Atmospheric Chemistry Flashcards
(L1)
In what layers does temperature decrease in the atmosphere?
Decreases in Troposphere and Mesosphere
In what layers does temperature increase in the atmosphere
Increases in the Stratosphere and Thermosphere
What are they layers of the atmosphere separated by?
Troposphere, Stratopause, Mesopause
Which gases decrease in concentration as elevation increases?
Methane, carbon dioxide and oxygen
Describe the concentration of ozone in the atmosphere
it increases until around 40km (ozone layer) and then decreases
What is the mixing ratio?
The gas density divided by the total density reported at % or ppm
Which components of the atmosphere are well mixed?
Oxygen and Carbon Dioxide
What species are absorbed by UV?
Ozone
what species are absorbed by visible radiation?
sunlight at the top of the atmosphere
What species are absorbed by infrared radiation?
5250C blackbody and radiation at sea level
Which region is most radiation absorbed.
visible region
What is energy proportional and inversely proportional to?
frequency; wavelength
does breaking bonds produce or require energy?
requires energy
Is water well mixed?
Water decreases significantly over the troposphere (cold trap) and then becomes well mixed in the stratosphere
what wavelength is required to break up ozone?
less than 290nm
what is photolysis?
The process of breaking bonds with light
what do photolysis reactions depend on?
local temperature and pressure along with the wavelength of the absorbed light.
What do photochemical reactions start with?
Photo-excitation
Give the reaction for the Breakdown of ozone by sunlight radiation
O₃ + hv -> O + O₂
State the reaction for Photo-excitation.
A + hv -> A* where A* is the excited state of A
what happens after photo-excitation?
the radical undergoes different reactions depending on how much energy is absorbed
what are the 4 main reactions that could occur?
Ionisation, Dissociation, fluoresence and collisional relaxiation
Give the equation for Ionization.
A* -> A⁺ + e ⁻
Give the equation for dissociation.
A* -> B + C
Give the equation for fluoresence.
A* -> A + hv
Give the equation for Collisional relaxation.
A* + M -> A + M
What does the rate of formation of A* depend on?
How much of the Neutral gas A exists and how much Photons are at that location and how those photons interact with the A.
What does the chemical rate constant ja incorporate?
Absorption cross section Quantum Yield and Spectral Actinic Flux (I)
What is the absorption cross section?
number of photons absorbed by a molecule at a particular wavelength
What is the Quantum Yield?
The efficiency at which absorbed photons result in the molecular process of interest.
What is the spectral actinic flux?
Density of photons in the atmosphere at a particular wavelength. Measured at a given height.
What is the competition between subsequent processes determined by?
Quantum Yield
What is responsible for the huge difference between UV radiation at the top of the atmosphere and the ground?
Ozone
Which Photons are absorbed by molecules?
Molecules absorb light energy in discrete wavelengths (E=hv=hc/), (square = lamda/wavelength ), molecules absorb photons within a given radii of their centres.
what is a cross-section?
The proportional area blocked by the molecule at each wavelength.
What wavelength does ozone optinumly abdorb
250nm
What do photo chemical processes depend on?
temperature (absorption cross sections and quantum yields), pressure (Collisional relaxation vs fluorescent timescale) and altitude (actinic flux)
What is the Beer Lambert Law that brings these all together?
I (l)=Ioe(-Nl)
what is the equation for optical depth (t)
aNI
If a certain wavelength has an optical depth (t)? of over 1, what does that imply
complete absorption of that wavelength in a column of gas.
What does ttotal equal?
+(tgas+twater+tScattering by gas +aeorsols)
What is column density?
The total amount of gas in the vertical column collapsed down into an effective area.
What is column density measured in?
Molecules/cm2
How do we work out optical depth using cross section and column density?
Multiply them together
What are the 2 naturally occuring stable isotopes of Carbon
12C and 13C
How is Carbon-14 formed?
Cosmic rays slam into the most dominant component of our atmosphere 14N + n -> 14C + p
What is the half life of 14C -> 14N
5730 years
Where is Plank’s blackbody radiation most intensly as temperature increases?
To shorter wavelengths
How do we work out the total flux from a blackbody radiator?
We multiply the steffan-boltzman constant (a) with temperature^4
What is the equation for energy balance?
Energy absorbed S(a-A)/4=energy emmited aTeff^4piR^2
What is S?
incoming sunlight
What is A?
Planetary albedo, the fraction of light that is reflected
What is R?
