Lecture 2: Atmosphere 2 Flashcards
EARTH’S ‘SECOND ATMOSPHERE’
Formed 4.0–3.3 billion years ago
• Earth cooled down and developed a solid crust
- Convective magma activity contributed to the formation of magnetic field capable of deflecting solar wind and cosmic rays
- Volcanic activity with outgassing of H2O, CO2, NH3, and sulphates
• Atmosphere likely contained CO2, H2O, N2 and other volcanic gases
but still no oxygen (anoxic atmosphere)
• Very high atmospheric temperatures (> 100o C) due to volcanic & tectonic activity, frictional heating from meteorite impacts, and strong greenhouse effect (c.f. Planet Venus today)
FORMATION OF OCEANS
Once temperatures had cooled down below 100 oC condensation of water vapour and precipitation become possible (3.8-3.9 billion years ago)
• Expanding oceans served as a sink for much of the atmospheric CO2
• Anaerobic bacteria began to develop in the deep ocean near volcanic hydrothermal vents
THE “OXYGEN CATASTROPHE”
3.8-3.5 billion years ago prokaryote cyanobacteria (“blue-green algae”) appeared as first O2 producing organisms (photosynthesis)
• 3.5 billion year-old stromatolites are found today in Australia and Southern Africa
• They must have lived deep enough (> 5m) to be protected from harmful radiation but still within
the range of sunlight
• 1.3 billion years ago eukaryote cells (“green algae”) were more efficient at producing oxygen
• 1 billion year ago ocean was saturated with oxygen and O2 could subsequently accumulate in the atmosphere, eventually forming an ozone layer
• Dissolved oceanic oxygen was toxic to anaerobic organisms
EARTH’S ‘THIRD ATMOSPHERE’
With the development of more sophisticated forms of plant life oxygen levels in the atmosphere continued to increase (photosynthesis)
CO2 + H2O + sunlight = organic compounds + O2
• Photodissociation of H2O played probably only a minor role in O2 increase
• Present-day atmosphere (oxygen- nitrogen) formed approximately 400 Million years ago
• Formation of ozone layer and protection from harmful solar radiation were an essential requirement for the formation of plant life on land
VERTICAL STRUCTURE OF THE ATMOSPHERE
Pressure & Density
1 cm
Atmospheric pressure is the consequence of the atmosphere’s own weight.
p = r gh
pressure p is dependent on density, gravity
and height
Air is a compressible medium, therefore the density r is not constant with height!
ATMOSPHERIC VERTICAL PROFILE: PRESSURE
In the global average atmospheric pressure decreases near-exponentially in vertical direction.
90% of the atmospheric mass is below 10 km.
99.9% of the atmospheric mass is below 50 km.
ATMOSPHERIC VERTICAL PROFILE: TEMPERATURE
Temperature is the most often used criterion to subdivide the atmosphere into vertical layers with specific properties.
A BIT OF THERMODYNAMICS…
An “air parcel” expands and cools when rising vertically
• Conversely, air that descends is compressed and gets warmer
• The physical properties of air as a homogeneous mixture of gases are described by various thermodynamic laws
ALTERNATIVE DESCRIPTIONS OF VERTICAL STRUCTURE
Layering by Functionality:
Upper layer is the Ionosphere (> 50 km), consists of Ions (electrically charged atoms & molecules) created by solar UV radiation
Layer below is the Ozone Layer
(~20-50 km), sometimes also referred to as Ozonosphere
ALTERNATIVE DESCRIPTIONS OF VERTICAL STRUCTURE 2
Layering by Composition:
Homosphere (surface to ~80 km) Mixing ratio of oxygen to nitrogen, as well as long-lived trace gases constant. The homosphere extends from sea level to the mesopause.
Heterosphere (above ~80 km)
From this altitude molecular separation occurs due to gravitation with the heaviest molecules at the bottom and the lightest molecules at the top
COMPOSITION OF THE ATMOSPHERE: AIR
Air consists of 78% Nitrogen and 21% Oxygen. The remaining 1 percent is mainly Argon. The small remaining fraction consists of trace gases, which include greenhouse gases of which carbon dioxide is the most abundant.
The abundance of water vapour is mainly determined by physical properties (temperature) and is highly variable.
COMPOSITION OF THE ATMOSPHERE: AEROSOLS
Particulates in the atmosphere are suspended solids or liquid droplets other than H2O droplets or ice crystals.
Aerosol describes the mixture of air with suspended particulates – these particulates are sometimes called aerosol particles.
COMPOSITION OF THE ATMOSPHERE: AEROSOLS 2
Particulates in the atmosphere can result in reduction of visibility (haze). They can either scatter incoming light or, like greenhouse gases, absorb outgoing thermal radiation.
Particulates are grouped in classes depending on their diameter. Typical classification would be:
o PM10 : particles with diameter ≤ 10 μm o PM2.5 : particles with diameter ≤ 2.5 μm
