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

Photochemical smog (also known as Los Angeles-type smog) is a complex mixture of air pollutants formed when primary pollutants react in the atmosphere in the presence of sunlight. The key primary pollutants involved are:

• Nitrogen Oxides (NO$_x$): Primarily nitric oxide (NO) and nitrogen dioxide (NO$_2$), mainly from vehicle exhaust and industrial combustion.
• Volatile Organic Compounds (VOCs): A wide range of carbon-based chemicals that evaporate easily, including hydrocarbons. Sources include vehicle exhaust, industrial emissions, solvents, and natural emissions from vegetation.

These primary pollutants undergo a series of complex photochemical reactions initiated by sunlight, leading to the formation of secondary pollutants, which constitute photochemical smog. The most prominent secondary pollutants are:

• Ozone (O$_3$): Ground-level ozone is a major component and a strong oxidant.
• Peroxyacetyl Nitrate (PAN) and other peroxyacyl nitrates.
• Aldehydes.
• Secondary particulate matter.

The question asks for "primary pollutant responsible". Both NO$_x$ and VOCs are primary pollutants. Option (c) lists "Ozone (O$_3$) and volatile organic compounds (VOCs)". Ozone is a secondary pollutant (a product of the reactions), while VOCs are primary pollutants. Option (d) lists "nitrogen oxides (NO$_x$)", which is also a primary pollutant. This question is slightly ambiguously worded if it's asking for *only* primary pollutants. However, photochemical smog is characterized by high levels of ozone, which is formed from reactions involving VOCs and NOx. If the question implies "key ingredients and products," then ozone and VOCs (and NOx) are all central. Given the options: Option (a) Sulfur dioxide (SO$_2$) is a primary pollutant associated with industrial smog (London-type smog) and acid rain, not primarily photochemical smog. Option (b) Particulate matter (PM) can be primary or secondary. While secondary PM is part of photochemical smog, it's not the sole primary precursor in the way NOx and VOCs are. Option (c) Ozone (O$_3$) is a major component and product of photochemical smog (a secondary pollutant), and VOCs are key primary pollutants (precursors). This option combines a product with a precursor. Option (d) Nitrogen oxides (NO$_x$) are key primary pollutants (precursors). Often, photochemical smog is described as being formed from NOx and VOCs, resulting in ozone, PAN, etc. If the question is interpreted as "Which of these options best describes the chemical soup involved, considering both precursors and key products specific to photochemical smog?", option (c) might be chosen because ozone is the hallmark secondary pollutant, and VOCs are essential primary precursors. If the question strictly means "primary pollutants that are responsible", then both VOCs (from option c) and NOx (option d) are correct. Since only one option can be correct and (c) includes the main product that defines photochemical smog (ozone) along with a primary precursor (VOCs), it might be considered the intended answer in some contexts. The image highlights (c) as correct. This implies the question is likely looking for a combination of a key resultant pollutant (ozone) and a key precursor (VOCs). Alternatively, if the question intends to ask which combination of substances are central to photochemical smog formation and its composition, then (c) highlights ozone (the main characteristic secondary pollutant) and VOCs (a key primary pollutant). \[ \boxed{\text{Ozone (O}_3\text{) and volatile organic compounds (VOCs)}} \]