Earth Systems Science

🌈Earth Systems Science Unit 8 – Atmosphere: Composition and Structure

The atmosphere is Earth's protective blanket, a mix of gases held by gravity. It's mostly nitrogen and oxygen, with trace gases like CO2. This gaseous envelope shields us from harmful radiation, regulates temperature, and enables life-sustaining processes like the water cycle. The atmosphere has distinct layers, each with unique properties. From the weather-filled troposphere to the auroral thermosphere, these layers play crucial roles in Earth's systems. Understanding the atmosphere's composition, structure, and functions is vital for grasping climate change and environmental challenges.

What's the Atmosphere?

  • Gaseous envelope surrounding Earth held in place by gravity
  • Composed of various gases including nitrogen (~78%), oxygen (~21%), and trace gases (argon, carbon dioxide, water vapor)
  • Extends from Earth's surface to the exosphere, gradually thinning with increasing altitude
  • Protects life on Earth by absorbing harmful ultraviolet radiation from the sun
  • Plays a crucial role in regulating Earth's temperature through the greenhouse effect
  • Enables the water cycle by storing and transporting water vapor
  • Provides oxygen for respiration and carbon dioxide for photosynthesis
  • Influences weather patterns and climate through atmospheric circulation

Layers of the Atmosphere

  • Troposphere: lowest layer, contains ~80% of atmospheric mass, where most weather phenomena occur
    • Extends from Earth's surface to an average height of ~12 km
    • Temperature decreases with altitude at a rate of ~6.5°C/km (lapse rate)
  • Stratosphere: layer above the troposphere, contains the ozone layer
    • Extends from the tropopause to an altitude of ~50 km
    • Temperature increases with altitude due to absorption of UV radiation by ozone
  • Mesosphere: layer above the stratosphere, where meteors burn up
    • Extends from the stratopause to an altitude of ~85 km
    • Temperature decreases with altitude, reaching the coldest point in the atmosphere at the mesopause
  • Thermosphere: layer above the mesosphere, where auroras occur
    • Extends from the mesopause to an altitude of ~600 km
    • Temperature increases with altitude due to absorption of high-energy radiation
  • Exosphere: outermost layer, gradually merges with outer space
    • Extends from the thermopause to ~10,000 km above Earth's surface
    • Extremely low density, composed mainly of hydrogen and helium atoms

Key Atmospheric Components

  • Nitrogen (N2): most abundant gas, essential for life as a component of amino acids and proteins
  • Oxygen (O2): second most abundant gas, crucial for respiration in many organisms
    • Produced by photosynthesis in plants and some microorganisms
    • Ozone (O3), a form of oxygen, absorbs harmful UV radiation in the stratosphere
  • Argon (Ar): third most abundant gas, an inert noble gas
  • Carbon dioxide (CO2): a greenhouse gas that absorbs and re-emits infrared radiation, contributing to Earth's warming
    • Concentration has increased due to human activities (fossil fuel combustion, deforestation)
  • Water vapor (H2O): a greenhouse gas that varies in concentration, influences weather and climate
  • Trace gases: gases present in small amounts (e.g., neon, helium, methane, krypton, xenon)
    • Some trace gases, such as methane and nitrous oxide, are potent greenhouse gases

How the Atmosphere Formed

  • Early Earth's atmosphere was composed mainly of hydrogen and helium, which escaped due to the planet's weak gravity and high temperature
  • Volcanic outgassing released gases (water vapor, carbon dioxide, nitrogen, sulfur dioxide) that formed the secondary atmosphere
    • Water vapor condensed to form oceans as Earth cooled
    • Carbon dioxide dissolved in oceans and was stored in carbonate rocks
  • Photosynthesis by early microorganisms (cyanobacteria) began producing oxygen ~2.4 billion years ago
    • Oxygen accumulated in the atmosphere, leading to the Great Oxidation Event
    • Ozone layer formed in the stratosphere, shielding Earth from harmful UV radiation
  • Nitrogen became the most abundant atmospheric gas due to its stability and low reactivity
  • Atmospheric composition has continued to evolve over time due to biological and geological processes

