Lecture 4 Atmospheric Composition and Structure

Atmospheric Composition

Overview

  • Date of Presentation: September 5, 2025

  • Purpose: Review the fundamentals of planetary energy balance, the composition and structure of the atmosphere.

Plan For Today

  1. Review Planetary Energy Balance

  2. Atmospheric Composition

    • What is the composition of the atmosphere?

    • Why do greenhouse gases trap energy?

  3. Atmospheric Structure

    • What is the structure of the atmosphere?

    • Defining different layers

Energy Balance Review

Electromagnetic (EM) Radiation

  • Solar Radiation Input: 342 W/m² at a distance of 150 million kilometers (Earth's average distance from the sun).

    • Reflected By Ice and Clouds: 103 W/m²

    • Energy is lost via the albedo effect.

  • Longwave Radiation:

    • Effective Temperature (TE) is calculated using the Stefan-Boltzmann Law:
      TE = igg( rac{Input}{ ext{σ}} igg)^{ rac{1}{4}} = 255 ext{K}

    • Note: Treating Earth as a perfect emitter yields a temperature too cold for life.

Greenhouse Effect

Contribution of Greenhouse Gases

  • Without atmosphere: Earth's surface temperature would be approximately 255 K (-18°C).

  • Actual Surface Temperature: 15°C (288 K).

  • Greenhouse Contribution Calculation:

    • Difference between actual and effective temperatures:
      T<em>extactual−T</em>exteffective=33extKT<em>{ ext{actual}} - T</em>{ ext{effective}} = 33 ext{K}

  • The greenhouse effect is due to the difficulty of longwave radiation escaping back into space.

  • Mechanism:

    • Longwave radiation absorbed by greenhouse gases (GHGs), then re-emitted in all directions, including back to Earth (referred to as counter-radiation).

Greenhouse Gases

  • Significant greenhouse gases include:

    • Carbon Dioxide (CO₂)

    • Ozone (O₃)

    • Water Vapor (H₂O)

    • Methane (CH₄)

    • Chlorofluorocarbons (CFCs)

  • Function: They absorb longwave radiation and subsequently re-radiate it to the Earth's surface.

Radiation Flux Chart

  • Radiation flux in terms of W/m²/μm is shown, differentiating between ultraviolet, visible, and infrared emissions.

  • Emission from the Earth (~15°C) and absorption characteristics of different greenhouse gases are described.

Chemical Composition of the Atmosphere (2025)

Major Gases

  • Nitrogen (N₂): 78.084%

  • Oxygen (O₂): 20.95%

  • Argon (Ar): 0.934%

  • Trace Gases:

    • Carbon Dioxide (CO₂): 0.037%

    • Other trace gases include Neon (Ne), Helium (He), Methane (CH₄), Nitrous oxide (N₂O), with their respective concentrations detailed as follows:

    • Neon (Ne): 18 ppm

    • Helium (He): 5 ppm

    • Methane (CH₄): 2 ppm

    • Nitrous Oxide (N₂O): 338 ppb (as of April 2024)

    • Carbon Monoxide (CO): 40-70 ppb

    • Ozone (O₃): Trace - 4%

    • Water Vapor (H₂O): Varies from 0.00001% to 4%

    • Trillions of other gases are present in very minute concentrations.

Important Greenhouse Gases

  • Water Vapor (H₂O)

  • Carbon Dioxide (CO₂)

  • Methane (CH₄): Also known as natural gas.

  • Nitrous Oxide (N₂O): Known for its role in smog.

  • Ozone (O₃)

  • CFC-11 (CCl₃F) and CFC-12 (CCl₂F₂): Refrigerants.

Significance of Greenhouse Gases

  1. Concentration: Amount of gas in the atmosphere.

  2. Absorption Potential: Ability of the gas to absorb electromagnetic radiation.

  3. Lifetime: Duration the gas remains in the atmosphere post-emission.

    • Examples:

    • CO₂ concentration: 426 ppm (0.04%)

    • Methane concentration: 2 ppm (0.0002%)

Absorption Characteristics of Greenhouse Gases

Mechanism of Absorption

  • GHGs absorb EM radiation predominantly in longwave bands.

