🌡️Intro to Climate Science Unit 2 – Atmosphere: Composition and Structure

What's the Atmosphere?

• Gaseous envelope surrounding the Earth held in place by gravity
• Composed of various gases, primarily nitrogen (78%) and oxygen (21%)
• Extends from the Earth's surface to the exosphere, gradually thinning with altitude
• Protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes
  • Ozone layer in the stratosphere absorbs harmful UV radiation
• Plays a crucial role in the water cycle by transporting water vapor and enabling precipitation
• Influences weather patterns and climate through atmospheric circulation and heat distribution
• Provides the air we breathe and is essential for supporting life on Earth

Layers of the Atmosphere

• Troposphere: Lowest layer, extends from Earth's surface to an average height of 12 km
  • Contains approximately 75% of the atmosphere's mass and 99% of its water vapor
  • Most weather phenomena occur in this layer (clouds, precipitation, storms)
• Stratosphere: Layer above the troposphere, extends from 12 km to 50 km
  • Contains the ozone layer, which absorbs harmful UV radiation from the sun
  • Relatively stable with little vertical mixing, allowing for long-distance travel of particles
• Mesosphere: Layer above the stratosphere, extends from 50 km to 80 km
  • Coldest part of the atmosphere, with temperatures dropping to -90°C
  • Meteors burn up in this layer due to friction with atmospheric gases
• Thermosphere: Layer above the mesosphere, extends from 80 km to 600 km
  • Temperatures can reach up to 2,000°C due to absorption of high-energy radiation
  • Aurora borealis and aurora australis occur in this layer
• Exosphere: Uppermost layer, extends from 600 km to 10,000 km
  • Transition zone between Earth's atmosphere and outer space
  • Extremely low density, with particles rarely colliding

Key Players: Atmospheric Gases

• Nitrogen ($N_2$): Most abundant gas in the atmosphere (78%)
  • Relatively inert and does not contribute directly to the greenhouse effect
  • Essential for life as it is a crucial component of amino acids and proteins
• Oxygen ($O_2$): Second most abundant gas in the atmosphere (21%)
  • Vital for respiration in many living organisms
  • Produced by photosynthesis in plants and algae
• Argon (Ar): Third most abundant gas in the atmosphere (0.93%)
  • Inert gas that does not participate in chemical reactions
• Carbon dioxide ($CO_2$): Trace gas with a concentration of about 0.04%
  • Potent greenhouse gas that absorbs and re-emits infrared radiation
  • Concentration has increased significantly due to human activities (fossil fuel combustion, deforestation)
• Water vapor ($H_2O$): Concentration varies depending on location and atmospheric conditions
  • Most abundant greenhouse gas, absorbing and re-emitting infrared radiation
  • Plays a crucial role in the water cycle and heat distribution in the atmosphere
• Ozone ($O_3$): Trace gas found primarily in the stratosphere
  • Absorbs harmful UV radiation, protecting life on Earth
  • Can be harmful to human health and vegetation when present in high concentrations near the surface

Pressure and Temperature Profiles

• Atmospheric pressure decreases exponentially with altitude
  • Pressure at sea level is approximately 1013 hPa (hectopascals)
  • Pressure halves for every 5.5 km increase in altitude
• Temperature profile varies with altitude and is influenced by atmospheric composition and solar radiation
  • Troposphere: Temperature decreases with altitude at a rate of ~6.5°C/km (lapse rate)
    • Caused by adiabatic cooling as air expands and rises
  • Stratosphere: Temperature increases with altitude due to absorption of UV radiation by the ozone layer
  • Mesosphere: Temperature decreases with altitude, reaching the coldest point in the atmosphere
  • Thermosphere: Temperature increases significantly with altitude due to absorption of high-energy radiation
• Tropopause: Boundary between the troposphere and stratosphere
  • Marked by a sharp change in the temperature lapse rate
  • Acts as a barrier to vertical mixing between the two layers
• Stratopause, mesopause, and thermopause: Boundaries between the stratosphere and mesosphere, mesosphere and thermosphere, and thermosphere and exosphere, respectively

Atmosphere's Role in Climate

• Greenhouse effect: Atmospheric gases (primarily $CO_2$, $H_2O$, and $CH_4$) absorb and re-emit infrared radiation
  • Traps heat near the Earth's surface, warming the planet and making it habitable
  • Increasing greenhouse gas concentrations due to human activities are causing global warming
• Atmospheric circulation: Large-scale movement of air driven by uneven heating of the Earth's surface
  • Hadley, Ferrel, and Polar cells redistribute heat from the equator to the poles
  • Jet streams, high-altitude fast-moving air currents, influence weather patterns and air travel
• Hydrological cycle: Atmosphere plays a crucial role in the continuous movement of water on Earth
  • Evaporation from oceans, lakes, and land surfaces adds water vapor to the atmosphere
  • Condensation of water vapor forms clouds and precipitation, returning water to the surface
• Albedo: Atmosphere influences the Earth's reflectivity (albedo) through clouds and aerosols
  • Clouds can reflect incoming solar radiation, cooling the Earth's surface
  • Aerosols (tiny particles suspended in the atmosphere) can scatter or absorb solar radiation, affecting the Earth's energy balance

