Japanese Radiation: Movement Through the Atmosphere
Source Newsroom: University of Maryland, College Park
Newswise — COLLEGE PARK, Md. - Predicting what happens to radioactive materials released by Japan's crippled nuclear power plants is even more difficult than forecasting the weather, and depends on several key factors, University of Maryland atmospheric scientists explain.
Factors determining the distribution of hazardous material include:
*Altitude to which radioactive or toxic materials are lofted (plume height);
*Day-to-day variability in forecast winds;
*Amount and nature of radioactive materials emitted;
*Removal and dilution of radioactive materials (by dispersion, wash-out by rain, or contact with the ocean, for example).
University of Maryland atmospheric science researchers Tim Canty , Jeff Stehr, Russell Dickerson and Ross Salawitch have been examining atmospheric patterns over the past week using a tool developed by the National Oceanic and Atmospheric Administration (NOAA) - the HYSPLIT mode.
These simplified models of atmospheric transport provide reasonable pictures of the long-range movement of potentially hazardous materials, and also provide guidance on which variables need to be monitored, the Maryland researchers say.
"Projected air mass patterns have varied dramatically from day to day, and it's these changing conditions that control the dispersal of radiation," says Maryland's Tim Canty.
NOAA has not reviewed the results and these model calculations.
KEY FACTORS IN ATMOSPHERIC MOVEMENT OF RADIOACTIVE MATERIALS
PLUME HEIGHT: The altitude where rapid, local upward motion of the escaped radiation ceases. When upward motion stops, horizontal winds take over (entrainment).
"In general, the higher the radioactive plume, the farther and faster it will travel," explains Jeff Stehr. "The ground-hugging winds tend to keep radiation localized. At higher levels, winds tend to move on a fast track that can transport material longer distances."
EMISSIONS: Large smoke or dust particles will settle out locally; gases and small particles will not. The longest lived materials - those most likely to have a large-scale impact - are particles between 0.1 and 1.0 micrometers in diameter. Materials of this size, characteristic of atmospheric pollution or haze, generally remain airborne until removed by precipitation.
The radioactive decay ranges broadly as well. In Chernobyl, the main radioactive materials included Iodine 131 with a half-life of eight days and Cesium 137 with a half-life of 30 years. As yet, only limited information is available on the nature and magnitude of emissions, though Japanese and American monitors are in the area.
ATMOSPHERIC REMOVAL OF RADIONUCLIDES: Long-lived radioactive materials are removed from the atmosphere by precipitation or contact with the ocean or land. "Precipitation is expected to be an important pathway for removal of radiation," says Canty. "We know rain efficiently removed such materials released by the Chernobyl accident."
Also, background air continuously mixes with polluted air, causing steep drops in concentration as radiation is transported away from a localized source. "Radioactive material will dissipate just as smoke from fireworks spreads in the sky", adds Stehr.
The rapid vertical motion associated with convection, which generally occurs in low pressure weather systems, is another process that leads to a decline in radiation concentration levels.
"The notion that radiation will remain at one altitude is a misnomer that would apply only to tracks that transit the Pacific without encountering convection", says Salawitch.
REACHING NORTH AMERICA
The level of radiation reaching North America depends on many factors, including the type of radioactive material released, whether it is in the gaseous or particulate form, the height of the radioactive plume, overall weather patterns, and precipitation and dilution as the material crosses the Pacific.
Prevailing winds show that plumes originating over Fukushima generally take at least five to seven days to reach North America. The majority of the radiation, upon reaching North America, is expected to reside at an altitude well above the surface and below where commercial airplanes fly at cruise altitude.
Significant amounts of radiation will likely be removed by precipitation or contact with the ocean. Also, radiation concentration levels will be reduced many orders of magnitude by atmospheric mixing, the team says.
"Calculations such as those in the dissipation figure are the basis for statements by many scientists that radiation will be diluted, to levels below thresholds of concern for human health, by the time these air masses reach the North America", says Russell Dickerson. "If there is widespread public concern, airborne measurements of atmospheric radionuclides using small gamma ray spectrometers, at projected locations of plumes, could be used by authorities to re-assure the public that the health risk is minimal".
Satellite imagery provides visual evidence of Asian dust storms, originating from the Gobi desert, crossing the Pacific and depositing material in North America. Transport of this material occurs on what is called the warm conveyor belt, a wedge of warm air that is lofted to very high altitude (above 5 km) and rides across the Pacific over a region of cold air.
While precipitation is generally associated with the initial lofting of warm conveyor belt air masses, there are times when it occurs with little rainfall. "Perhaps this poses the largest risk of widespread dispersal of the emitted material," says Salawitch. "While this is unlikely, it should be monitored."
SEASONAL WESTERN PACIFIC WEATHER
"It's an active time of year in the region - storm systems regularly push off the coast of East Asia," Stehr says. "This means that Japan is unlikely to have extended periods of stagnation that could trap radiation and increase exposures. Generally, prevailing winds disburse pollution away from Japan. On some days, however, weather patterns tend to re-circulate air from Japan over the ocean and back toward the Japanese coast. Obviously, this issue is being closely monitored by Japanese authorities."