  • The specific wavelength absorbed and current saturation levels impact the absorption effectiveness of each gas molecule.

Physical Effects of Absorption

  1. GHGs vibrate, bend, and stretch when exposed to specific wavelengths of electromagnetic radiation.

  2. Greater electric asymmetry increases absorption potential, and vice-versa.

  3. Importance of saturation overlap between absorption bands.

Column Air Example

  • Visualizing two columns of air—one with low GHG concentration and another with high concentration.

    • Hot air prefers to rise while cold air remains low.

    • As emission and absorption occur throughout the column, the temperature gradients create warming effects.

Lifetime of Greenhouse Gases

Variability in Atmospheric Lifetimes

  • GHGs have widely different atmospheric lifetimes:

    • Carbon Dioxide (CO₂): > 1,000 years for over 25% of molecules.

    • Methane (CH₄): ~10 years due to atmospheric oxidation.

  • Reference to a class discussing more comprehensive carbon cycle dynamics entitled “Carbon is Forever.” (Inman 2008)

Summary of Non-CO₂ Gases

  • Tabulated details on various gases including:

    • CFC-11: Lifetime = 52 years, Efficiency: 0.29 W/m²/ppb

    • Methane: Lifetime = 12 years, Efficiency: 5.7 × 10⁻⁴ W/m²/ppb

    • Nitrous Oxide (N₂O): Lifetime = 109 years, Efficiency: 3 × 10⁻³ W/m²/ppb

  • Absorption in the atmospheric window is most significant for climate.

Atmospheric Structure

General Structure

  • Layers are defined:

    1. Troposphere

    2. Stratosphere

    3. Mesosphere

    4. Thermosphere

    5. Exosphere

Hydrostatic Balance

  • As pressure in a water column increases with depth:

    • The relationship is described by the equation:
      extPressure=racextForceextArea[Pa]ext{Pressure} = rac{ ext{Force}}{ ext{Area}} [Pa]

  • Pressure decreases exponentially with altitude in the atmosphere with implications on weather and climate.

Troposphere Characteristics

  • Extends to about 20 km altitude.

    • Characterized by turbulence and well-mixed air.

    • Contains over 80% of the atmosphere's mass.

    • Uniform concentration of gases: N₂ (78%), O₂ (21%), CO₂ (<0.1%)

Pressure, Height, and Temperature Relationship

  • In the troposphere, temperature generally decreases with height.

  • This rate of temperature decline is known as the “Environmental Lapse Rate.”

    • Misalignment with common assumptions of pressure and temperature relationship due to variations in thermal dynamics.

Stability in the Atmosphere

  • The phenomenon of stability is examined through temperature inversions existing in the troposphere.

  • Example: warmer air at the surface creates a stable upper atmosphere scenario.

Stratospheric Changes

Transition to the Stratosphere

  • Pressure and temperature dynamics indicate that as one ascends into the stratosphere, a pressure drop should accompany decreased temperature; however, this is not observed.

  • The stratosphere includes about 20% of Earth's atmospheric mass and is noted as the region wherein ozone forms due to UV radiation interactions.

Absorption Mechanism in the Stratosphere

  • Incoming solar radiation affects molecular oxygen leading to the formation of ozone (O₃); this contributes to the protective ozone layer.

Additional Atmospheric Layers

  • Mesosphere:

    • Located below the blackness of space, characterized by decreasing temperatures with altitude.

  • Thermosphere:

    • Above 90 km, high temperatures due to the absorption of radiation by nitrogen and oxygen.

  • Exosphere:

    • At heights above 500 km, gas molecules may escape the atmosphere.

  • The significance of greenhouse gases and their interactions with the atmospheric components play a crucial role in regulating Earth's climate and maintaining terrestrial life. This lecture lays the groundwork for deeper discussions regarding environmental science, climatology, and atmospheric studies.