Unit 3: Atmospheric chemistry

Composition of atmosphere

The atmosphere is a mixture of gases. Its composition in dry air is relatively constant:

The atmosphere is also structured in layers based on temperature:

• Troposphere (0-12 km): Where we live and where weather occurs. Temperature decreases with altitude. Contains 90% of atmospheric mass.
• Stratosphere (12-50 km): Contains the Ozone Layer. Temperature increases with altitude (due to ozone absorbing UV).
• Mesosphere (50-85 km): Temperature decreases again.
• Thermosphere (>85 km): Temperature increases dramatically due to solar radiation.

Photochemical reactions in atmosphere

A photochemical reaction is a chemical reaction that is initiated by the absorption of energy in the form of light (photons, hν).

These reactions are the most important processes in the atmosphere. They drive both the creation of protective ozone and the formation of harmful smog.

Key Example: Ozone Formation
High-energy UV light (hν) from the sun splits an oxygen molecule (O₂) in the stratosphere:

Step 1 (Photolysis): O₂ + hν (UV light) → O + O

A free oxygen atom (O) then combines with another oxygen molecule (O₂) to form ozone (O₃):

Step 2 (Formation): O + O₂ → O₃

This O₃ molecule then forms the ozone layer, which in turn absorbs other UV rays, protecting life below.

Smog formation

Smog (a portmanteau of "smoke" and "fog") is a type of air pollution. There are two main types:

Types of smog (Sulphur smog and Photochemical smog)

1. Sulphur Smog (London Smog or Industrial Smog)

• Main Source: Burning high-sulfur fossil fuels, especially coal.
• Key Ingredients: Sulfur dioxide (SO₂), particulate matter (soot, ash), and high humidity (fog).
• Weather Conditions: Cold, damp, and still (often in winter), with a temperature inversion trapping pollutants near the ground.
• Chemistry:
  1. Burning coal: S + O₂ → SO₂
  2. SO₂ reacts with oxygen (slowly) or catalysts in soot: 2SO₂ + O₂ → 2SO₃
  3. SO₃ reacts with water droplets in fog: SO₃ + H₂O → H₂SO₄ (Sulfuric Acid)
• Effects: Forms an acidic, gray-colored air. Causes severe respiratory problems.

2. Photochemical Smog (Los Angeles Smog or Brown-Air Smog)

• Main Source: Exhaust from vehicles.
• Key Ingredients: Nitrogen oxides (NOx), Volatile Organic Compounds (VOCs), and Sunlight.
• Weather Conditions: Warm, sunny, dry, and still (often in summer).
• Chemistry (Very Complex):
  1. Cars release NO: N₂ + O₂ (in engine) → 2NO
  2. NO reacts with O₂ in the air: 2NO + O₂ → 2NO₂ (Nitrogen dioxide, a brown gas that gives the smog its color)
  3. Sunlight breaks down NO₂: NO₂ + hν (Sunlight) → NO + O
  4. The free oxygen atom (O) forms ground-level ozone: O + O₂ → O₃ (Ozone)
  5. VOCs react with all this to form other pollutants like PANs (Peroxyacetyl nitrates).
• Effects: Forms a hazy, brownish-red air. Ozone (O₃) is the main pollutant. It is highly toxic, causing respiratory issues and damaging plants.

Aerosols

Aerosols are tiny solid particles or liquid droplets suspended in the atmosphere. They are also known as Particulate Matter (PM).

• Natural Sources: Sea salt spray, dust (from soil erosion), pollen, volcanic ash, smoke from forest fires.
• Anthropogenic (Human) Sources: Soot (from burning), fly ash (from industry), sulfates and nitrates (from chemical reactions of SO₂ and NOx).

Environmental Effects:

• Health: Fine particles (especially PM₂.₅, smaller than 2.5 micrometers) can be inhaled deep into the lungs and enter the bloodstream, causing respiratory and cardiovascular diseases.
• Climate:
  • Cooling Effect: Most aerosols (like sulfates) are light-colored and reflect sunlight back to space (increasing albedo).
  • Warming Effect: Some aerosols (like black carbon/soot) are dark and absorb sunlight, warming the atmosphere.

Acid rain

Acid rain is any form of precipitation (rain, snow, fog) that is unusually acidic, meaning it has a pH lower than 5.6 (the pH of normal, unpolluted rain, which is slightly acidic due to dissolved CO₂).

Reactions of NO₂ and SO₂

The primary causes of acid rain are sulfur dioxide (SO₂) and nitrogen oxides (NOx) released from burning fossil fuels (power plants, factories, and cars).

Sulfuric Acid Formation:

SO₂ is released and reacts with oxygen in the atmosphere to form sulfur trioxide (SO₃), a reaction often catalyzed by dust particles or sunlight.

2SO₂ + O₂ → 2SO₃

Sulfur trioxide then dissolves in water droplets to form sulfuric acid (H₂SO₄), a strong acid.

SO₃ + H₂O → H₂SO₄

Nitric Acid Formation:

Nitrogen oxides (e.g., NO₂) react in the atmosphere with hydroxyl radicals (•OH) or water to form nitric acid (HNO₃), another strong acid.

NO₂ + •OH → HNO₃

Free radicals and ozone layer depletion

The Ozone Layer is a region in the stratosphere (not ground-level) with a high concentration of ozone (O₃). It is vital for life as it absorbs >95% of the Sun's harmful high-energy UV-B radiation.

Free Radicals

A free radical is a highly reactive atom or molecule that has an unpaired valence electron. Radicals are "desperate" to find another electron to pair with, so they react with almost anything, often starting a chain reaction.

In the stratosphere, natural radicals (like •OH, •NO) and human-made radicals (like •Cl and •Br) destroy ozone.

Role of CFCs in ozone depletion

CFCs (Chlorofluorocarbons) are man-made chemicals once used as refrigerants, aerosol propellants, and solvents (e.g., Freon). They are non-toxic, non-flammable, and very stable.

Their stability is the problem: they do not break down in the troposphere. They slowly drift up to the stratosphere.

The Ozone Depletion Process:

1. Step 1 (Photolysis): In the stratosphere, high-energy UV light (which is abundant there) breaks the stable CFC molecule, releasing a chlorine free radical (•Cl).

   CFCl₃ (CFC) + hν (UV light) → CFCl₂ + •Cl

2. Step 2 (Catalytic Cycle): This single chlorine radical then begins a catalytic cycle that destroys thousands of ozone molecules.
   Reaction A: The chlorine radical steals an oxygen atom from ozone (O₃), destroying it.

   •Cl + O₃ → ClO + O₂

   
   Reaction B: The resulting ClO molecule reacts with a free oxygen atom (O), which regenerates the original chlorine radical.

   ClO + O → •Cl + O₂