6.1D Atmosphere system (HL) Notes

1. The atmosphere is a dynamic, constantly changing system influenced by physical and chemical processes that operate from the Earth’s surface up into the upper layers.
2. These processes shape atmospheric composition, temperature structure, circulation patterns, and the formation or destruction of key gases such as ozone.
3. Because the atmosphere receives continuous inputs of solar radiation, aerosols, gases, and energy, it continually adjusts through feedback mechanisms and exchanges with other Earth systems.

Physical Processes in the Atmosphere

1. Global Warming

1. Global warming is caused by an increase in greenhouse gas concentrations, which enhances the natural greenhouse effect.
2. Human activities such as burning fossil fuels, deforestation, and industrial agriculture add excess carbon dioxide, methane, and nitrous oxide to the atmosphere.
3. This increases radiative forcing, traps more heat, and alters atmospheric circulation, humidity levels, and climate patterns.

2. Air Movements: Temperature and Pressure Differences

1. Warm air rises because it becomes less dense.
2. Cold air sinks because it is more dense.
3. This difference creates convection currents, the basis of global wind systems.
4. These convection currents drive:
   1. the tricellular model of circulation (Hadley, Ferrel, Polar cells),
   2. prevailing winds such as trade winds and westerlies,
   3. and large-scale weather systems (cyclones, jet streams, monsoons).
5. Air moves from high-pressure regions to low-pressure regions.
6. These pressure differences are generated by temperature contrasts, such as:
   1. warm equatorial air rising and creating low pressure,
   2. cold polar air sinking and creating high pressure.

Chemical Processes in the Atmosphere

Ozone Production

1. The stratosphere contains molecular oxygen (O₂) which absorbs high-energy ultraviolet (UV) radiation from the Sun.
2. This splits O₂ into two free oxygen atoms (O).
3. A free oxygen atom then combines with another O₂ molecule to form ozone (O₃).

The Ozone-Oxygen Cycle (Dynamic Equilibrium)

1. UV radiation splits O₂ into O + O.
2. A free oxygen atom (O) collides with O₂ forming O₃.
3. Another UV photon can break O₃ into O₂ + O.
4. The oxygen atom can rejoin O₂ again.
5. This creates a dynamic equilibrium, maintaining stable ozone levels in the stratosphere.

Atmospheric System as an Open System

1. Inputs include solar radiation, volcanic gases, aerosols, and water vapour.
2. Outputs include infrared radiation, precipitation, and gas exchange with the biosphere and hydrosphere.
3. Flows include wind, convection, and chemical transformations.
4. Storages include atmospheric gases, clouds, and aerosol particles.

Atmospheric Thinning with Altitude and the Standard Lapse Rate

1. The distribution of atmospheric gases is heavily influenced by gravity.
2. Molecules are pulled toward the Earth’s surface, creating higher density and pressure at low altitudes.
3. Because gravitational force weakens with distance from Earth, the atmosphere becomes thinner at higher altitudes.
4. Temperature also changes predictably with height due to the standard lapse rate.

Atmospheric Density and Pressure

1. Near the Earth’s surface, air pressure is high because the mass of the atmosphere above compresses the lower layers.
2. At higher altitudes, there is less air above, so pressure and density are reduced.
3. As altitude increases:
   1. Oxygen availability decreases,
   2. Air becomes thinner,
   3. Temperature generally declines.

Why the Atmosphere Thins with Altitude

1. Gravity’s Influence

1. Gravity pulls gas molecules downward.
2. The gravitational force decreases with height because it is inversely proportional to distance.
3. As a result, atmospheric density is greatest near the surface and declines upward.

2. Pressure from Layers Above

1. Lower layers of the atmosphere support more weight from the air above.
2. At higher altitudes, there is less weight pressing down, so gas molecules spread out more.

Temperature Change with Altitude

1. The standard lapse rate states that temperature decreases by about one degree Celsius for every 100 metres in altitude (or 6.5 °C per kilometre).
2. This occurs because rising air expands in lower pressure and cools.
3. Different lapse rates occur depending on humidity and atmospheric stability:
   1. Dry air cools faster.
   2. Moist air cools more slowly.

Consequences of Atmospheric Thinning

1. Lower temperatures at high altitudes lead to snow and ice accumulation on mountain peaks.
2. Biodiversity decreases with altitude because fewer organisms can tolerate low temperatures and low oxygen levels.
3. Weather processes, such as cloud formation, depend heavily on how temperature and pressure change with altitude.