtures
- Deposition rates
- Effects of rain or fog
- Releases into water bodies
- Duration limited to 1 hour
- Release area limited to 10 km.
- Minimum distance to source is 100 m.

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Dispersion Model Results

- Footprints (concentration isopleths)
- Dose or dose rate
- Deposition isopleths
- Probabilities of exposure

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## ALOHA Users

- Emergency managers & responders
- Emergency planners
- Toxicologist
- Health Physicits

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## ALOHA Scenario

- In a transportation accident near Baton Rouge, LA, a 9000 Gallon tank truck carrying ammonia overturns and shears off a flange whose diameter is 4 inches. The size of the tank is 24 feet long and 8 feet in diameter. The tank contains liquid, which is stored at ambient temperature. The fill density of the tank is 75% by volume. The sheared-off flange creates a circular opening of about 3 inches in diameter, and it is located at 30% of the way to the top of the tank.
- At the time of the accident, the wind direction is NE (i.e. blowing from NE), the wind speed is 5 m/s, measured at 10 m height. The accident occurred on a highway near a small village. The weather is partly cloudy, 50% relative humidity, and the temperature is 12 degrees Celsius.

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA

## Evaluating Hazard Models

- Quality
- Do the results accurately reflect the event?
- Are the limitations of the model clearly stated?
- Is information presented in an orderly manner?
- Timeliness
- Is the information provided when needed?
- Is the information up to date?
- If the scenario changes, does the model reflect the change?
- Completeness
- Is the model complete?
- Does the user have access to past model runs?
- Does the model provide appropriate information?
- Does the model give too much information?

- Session 11: Modeling Dispersion of Chemical Hazards, using ALOHA