13.2 Smog

Smog is a type of air pollution resulting from the combination of fog and smoke or other atmospheric pollutants. It appears as a hazy, often yellowish or brownish, layer in the atmosphere that significantly reduces visibility and poses serious health risks. Smog forms when pollutants from industrial emissions, vehicle exhaust, and natural sources react chemically in the presence of sunlight.

13.2.1 Types of Smog

There are two primary types of smog:

Photochemical Smog:

• Typically forms in sunny, dry, urban areas with high traffic density.
• Results from photochemical reactions between primary pollutants such as nitrogen oxides ($N{O}_x$) and volatile organic compounds (VOCs) in the presence of strong sunlight.
• A key component is ground-level ozone ($O_3$), which is harmful to human health and plants.
• Also known as "brown smog" or "oxidizing smog."

Industrial Smog:

• Also known as "black smog," "London smog," or "reducing smog."
• Results from the heavy use of fossil fuels, particularly coal, in industrial and power generation facilities.
• Main components are sulphur dioxide ($S{O}_2$) and particulate matter (such as soot).

Composition of Photochemical Smog

The primary and secondary pollutants that constitute photochemical smog include:

• Nitrogen oxides ($N{O}_x$)
• Volatile organic compounds (VOCs)
• Ozone ($O_3$)
• Particulate matter (PM)
• Peroxyacetyl nitrate (PAN)

General Mechanism of Photochemical Smog Formation

The formation of photochemical smog involves a complex series of reactions initiated by sunlight.

1. Photodissociation of $N{O}_2$:

Sunlight splits nitrogen dioxide into nitric oxide and atomic oxygen.

${\mathrm{NO}}_2+\text{sunlight}\to \mathrm{NO}+\mathrm{O}$

2. Ozone Formation:

The highly reactive atomic oxygen combines with molecular oxygen to form ozone.

$\mathrm{O}+{\mathrm{O}}_2\to {\mathrm{O}}_3$

3. Ozone Destruction:

Ozone reacts with nitric oxide, reforming nitrogen dioxide. This reaction typically keeps ozone levels in check.

$\mathrm{NO}+{\mathrm{O}}_3\to {\mathrm{NO}}_2+{\mathrm{O}}_2$

4. Role of VOCs:

Volatile organic compounds (represented by R) react with other species to form highly reactive organic radicals (ROx). These radicals react with NO, preventing it from destroying ozone. This allows ozone to accumulate to harmful levels.

$\mathrm{NO}+\mathrm{RO}\cdot \to {\mathrm{NO}}_2+\text{products}$

5. Formation of PAN:

Nitrogen dioxide also reacts with organic fragments to form other harmful secondary pollutants such as peroxyacetyl nitrate (PAN).

${\mathrm{NO}}_2+\mathrm{RCOO}\cdot \to \mathrm{PAN}$

13.2.2 Hazardous Effects of Smog

Health Effects:

• Respiratory Problems: Smog can cause or worsen respiratory conditions such as asthma, bronchitis, and pharyngitis, leading to coughing, irritation, and breathing difficulties.
• Cardiovascular Issues: Long-term exposure is linked to heart problems, including increased risk of heart attacks and arrhythmia.
• Eye Irritation: Components of smog (especially PAN and Ozone) can cause redness, itching, and watering of the eyes.

Environmental Impacts:

• Reduced Visibility: The haze from smog reduces visibility, increasing the risk of accidents in road, air, and maritime transport.
• Vegetation Damage: Ozone in smog damages crops, forests, and other vegetation, leading to reduced agricultural yields.
• Global Warming: Some pollutants in smog, such as methane (a VOC), are potent greenhouse gases that contribute to climate change.
• Aesthetic Degradation: Smog creates a persistent haze that spoils scenic views and lowers the overall aesthetic quality of urban and natural environments.