Radiation and Atmospheric Behavior

1. Radiation and Atmospheric Behavior Global Environmental Change – Lecture 3 Spring 2014

2. What is Radiation? • Radiation describes a process in which energetic particles or waves travel through a medium or space • Radiation is often referred to as electromagnetic radiation (EMR) • It comprises both electric and magnetic field components • These components oscillate in phase perpendicular to each other, and usually perpendicular to the direction of energy propagation

3. EM Radiation • Relation between electric field, magnetic field, and direction of propagation

4. Classification of Radiation • Electromagnetic radiation is classified into several types according to the frequency, or alternatively its related property wavelength, of the wave

5. Frequency • Frequency is the number of occurrences of a repeating event per unit time. • The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency • It is usually measured in the unit Hertz, formerly known as cycles per second

6. Wavelength • Wavelength of a sinusoidal wave is the spatial period of the wave, the distance over which the wave's shape repeats • It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or null crossings

7. Wavelength Diagram • Wavelength of a sine wave, λ, can be measured between any two points with the same phase, such as between crests, or troughs, or corresponding null crossings as shown

8. Relationship of Frequency and Wavelength • λ = c/ν • c is the speed of light in vacuum, a fundamental constant of nature (cm/s) • ν is the frequency, measured in Hertz (Hz) • λ is the wavelength (cm/cycle) • c = 29,979,245,800 cm/sec (29.979 x 109 cm/sec)

9. Electromagnetic Spectrum • The EM spectra ranges from long waves with low energy to short wave X-ray and γ rays with high energy

10. Size and Sources

11. Radiation Interactions with Matter • If an atom absorbs a photon of electromagnetic radiation and remains intact, there is a strong tendency for it to return to its ground state • All physical systems will tend to move to lower energy levels, much as water runs downhill

12. Absorption of Radiation • It is the absorption of radiation by matter that is of concern in an effect that has come to be known as the Greenhouse Effect • The name comes from the use of glass structures to grow plants during times when temperatures are below the normal range for plant growth

13. Greenhouse Giant Amazon waterlilies in a large greenhouse at the Saint Petersburg Botanical Garden, Russia

14. Why Do Greenhouses Work? • Glass is essentially transparent to visible radiation • Light striking the Greenhouse enters freely • Light is absorbed by interaction with matter inside the Greenhouse • Various forms of interaction transform the visible light radiation to heat, in the form of bond vibration and stretching

15. Re-radiation • The warmed structures and plants inside the greenhouse re-radiate this energy in the infra-red, to which glass is partly opaque, and that energy is trapped inside the glasshouse • Although there is some heat loss due to conduction, there is a net increase in energy (and therefore temperature) inside the greenhouse • Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall

16. Temperature Scales • Scientists (and most of the world) use the Celsius temperature scale, a temperature scale that is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale two years before his death • From 1744 until 1954, 0°C was defined as the freezing point of water and 100°C was defined as the boiling point of water, both at a pressure of one standard atmosphere

17. Definition of Celsius Scale • By international agreement, the unit "degree Celsius" and the Celsius scale are currently defined by two different points: absolute zero, and the triple point of VSMOW (Vienna Standard Mean Ocean Water - specially prepared water) • This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature (symbol: K)

18. Relation to Absolute Temperature Scale • Absolute zero, the hypothetical but unattainable temperature at which matter exhibits zero entropy, is defined as being precisely 0 K and −273.15°C • The temperature value of the triple point of water is defined as being precisely 273.16 K and 0.01°C • The triple point is where water, ice, and water vapor coexist

19. Infrared Blanket • If the earth's atmosphere were transparent to infrared radiation, the earth would lose heat rapidly and would have a low average temperature • This temperature would be about 254 K, or about -19°C • While life would probably exist at these temperatures, it would be difficult and life on earth would likely be much different from life as we know it • Fortunately, some gases in the earth's atmosphere absorb some outgoing infrared radiation • These gases as known as Greenhouse gases, often denoted GHG

20. Polyatomic Gases • The most abundant polyatomic gas is water • Water is the most important greenhouse gas in the sense that it accounts for the major portion of the natural greenhouse effect • Other polyatomic gases in the atmosphere • Carbon dioxide (CO2) • Methane (CH4) • Nitrogen gases (NOx) • Sulfur Gases (H2S, DMS or (CH3)2S, SO2, SO3) • Chlorofluorocarbons (CFC’s)

21. Absorption of IR Radiation • IR radiation is absorbed by polyatomic molecules • It excites rotational and vibrational states and raises the molecules to a higher energy state • They return to the ground state by radiating IR radiation in all directions • Some of this radiation is directed at the ground and will likely be reabsorbed by the ground • Other rays are directed sideways, or upward • These rays will likely encounter other greenhouse gas molecules before escaping from the atmosphere

22. Dipole Moments • Polyatomic gases which absorb IR radiation must possess one or more moving electric dipole moments • The electric dipole moment is a measure of the separation of positive and negative electrical charges in a system of electric charges, • It is a measure of the charge system's overall polarity

23. Dipole Moment Diagrams Moving charges generate a changing electric field around the ions A water molecule possesses a dipole, with the oxygen (red) being megative, and the hydrogens (blue) being positive

