MEV 013: Unit 05 - Atmospheric Chemistry

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UNIT 5: ATMOSPHERIC CHEMISTRY

5.0 Introduction

Atmospheric chemistry is the study of the chemical composition of the Earth's atmosphere and the reactions and interactions that take place among atmospheric components. This unit provides insight into the origin, structure, and chemical processes occurring in the atmosphere, which are essential for understanding air quality, climate change, and environmental sustainability.

5.1 Objectives

After studying this unit, you will be able to:

  • Understand the origin and composition of the atmosphere.
  • Explain the concept of atmospheric stability and lapse rates.
  • Describe key chemical and photochemical reactions in the atmosphere.
  • Identify the roles of ions, free radicals, and particles in atmospheric chemistry.
  • Understand the environmental and health impacts of atmospheric particulates.

5.2 Origin of Atmosphere

The Earth's atmosphere has evolved through three main stages:

  1. Primordial Atmosphere: Composed mainly of hydrogen and helium, lost due to solar winds.
  2. Secondary Atmosphere: Formed through volcanic outgassing, rich in CO₂, H₂O, NH₃, and CH₄.
  3. Modern Atmosphere: Developed as photosynthetic organisms increased oxygen levels, resulting in a nitrogen-oxygen rich atmosphere with trace gases.

5.3 Composition of Atmosphere

The Earth's atmosphere is composed of:

Gas

Percentage

Nitrogen (N₂)

78.08%

Oxygen (O₂)

20.95%

Argon (Ar)

0.93%

Carbon Dioxide (CO₂)

~0.04%

Other Gases (Ne, He, CH₄, Kr, H₂, etc.)

Trace amounts

Water vapor varies from 0–4% depending on location and weather.

5.4 Structure of Atmosphere

The atmosphere is divided into layers based on temperature gradients:

  1. Troposphere (0–12 km): Weather and clouds occur here; temperature decreases with altitude.
  2. Stratosphere (12–50 km): Contains the ozone layer; temperature increases with altitude.
  3. Mesosphere (50–80 km): Coldest layer; meteors burn up here.
  4. Thermosphere (80–700 km): High temperatures due to solar radiation.
  5. Exosphere (>700 km): Merges into space.

5.5 Atmospheric Stability

5.5.1 Atmospheric Lapse Rate

  • Lapse Rate: The rate at which atmospheric temperature decreases with height.
  • Normal Lapse Rate: About 6.5°C/km in the troposphere.

5.5.2 Types of Lapse Rates

  1. Environmental Lapse Rate (ELR): Actual decrease in temperature with height.
  2. Dry Adiabatic Lapse Rate (DALR): ~10°C/km – applies to unsaturated air.
  3. Moist Adiabatic Lapse Rate (MALR): ~6°C/km – applies to saturated air.

5.5.3 Lapse Rates and Atmospheric Stability

  • Stable Atmosphere: ELR < MALR → suppresses vertical motion.
  • Unstable Atmosphere: ELR > DALR → enhances convection.
  • Neutral Atmosphere: ELR = DALR → neither suppresses nor enhances.

5.5.4 Importance of Understanding Atmospheric Stability

Stability affects:

  • Weather (e.g., cloud formation, storms)
  • Air Pollution Dispersion
  • Aviation and Climatology

5.6 Chemical and Photochemical Reactions of Atmosphere

Atmospheric reactions are driven by solar radiation, initiating processes like photodissociation and free radical formation.

5.6.1 Photochemistry of the Atmosphere

  • Photodissociation: Molecules absorb UV radiation and break apart, e.g.,
    • O₂ + hv → 2O
  • These reactions are essential in ozone formation and degradation.

5.7 Distribution of Species in Atmosphere

5.7.1 Ions in the Atmosphere

  • Produced by cosmic rays and UV radiation.
  • Common in the ionosphere.
  • Important for radio communication and electrical conductivity.

5.7.2 Free Radicals in the Atmosphere

  • Highly reactive species with unpaired electrons (e.g., OH•, HO₂•).
  • Drive important reactions such as ozone formation, NOx transformation, and VOC degradation.

5.8 Reactions of Atmospheric Oxygen

  • O₂ is stable but reacts when energized by UV or other radicals:
    • O₂ + hv → 2O (atomic oxygen)
    • O + O₂ → O₃ (ozone)
  • These reactions regulate ozone concentration in the stratosphere.

5.9 Reactions of Atmospheric Ozone

  • Formation: O₂ + hv → 2O, followed by O + O₂ → O₃
  • Depletion: O₃ + NO → NO₂ + O₂
  • Ozone protects life by absorbing harmful UV-B and UV-C radiation.

5.10 Reactions of Nitrogen Oxides (NOx)

  • Emitted from combustion, vehicles, and lightning.
  • Participate in ozone formation and smog development:
    • NO₂ + hv → NO + O
    • O + O₂ → O₃
  • NOx is also a precursor of acid rain.

5.11 Particles in the Atmosphere

Particulate matter (PM) includes solid and liquid particles suspended in air.

5.11.1 Characteristics of Particulates

  • Size: PM10 (<10 µm), PM2.5 (<2.5 µm)
  • Composition: Dust, soot, sulfate, nitrate, organics, metals
  • Source: Natural (dust storms, volcanoes) and anthropogenic (vehicles, industry)

5.11.2 Chemical Reactions of Atmospheric Particles

  • Particles can adsorb gases and act as surfaces for heterogeneous reactions, altering atmospheric chemistry.
  • Example: SO₂ + H₂O on particle → H₂SO₄ (sulfuric acid aerosol)

5.11.3 Aerosols

  • Tiny particles or droplets suspended in air.
  • Influence climate by scattering sunlight and acting as cloud condensation nuclei (CCN).

5.11.4 Black Carbon

  • Produced from incomplete combustion (e.g., diesel engines, biomass burning).
  • Absorbs sunlight → contributes to global warming and glacier melting.

5.11.5 Effects of Particulate Matter on Human Health

  • PM2.5 and PM10 penetrate the lungs and bloodstream.
  • Health effects:
    • Respiratory diseases (asthma, bronchitis)
    • Cardiovascular problems
    • Premature deaths

5.12 Let Us Sum Up

Atmospheric chemistry deals with the origin, structure, and chemical dynamics of Earth's atmosphere. Key chemical processes such as photodissociation, oxidation, and free radical reactions shape our climate and air quality. Particulate matter, nitrogen oxides, and ozone play major roles in atmospheric reactions and directly impact human health and ecosystems. A detailed understanding of atmospheric stability and chemistry is crucial for addressing environmental challenges like smog, acid rain, and global warming.

5.13 Keywords

  • Lapse Rate-Rate of temperature decrease with altitude
  • Photodissociation-Breakdown of molecules by light (UV radiation)
  • Free Radicals-Highly reactive molecules with unpaired electrons
  • Ozone-Triatomic oxygen molecule (O₃), protects Earth from UV radiation
  • Nitrogen Oxides (NOx)-Pollutants that contribute to ozone formation and acid rain
  • Particulate Matter-Solid/liquid particles suspended in air; harmful to health
  • Aerosols-Suspensions of fine solid or liquid particles in air
  • Black Carbon-Soot from incomplete combustion; a major warming agent
  • Atmospheric Stability-Resistance of the atmosphere to vertical motion

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