Question

Which of the following atmospheric pollutants is known to contribute to the formation of ozone in troposphere?

• A. Sulfur dioxide (SO$_2$)
• B. Oxides of nitrogen \& volatile organic compounds
• C. Carbon monoxide (CO)
• D. Carbondioxide (CO$_2$)

Hint:

• Tropospheric ozone (ground-level ozone) is a secondary pollutant formed by photochemical reactions.
• The main primary pollutant precursors are:
  • Nitrogen Oxides (NO$_x$)
  • Volatile Organic Compounds (VOCs)
• These react in the presence of sunlight to produce ozone.
• This process is central to the formation of photochemical smog.

Answer 1

The Correct Option is B

Ozone (O$_3$) in the troposphere (ground-level ozone) is a secondary air pollutant, meaning it is not directly emitted but is formed through chemical reactions in the atmosphere involving primary pollutants. The formation of tropospheric ozone is a key component of photochemical smog.
The primary pollutants that contribute to its formation are: 
Option (A) Oxides of Nitrogen (NO$_x$): This includes nitric oxide (NO) and nitrogen dioxide (NO$_2$). Major sources are combustion processes, such as in vehicle engines and industrial facilities. 
Option (B) Volatile Organic Compounds (VOCs): These are a diverse group of carbon-based chemicals that evaporate easily at room temperature. Sources include vehicle exhaust, industrial emissions, solvents, paints, and natural emissions from vegetation. In the presence of sunlight (UV radiation), NO$_x$ and VOCs undergo a complex series of photochemical reactions that lead to the formation of ozone. A simplified overview of the cycle involves:

• NO$_2$ + Sunlight (UV) $\rightarrow$ NO + O (atomic oxygen)
• O + O$_2$ (molecular oxygen) $\rightarrow$ O$_3$ (ozone)
• VOCs react with hydroxyl radicals (OH) and NO to regenerate NO$_2$ and produce other pollutants like peroxyacyl nitrates (PANs), thus fueling the ozone production cycle.

Option (b) "Oxides of nitrogen & volatile organic compounds" correctly identifies the key precursor pollutants. Other options: Option (a) Sulfur dioxide (SO$_2$): Primarily contributes to industrial smog and acid rain, not directly to tropospheric ozone formation in the same way as NOx and VOCs. Option (c) Carbon monoxide (CO): While CO can participate in atmospheric chemistry and indirectly influence ozone levels by reacting with OH radicals (thus affecting the VOC oxidation pathways), NOx and VOCs are the direct and primary precursors. Option (d) Carbon dioxide (CO$_2$): A major greenhouse gas, but not a direct precursor for tropospheric ozone formation. \[ \boxed{\text{Oxides of nitrogen \& volatile organic compounds}} \]