24. Dipole Moment Video GLY 6746

25. IR Absorption • Each polyatomic gas adsorbs IR radiation at a discrete set of wavelengths • Combinations of gases are more effective at absorbing across the electromagnetic spectrum than any single gas • Some gases are much more effective than others

26. Liquid Water Absorption Spectra

27. CO2 IR Absorption

28. MethaneIR Absorption

29. Combined IR Absorption

30. Atmospheric Windows

31. Effect of Concentration • As the concentration increases, so does absorption • Eventually, the absorption is nearly 100% (saturation) • Further increases in concentration have little effect on absorption, or so it was long thought • The next slide illustrates the effect for C2F6

32. C2F6 Transmissivity vs. Wavenumber and Concentration Wavenumber (k) is defined as where λ is the wavelength • Animation made from a sequence of still images (double-click to play)

33. Svante August Arrhenius • Arrhenius, a Swedish scientist, published an article in April, 1896, called “On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground” • This was one of the first times anyone realized that atmospheric gases might influence temperature • In 1881,Arrhenius had received a fourth class doctorate, later upgraded to third class after his defense, so was not highly regarded at the time • In 1903 his later work in the same area as his doctorate was recognized by the Nobel Prize in Chemistry

34. Knut Angström • Angström, a Physics professor at Uppsela University, challenged Arrhenius work in the Monthly Weather Review of the American Meteorlogical Society • “He infers, therefore, that a layer so thick as to be equivalent to that contained in the earth’s atmosphere will absorb about 16 per cent of the earth’s radiation, and that this absorption will vary very little with any changes in the proportion of carbon dioxid (sic) gas in the air”

35. Error • For many years, this stopped consideration of Arrhenius’ idea • But that was a flaw in Angström’s approach • This error is described in a RealClimate web page, written by Spencer Weart • http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument

36. Fluid Properties • Before investigating this error, we need to think about the properties of fluid • Incompressible • Compressible • Compressibility is a measure of the relative volume change of a fluid or solid as a response to a pressure change • Objects may be said to be compressible or incompressible, depending on the degree of volume change they experience per unit of pressure

37. Compression of Water • Water is often said to be incompressible • At a depth of 4 km, with pressures are around 40 megapascals, water has a volume decrease of 1.8% • At 0º C, the compressibility is less than one part in a billion per Pascal • (One atmosphere is 101,000 Pascals)

38. Linear Pressure Response • As the figure shows, this means that water shows a linear response to an increase in pressure

39. Non-Linear Pressure Response • The figure is a graph of the actual change in pressure with increasing altitude, and is clearly non-linear • At an altitude of 8 kilometers, pressure is half as much as at sea-level • This is because the atmosphere is compressible Vertical scale is km

40. Compressible vs. Incompressible • The figure shows a response to pressure by a compressible substance (air), and an incompressible substance, water • There is more air per meter at low altitude than at higher altitude • The amount of water per meter does not depend on the depth to a significant extent

41. Exponential Function • The change in pressure with altitude is an example of an exponential function • Q = ekx, where: • Q = quantity in question • k is a constant, which may be positive or negative • x is a variable • e is an irrational number (sometimes called Euler’s number) equal to 2.7182818284590452353602874713527…. , and is the base of the natural logarithms

42. Change of Pressure with Altitude • Pressure clearly decays (grows smaller) with altitude • We can calculate the change in pressure as follows • P(z) = 1 atm • e-z[km]/8 km • z is the height above the ground, measured in kms

43. Temperature • Temperature is related to the average kinetic energy of the molecules in the volume under consideration • The faster molecules move, the higher the temperature • It does not matter how many molecules there are per unit volume

44. Heat Content (Enthalpy) • The heat content is equal to the energy required to create a system, plus the energy required to displace the surroundings, creating room for the system • If a gas is compressed, it warms up – we did work on the system to compress it, which added energy • If a gas expands, it cools down – the gas expanded, doing work on the universe

45. Adiabatic Change • Adiabatic change refers to change with no change in heat content • Adiabatic expansion – a gas occupies a bigger volume, but the molecules move slower • Adiabatic compression - a gas occupies a smaller volume, but the molecules move faster

46. Lapse Rate • As gas rises in the atmosphere, it expands, because pressure is less • If conditions are adiabatic, the gas will behave as shown in the diagram, depending on how much water it holds

47. Lapse Rate Definition • The lapse rate is defined as the change with height of an atmospheric variable • The variable is usually temperature • The adiabatic lapse rate is the change with constant heat content

48. Phase Changes • Substances, such as water, can exist in any of three phases • Gas (Water vapor) • Liquid • Solid (Water ice) • A change in phase involves heat • Water vapor → Water + heat • Ice + heat → Water

49. Latent Heat • If you stick your hand in an oven at 100º C for a short time, you will not be burned • If steam from a kettle contacts your hand, you probably will be • Steam has extra energy, called latent heat • When the steam hits your hand, some of it condenses, transferring energy to your hand, and burns you

50. Vapor Pressure • Water molecules in the air contribute to the total pressure within a system • The pressure is known as the vapor pressure • Vapor pressure is primarily a function of the temperature • The higher the temperature, the higher the vapor pressure