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

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