Human Impact on Atmospheric Composition

• Fossil fuel combustion: Burning of coal, oil, and natural gas releases $CO_2$ and other pollutants into the atmosphere
  • Increases atmospheric $CO_2$ concentration, contributing to global warming
  • Releases nitrogen oxides ($NO_x$) and sulfur dioxide ($SO_2$), which can form acid rain and affect air quality
• Deforestation: Clearing of forests for agriculture, logging, or urban development
  • Reduces the amount of $CO_2$ absorbed by plants through photosynthesis
  • Releases stored carbon into the atmosphere when trees are burned or decompose
• Agriculture: Livestock farming and rice cultivation release methane ($CH_4$), a potent greenhouse gas
  • Application of nitrogen-based fertilizers can lead to emissions of nitrous oxide ($N_2O$), another greenhouse gas
• Industrial processes: Manufacturing and industrial activities release various pollutants into the atmosphere
  • Chlorofluorocarbons (CFCs) used in refrigerants and aerosol sprays have depleted the ozone layer
  • Particulate matter (PM) from factories and power plants can affect air quality and human health
• Transportation: Vehicles powered by fossil fuels emit $CO_2$, $NO_x$, and other pollutants
  • Contributes to urban air pollution and smog formation
  • Aviation emissions, including $CO_2$ and contrails, have a warming effect on the climate

Cool Atmospheric Phenomena

• Aurora (aurora borealis and aurora australis): Colorful light displays in the polar regions
  • Caused by the interaction of charged particles from the sun with the Earth's magnetic field and atmosphere
  • Typically occurs in the thermosphere, with colors depending on the type of atmospheric gas and the energy of the particles
• Noctilucent clouds: Rare, high-altitude clouds visible during twilight
  • Form in the mesosphere at altitudes of around 80 km
  • Composed of tiny ice crystals that reflect sunlight, making them visible from the ground
• Mirages: Optical illusions caused by the refraction of light in the atmosphere
  • Inferior mirages (e.g., "water on the road") occur when a layer of hot air is close to the ground, bending light from the sky
  • Superior mirages (e.g., "floating" ships or buildings) occur when a layer of cold air is below a layer of warm air, bending light from objects below the horizon
• Atmospheric optics: Various optical phenomena caused by the interaction of light with the atmosphere
  • Rainbows form when sunlight is refracted and reflected by water droplets in the atmosphere
  • Halos, sun dogs, and light pillars are caused by the refraction and reflection of light by ice crystals in the atmosphere
• Atmospheric waves: Oscillations in the atmosphere that can propagate over large distances
  • Gravity waves are caused by the displacement of air parcels in a stable atmosphere (e.g., by mountains or convection)
  • Rossby waves are large-scale meanders in the jet stream that influence weather patterns and heat distribution

Why Should We Care?

• Atmosphere sustains life on Earth by providing oxygen for respiration and $CO_2$ for photosynthesis
• Atmospheric composition and dynamics influence weather patterns and climate
  • Understanding atmospheric processes is crucial for accurate weather forecasting and climate modeling
  • Changes in atmospheric composition (e.g., increased greenhouse gases) can have significant impacts on global climate and ecosystems
• Air quality affects human health, with poor air quality linked to respiratory and cardiovascular diseases
  • Monitoring and regulating atmospheric pollutants is essential for protecting public health
  • Developing clean technologies and reducing emissions can improve air quality and mitigate climate change
• Atmosphere plays a vital role in the water cycle, enabling the distribution of freshwater resources
  • Changes in atmospheric circulation patterns can affect regional water availability and increase the risk of droughts or floods
• Atmospheric research advances our understanding of Earth's systems and helps us prepare for and mitigate potential risks
  • Studying the atmosphere's role in climate change informs policy decisions and adaptation strategies
  • Monitoring atmospheric composition and dynamics is crucial for predicting and responding to natural hazards (e.g., hurricanes, air pollution events)
• Appreciating the beauty and complexity of atmospheric phenomena fosters a sense of connection to the natural world
  • Engaging the public in atmospheric science can promote environmental stewardship and support for sustainable practices