MEV 013: Unit 09 - Chemistry of Air Pollution-II

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UNIT 9: CHEMISTRY OF AIR POLLUTION – II

9.0 Introduction

Air pollution includes both inorganic and organic pollutants. While UNIT 8 discussed major inorganic pollutants, this unit focuses on organic air pollutants, including hydrocarbons and various oxygen-, halogen-, sulphur-, and nitrogen-containing compounds. These compounds contribute to harmful phenomena such as photochemical smog and ozone layer depletion. This unit also explores the chemical mechanisms underlying these atmospheric reactions and the environmental impacts they produce.

9.1 Objectives

After completing this unit, you will be able to:

  • Identify various types of organic air pollutants.
  • Understand the atmospheric reactions of organic compounds.
  • Explain the formation and effects of photochemical smog.
  • Describe the chemistry of ozone depletion in different regions.
  • Recognize the role of radicals like chlorine and bromine in atmospheric reactions.

9.2 Sources of Organic Air Pollutants

Organic pollutants are compounds that contain carbon and are released into the atmosphere through natural and anthropogenic activities.

9.2.1 Hydrocarbons as Pollutants

Hydrocarbons (HCs) are organic compounds composed of hydrogen and carbon.

Sources:

  • Vehicular exhaust
  • Fuel evaporation
  • Industrial solvents
  • Biomass burning

Types:

  • Saturated hydrocarbons (alkanes): e.g., methane
  • Unsaturated hydrocarbons: e.g., ethylene, acetylene
  • Aromatic hydrocarbons: e.g., benzene, toluene, xylene (BTX)

Environmental Impact:

  • Act as precursors for photochemical smog.
  • Carcinogenic (e.g., benzene).
  • Reactive with nitrogen oxides under sunlight.

9.2.2 Oxygen-Containing Organic Pollutants

These include aldehydes, ketones, alcohols, and organic acids.

Common examples:

  • Formaldehyde (HCHO): from combustion and atmospheric oxidation
  • Acetaldehyde: vehicle exhaust, biomass burning

Effects:

  • Eye, nose, and throat irritation
  • Involved in ozone and smog formation

9.2.3 Organohalide Compounds

These contain carbon bonded to halogen atoms (Cl, F, Br, I).

Examples:

  • Chlorofluorocarbons (CFCs): used in refrigerants and aerosols
  • Trichloroethene (TCE) and tetrachloroethene (PCE): industrial solvents

Environmental Concerns:

  • Major role in ozone layer depletion
  • Long atmospheric lifetimes
  • Some are greenhouse gases

9.2.4 Organosulphur and Organonitrogen Compounds

Organosulphur compounds: e.g., thiols, sulphides
Organonitrogen compounds: e.g., amines, nitro-compounds

Sources:

  • Petrochemical industries
  • Waste treatment plants
  • Fertilizer production

Impacts:

  • Odor nuisance
  • Precursor to acid rain
  • Toxicity to human and ecological health

9.3 Reactions of Organic Compounds in Atmosphere: Photochemical Smog

Photochemical smog is a brownish haze formed by reactions between sunlight, NOₓ, and volatile organic compounds (VOCs).

9.3.1 Understanding Reactions During Photochemical Smog

Key steps include:

  1. Initiation:
    • NO₂ → NO + O (under UV light)
    • O + O₂ → O₃ (ozone formation)
  2. Propagation:
    • VOCs react with OH• radicals forming peroxy radicals (RO₂•)
    • RO₂• reacts with NO → NO₂ + RO•
  3. Amplification:
    • Regenerated NO₂ leads to more ozone production

9.3.2 Reactions of Hydrocarbons with O, O₃, and OH•

  • Alkanes:
    RH + OH• → R• + H₂O
  • Alkenes and Aromatics:
    React more rapidly forming aldehydes, ketones, and PAN (Peroxyacetyl Nitrate)
  • Reaction with ozone:
    O₃ + alkene → carbonyl compounds + radicals

9.3.3 Chain Terminating Reactions

These reactions reduce the formation of ozone and smog:

  • HO₂• + RO₂• → ROOH + O₂
  • NO₂ + OH• → HNO₃

These lead to the removal of reactive species and slow down the cycle.

9.3.4 Compounds That Readily Undergo Photodissociation in Atmosphere

  • NO₂ → NO + O (λ < 400 nm)
  • O₃ → O₂ + O(¹D)
  • Aldehydes → radicals + CO

These reactions are sunlight-dependent and drive daytime smog formation.

9.3.5 Aerosols in Atmospheric Smog

Aerosols are fine particles or liquid droplets suspended in air. In smog:

  • Formed from oxidation of SO₂, NOₓ, and VOCs
  • Act as condensation nuclei for water vapor
  • Reduce visibility and affect respiratory health

9.3.6 Effects of Photochemical Smog

  • Health: Eye irritation, asthma, reduced lung function
  • Vegetation: Damages crops, inhibits photosynthesis
  • Materials: Corrosion of rubber, paints, and metals
  • Aesthetics: Haze and reduced visibility

9.4 Ozone Layer and Its Depletion

The stratospheric ozone layer (15–35 km altitude) shields Earth from harmful UV-B radiation. Human activities have introduced chemicals that degrade this protective layer.

9.4.1 Chlorine and Bromine Radicals as Catalysts

Mechanism:

  • CFCs are photolyzed:
    CFCl₃ + UV → CFCl₂ + Cl•
  • Cl• + O₃ → ClO• + O₂
  • ClO• + O → Cl• + O₂
  • Net reaction: O₃ + O → 2O₂

Bromine from halons and methyl bromide is even more efficient in destroying ozone.

9.4.2 Antarctic Ozone Hole

  • Observed since the 1980s
  • Occurs in spring (Sept–Nov) due to:
    • Polar stratospheric clouds (PSCs)
    • Low temperatures allowing Cl₂ and Br₂ to accumulate
    • Sunlight releases active radicals, triggering ozone destruction

9.4.3 Ozone Destruction in Arctic Region

  • Less severe than Antarctica but still significant
  • Arctic winters are shorter and warmer, reducing PSC formation
  • However, ozone thinning is increasing due to climate change

9.4.4 Ozone Destruction in Non-Polar Regions

  • Occurs via slow but continuous degradation
  • CFCs and related compounds persist in atmosphere
  • Global measures like Montreal Protocol have reduced emissions

9.5 Let Us Sum Up

This unit explored various organic air pollutants and their behavior in the atmosphere. Photochemical smog, largely driven by hydrocarbons and nitrogen oxides under sunlight, has serious health and environmental consequences. Additionally, the depletion of the ozone layer—particularly over polar regions—is linked to halogenated organic compounds. Understanding these chemical processes is vital to formulating air quality policies and environmental regulations.

9.6 Key Words

  • VOCs-Volatile Organic Compounds that contribute to smog.
  • Photochemical Smog-Haze formed by reaction of sunlight with NOₓ and VOCs.
  • PAN-Peroxyacetyl nitrate, an eye and respiratory irritant.
  • CFCs-Chlorofluorocarbons, ozone-depleting compounds.
  • Ozone Hole-Region of depleted stratospheric ozone, notably over Antarctica.
  • Radical-Highly reactive species with unpaired electrons.

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