Air pollution that occurs in sunlight is
(a) acid rain
(b) oxidising smog
(c) fog
(d) reducing smog

Q: Why is photochemical smog called oxidising smog?

It is called oxidising smog because it contains high concentrations of strong oxidising agents like ozone ($O_3$) and peroxyacetyl nitrate (PAN), which are formed by the photochemical reaction of primary pollutants.

Q: What is classical smog, and how is it different?

Classical smog occurs in cool, humid climates and is primarily a mixture of smoke, fog, and sulfur dioxide ($SO_2$). Because $SO_2$ is a reducing agent, classical smog is called reducing smog, which is the exact opposite of photochemical smog.

Q: What are the primary pollutants that cause photochemical smog?

The primary pollutants are nitrogen oxides ($NO_x$) and unburnt hydrocarbons. These are mostly released into the atmosphere from automobile exhausts and fossil fuel combustion.

Q: Why does photochemical smog often have a brown color?

The distinct brown haze of photochemical smog is primarily due to the presence of nitrogen dioxide ($NO_2$), which is a brown-colored gas formed during the complex chain of photochemical reactions.

Q: Can photochemical smog occur in the winter?

It is much less likely. Photochemical smog requires warm, dry, and sunny conditions to provide the necessary UV radiation ($h\nu$) to drive the photochemical reactions. Winter conditions usually favor classical smog.

Question

Air pollution that occurs in sunlight is

Options:

(A) acid rain

(B) oxidising smog

(C) fog

(D) reducing smog

Sigma Percentile · Accepted Answer

### The Tale of Two Smogs

When we talk about severe air pollution in urban environments, the word **smog** immediately comes to mind. Coined as a portmanteau of 'smoke' and 'fog', smog is a visible manifestation of toxic air. However, not all smog is created equal. In environmental chemistry, we broadly classify smog into two distinct categories based on their chemical nature and the conditions required for their formation: **Classical Smog** and **Photochemical Smog**.

Classical smog, often associated with the infamous Great Smog of London, occurs in cool, humid climates. It is primarily a mixture of smoke, fog, and sulfur dioxide ($SO_2$). Because $SO_2$ is a reducing agent, classical smog is chemically referred to as **reducing smog**.

### The Role of Sunlight: Photochemical Smog

The question specifically asks about the air pollution that occurs in **sunlight**. This is the ultimate giveaway. Photochemical smog, as the prefix 'photo' implies, is driven by the energy from sunlight ($h
u$).

Unlike its classical counterpart, photochemical smog thrives in warm, dry, and sunny climates—think of cities like Los Angeles or heavily congested metropolitan areas during the summer. The intense solar radiation acts as a catalyst, initiating a cascade of complex chemical reactions in the atmosphere.

### The Chemical Recipe: From Exhaust to Oxidants

So, what are the ingredients for this toxic soup? The primary culprits are **nitrogen oxides ($NO_x$)** and **unburnt hydrocarbons**. These primary pollutants are continuously pumped into the atmosphere by automobile exhausts and industrial emissions.

When sunlight strikes these primary pollutants, it triggers a photochemical chain reaction. The nitrogen dioxide ($NO_2$) absorbs sunlight and breaks down, eventually leading to the formation of highly reactive secondary pollutants. The most prominent among these are **ozone ($O_3$)** and **peroxyacetyl nitrate (PAN)**, which has the chemical formula $CH_3-C(=O)-O-O-NO_2$.

### Conclusion: Oxidising vs Reducing

Because ozone and PAN are exceptionally strong oxidising agents, the resulting photochemical smog is highly oxidising in nature. It often appears as a brownish haze due to the presence of $NO_2$ gas.

Therefore, the air pollution that occurs in sunlight is unequivocally **oxidising smog**. Understanding this fundamental difference not only helps in solving such direct questions but also builds a strong foundation for tackling complex assertion-reasoning problems in environmental chemistry.

Animated Solution for Chemistry - Environmental Chemistry: Air pollution that occurs in sunlight is

Select Answer:

Visualized Solution

$Types of Smog$

$Role of Sunlight$

$Primary Pollutants$

$Formation of Oxidising Smog$

$Final Conclusion$

$The Way Forward$

The Sigma Insight: Atmospheric Pollution

The Tale of Two Smogs

The Role of Sunlight

The Chemical Recipe

Conclusion

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