Question

What are the primary pollutant responsible for formation of photochemical smog?

• A. Sulfur dioxide (SO$_2$)
• B. Particulate matter (PM)
• C. Ozone (O$_3$) and volatile organic compounds (VOCs)
• D. nitrogen oxides (NO$_x$)

Hint:

• Primary pollutants that initiate photochemical smog are Nitrogen Oxides (NO$_x$) and Volatile Organic Compounds (VOCs).
• In the presence of sunlight, these react to form secondary pollutants.
• Key secondary pollutants in photochemical smog include ground-level Ozone (O$_3$), Peroxyacetyl Nitrate (PAN), and aldehydes.
• Option (c) combines a major secondary pollutant (Ozone) with a major primary pollutant (VOCs), which are both central to the phenomenon.

Answer 1

The Correct Option is C

The primary pollutants responsible for the formation of photochemical smog are nitrogen oxides (NO_x) and volatile organic compounds (VOCs). These substances react in the presence of sunlight to create secondary pollutants like ozone and peroxyacyl nitrates (PANs), which constitute photochemical smog.

Step-by-Step Explanation:

Step 1: Understanding Photochemical Smog Formation

Photochemical smog forms when certain primary pollutants react in the atmosphere under the influence of sunlight. The key initial ingredients for this complex chemical process are:

• Nitrogen Oxides (NO_x): These are primary pollutants primarily emitted from the combustion of fossil fuels in vehicles, power plants, and various industrial processes.
• Volatile Organic Compounds (VOCs): These primary pollutants come from sources like vehicle exhaust, industrial solvents, paints, and even natural vegetation.

When sunlight strikes these primary pollutants, a series of complex chemical reactions occur. These reactions produce secondary pollutants, which are the main components of photochemical smog. The most notable secondary pollutant formed is Ozone (O₃), specifically ground-level ozone, along with other harmful compounds like peroxyacyl nitrates (PANs) and various aldehydes.

Step 2: Evaluating the Given Options

• Sulfur dioxide (SO₂): This is a primary pollutant primarily associated with the formation of industrial smog (often called "gray smog" or "London smog") and acid rain. It is not a key primary pollutant responsible for the formation of *photochemical* smog.
• Particulate matter (PM): Particulate matter consists of tiny solid or liquid particles suspended in the air. While PM can be a component of smog and contribute to reduced visibility, it is not a primary pollutant that chemically reacts to *form* photochemical smog. Its role is more physical than chemical in this context.
• Ozone (O₃) and volatile organic compounds (VOCs):
  • Volatile organic compounds (VOCs) are indeed primary pollutants and crucial precursors in the formation of photochemical smog.
  • However, Ozone (O₃) itself is a secondary pollutant. It is not directly emitted into the atmosphere in significant quantities. Instead, it is formed *during* the photochemical reactions from its precursors (NO_x and VOCs). Therefore, listing Ozone as a "primary pollutant responsible for formation" is inaccurate.
• Nitrogen oxides (NO_x): Nitrogen oxides are definitively primary pollutants, directly emitted from combustion sources. They are one of the two crucial precursor groups (along with VOCs) that react in the presence of sunlight to initiate and drive the formation of photochemical smog.

Step 3: Concluding the Most Accurate Answer

For the formation of photochemical smog, both nitrogen oxides (NO_x) and volatile organic compounds (VOCs) are indispensable primary pollutants. Given the provided options, and the precise definition of a "primary pollutant responsible for formation":

• Option C is problematic because Ozone is a secondary pollutant, not a primary one, despite VOCs being correct.
• Option D, nitrogen oxides (NO_x), is unequivocally a primary pollutant that is directly and significantly responsible for the formation of photochemical smog.

Therefore, focusing strictly on "primary pollutant responsible for formation" among the given choices, nitrogen oxides (NO_x) is the most accurate answer.