Dispersal and Fate of Chemical Warfare Agents
Once a chemical weapon has detonated, it creates a "primary cloud," a solid or liquid aerosol cloud. The cloud then settles to the ground, landing on individuals and creating ground contamination. The ground contamination then has a finite lifetime; it can injure from direct contact or from contact with a "secondary cloud" of agent that has evaporated from the ground contamination. Eventually, the agent contamination disappears as the agent reacts chemically or is diluted below toxic levels by physical action.
Primary Cloud
The factors that affect the danger from the primary cloud are related to the local weather. Factors diminishing the danger of the primary cloud include:
- Variable wind direction, which causes dilution by redirection of the cloud
- Wind velocity over 6 m s-1, which causes dilution by turbulence
- Unstable air, which causes dilution by turbulence
- Temperature below 0ÉC, which causes less evaporation from liquid or solid aerosol particles; aerosol particles settle to the ground more quickly than does agent vapor
- Precipitation, which washes both aerosol particles and vapor out of the atmosphere
Factors increasing the danger of the primary cloud tend to be the opposites of those previously cited:
- Steady wind direction
- Wind velocity under 3 m s-1
- Stable air (inversion)
- Temperature above 20ÉC
- No precipitation
Ground Contamination
Once the primary cloud settles to the ground, factors that decrease the danger of ground contamination include:
- High ground temperature, which causes decomposition of some agents
- High wind velocity, which dilutes the agent
- Unstable air, which dilutes the agent
- Heavy precipitation, which dilutes the agent, hydrolyzes the agent, and washes the agent into the soil
Factors that increase the danger of ground contamination are related to the stability of the air. Factors that characterize stable air include:
- Low wind velocity
- Temperature inversion
Fate of Dispersed Chemical Agent
Persistency describes duration of an area’s toxic contamination. Persistent agents are generally considered to be those where contamination lasts a day or more. Non-persistent (or volatile) agents are generally considered to be those where contamination dissipates in a matter of minutes or hours. In some cases, thickeners can be added to non-persistent agents to extend their persistence.
Eventually, agent contamination disappears as the agent reacts chemically or is diluted below toxic levels by physical action. Chemical reactions that affect the persistence of agent in the environment include:
- Hydrolysis with water in the environment
- Photochemical reactions with sunlight
- Thermochemical decomposition
- Other reactions with compounds present in the environment
Physical transformations of chemical agents are dominated by volatilization, which is a function of the vapor pressure of the agent. Data on the physical properties and rates of chemical reactions for various agents are can be found by following the links on the Chemistry of Lethal Chemical Warfare (CW) Agents and Chemistry of Other Military Chemical Compounds pages.
Calculating the precise lifetime of a chemical warfare agent in the field can be very complex. It requires the parameters from the agent, plus climate information from the specific location, including temperature, humidity, rainfall, and solar flux, among others. Nonetheless, one can assume some average values for humidity, rainfall, and solar flux, and give approximate lifetimes that indicate the degree of persistence under generalized winter and summer conditions.
