Chapter 20 Study Notes: Chemistry in the Atmosphere

Chapter 20 Study Notes: Chemistry in the Atmosphere

20.1 Earth's Atmosphere

  • Major Points:

    • Overview of Earth's atmosphere as a chemically active system, primarily consisting of nitrogen (N2) and oxygen (O2).

    • Comparison with the atmospheres of other planets, e.g., Mars and Jupiter.

    • Historical composition of Earth's atmosphere and the evolution towards current states.

  • Learning Objectives:

    • Summarize the process of nitrogen fixation.

    • Categorize the regions of Earth's atmosphere.

Composition of Earth's Atmosphere

  • Unique characteristics compared to other planets:

    • Mars: 90% carbon dioxide, thin atmosphere.

    • Jupiter: No solid surface, 90% hydrogen, 9% helium, 1% other.

  • Historical atmospheric changes:

    • Initial composition (3-4 billion years ago): Mainly ammonia (NH3), methane (CH4), and water (H2O), very little free oxygen (O2).

    • Role of ultraviolet (UV) radiation from the sun in initiating chemical reactions for the evolution of life on Earth.

Photosynthesis and Nitrogen Fixation

  • Photosynthesis is fundamental for producing oxygen (O2) and incorporation of carbon into living organisms:

    • Equation: CO2 + H2O → C6H12O6 + O2.

  • Nitrogen fixation process: Conversion of atmospheric nitrogen (N2) into usable compounds for plants (nitrates).

  • Important Notes:

    • Major by-products of nitrogen fixation include nitrates, which are essential for plant nutrients.

    • Nitrogen cycle illustrated in Figure 20.1 shows processes like fixation and denitrification.

Atmospheric Composition at Sea Level

  • Table 20.1: Shows composition of dry air:

    • Nitrogen (N2): 78.08%

    • Oxygen (O2): 20.95%

    • Argon (Ar): 0.93%

    • Other trace gases.

  • Total mass of the atmosphere: approximately 5.3imes10185.3 imes 10^{18} kg.

  • Process: Lightning converts atmospheric nitrogen to nitric oxide (NO)

    • Approx. 30 million tons of HNO3 produced annually through this process, subsequently forming nitrates in soil.

Oxygen Cycle

  • Distinct processes for oxygen release:

    • Photosynthesis and photodecomposition of water vapor.

  • Roles of Ozone (O3):

    • Absorbs harmful UV radiation, maintaining a balance of gases in the atmosphere.

Layers of the Atmosphere

  • Troposphere:

    • Contains 80% of atmospheric mass; all weather phenomena occur here.

    • Extent: About 10 km.

  • Stratosphere:

    • Contains ozone, temperature increase with altitude due to UV absorption.

    • Exothermic chemical reactions increase temperature.

  • Mesosphere: Low concentrations of gases, temperature decreases with altitude.

  • Thermosphere (or Ionosphere): High temperatures; ionized gases reflecting radio waves.

20.2 Phenomena in the Outer Layers of the Atmosphere

  • Learning Objective:

    • Outline the formation of aurora borealis and aurora australis.

Aurora Phenomena

  • Caused by solar flares ejecting particles into space; results in beautiful light displays when particles hit Earth’s upper atmosphere.

  • Mechanism:

    1. Solar particles collide with atmospheric gases, ionizing them.

    2. Excited gases emit light upon returning to ground state.

  • Color Emissions:

    • Green from excited oxygen at 558 nm.

    • Red emissions around 630-636 nm.

    • Blue and violet colors from ionized nitrogen (NONO).

Human-induced Glow in Space Shuttles

  • Unique phenomenon observed during shuttle missions at high altitudes (about 300 km).

  • Glow caused by interaction between fast-moving shuttles and atmospheric oxygen, producing species like NO and NO2.

    • Equation: O + NO → NO2* + hv (where hv is emitted photon).

20.3 Depletion of Ozone in the Stratosphere

  • Learning Objective:

    • Evaluate the role of ozone in the atmosphere and the impact of chlorofluorocarbons (CFCs).

