Week 1 - Introduction on atmosphere, ozone, greenhouse gas

Atmospheric density

The number of molecules in a given space

• atmosphere is denser at the surface

Atmospheric pressure

Amount of force exerted per unit surface area

• atmospheric pressure is higher at surface (more air pushing down)

Temperature in the atmospheres

In troposphere and mesosphere: temperature decreases with height due to reduced pressure

In stratosphere and thermosphere: temperature increases with height - gasses at higher altitudes absorb energy before it passes further down

Ideal gas equation

pV = nRT

P = pRT

P is pressure in Pascals

p is density in kg m^-3

T is temperature

R is the universal gas constant: 287 J kg^-1 K^-1

Increased P = increased T or increased density

Troposphere

• Layer where most action occurs

• “Tropo” means turning

• Whole land surface

• 80% of mass of the atmosphere is water vapour

Stratosphere

• Layer contains about 20% of atmosphere’s mass

• Has some water vapour, however less than troposphere

• Temperature increases with height due to ozone absorbing shortwave solar radiation (ozone in stratosphere is good)

• Clouds cannot form in the stratosphere (except for nacreous clouds in polar regions due to chemical interactions and waves generated in the lower atmosphere which propagate upwards)

Mesosphere

• Layer where meteors burn up

• Lower ozone concentration compared to stratosphere (so temp declines with height again)

Thermosphere

• Temperature rise with height

• Ions and electrons are excited by solar radiation and there is greater absorption at higher levels

• Above 100km there is no effective mixing. Where space begins

Ionosphere

• Encompasses parts of thermosphere and mesosphere

• Atoms are stripped of their electrons so they become charged - absorption of energy due to cosmic rays and solar winds

• Results in Aurora Borealis and Aurora Australis

Composition of Earth’s atmosphere

• 78% Nitrogen

• 21% Oxygen

• 1% Argon

• Trace amounts of water vapour, carbon dioxide, other gasses

• Also aerosols ie pollutants and other particulates

(No other planet with O2 % like Earth is known right now - high concentration of O2 would burn quicker, low concentration would not sustain life, 21% is perfect)

Greenhouse effect

• Radiation from the sun is shortwave, earth emits long wave radiation

• Due to the lower temperature of the earth compared to the sun

• Greenhouse effect depends on ability to absorb energy and the exact wavelengths that absorb

• Chlorofluorocarbons (CFCs) are banned but were used in cooling systems

• Only ozone has infrared absorption

Ozone hole

• Ozone (O3) most concentrated in stratosphere

• Protects from ultraviolet radiation

• Ozone in troposphere is pollutant

• Ozone hole caused by ozone and chlorine or bromine introduced by CFCs and Halons

Ozone hole ingredients

• Cold descending air => vortex ‘cutting off’ air from horizontal mixing

• Polar Stratospheric Clouds - heterogeneous chemistry

• Chlorine and Bromine (CFCs + Halons)

• Sunlight to energise reactions leading to ozone depletion and cause chemical release from the clouds once the vortex has been established

Sudden Stratospheric Warmings (SSWs)

• Occur in the Northern Hemisphere on average once every two winters and can trigger severe cold weather

• Can also occur in Southern Hemisphere but are much less common - associated with hotter, drier conditions in Australia

  • In 2019, this contributed to the early start of the bushfire season, but also reduced the size of the ozone hole

• Normally in stratosphere in winter - strong westerly winds