Atmospheric Pollution

• Primary Pollutants: Emitted directly from identifiable sources into the atmosphere:
  • Carbon Monoxide ($\text{CO}$): Incomplete combustion of fossil fuels; silent killer (binds to hemoglobin $250\times$ more strongly than $\text{O}_2$ to form toxic carboxyhemoglobin, causing asphyxiation).
  • Sulphur Dioxide ($\text{SO}_2$): Combustion of sulphur-rich coal and oil, roasting of sulphide ores.
  • Oxides of Nitrogen ($\text{NO}, \text{NO}_2$): High-temperature vehicle engines ($N_2 + O_2 \to 2NO$).
  • Particulate Matter (PM 2.5, PM 10): Soot, fly ash, dust causing asthma, silicosis, and bronchitis.
• Secondary Pollutants: Formed in the atmosphere through chemical reactions between primary pollutants and sunlight/moisture:
  • Acid Rain: $\text{H}_2\text{SO}_4$ and $\text{HNO}_3$.
  • Photochemical Smog: Formed by reaction of $\text{NO}_x$ and unburnt hydrocarbons under sunlight, containing toxic Peroxyacetyl Nitrate (PAN) and ground-level Ozone ($\text{O}_3$).

A. Chemical Formation:

Normal unpolluted rainwater is weakly acidic ($\text{pH} \approx 5.6$) due to dissolved atmospheric carbon dioxide forming carbonic acid: $\text{CO}_2 + \text{H}_2\text{O} \to \text{H}_2\text{CO}_3$.

When atmospheric $\text{SO}_2$ and $\text{NO}_x$ dissolve, the $\text{pH}$ plunges below $5.6$ (often to $3.5-4.0$):

1. Sulphur dioxide oxidation: $$2\text{SO}_2(g) + \text{O}_2(g) \xrightarrow{\text{dust/soot}} 2\text{SO}_3(g)$$ $$\text{SO}_3(g) + \text{H}_2\text{O}(l) \to \mathbf{\text{H}_2\text{SO}_4(aq)} \quad (\text{Sulphuric Acid})$$
2. Nitrogen dioxide reaction: $$4\text{NO}_2(g) + 2\text{H}_2\text{O}(l) + \text{O}_2(g) \to \mathbf{4\text{HNO}_3(aq)} \quad (\text{Nitric Acid})$$

B. Marble Cancer:

Historical monuments made of marble or limestone ($\text{CaCO}_3$) are corroded and yellowed by acid rain:

The resulting calcium sulphate ($\text{CaSO}_4$) is slowly washed away by rain, leaving pits and crumbling stone.

C. Ecological Consequences:

• Acidification of lakes ($\text{pH} < 4.5$) leaches toxic aluminium ions ($\text{Al}^{3+}$) from soil, causing mucus to clog fish gills, resulting in mass fish kills.
• Leaches vital plant nutrients (calcium, magnesium, potassium) from soil, stunting agricultural crops and killing forests.

A. The Stratospheric Ozone Shield:

Ozone ($\text{O}_3$) in the stratosphere ($15-30\text{ km}$) absorbs lethal, high-energy UV-B and UV-C solar radiation ($200-315\text{ nm}$):

B. Depletion by Chlorofluorocarbons (CFCs):

Synthetic CFCs (Freons: $\text{CFCl}_3, \text{CF}_2\text{Cl}_2$) from old refrigerants and aerosol sprays drift into the stratosphere. High-energy UV radiation photolytically cleaves them, liberating reactive atomic chlorine radicals ($\text{Cl}^\bullet$):

Catalytic Chain Reaction: The chlorine radical $\text{Cl}^\bullet$ is regenerated! A single chlorine atom destroys over 100,000 ozone molecules before being removed! This caused the Antarctic Ozone Hole. Consequence: Increased cataracts, skin carcinomas, and phytoplankton mortality.

C. Industrial Pollution Control Devices:

• Automotive Catalytic Converter: Platinum/Palladium/Rhodium catalyst oxidizes unburnt hydrocarbons to $\text{CO}_2 + \text{H}_2\text{O}$, converts toxic $\text{CO}$ to $\text{CO}_2$, and reduces $\text{NO}_x$ to harmless $\text{N}_2$.
• Flue-Gas Scrubbers: Industrial factory chimneys spray slaked lime ($\text{Ca(OH)}_2$) slurry to absorb acidic $\text{SO}_2$ gas as $\text{CaSO}_3$.
• Electrostatic Precipitators: High voltage ($50\text{ kV}$) charges smoke particles, attracting them to collector plates to trap $99\%$ of fly ash and soot.

Problem: Explain chemically why unpolluted rainwater is naturally slightly acidic, and write the equation showing the origin of this natural acidity.

Solution:

Unpolluted atmospheric air naturally contains approximately $0.04\%$ carbon dioxide ($\text{CO}_2$).

As raindrops fall through the atmosphere, they dissolve a small amount of gaseous $\text{CO}_2$, reacting to form a weak, dilute solution of Carbonic Acid ($\text{H}_2\text{CO}_3$):

Carbonic acid dissociates weakly into hydrogen ions: $\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$.

This lowers the natural pH of rainwater from neutral ($7.0$) to slightly acidic ($\text{pH} \approx 5.6$). Only when pollutants like $\text{SO}_2$ and $\text{NO}_x$ push the pH below $5.6$ is it classified as Acid Rain.