Ozone Layer's Function

  • Ozone forms through photodissociation of O2 by UV radiation, critical for absorbing harmful solar radiation.

  • Dynamic Equilibrium: Formation and destruction maintain a stable concentration of ozone.

  • Chlorofluorocarbons (CFCs):

    • Historical uses in coolants, sprays, and insulation. Release leads to ozone layer depletion.

  • Mechanism of Ozone Destruction by CFCs:

    • CFCs decompose under UV light to release Cl radicals, which catalyze ozone destruction in numerous cycles.

    • One Cl atom can destroy up to 100,000 O3 molecules.

  • Evidence: Polar ozone holes identified in Antarctica and the Arctic, primarily due to CFCs and reaction with volcanic ash and ice.

International Response

  • Montreal Protocol: Agreement to phase out use of harmful ozone-depleting substances, showing nations' commitment to environmental protection.

20.4 Volcanoes

  • Learning Objective:

    • Identify the effects of volcanic eruptions on the atmosphere.

Volcanic Eruptions and Atmospheric Impact

  • Eruptions emit gases like N2, CO2, HCl, HF, H2S, and water vapor.

  • Major source of sulfur and effects on air quality:

    • Sulfur dioxide (SO2) can oxidize into sulfuric acid, leading to acid rain.

    • Eruptive aerosols impact global temperatures, leading to local cooling effects due to solar radiation absorption.

20.5 The Greenhouse Effect

  • Learning Objective:

    • Assess the greenhouse effect and its role in climate change.

Greenhouse Gases and Climate Control

  • CO2 plays a significant role in maintaining Earth's temperature. Its concentration is roughly 0.033% by volume.

  • Mechanism Similar to greenhouse glass: Acts as insulators trapping heat.

  • Without CO2, Earth’s average temperature could drop by 30°C.

  • Example: Venus, with 97% CO2, has extreme temperatures of about 730 K.

  • Thermal Radiation: CO2 and H2O absorb IR radiation, preventing heat loss from Earth to space.

20.6 Acid Rain

  • Learning Objective:

    • Understand how SO2 causes acid rain.

Causes and Effects of Acid Rain

  • Acid rain results from SO2 and NOx emissions, primarily from fossil fuel combustion.

  • Natural reagents for acid formation include metals and volcanic activities.

  • Acid Rain Equations:

    • Formation of H2SO4 via oxidation of SO2.

    • Corrosive effects on structures and ecosystems, with pH below 5.5 being typical in certain regions.

    • Major sources of SO2 are from industrial processes (smelting) and energy production.

20.7 Photochemical Smog

  • Learning Objective:

    • Describe the formation of photochemical smog.

Processing of Smog Creation

  • Formed from automobile exhaust in sunlight leading to primary pollutants (NO, hydrocarbons) converting into secondary pollutants (NO2, O3).

  • Mechanisms include:

    • Production and decomposition of NO to NO2 via sunlight.

    • Reaction sequences leading to ozone and harmful pollutants affecting air quality.

20.8 Indoor Pollution

  • Learning Objective:

    • Identify major indoor pollutants and their sources.

Types of Indoor Pollutants

  • Includes radon (radon-222), carbon monoxide (CO), carbon dioxide (CO2), and formaldehyde (CH2O).

  • Sources:

    • Radon sourced from soil and building materials.

    • CO from combustion of fuels and is lethal at high concentrations due to hemoglobin binding.

    • Importance of ventilation for pollutant removal, particularly with radon and CO2.

Health Concerns and Solutions

  • Radon associated with lung cancer; may require comprehensive home testing and remediation for high levels.

  • CO safety emphasized due to its toxicity; requiring effective ventilation and monitoring.

Summary of Concepts & Facts

  • Overview of chapters covering atmospheric chemistry, ozone, and effects of pollution in the atmosphere.

Key Words

  • Greenhouse effect, ionosphere, mesosphere, nitrogen fixation, photochemical smog, stratosphere, thermosphere, troposphere.