The radius of the Earth
What is effective Temperature?
The temperature of a blackbody that would emit the same total amount of radiation as the star or planet in question.
How do we work out the surface temperature?
Tsurface= Teffective + Greenhouse
Which planet has the greatest effective Temperature?
Earth, then Venus, then Mars
Which planey jas the greatest Greenhouse effect?
Venus, then Earth, then Mars
Which Planet has the greatest surface temperature?
Venus, then Earth, then Mars
Which Molecule in the atmosphere absorbs and scatters the most radiation?
Water Vapour
(L2)
Describe the faint young sun paradox.
The sun was 30% dimmer (small Teffective) when Earth was formed, in the absence of atmospheric evolution, the early earth would have been frozen over which it wasn’t. Implying the atmosphere must have evolved with time containing more greenhouse in the past.
What is the equation for atmospheric gas?
d/dt(gas) = Fsource - Fsink
Why do atmospheres stick to planets?
Gravity (smaller planets have larger atmospheric loss)
How do we work out escape velocity?
Make kinetic energy (1/2mvesc^2)= GmMp/R and solve for Vesc=(2GMp/R)^1/2
Explain Earth’s red glow.
It is Hydrogens geo-corona (escape)
How fast is hydrogen escaping?
3kg/s
By what means is hydrogen escaping?
50% from H2O photolysis, 50% from microbial CH4
What planets can retain gas over the age of the solar system?
Over the dashed line- Neptune, Uranus, Saturn, Jupiter.
What are atmospheric sinks?
Atmospheric escpae, reactions with surface (rain, particulates), photochemical destruction.
What are atmospheric sources?
Volcanic activity
What volatile species are in equilibrium with molten rock?
C/O/H from a melt. Oxygen fugacity of Earth mantle(constant from 4.4Ga-present)
What are the 2 stages of mantle evolution?
- Outgassing of Hydrogen, methane and anommonia. FeO, silicates in magma.
- Solidification of Fe Core. FeO silicates in the mantle and outgassing of water, carbon dioxde, nitrogen, hydrogen and carbon monoxide exhumed.
What are the implications of this?
H2:H2O, CO:CO2, H2S:SO2 ratios from volcanoes have been more or less constant over Earths History
What is the Heat source for Volcanic activity?
Generation through radioactive decay (U238, Th235, Th232, K40)
What does this suggest about heat flow and hence volcanic activity?
Heat flow was likely slightly higher during the Hadean than present. Only slightly higher out-gassing rates throughout most of Earths history therefore volcanic CH4, CO2 and water production weren’t drastically different.
Discuss early earth atmosphere.
No life- only traces of oxygen in the form of water vapour. H2 and CO2 outgassing. Early methanogens created methane and H2O from these constituents. As methane contributes more to the greenhouse effect per molecules than carbon dioxide helping Faint Young Sun Paradox.
What were the Early atmospheric reactions?
Methanogenesis at hydrothermal vents, elemental sulphur reduction, fermentation and anoxygenic phtotsynthesis.
Give the equation for methanogenesis?
H2 + CO2 -> CH4 + H2O
Give the equation for elemental sulphur reduction.
S˚+H2 ->H2S
Give the equation for anoxygenic photosynthesis
2H2 + CO2 + hv -> CH2O + H2O
2H2S + CO2 + hv -> CH2O + H20 +2S
Give the equation for fermentation?
2CH2O -> CH4 + H2O
How was oxygen first produced?
Cyanobacteria -> CO2 + H2O + sunlight -> CH2O + O2 + energy
What is the prominent sink of methane?
Oxidation
To summarise, what were the main reactions in the early Archean and why were they Limited?
Coupled anoxygenic photosynthesis-methanogenesis. AP is substrate limited so primary productivity was low.
To summarise, what were the main reactions in the late Archean?
Coupled oxygenic photosynthesis-methanogenesis
Which are the only 2 gases in the atmosphere that absorb wavelengths over 300nm?
Ozone and SO2
Why is ther no SO2 photolysis in the modern atmosphere?
Because SO2 is only in the Troposphere, ozone on the other hand can be found in the Stratosphere.
(L3)
Where is Stratospheric ozone produced?
In the Troposphere
is it formed by natural or anthropogenic means and how?
Anthropogenic, due to complex chemical reactions whose products settle on pollution.
What was the first evidence of ozone?
In the 1910’s, observations made by UV spectroscopy suggested absorption in the stratosphere which no molecule known at the time could expplain.
