week 3 Flashcards
(18 cards)
EM radiation’s interaction with matter
- the electric and magnetic fields of EM radiation can interact with the electric forces binding matter together, causing it to be absorbed, reflected, scattered etc.
- atomic and molecular properties are “quantised”, i.e. only specific values are allowed
- only EM radiation of the right frequency can interact with matter and change its state
- UV and visible light -> atomic transitions / chemical reactions
- IR -> molecular vibration and rotation
describe molecular absorption spectra
- like atoms, molecules only absorb at specific frequencies but as they have a richer set of possible states, they absorb at many frequencies
- in the earth’s atmosphere various processes broaden the absorption spectra of molecules e.g:
- collisions/pressure broadening
- doppler shift
molecules that are not good absorbers of EM radiation
- to absorb or emit EM radiation at IR frequencies an electric dipole must be set up i.e. a difference in electric charge across the molecule
- N2, O2, etc. - symmetric electric fields cancel -> no IR absorption
- CO2 - symmetric but can produce an electric dipole if distorted -> IR absorption for vibrations
- H2O - has a dipole in resting state -> IR absorption for vibrations and rotations
difference between Planck, Wien and Stefan-Boltzmann law
planck’s law - the intensity of black-body radiation as a function of wavelength and temperature, i.e. the shapes of the black-body radiation curves. it is an idealisation, real bodies don’t absorb or emit all wavelengths of light
wien’s law - gives peak location
stefan-boltzmann law - gives total power (area under curve)
thermal emission from real bodies
- thermal emission from real bodies is a product of Planck’s law and body’s emissivity:
Blambda(lambda, T) x E(lambda) - planck’s law sets the maximum thermal emission for the body
- the emissivity sets the fraction of this maximum that is realised at each frequency/ wavelength i.e. the efficiency with which it emits
what is kirchoff’s law
- absorptivity (alambda) = emissivity (Elambda)
- emissivity and absorptivity are a fraction i.e. range between 0 and 1
- things are as good at absorbing EM radiation of a specific wavelength as they are at emitting it -> absorption lines = emission lines
how do we measure constituents of the earth’s atmosphere
- as the atmosphere is an “ideal gas”, we measure constituents by fraction of molecules/volume in air
- concentrations of gases in the atmosphere expressed as parts per million by volume (ppmv)
describe water vapour and the atmospheric window
- water vapour is the dominant greenhouse gas in the Earth’s atmosphere
- water vapour absorbs strongly in many bands but there are gaps where it absorbs weakly
- between ~8 and ~14 um, around the peak of eath’s black-body spectrum, water is only weakly absorbing of IR radiation. this band is known as the atmospheric window
- gases which absorb wavelengths in the atmospheric window can have a greenhouse effect
- CO2 absorbs over a narrower range of frequencies than water but it has a strong absorption band in the atmospheric window
- greenhouse gases are most effective if they absorb frequencies that other GHGs don’t
describe the absorption of light by the atmosphere
-some frequencies of incoming light are absorbed by the atmosphere
- water vapour absorbs some bands of the near-infrared
- ozone absorbs UV strongly
what is radiative forcing
- radiative forcing (Wm^-2) is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the earth system
- a positive radiative forcing will cause the earth to absorb more energy and warm
- as it warms, it will emit more longwave radiation (Stefan-Boltzmann law) and eventually a new equilibrium temperature will be reached when incoming and outgoing energy balances again
describe the radiative forcing of CO2 and band saturation
at current concentrations, increasing CO2 concentration leads to diminishing increases in radiative forcing due to band saturation, when the frequencies most effectively absorbed have all been absorbed
- increasing CO2 increases absorption at tails of distribution
- CO2 radiative forcing does not increase linearly with CO2 concentration
describe the radiative forcing of CO2
- CO2 radiative forcing is proportional to the number of doublings of the CO2 concentration i.e., it follows a logarithmic function
- RFco2 = 3.71 x ln(C/C0)/ln(2) x Wm^-2
- where C is the new CO2 concentration and C0 is the old one
ideal gas assumptions
- molecules are point particles
- have elastic interactions
- this assumption works very well for atmospheric conditions - breaks down at very high pressure, low temperature
equation for kinetic energy
- kinetic energy (Ek) of an ideal gas is proportional to temperature
- Ek = 3/2 nRT
- where n is the number of moles of gas, R is the ideal gas constant, T is the temperature in Kelvin
- 1 mole of carbon-12 weighs 12 grams
- 1 mole = 6.022x10^23 atoms
- R = 8.315 Jmol^-1K^-1
equation for force and pressure
- force = mass x acceleration (kg ms^-2)
1 newton (N) = 1 kg ms^-2 - pressure = force / area (Nm^-2)
1 pascal (Pa) = 1 Nm^-2
pressure against box determined by: - number of collisions over area
- kinetic energy of colliding particles
what is the ideal gas law
PV = nRT
describe the pressure of water
- it is effectively incompressible
- pressure is proportional to depth as the weight of water above you increases monotonically
- every 10m deeper, pressure rises by 1atm
describe the pressure of air
- is compressible
- air at the bottom is compressed by the weight of air above
- pressure falls exponentially with altitude
P = A0e^-z(km)/8, A0 = 1atm - the scale height on earth is 8km
