Japanese Radiation: Atmospheric Transport and Removal
Source Newsroom: University of Maryland, College Park
Newswise — COLLEGE PARK, Md. – What happens to radioactive particles and gases released from Japan’s crippled nuclear power plants is even more difficult to predict than the weather, and will depend on several key factors, explain University of Maryland atmospheric scientists.
Factors determining the distribution of hazardous material include:
• Altitude to which radioactive or toxic materials are lofted
• Day-to-day variability in forecast winds
• Amount and nature of materials emitted
• Removal and dilution of radioactive materials (by dispersion, wash-out by rain, or contact with the ocean, for example)
Worldwide, scientists are using publicly accessible meteorological tools to track how released radiation might be transported through the atmosphere. At the University of Maryland, atmospheric science researchers Tim Canty, Jeff Stehr, Russell Dickerson and Ross Salawitch have examined atmospheric patterns this week using the National Oceanic and Atmospheric Administration (NOAA) HYSPLIT model. NOAA has not reviewed the results and these model calculations do not consider the decay of radioactive compounds.
“Projected air mass patterns vary dramatically from day to day, and it’s these changing conditions that control the dispersal of radiation,” says Tim Canty.
TRAJECTORIES: MARCH 14 THROUGH 17
This trajectory figure shows projections of air parcels originating over the Fukushima nuclear plant on March 14, 15, 16, and 17. The figure legend indicates when parcels are expected to reach North America.
Though these are simplified models of atmospheric transport, the Maryland scientists say they provide reasonable pictures of the long-range movement of potentially hazardous materials, and also provide guidance on which variables need to be monitored.
Note: The figures in this release will be updated daily at http://www.atmos.umd.edu/~tcanty/hysplit
INTERPRETING THE TRAJECTORIES
Plume height: The altitude where rapid, local upward motion of the escaped radiation ceases. When upward motion ceases, horizontal winds take over. “This is a critical factor, says Ross Salawitch. “Plume height cannot be estimated in a reliable manner and must be directly measured. In lieu of such measurements, we have treated plume height as an adjustable parameter.” The legend on the right side of the trajectory figure indicates plume height in units of kilometer (1 km = 3281 feet).
“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.”
Track Uncertainty: Roughly speaking, the uncertainty of the location of radiation is equal to 20 percent of the distance along the track from the start point.
“We have placed error bars on the figures, at 24 hour intervals, to represent this approximate uncertainty in air parcel location,” says Canty. “This uncertainty is likely to be more realistic for the first few days for parcels that are lofted and not particularly meaningful 7 days out.”
When simulations are repeated using daily updates to the meteorological fields, for the same specified initial conditions, track positions vary. “Generally the further along the trajectory, the more variability we see, because wind patterns are inherently better known a day or two from now than a week from now,” says Salawitch. Wind fields used by HYSPLIT are updated as atmospheric measurements are obtained.
As yet, only limited information is available on the nature and magnitude of emissions. Japanese and American monitors are in the area. Large smoke or dust particles will settle out locally; gases and small particles will not. The longest lived materials, and thus the most likely to have a large-scale impact, are particles between 0.1 and 1.0 micrometers (10-7 and 10-6 meter) in diameter. Such aerosols, characteristic of atmospheric pollution or haze, generally remain airborne until removed by precipitation. The half-life with respect to radioactive decay varies broadly as well. In Chernobyl, the main radionuclides included iodine 131 with a half-life of 8 days and cesium 137 with a half-life of 30 years.
REMOVAL OF RADIONUCLIDES
Long-lived radionuclides (radioactive material) are removed from the atmosphere by precipitation or contact with a surface such as the ocean or land. The HYSPLIT model estimates precipitation along the projected tracks. The bottom panel of the trajectory figure shows the altitude of the tracks, with dotted lines representing encounters with precipitation.
“Precipitation is expected to be an important pathway for removal of radiation” says Canty. “Without knowing the chemical composition of the material being released, it is difficult to quantitatively estimate the efficiency of radionuclide removal by precipitation. However, we know rain efficiently removed radionuclides released by the Chernobyl accident.”
Health hazards are commonly quantified in terms of particulate and gaseous concentrations, represented as mass per volume of air. 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,” says Stehr.
The University of Maryland team has used HYSPLIT to produce an animated dissipation figure. The contours show atmospheric concentrations resulting from the release of one unit “mass” of material over Fukushima, between 0 and 2 km altitude on 15 March 2011. Removal processes due to rain and contact with surfaces are not considered; the modeled concentrations decline solely due to atmospheric mixing. The model stops reporting values of concentration when levels fall below a threshold of 10−17 mass per cubic meter or when air parcels cross the 159ºE longitude line. “This image allows scientists to relate projected atmospheric concentrations to the amount of material released”, says Stehr. “For instance, if a ton of material were to be released, the light blue color indicates where atmospheric concentrations of 0.01 nanogram per cubic meter of material would result (1 metric ton = 1000 kg; 1000 kg × 10−17 meter−3 = 0.01× 10−9 g meter−3, the same as 0.01 nanogram per cubic meter).”
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. When air parcels encounter convection, radiation will be distributed throughout the height range of the strong vertical winds, causing further drops in concentration. “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.
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 5 to 7 days to reach North America. The majority of the radiation, upon reaching North America, would be expected to reside at an altitude well above the surface and below where commercial airplanes fly at cruising altitude. Significant amounts of radiation will be removed by precipitation or contact with the ocean. Otherwise, radiation concentration levels will be reduced many orders of magnitude by atmospheric mixing.
“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 commercial available gamma ray spectrometers, at projected locations of plumes, could be used to verify that the public 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 such lofting occurs with little rainfall (i.e., when the underlying air is especially dry, such as over the Gobi desert). In this case, lofted material travels at high altitude and deposits where it next encounters rain. “Entrainment of the Fukushima plume in a warm, conveyor belt circulation that is too dry to precipitate perhaps poses the largest risk of widespread dispersal of the emitted material. While this is unlikely, it should be monitored,” says Salawitch.
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.”
This team specializes in the quantification of human activity on atmospheric composition.
The figures in this release will be updated daily at www.atmos.umd.edu/~tcanty/hysplit