External Chemical Attack
One way of analyzing a chemical attack on a major event is to use an internal-versus-external comparison. An internal attack is one using a device or materials inside a venue or perimeter, while an external attack is perpetrated outside of security perimeters with the hope of affecting the event. External attack is the form of CBRN terrorism that most closely resembles military chemical warfare. External attack may take the form of a simple gas or vapor release, in the hopes that winds will carry the chemical weapon to the target. Such attacks, as shown in the First World War, tend to have poor efficiency. Another tactic may be to use some type of munition, such as a mortar shell or a rocket. The efficiency of such an attack will depend on a wide variety of factors.
By their very definition, the materials used in an external attack begin their scenario at a distance from the target. While this may obviate the terrorists' need to smuggle something through security screening, it also means that more sophisticated and/or bigger devices may be needed. Chemical weapons require an effective concentration, and standoff distance leads to diffusion and dilution, so larger quantities of agent may be needed to compensate. External releases are often dependent on weather conditions. The extent to which the perpetrators understand these factors will have a direct bearing on the effectiveness of such an attack.
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