Signatur:
ZSP-202-121
In:
Research report / Cold Regions Research and Engineering Laboratory, 121
Beschreibung / Inhaltsverzeichnis:
Abstract: Stable 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 super cooled 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 H20 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 difference in size are shown to result from the differences in cooling rates over five orders of magnitude. Most of the ice fog crystals have settling rates which are smaller than the upward velocity of air over a 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 center 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 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 is further worsened, because the mechanisms for cleaning the air are virtually eliminated while all activities which pollute the air are increased.
Materialart:
Schriftenreihen ausleihbar
Seiten:
v, 118 Seiten
,
Illustrationen
Serie:
Research report / Cold Regions Research and Engineering Laboratory 121
URL:
https://hdl.handle.net/11681/5972
Sprache:
Englisch
Anmerkung:
CONTENTS
I. Introduction
II. Air pollution
Types of air pollution
Temperature in versions
Low temperature air pollution
III. Sources of pollution-water
Combustion products
Cooling water from power plants ·
Miscellaneous sources
IV. Sources of pollution other than water
Electrical conductance and particulates
Combustion products
Summary
V. Economic growth and ice fog
VI. General physical properties of ice fog
Optical properties
Cooling rate of exhaust gases
Development of a typical ice fog
The effect of freezing droplets on the growth rate of ice fog
VII. Structure of the polluted air layer
Volume
Temperature distribution and convection in Fairbanks air
VIII. Mass budget of ice fog
Ice fog precipitation rates
Density of ice fog
Ice fog evaporation rates
Use of the mass budget equation
Summary of the mass budget
IX. Air pollution aspects of ice fog
Air pollution
Remedial action
Ice fog probability
Literature cited
Appendix A. :The effect of suspended ice crystals on radiative cooling
Appendix B. Nucleation and freezing of supercooled water droplets
Abstract
Standort:
AWI Archiv
Zweigbibliothek:
AWI Bibliothek
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