Chemical Survival Overview
The field behavior of chemical agents is dependent on weather variables such as wind, temperature, air stability, humidity, and precipitation. The influence of each variable depends upon the synoptic situation and is locally influenced by topography, vegetation, and soil.
Chemical agents may appear in the field in different forms: vapors, aerosols, or liquids. To understand the impact of chemical agents on the battlefield, the soldier must also understand how these agents are affected by weather and terrain. The following paragraphs give an overview of the basic characteristics of chemical agents and how weather and terrain influence and have specific effects on them.
Basic Characteristics
Vapors and small particles are carried by the winds, while any large particles and liquid drops fall out in a ballistic-like trajectory and are quickly deposited on the ground. Many agents give off vapors that form vapor clouds. The speed at which an agent gives off vapors is called volatility. Agents may be removed naturally from the air by falling out (large particles fall out much more quickly), by sticking to the ground or vegetation, or by being removed by precipitation. Once deposited upon vegetation or other ground cover, volatile agents may be re- released to the atmosphere for further cycles of travel and present a hazard until sufficiently diluted or decontaminated.
During approximately the first 30 seconds, the size and travel of an agent are determined primarily by the functioning characteristics of the munition or delivery system. Thereafter, the travel and diffusion of the agent cloud are determined primarily by weather and terrain. For example, in high temperatures, volatile agents produce maximum agent vapor in 15 seconds. Light winds and low turbulence allow high local concentrations of agents. High winds and strong turbulence reduce the concentration and increase the area coverage by more quickly carrying away and diffusing the agent cloud.
Vapors
When a chemical agent is disseminated as a vapor from a bursting munition, initially the cloud expands, grows cooler and heavier, and tends to retain its form. The height to which the cloud rises, due to its buoyancy, is called the height of the thermally stabilized cloud. If the vapor density of the released agent is less than the vapor density of air, the cloud rises quite rapidly, mixes with the surrounding air, and dilutes rapidly. If the agent forms a dense gas (the vapor density of the released agent is greater than the vapor density of air), the cloud flattens, sinks, and flows over the earth’s surface. Generally, cloud growth during the first 30 seconds is more dependent upon the munition or delivery system than upon surrounding meteorological conditions.
Nevertheless, the height to which the cloud eventually rises depends upon air temperature and turbulence. These determine how much cooler, ambient air is pulled into the hot cloud (and, hence, determines its rate of cooling). The agent concentration buildup is influenced by both the amount and speed of agent release and by existing meteorological conditions.
Shortly after release, the agent cloud assumes the temperature of the surrounding air and moves in the direction and at the speed of the surrounding air. The chemical cloud is subjected to turbulence forces of the air, which tend to stretch it, tear it apart, and dilute it. The heavier the agent, the longer the cloud retains its integrity. Under conditions of low turbulence, the chemical agent cloud travels great distances with little decrease in agent vapor concentration. As turbulence increases, the agent cloud dilutes or dissipates faster.
Aerosols
Aerosols are finely divided liquid and/or solid substances suspended in the atmosphere. Sometimes dissolved gases are also present in the liquids in the aerosols. Chemical agent aerosol clouds can be generated by thermal munitions and aerosol spray devices or as by-products of liquid spray devices and bursting munitions.
Airborne aerosols behave in much the same manner as vaporized agents. Initially, aerosol clouds formed from thermal generators have a higher temperature than clouds formed from other types of munitions. This may cause some initial rise of the cloud at the release point. Aerosol generated clouds are heavier than vapor clouds, and they tend to retain their forms and settle back to earth. Being heavier than vapor clouds, they are influenced less by turbulence. However, as the clouds travel downwind, gravity settles out the larger, heavier particles. Many particles stick to leaves and other vegetative surfaces they contact.
Liquids
When a chemical agent is used for its liquid effect, evaporation causes the agent to form into vapor. Depending upon volatility, vapor clouds are usually of low concentration, have about the same temperature as the surrounding air, and tend to stay near the surface because of high vapor density. Additionally, vapor density governs the extent that the vapor will mix with the air.
Liquid agents with high vapor density impact at ground level with very little evaporation of the agent. These agents are termed persistent agents. While drops are airborne, and after impacting, the liquid continues to evaporate. Agent vapor pressure will govern the rate at which the liquid will evaporate at a given temperature and pressure. Initial concentrations are lower, since the vapor source is not instantaneous as a vapor agent is but evolves over a long period (until the liquid source is gone).
Liquid agents may be absorbed (soaked into a surface) and adsorbed (adhered to a surface), and they may also evaporate. Once the liquid is no longer present on the surface, desorption (going back into the air) begins. The vapor concentration over areas contaminated with a liquid agent tends to be less than with newly formed vapor clouds, and downwind agent concentrations are not nearly as great as with other types of agents.