SLM10-Chemistry-of-atmosphere

Chemistry of Atmosphere

Learning Outcomes

  • Key Skills:

    • Explain the different regions of the atmosphere based on temperature profile.

    • Discuss temperature changes within the atmosphere.

    • Analyze the composition of the atmosphere and calculate concentrations from partial pressure.

    • Explain important atmospheric photochemical reactions.

    • Seek knowledge on the stratosphere's chemical processes and ozone layer depletion.

    • Understand the consequences of fossil fuel combustion leading to acid rain and photochemical smog.

    • Evaluate the role of greenhouse gases in global warming and climate change.

Overview of Earth's Atmosphere

  • Significance:

    • Earth is the only planet known to sustain life with an atmosphere.

    • The atmosphere contains the air we breathe and shields us from solar radiation.

    • It regulates temperature: warming during the day and cooling at night.

    • The atmosphere is a thin mixture of gases, whose temperature varies with altitude.

Regions of the Atmosphere by Temperature Profile

1. Troposphere

  • Characteristics:

    • Closest layer to Earth's surface, extends up to about 10 km (4-12 miles).

    • Contains approximately 75% of the atmosphere's mass.

    • Weather phenomena like clouds and precipitation occur here.

    • Temperature decreases with altitude at approximately 6.5°C/km.

    • Lower section known as the boundary layer.

2. Stratosphere

  • Characteristics:

    • Begins above the troposphere, stretching from 10 km to about 50 km (31 miles).

    • Contains a high concentration of ozone that absorbs harmful UV radiation.

    • Air is dry and significantly less dense compared to sea level.

    • Jet aircraft typically cruise here due to the thin atmosphere.

3. Mesosphere

  • Characteristics:

    • Extends from 50 km to 85 km (31 to 53 miles).

    • Mesopause is the coldest layer, averaging -130°F (-90°C).

4. Thermosphere

  • Characteristics:

    • Ranges from 85 km to between 500 km and 1,000 km.

    • Temperatures can soar to 2,700°F (1,500°C) due to solar activity.

    • Contains the ionosphere, which reflects radio waves and is the site of auroras.

5. Exosphere

  • Characteristics:

    • Outermost layer starting at about 500 km extending to 10,000 km.

    • Meteorologically thin, merging into outer space.

    • Mainly composed of hydrogen and helium atoms.

6. Magnetosphere

  • Characteristics:

    • Earth's magnetic field traps charged particles (electrons and protons).

    • Houses the Van Allen radiation belts.

    • Charged particles spiral along magnetic field lines.

Atmospheric Gases Composition

  • Main Constituents:

    • Nitrogen: 78 %

    • Oxygen: 21 %

    • Argon: 0.93 %

    • Carbon Dioxide: 0.04 %

    • Trace Gases: Include neon, helium, methane, and hydrogen, with varying small percentages.

  • Effects of Solar Radiation: Air chemistry is influenced significantly by energetic particles from the sun—especially at altitudes above 80 km.

Key Chemical Processes

Photochemical Reactions

  • Types:

    1. Photodissociation:

      • Photon absorption results in the breaking of chemical bonds without forming ions.

      • Example: Photodissociation of O2 at altitudes above 120 km requires 495 kJ/mol energy.

    2. Photoionization:

      • Absorption of photons leads to electron ejection.

      • Creates positively charged ions (cations).

Ozone Layer and its Depletion

  • Ozone Dynamics:

    • Forms a protective layer that absorbs UV radiation, particularly wavelengths between 240 nm and 310 nm.

    • Depletion Factors:

      • Natural nitrogen oxides and synthetic CFCs (e.g., CCl3F) play significant roles in ozone level reduction.

      • Recognized by chemists such as Rowland and Molina in the 1970s, contributing to environmental policy.

Acid Rain and Its Impacts

  • Causes:

    • Emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from burning fossil fuels.

  • Effects of Acid Rain:

    1. Weakens plant structure and inhibits growth.

    2. Alters soil and aquatic habitats, threatening ecosystems.

    3. Causes health issues related to respiratory systems.

    4. Deteriorates historical and architectural stonework.

Photochemical Smog

  • Formation:

    • Results from nitrogen oxides reacting with sunlight, exhibiting as a brown haze.

  • Primary Pollutants:

    • Emissions from vehicles (NO, CO, unburned hydrocarbons).

  • Impacted Areas:

    • Prominent in densely populated urban regions, notably Los Angeles, Mexico City, and Beijing.