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Plume Dispersion

       One of the greatest threats and greatest assistance to the civilian community at large, from a hazardous materials spill is “plume dispersion”. Plume dispersion is the natural dissipation and dilution of a contaminants vapor cloud in the atmosphere. Plume dispersion calculations can be as inaccurate as the weather for a variety of reasons. Wind itself is a function of balance between temperature, cloud formation, and topography. In any given geographic area “wind” is caused by the heating and cooling of the earths’ surface. As the sunsets and the earth begins to cool, warmer air over the near constant ocean temperature rushes towards the shore which now becomes a cooler location of low pressure. The reverse wind current occurs as the sun heats the earth during the day. This action of temperature rise and fall occurs every morning and night near ocean coastlines. This phenomenon, known as a “sea breeze” affects other areas of the country in lesser degrees. The effect of this function of physics, is what we call “Wind”.

     Wind affects plumes and the spread of contaminant through four pathways. 1] Speed and direction, 2] low level inversions, 3] ground “roughness” or topography, and 4] atmospheric heating, all affect hazardous plumes. As you can see in drawing one, low speeds of wind affect plume dispersion in a number ways. A weak breeze leaves an atmospheric contaminant more concentrated, but at the same time reduces the area of contamination. The benefit here may be a lower volume of evacuation to contend with. Depending on the contaminant, monitoring, and chemical characteristics, you will be able to determine how the contaminant disperses, and if it will become less toxic.

     While high winds disperse contaminants, they often increase evacuation volumes due to a longer downwind contaminant plume that maintains a concentration above some PELs {Permissible Exposure Limits}. Sometimes, high winds can prevent plumes from dilution by forcing them low to the ground. Unfortunately, this area is where humans exist. These types of air currents can accelerate evacuation concerns, personnel increases, close roads and restrict entry to homes. Down-wind plume perimeter monitoring is required to insure correct exposure data for the chemicals toxic limit.

     Moderate winds often cause the greatest concern. If your toxic contaminant resists dilution when mixed with air, you can be faced with parts of both disadvantages. Moderate winds combine a firm directional flow with toxic concentration sometimes remaining intact. Strong concentration, above PEL limits, maintained over a moderate area of flow will maximize your evacuation concerns. Additionally, a wider lateral of contamination may increase downwind of the vapor release.

     A thermal inversion occurs when cool air moves in low over the ground surface where the land is still warm from the previous days’ heat. In the Midwest, this is commonly seen as fog early in the fall as cooler winter winds begin to move in over a warm summer earth. The interface seen between these layers is an obvious sharp line. This continues for a few weeks until the ground finally cools and begins to accept the coming winter. If a hazardous plume becomes trapped under this formation, it can maintain a strong concentration over a longer distance.

     Balancing between toxin and air currents is the local topography. Any structure, natural rise, or depression can influence your evacuation concerns. This is one of the reasons facilities producing hazardous materials have large geographic properties. In the event of a leak, the hope is that plume dispersion will weaken below the materials PEL before, it blows off-site. Obstacles in the path of a plume shape the plume by their aerodynamic shape. The contaminant plume either flows effortlessly around it, or slams into it causing “mixing”. For most contaminants, this mixing assists in dispersion and dilution. If you have local topography or conditions such as mountains, valleys, or sea breezes, this may benefit your clean-up and minimize evacuation during a toxic plume release. If this is not the case, your mixing topography usually consists of human dwellings. For the Hazardous Materials Technician, contamination spread and evacuation can be limited by two techniques in urban areas.

     While responders with specialized equipment are being contacted, first responder haz mat “Technicians” can identify the contaminant. Once ID is confirmed, vaporization may be reduced by immediate application of medium expansion Foam. Second, in-place sheltering can begin. This evacuation procedure should be mandatory for most communities. Man-power shortages make full scale evacuation highly problematic. For this to be useful and functional, a pre-plan and constant public education must be consistent for civilians to safely implement. With proper shutdown of ventilation intakes in dwellings, you eliminate plume contact from occupants. Quality Foam application can slow or eliminate vaporization on some products, shortening the overall time required for in-place sheltering.

     Weather conditions play a vital role when mitigating or dispersing hazardous plumes. Always consult current weather conditions at the response site. If you have the luxury of continuing weather data throughout the incident you will be able to adapt your operations for plume dispersion changes.

                                                                      Haz Mat Mike



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