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dc.contributor.authorBenson, Carl S.
dc.description.abstractStable pressure systems over interior Alaska sometimes produce prolonged, extreme (below -40°C) cold spells at the surface. The meteorological conditions responsible for two such cold spells are discussed in detail in Appendix A, where it is shown that the rate of radiative cooling of the air is enhanced by suspended ice crystals which are themselves a result of the initial cooling. Radiation fogs formed during the onset of cold spells are generally of short duration because the air soon becomes desiccated. These fogs consist of supercooled water droplets until the air temperature goes below the "spontaneous freezing point” for water droplets (about -40°C); the fog then becomes an ice crystal fog, or simply "Ice Fog". During the cooling cycle water is gradually condensed out of the air until the droplets freeze. At this point there is a sharp, discontinuous decrease in the saturation vapor pressure of the air because it must be reckoned over ice rather than over water. The polluted air over Fairbanks allows droplets to begin freezing at the relatively high temperature of -35°C. Between -35 and -40°C the amount of water vapor condensed by freezing of supercooled water droplets is 3 to 5 times greater than the amount condensed by 1°C of cooling at these temperatures. This results in rapid and widespread formation of ice fog (Appendix B) which persists in the Fairbanks area as long as the cold spell lasts. The persistence of Fairbanks ice fog depends on a continual source of moisture (4.. 1 x 10^6 Kg H2O per day) from human activities within the fog. Ice fog crystals are an order of magnitude smaller than diamond dust, or cirrus cloud crystals, which in turn are an order of magnitude smaller than common snow crystals (0.01, 0.1 and 1 to 5-mm respectively). The differences in size are shown to result from differences in cooling rates over 5 orders of magnitude. Most of the ice fog crystals have settling rates which are slower than the upward velocity of air over the city center. The upward air movement is caused by convection cells driven by the 6°C "heat island" over Fairbanks. This causes a reduced precipitation rate which permits the density of ice fog in the city center to be three times greater than that in the outlying areas. The inversions which occur during cold spells over Fairbanks begin at ground level and are among the strongest and most persistent in the world. They are three times stronger than those in the inversion layer over Los Angeles. Thus, the low-lying air over Fairbanks stagnates and becomes effectively decoupled from the atmosphere above, permitting high concentrations of all pollutants. The combustion of fuel oil, gasoline, and coal provides daily inputs of: 4.1 x 10^6 kg CO2 ; 8.6 x 10^3 kg SO2 ; and 60, 46 and 20 kg of Pb, Br and Cl respectively, into a lens-like layer of air resting on the surface with a total volume less than 3 x 10^9 m^3. The air pollution over Fairbanks during cold spells couldn't be worse, because the mechanisms for cleaning the air are virtually eliminated while all activities which pollute the air are increased.en_US
dc.description.sponsorshipThis study was supported by Grants DA-ENG-11-190 61 G3, DA-ENG-27-021-62-G5 and DA-AMC-27-021-64-G8 from the Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, and by State of Alaska funds.en_US
dc.description.tableofcontentsIntroduction : Acknowledgements -- Air Pollution : Types of air pollution ; Temperature Inversions ; Low Temperature Air Pollution -- Sources of Pollution, I Water : Combustion Products ; Cooling Water from Power Plants ; Miscellaneous Sources ; Summary of Man-made Water Sources for the Fairbanks Atmosphere -- Sources of Pollution, II Products Other Than Water : Electrical Conductance and Particulates ; Combustion Products ; Summary of Pollutants Other Than Water -- Economic Growth and Ice Fog -- General Physical Properties of Ice Fog : Optical Properties ; Cooling Rate of Exhaust Gases ; Development of a Typical Ice Fog -- Structure of the Polluted Air Layer : Volume ; Temperature Distribution and Convection in Fairbanks Air -- Mass Budget of Ice Fog : Ice Fog Precipitation Rates ; Density of Ice Fog ; Ice Fog Evaporation Rates ; Use of the Mass Budget Equation -- Air Pollution Aspects of Ice Fog : Air Pollution ; Remedial Action -- References Cited -- Appendices : Appendix A -- Appendix Ben_US
dc.publisherGeophysical Institute at the University of Alaskaen_US
dc.sourceGeophysical Instituteen_US
dc.titleIce Fog: Low Temperature Air Pollution; Defined with Fairbanks, Alaska as type localityen_US

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