Greenhouse gases Flashcards
How does data from ice cores show atmospheric CO₂ changes since 1950
Before 1950 around 300ppm
Now around 400ppm
Which are the top 3 sectors contributing to global GHG emissions
- Fossil fuels and industrial processes (65%)
- Argiculture (16%)
- Forestry and other land use (11%)
Why does increasing CO₂ levels lead to increasing global temperatures
- GHGs like CO₂ absorb infrared radiation which traps heat
- This is due to their molecular bonds altering energy levels when hit by infrared radiation of certain wavelengths
Solar radiation hits the earth as …….. radiation
And leaves the earth’s surfaces as …….. radiation
Shortwave (UV)
Longwave (IR)
Increasing levels of GHGs results in…
a reduced amount of energy the Earth’s surface radiates to space, thus warming the planet
The shorter the wavelenght (λ) of a wave
The higher energy it is
True of false
Electromagnetic waves don’t need molecules like sound to travel
true
The earths ozone prevents what type of waves from entering the troposphere
High energy waves: gamma, x-rays and UV
The atmosphere absorbs this energy
Which energy wavelengths does penetrate the atmosphere
Energy in the visible part of the spectrum
Energy absorbed by the atmosphere is inversely correlated to
energy intensity (i.e. the wavelength)
What is the atmospheric window
it is a band of IR radiation which can freely pass through to the Earth’s surface - important to avoid the atmosphere being to warm
(this is opposite to an absorption band which is the portion of the electromagnetic spectrum that is blocked)
What is the difference between UV-A, B and C
- wavelength decreases in size from UV-A to UV-C
- UV-C is totally absorbed by ozone in the atmosphere
- UV-B is 90% absorbed by ozone (this is what causes sunburn
What is the radiation balance of the earth assumed to be?
Amount of heat absorbed = heat radiation emitted
(climate change is a difference in this balance)
What is a Radiative forcing (W m‾²)
- The influence a given climatic factor has on the amount of downward-directed radiant energy impingging upon Earth’s surface (strongest for water vapour)
- Imbalance on the steady state
What is Radiative efficiency (aₓ)
The efficiency is determined by the capacity of each molecule (e.g. CO₂, CH₄) to absorb long IR radiation
What is GWP
Global warming potential
(no units)
Which of the following molecules has the highest radoative efficiency?
CO₂, CH₄, NO₂
NO₂
Molecular vibrations can derermine the radiative efficiency of molecule?
What are molecular vibrations?
- to absorb IR the electrical dipole moment of the molecule must change when electrons are displaced
What is a dipole moment?
Tension between a positive and negetive pole within the molecule
This can only occcur in molecules which are heteronuclear diatomic - unsymmetrical distribution of change
If a vibrating molecules absorbs radiation of a matching frequency ….
… it becomes more ‘energetic’
amplitude of vibration increases
A molecule can lose vibrational energy by ….
… re-emitting radiation (energy) and return to the original state
(this repeats and this is what allows the atmosphere to heat up)
CO₂ is a linear triatomic molecule
Which vibration modes are IR active?
Asymmetrical stretching (v3)
Symmetrical bending (v2)
Why is the symmetrical stretching (v1) mode of CO₂ IR inactive
Due to there being no change in the dipole moment
SO₂ is a bent triatomic molecule
Which vibrational modes are IR active?
- Symmetrical stretching (v1)
- Symmetrical bending (v2)
- Asymmetrical stretching (v3)
Name some other non-linear triatomic molecules
H₂O or O₃
……….. can cause absorption of ……. giving rise to a shift in the ……….. level
IR radiation can cause absorption of photons giving rise to a shift in the vibrational level
What peaks would pure CO₂ produce on a IR
Molecules absorb radiation of different wavelengths depending on their ………..
vibrational enery
This means ozone, CO₂ and H₂O will all produce peaks in different places
Which molecule is the largest GHG absorber?
water
Radiative efficiency is determined by?
How many vibrational states a molecule has
Specific vibration states (determined by the molecular structure) controls ….
how much IR energy a molecule can absorb
The longer the lifetime of a molecule in the atmosphere, the stronger the GHG it is
What determines lifetime?
The sinks of the Greenhouse gases
e.g. short lifetime of methane is due to its high reactivity with OH radical
How does ozone react with an OH radical?
O₃ + OH → HO₂ + O₂
(occurs in the stratosphere)
What is straospheric photolysis (tip: includes N₂O)
N₂O → N₂ + O.
(major sink for N₂O)
What is Tropospheric photolysis (tip: includes ozone)
O₃ + OH → O₂ + O.
Lifetime (τ) is defined as
τ = 1/k’
it is the time taken for a species concentration too drop by a factor of 1/e
(k is the rate constant)
The lifetime of methane is dependent on its reaction with OH. radical
Hence the rate of reaction = k[OH][CH₄]
how can this equatin be simplifed?
= k’[CH₄]
assumes that OH concentration is much higher than methanes
What are the units of k
cm³ molecule‾¹ s‾¹
How would you work out the lifetime of a GHG
τ = 1/k[OH]
if the lifetime of methane is given by: τ = 1/k’
if k = 6 x 10‾¹⁵ cm³ molecules‾¹ s‾¹
and [OH] = 1 x 10⁶ molecules cm‾³
give τ
τ(CH₄) = 1/(6 x 10‾¹⁵ x 1 x 10⁶) = 5.3
The longer the lifetime of a GHG….
…. the further the distance it can travel
What is Gloval Warming potential (GWP) for a GHG, and how is it worked out?
- Ratio of the time-intergrated radiative forcing from the instantaneous release of 1kg of a trace substrance relative to that of 1kg of a reference gas (CO₂) - of which the GWP is 1
- It is worked out by combining a GHGs radiative efficiency and its lifetime in the atmosphere
So what factors make a GHG potent
Radiative efficiency (how much radiation it can absorb)
Lifetime
Concentration
50% of incoming short wave radiation reaches the surface
How much gets scattered back into space
30%
How much of the incoming solar radiation is taken up by the oceans and the land
70% taken up by the oceans
30% taken up by the land
What other factors apart from global warming affect the Earth energy balance
- Albdeo (reflectivity) by clouds, ice etc
Radiative forcing can be a measure of?
Imbalance of the energy budget
(values of change are relative to preindustrial conditions defined at 1750)
In Simple terms, radiative forcing is
the rate of energy change per unit area
If the sun suddenly put out more energy, this is a …….. radiative forcing
positive
If we put more CO₂ or other GHG into the atmosphere that traps the radiation, this is a ……. radiative forcing
Positive
If there are more clouds or desert, this is a ……. radiative forcing
negative radiative forcing
What is the difference between ozone in the troposphere vs straosphere?
positive radiative effect in the troposphere
negative radiative effect in the stratosphere
Both methane and nitrous oxide have really high Global warming potentials, why are we still more concerned about CO₂
Because concentrations are significantly lower
