Topic 2 - Stratospheric Chemistry Flashcards
What were the features of the very early atmosphere?
It consisted of H2 and He.
There was enhanced solar wind, which removed most of the Earth’s atmosphere.
There were additional losses from overcoming the gravitational field.
What were the features of Earth’s second atmosphere?
Second atmosphere was formed from rocks outgassing.
Prebiotic atmosphere containing mainly of H2, CH4, NH3, H2S, H2O, CO, CO2, NO2 and SO2.
What were the features of the prebiotic atmosphere?
The prebiotic atmosphere was transparent to radiation down to wavelengths <= 200 nm.
Strong, short wavelength UV radiation field present on the surface of the Earth.
UV radiation can damage the molecules of life, which means that early life must have been UV robust or protected from UV radiation.
Very little O2 and thus reducing atmosphere.
What were the features of early life?
Initial formation of the molecules of life (amino acids) occurred as a result of lightning and UV impingent on the early atmosphere.
Organisms exist today which can use CO2, H2, water, organic molecules, various ions and sunlight as energy sources.
What were the features of the Great Oxygenation Event?
Bacteria evolved which could produce a range of different molecules, leading to the accumulation of nitrogen.
Early organisms did produce oxygen but not enough for it to build up as it reacted to form various minerals in rocks, such as iron oxides.
About 2.5 billion years ago the ‘Great Oxygenation Event’ occurred after the rocks became saturated with oxygen.
Oxygen eventually accumulated in the atmosphere.
How did ozone concentration increase?
The concentration of ozone is linked with the concentration of oxygen.
What are three classifications of UV light and their wavelengths?
UV-A (320-400 nm)
UV-B (290-320 nm)
UV-C (200-290 nm)
How does oxygen absorb UV radiation?
Oxygen absorbs UV radiation below 195 nm (Herzberg continuum).
There is also a weak absorption band above 200 nm (Schumann-Runge band).
How does oxygen dissociate when absorbing UV light with a wavelength of < 175 nm?
O2 → O(3P) + O(1D)
How does oxygen dissociate when absorbing UV light with a wavelength of between 175 nm and 242 nm?
O2 → 2O(3P)
How does the O1D state decay?
By colliding with other molecules in the atmosphere.
It will relax into a 3P state.
How does the O atom production rate change with altitude?
The concentration of O2 decreases exponentially with altitude.
The O2 photolysis rate increases with altitude due to increased UV intensity.
This causes an optimum altitude for O atom production which is in the stratosphere.
How does ozone interact with radiation?
Ozone absorbs UV radiation, particularly strongly below 250 nm.
At λ < 1180 nm: O3 → O2 + O.
At λ < 411 nm: O3 → O(1D) + O2(3Σg-)
At λ < 310 nm: O3 → O(1D) + O2(1Δg)
What are the four stages of the Chapman Mechanism?
1) At λ < 242 nm: O2 → O + O
2) O + O2 –M→ O3
3) At λ < 1180 nm: O3 → O2 + O
4) O + O3 → 2 O2
How accurate is the Chapman cycle at predicting ozone concentration in the atmosphere?
It overpredicts ozone concentrations by a factor of two at all altitudes.
This is because it only models the effects of oxygen and nitrogen and does not include the effects of trace gases.
How does ozone lifetime vary with altitude?
Ozone lifetime decreases with ascending altitude from a few hours at the top of the stratosphere to more than a year at the bottom of the stratosphere.
In the troposphere, ozone could have a very long lifetime in addition to being a potent greenhouse gas.