Atmospheric Pressure and Density

  • Atmospheric pressure is the force exerted by the weight of the atmosphere per unit area
    • Decreases with increasing altitude as the amount of air above decreases
    • Standard atmospheric pressure at sea level is ~1013.25 hPa (hectopascals) or 1 atm (atmosphere)
  • Density of the atmosphere decreases with increasing altitude
    • Caused by the compressibility of gases under the weight of the overlying atmosphere
    • About half of the atmospheric mass is contained within the lowest ~5.5 km
  • Pressure and density variations influence atmospheric circulation patterns
    • High-pressure areas have descending air, while low-pressure areas have ascending air
    • Wind flows from high-pressure to low-pressure areas, deflected by the Coriolis effect

Temperature Variations in the Atmosphere

  • Temperature changes with altitude, primarily due to variations in solar radiation absorption and heat transfer mechanisms
  • Troposphere: temperature decreases with altitude (negative lapse rate) due to adiabatic cooling of rising air
    • Lapse rate is influenced by moisture content, with moist air having a lower lapse rate than dry air
  • Stratosphere: temperature increases with altitude (positive lapse rate) due to absorption of UV radiation by ozone
    • Ozone layer is crucial for protecting life on Earth from harmful UV radiation
  • Mesosphere: temperature decreases with altitude, reaching the coldest point in the atmosphere at the mesopause
    • Meteors burn up in this layer due to friction with atmospheric gases
  • Thermosphere: temperature increases with altitude due to absorption of high-energy radiation (X-rays and UV)
    • Auroras occur in this layer when charged particles from the sun interact with Earth's magnetic field

Atmosphere's Role in Earth Systems

  • Regulates Earth's temperature through the greenhouse effect
    • Greenhouse gases absorb and re-emit infrared radiation, warming the planet's surface
    • Without the greenhouse effect, Earth's average temperature would be ~33°C colder
  • Enables the water cycle by storing and transporting water vapor
    • Evaporation from oceans and land surfaces adds water vapor to the atmosphere
    • Condensation of water vapor forms clouds and precipitation, redistributing water across the planet
  • Influences weather patterns and climate through atmospheric circulation
    • Uneven heating of Earth's surface creates pressure gradients and wind systems
    • Hadley, Ferrel, and Polar cells transport heat and moisture between the equator and poles
  • Interacts with the biosphere by providing gases for photosynthesis and respiration
    • Plants and other photosynthetic organisms absorb CO2 and release O2
    • Animals and other respiring organisms consume O2 and release CO2
  • Plays a role in the carbon cycle by storing and exchanging carbon with the biosphere and oceans
    • Atmospheric CO2 is absorbed by oceans and converted into carbonate rocks
    • Weathering of carbonate rocks and volcanic outgassing return CO2 to the atmosphere

Current Issues and Future Challenges

  • Climate change: increasing atmospheric greenhouse gas concentrations due to human activities
    • Burning of fossil fuels, deforestation, and land-use changes contribute to rising CO2 levels
    • Warmer global temperatures lead to changes in weather patterns, sea-level rise, and ecosystem disruptions
  • Air pollution: emission of harmful substances into the atmosphere from human activities
    • Particulate matter, nitrogen oxides, sulfur dioxide, and volatile organic compounds can impact human health and the environment
    • Acid rain, smog, and respiratory issues are some consequences of air pollution
  • Ozone depletion: thinning of the ozone layer due to the release of ozone-depleting substances (e.g., CFCs)
    • Montreal Protocol has helped reduce the production and emission of these substances
    • Recovery of the ozone layer is expected to take several decades
  • Geoengineering proposals: intentional, large-scale interventions to counteract climate change
    • Examples include solar radiation management (e.g., stratospheric aerosol injection) and carbon dioxide removal (e.g., direct air capture)
    • Potential risks and unintended consequences need to be carefully considered
  • Improving atmospheric monitoring and modeling to better understand and predict future changes
    • Satellite observations, weather balloons, and ground-based instruments provide valuable data
    • Climate models simulate the complex interactions between the atmosphere and other Earth systems


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.