By Vance Bennett
When you use the DOT Emergency Response Guidebook, one piece of advice you want is how far downwind the bad stuff will go. That will give you an idea of how large of an area to evacuate or to have people shelter in place. The ERG calls this the protective action distance. This piece of information is found in Tables 1 and 3 of the green bordered pages.
You will need two basic pieces of information in order for that section to give you any sound recommendations. It will ask for the size of the spill (large or small) and the time of day (day or night). It defines a small spill as anything less than 55 gallons. A large spill is more than 55 gallons. Day and night are the times between sunset and sunrise.
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When you look in the green-bordered pages, ERG gives you four sets of protective action distances. For example, the protective action distances for hydrogen sulfide are:
· Small spill: Day: 0.1 miles, Night: 0.3 miles
· Large spill: Day: 1.5 miles, Night: 4 miles
Why is the area affected so much larger at night?
The different distances for day and night are primarily determined by atmospheric stability. That’s the tendency of the atmosphere to resist or enhance vertical motion, especially in a low-lying atmosphere.
A stable atmosphere inhibits the vertical mixing. A neutral atmosphere neither enhances nor inhibits vertical mixing. And an unstable atmosphere enhances vertical mixing and turbulence. Hazmat releases, unlike some other forms of air pollution, take place near ground level. Because of this, how far the vapor or gas spreads is largely determined by atmospheric stability, which varies by time of day.
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Does the ERG explain this? Where can I find this information?
The obvious place to look is in the ERG white pages. Unfortunately, finding it isn’t obvious. As it turns out, the information is in two different places.
The first place this is addressed is on Page 287. It has a brief, one-sentence description of the reason for different distances for day and night. It says, “Since atmospheric mixing is less effective at dispersing vapor plumes during nighttime, day and night were separated in the analysis.”
That’s it?
Fortunately the ERG gives us a more complete explanation on page 347 where it says:
“The protective-action distance is divided into daytime and nighttime incidents because varying atmospheric conditions affect a hazardous area’s size. In fact, the quantity or concentration of the material’s vapor poses problems, not its mere presence. During the night, the air is generally calmer. This causes the vapor to disperse less and therefore creates a greater toxic zone. In daytime, the atmosphere is more active, so the vapor disperses more. As a result, there is a lower concentration of vapor in the surrounding air and the area that reaches toxic levels is smaller. Daytime is after sunrise and before sunset. Nighttime is between sunset and sunrise.”
That’s a lot to digest. Let’s break it down into smaller chunks.
The varying atmospheric conditions the ERG refers to are the daily changes in atmospheric stability. These fluctuations are referred to as diurnal variation. Diurnal is a meteorological term that relates to the variation in temperature between night and day.
There are other factors that can affect atmospheric stability such as cloud cover, relative humidity, wind speed, soil type and topography but the most important is heating and cooling at the earth’s surface. The recommended protective action distances in the ERG don’t take these other factors into account.
Night air is generally calmer because as the sun sets, the amount of radiant heat from the sun affecting the ground decreases. The ground cools rapidly as it loses its heat through radiation. A slight temperature inversion is created, which reduces the vertical movement of the air above the ground. Within a few hours after sunset, turbulent mixing is suppressed. The result is a calmer and more stable air mass.
When the sun rises, the increasing solar energy striking the ground causes the ground temperature to increase. The air in contact with and above the ground then warms by conduction and convection from the ground. The warm air near the ground becomes more buoyant and begins to rise, increasing the amount of turbulent mixing. This turbulence causes air to be stirred into the vapor cloud.
Turbulent mixing dilutes and disperses the hazardous gas or vapor. As the cloud disperses the concentration of the vapor or gas in the air decreases. The area affected by the release gets smaller.
This information can also be found in the ERG smartphone app, but the path to finding it is not obvious. You have to look in the section titled Reference Material that is located on the home page. The information you’re looking for is in the folder titled Protective Distance (it’s the middle of five). The wording is identical to that found in the printed version.
The explanation is rather complex, but the concept is simple. Daytime: unstable atmosphere, the toxic area is smaller. Nighttime: stable atmosphere, the toxic area is larger.
Now you know.
About the Author
Vance Bennett is retired from California Specialized Training Institute, the training branch of the Governor’s Office of Emergency Services. During his 30 years at CSTI he was an Emergency Management coordinator and instructor in the Hazardous Materials Section. He was the course manager for several CSTI courses including Hazmat Incident Commander, First Responder Operations and CSTI Instructor Certification. As the CSTI representative to the FIRESCOPE Hazardous Materials Subcommittee, he helped develop hazmat team typing and equipment standards for California hazmat teams. Vance was also a regular presenter at the Hazmat Continuing Challenge Workshop.
Prior to coming to work for CSTI, he was on active duty in the U. S. Coast Guard specializing in marine safety and pollution response. During that time he was involved in responses to hazmat incidents and oil spills including the EXXON Valdez and American Trader spills. He served in a variety of USCG units including the Pacific Strike Team before retiring in 2009.
