Radiation & Greenhouse Gases

Radiation & Greenhouse Gases Spring 2012, Lecture 4

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 perpendicular to the direction of energy propagation

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

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

Frequency Diagram • Sinusoidal waves of various frequencies • Bottom waves have higher frequencies, and shorter periods, than those above • Horizontal axis represents time

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 zero crossings

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 zero crossings as shown

Relationship of Frequency and Wavelength • λ = c/ν • c is the speed of light in a vacuum, a fundamental constant of nature (cm/s) • ν is the frequency, measured in cycles per second, now called hertz (cycles/sec) • λ is the wavelength (cm/cycle) • c = 29,979,245,800 cm/sec (29.979 x 109 cm/sec)

Radiation Classification Types • Classification types include (in order of increasing frequency and decreasing wavelength) • Radio waves • Microwaves • Infrared radiation • Visible light • Ultraviolet radiation • X-rays • Gamma rays

Visible Light • Visible light, as perceived by the human eye, lies between approximately 400 to 700 nanometers

What is a Greenhouse? • A greenhouse is a structure with a glass or plastic roof and frequently glass or plastic walls • Greenhouses heat up because incoming visible solar radiation from the sun is absorbed by plants, soil, and other things inside the building

Greenhouse Physics - 1 • Glass is transparent to this 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

Greenhouse Physics - 2 • 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

Incoming Radiation • Some trace gases are known as “greenhouse" gases because they function like the glass in a greenhouse • Incoming radiation strikes the earth and some is absorbed, with the remainder being reflected • The absorbed radiation heats the earth, which results in the earth reradiating energy, in the infrared portion of the spectrum

Temperature Scales • Scientists (and most of the world) uses 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

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 (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)

Relation to Absolute 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

Blackbody • A theoretical object that absorbs all electromagnetic radiation that falls on it • Since no light is reflected, the object is perfectly black at absolute zero (0 K) • As a blackbody is heated above 0 K, it begins to emit radiation • The wavelength of the emitted radiation decreases as temperature increases

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 • Although life might survive 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

Greenhouse Gas Properties • Greenhouse gases absorb infrared radiation that corresponds to the vibrational and rotational energy levels of their bonds • Normally these gases have three or more atoms (they are “polyatomic”)

Major Atmospheric Gases • The major gases in the atmosphere, nitrogen, oxygen, and argon, are either mono- or diatomic • They do not have any appreciable absorption in the infrared

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 • Water is followed by carbon dioxide (CO2) and methane (CH4)

Absorption of IR • IR radiation absorbed by polyatomic molecules 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

An IR Blanket • 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

Absorption • Each gas adsorbs 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

Atmospheric Windows • Amount of absorption at different wavelengths in the atmosphere, presented in terms of the half-absorption altitude

Half-Absorption Altitude • Half-absorption altitude is defined to be the altitude in the atmosphere (measured from the Earth's surface) where 1/2 of the radiation of a given wavelength incident on the upper atmosphere has been absorbed • Windows correspond to those regions where the half-absorption altitude is very small

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 • The next several slides illustrates the effect for C2F6

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)

Wavenumber vs. Wavelength • Diagram shows the relationship between wavelength in microns and the wavenumber in cm-1 • Note this is an inverse relationship • Visible light is between 0.4 to 0.7 microns • IR is > 0.7 microns

Blackbody Radiation vs. C2F6 Absorption • Diagram shows the affect of increasing temperature on the absorbance of IR radiation for one Freon compound NOTE: The X-axis is mislabeled; it is actually wavenumbers (cm-1)

IR Gas Spectra IR Absorption • The program (text above) demonstrates how four greenhouse gases actually absorb IR radiation • The incoming radiation, of the correct frequency, excites the moles as shown • Click the text to run the program - you will probably get a warning about a possible virus, but the program is known to be virus free

Heat Absorption by CO2 Human_produced_CO2_is_a_greenhouse_gas__and_thus__causing_global_warming

Natural Greenhouse Effect • The atmosphere naturally contains carbon dioxide, methane and nitrous oxide • These gases--together with water vapor--create the natural greenhouse effect • They trap some of the sun's energy and keep the Earth warm enough to sustain life

Cumulative Absorption • The animation shows the absorbance of carbon dioxide, methane, water, and nitrous oxide

Enhanced Greenhouse Effect • An increase in the natural process of the greenhouse effect, brought about by human activities, whereby greenhouse gases such as carbon dioxide, methane, chlorofluorocarbons and nitrous oxide are being released into the atmosphere at a far greater rate than would occur through natural processes

Carbon dioxide • Each year we add more than 30 billion tons of carbon dioxide to the air mainly by: • Burning fossil fuels • Cutting down and burning trees • Deforestation accounts for about 20 percent of the carbon dioxide increase from human activities

Deforestation • Until 50 years ago most of the carbon dioxide from deforestation was released from temperate zones • Now tropical deforestation is the largest source • Tropical forests are being burned and cut for farming, mining and raising cattle

Automobile Exhaust • Burning One Gallon of Gasoline Generates 22 Pounds of Carbon Dioxide • When gasoline is burned, the carbon in it combines with oxygen in the air to form carbon dioxide • Because the oxygen adds weight, the newly formed carbon dioxide weighs more than the original unburned fuel

Cars and Population Increase • There are over 1 billion motor vehicles in the world today • If present trends continue, the number of cars on Earth will reach 2.5 billion cars by 2050

Coal • It takes a pound of coal to generate the electricity to light a 100-watt bulb for 10 hours • For every pound of coal we burn, nearly three pounds of carbon dioxide go into the atmosphere

Good Pollution? • Burning coal and other fossil fuels also releases sulfur dioxide, polluting the air and forming a haze that blocks sunlight • Haze cools the climate, partly masking the greenhouse effect

Methane • Generated naturally by bacteria that break down organic matter in wetlands and in the guts of termites and some other animals • It also escapes from natural gas deposits • Methane gas escapes from garbage landfills and open dumps • It also leaks out during mining, extraction and transportation of coal, oil and natural gas

Adding Methane • Each year we add 350 to 500 million tons of methane to the air mainly by: • Raising livestock • Coal mining and drilling for oil and natural gas • Rice cultivation • Disposing of garbage in landfills • Burning forests and fields

New Source of Methane • In 2006, research has shown that permafrost melting in the arctic is releasing methane trapped in formerly frozen sediments • Permafrost melting is the result of global warming

Potential Source of Methane • A great deal of natural gas is trapped as a solid clathrate complex • These methane clathrates are found in bands under the coastal sediments offshore from continents in a number of areas • They represent a possibly large new source of energy • Utilization of this resource may resource in large releases of methane

Animals and Methane • Bacteria in the gut of cattle break down the food these animals eat, converting some of it to methane gas • Cattle can belch up to a half-pound of methane a day • Sheep, goats, buffalo and camels also belch methane • Rapidly growing world population produces greater demand for meat and dairy products • Number of cattle has doubled in the past 40 years

Methane Increase • The methane we release today could still trap heat more than a decade from now, because it stays in the atmosphere that long • Each molecule of methane traps heat more than 20 times more effectively than a carbon dioxide molecule

Global Carbon Flux

Nitrous Oxide • Nitrous oxide is released naturally from oceans and by bacteria in soils • Each year we add 7 to 13 million tons of nitrous oxide to the atmosphere mainly by: • Using nitrogen-based fertilizers • Disposing of human and animal wastes • Automobile exhausts

Radiation & Greenhouse Gases