Topic 1_Ch 2_"Composition of the Atmosphere"

Composition & Behaviour of the Atmosphere

• Chapter 2: Composition of the Atmosphere (to 2.5)

  • Evolution of the Atmosphere

    • All planets except Mercury have atmospheres.

    • Uncertainty exists regarding remnant gases; recent evidence suggests some may have been lost.

    • Early atmospheres formed and evolved over time.

    • Earth's atmosphere is unique and crucial to the Earth system, comprising five spheres:

      1. Primordial Atmosphere

      2. Evolutionary Atmosphere

      3. Living Atmosphere

      4. Modern Atmosphere

      5. Anthroposphere?

Sources of Earth's Primitive Atmosphere

• Probable Sources:

  1. Outgassing from molten rock and volcanic activity.

  2. Comets and asteroids (particularly ice/water vapor).

• Early atmosphere likely consisted mainly of water vapor and carbon dioxide, with trace amounts of nitrogen, sulfur, and other gases.

• Notable absence of oxygen in early atmosphere.

• Atmospheric Thinning:

  • Water vapor condensed into liquid water.

  • Carbon dioxide removal was gradual:

    • Initial removal through chemical weathering and the precipitation of carbonate rocks.

    • Later removal through photosynthesis and burial of organic matter.

The Gaia Hypothesis

• Proposed by James Lovelock in the 1970s:

  • The lithosphere, atmosphere, hydrosphere, and biosphere interact as a self-regulating Earth system.

  • Life did not evolve solely in response to environmental conditions but co-evolved with its environment.

  • The system works collectively to maintain ideal conditions for life.

Chemical Composition of Air

• Constant Gases:

  • Concentrations stable in lower atmosphere and well-mixed.

  • Examples: Nitrogen (78%), Oxygen (21%), and inert gases.

• Variable Gases:

  • Concentrations fluctuate over time and space with short residence times.

  • Examples include carbon dioxide, ozone, and water vapor.

Atmospheric Structure

• Heterosphere:

  • Exists above 80 km.

  • Characterized by molecular diffusion; heavier particles settle lower.

• Homosphere:

  • Exists below 80 km.

  • Dominated by turbulent mixing; constant gases are uniformly distributed.

Gases in the Climate System

• The atmosphere serves as both a reservoir with sources and sinks.

• Nitrogen Cycle:

  • Sink: Nitrogen fixation makes nitrogen available for organisms.

  • Source: Denitrification returns fixed nitrogen to N2 or nitrous oxide (N2O).

Oxygen Dynamics

• Source:

  • Produced by photosynthesis (CO2 + H2O ↔ CH2O + O2).

  • Plants convert carbon dioxide and water into carbohydrates and oxygen.

• Sinks:

  • Respiration: Converts carbohydrates and oxygen into energy, yielding CO2 and H2O.

  • Decomposition: Bacteria utilize oxygen, producing CO2.

  • Combustion: Fuel reacts with oxygen to release CO2.

  • Oxidation: Oxygen reacts with Earth's crust materials.

• Anthropogenic Sources:

  • Burning fossil fuels and alterations in land use.

Carbon Dioxide Dynamics

• Sources:

  • Respiration, decomposition, and combustion activities.

• Sinks:

  • Photosynthesis, atmosphere-ocean exchange, burial of organic carbon.

• Organic and Inorganic Processes:

  • Organic: Photosynthesis, respiration, decomposition.

  • Inorganic: Rock weathering, volcanism, and plate tectonics.

Carbonate–Silicate Cycle Breakdown

• Sink Process:

  • CO2 reacts with rainwater to form carbonic acid (H2CO3).

  • This acid reacts with calcium silicate rocks.

  • Marine organisms utilize calcium and bicarbonate to produce calcium carbonate for shells and skeletons.

• Source Process:

  • Carbonate sediments subduct into the Earth's crust, where high temperatures generate CO2.

  • Volcanoes emit CO2 into the atmosphere.

Water Vapour (H2O)

• Phase Existence:

  • Water can exist as solid, liquid, and gas.

• Hydrologic Cycle:

  • Involves phase changes, not chemical transformations.

  • In Atmosphere:

    • Source: Evaporation.

    • Sink: Condensation.

• Greenhouse Gas:

  • Water vapor significantly contributes to warming, potentially doubling the effect of carbon dioxide.

  • Water, ice, and clouds influence Earth’s reflectivity.

Ozone (O3) Functions

• Primary Roles:

  1. Absorbs ultraviolet radiation from the sun, protecting life on Earth from harmful radiation.

  2. Absorbs Earth's longwave radiation (acts as a greenhouse gas).

  3. Functions as a pollutant, harmful to rubber, plastic, plants, and animals.

Stratospheric Ozone Layer Formation

1. O2 molecules absorb UV radiation in the 0.1–0.2 µm range, leading to photodissociation, splitting into oxygen atoms.

2. New oxygen atoms collide with O2 molecules, forming O3.

• Ozone absorbs UV radiation in the 0.2-0.3 µm range, thus fully absorbing UV radiation with wavelengths <0.3 μm.

Atmospheric Aerosols

• Definition:

  • Solid or liquid particles suspended in air; land air contains about 10,000 aerosols per cubic centimeter.

• Primary Aerosols:

  • Direct emissions into the atmosphere, including:

    • Sea spray, dust, sand, pollen, spores, bacteria, viruses, soot, and ash.

• Secondary Aerosols:

  • Formed through gas-to-particle conversion, including:

    • SO2 to H2SO4, NO to HNO3, and VOCs to hydrocarbons.

• Sinks of Aerosols:

  • 80–90% are removed through precipitation via a multi-step process involving condensation and droplet growth until rain size is reached.