London Smog vs. Los Angeles Smog: A Comparative Study
The word "smog" was originally coined in the early 20th century by Dr. Henry Antoine Des Voeux to describe the suffocating mixture of smoke and fog. In environmental chemistry, "smog" is categorized into two distinct types based on their chemical composition, formation mechanisms, and environmental conditions. While London Smog is a byproduct of industrial coal combustion, Los Angeles Smog is a modern phenomenon driven by vehicular emissions and solar chemistry.
1. London Smog (Classical/Sulfurous Smog)
London Smog is characterized as primary pollution because it results directly from the emission of pollutants into the atmosphere, primarily through the combustion of fossil fuels (coal).
Key Characteristics
- Primary Pollutants: Sulfur dioxide (SO2) and particulate matter (soot).
- Weather Conditions: Occurs in cool, humid climates, often during winter mornings with temperature inversions.
- Chemical Nature: It is reducing in nature.
The Chemistry of London Smog
The main culprit is the high sulfur content in coal. When coal is burned, sulfur reacts with oxygen to form sulfur dioxide. In the presence of fog droplets, this gas can eventually form sulfuric acid, which is highly corrosive to lung tissue and infrastructure.
$$S + O_2 \rightarrow SO_2$$ $$2SO_2 + O_2 \rightarrow 2SO_3$$ $$SO_3 + H_2O \rightarrow H_2SO_4 \text{ (Sulfuric Acid)}$$
Historical Context: The "Great Smog of December 1952" in London led to approximately 4,000 to 12,000 premature deaths, prompting the Clean Air Act of 1956.
2. Los Angeles Smog (Photochemical Smog)
Unlike the London variety, Los Angeles Smog is a secondary pollution. It is not emitted directly from tailpipes or chimneys but is formed in the atmosphere through complex chemical reactions fueled by solar radiation.
Read Photochemical Smog
Key Characteristics
- Precursors: Nitrogen oxides (NOx) and Volatile Organic Compounds (VOCs) from vehicular exhaust.
- Weather Conditions: Occurs in warm, sunny, and dry climates.
- Chemical Nature: It is oxidizing in nature.
The Photochemical Reaction Mechanism
The process begins with the photolysis of Nitrogen Dioxide (NO2) by UV light, which releases a highly reactive atomic oxygen. This oxygen atom reacts with molecular oxygen to form Ozone (O3).
$$NO_2 + hv \text{ (sunlight)} \rightarrow NO + O$$ $$O + O_2 \rightarrow O_3 \text{ (Ground-level Ozone)}$$
The Nitrogen Oxide (NOx) then reacts with VOCs to form a cocktail of secondary pollutants, the most notorious being Peroxyacetyl Nitrate (PAN), which causes intense eye irritation and plant damage.
$$CH_3CO \cdot (\text{acyl radical}) + O_2 \rightarrow CH_3COO_2 \cdot$$ $$CH_3COO_2 \cdot + NO_2 \rightarrow CH_3COO_2NO_2 \text{ (PAN)}$$Comparative Summary
| Feature | London Smog (Classical) | Los Angeles Smog (Photochemical) |
|---|---|---|
| Primary Components | SO2, Particulates, H2SO4 | O3, PAN, NOx, VOCs |
| Major Source | Coal combustion (Industrial/Domestic) | Internal combustion engines (Vehicular) |
| Time of Day | Early morning (Winter) | Midday/Afternoon (Summer) |
| Atmospheric State | Reducing | Oxidizing |
| Visibility Effect | Grey/Black thick fog | Brownish/Yellow haze |
Health and Environmental Impact
Both types of smog pose significant risks, though through different pathways:
- Respiratory Issues: Both trigger asthma, bronchitis, and reduced lung capacity.
- Infrastructure: London smog (acidic) corrodes metals and limestone.
- Agriculture: Los Angeles smog (Ozone/PAN) inhibits photosynthesis and damages crop yields.
Understanding these differences is crucial for urban planning and environmental policy. While London smog is largely managed through fuel regulation, Los Angeles smog remains a modern challenge due to the global increase in vehicular traffic.
The Meteorological Catalyst: Temperature Inversion
A critical factor in both types of smog is a Temperature Inversion. Under normal conditions, air temperature decreases with altitude, allowing warm pollutants to rise and disperse. During an inversion, a layer of warm air sits above a layer of cooler air near the ground. This acts as a "lid," trapping pollutants and allowing their concentrations to reach lethal levels.
Advanced Mechanism: The Photochemical Cycle
For PG students, it is important to note that the formation of ozone is a cyclic process. However, the presence of Hydrocarbons (VOCs) disrupts this cycle by reacting with Nitric Oxide (NO), preventing it from breaking down the Ozone (O3) already formed. This leads to the accumulation of Ozone at ground level.
Quick Self-Assessment Quiz
- Which smog is known as "Reducing Smog"?
(Ans: London Smog, due to the presence of SO2 and H2SO4) - What is the primary organic eye irritant in Los Angeles smog?
(Ans: PAN - Peroxyacetyl Nitrate) - True or False: Photochemical smog requires high humidity.
(Ans: False. It requires dry, sunny conditions.) - Which chemical species starts the photochemical reaction?
(Ans: NO2, via photolysis by UV radiation)
Related Topics
Difference Between Primary and Secondary Smog
Ozone Depletion: Notes and MCQs
Ozone Layer Depletion / The Ozone Hole
How is ozone formed in the atmosphere
Global Warming and Ozone Depletion
Primary and Secondary Pollutants
Photochemical Smog