Which element acts as a catalyst in repeatedly breaking down ozone molecules in the stratosphere?

Which element acts as a catalyst in repeatedly breaking down ozone molecules in the stratosphere?

• A. Nitrogen
• B. Chlorine
• C. Oxygen
• D. Hydrogen

Answer: (B)

Chlorine acts as a catalyst in stratospheric ozone destruction because it participates in reactions that break ozone into oxygen while being regenerated to do it again and again. In sunlight, a chlorine atom reacts with ozone to form chlorine monoxide and oxygen: Cl + O3 → ClO + O2. The ClO then reacts with a free oxygen atom to release another O2 and restore the chlorine atom: ClO + O → Cl + O2. The net effect is O3 + O → 2 O2, and the chlorine atom is free to destroy many more ozone molecules. This regenerative cycle is what makes chlorine such an effective catalyst for ozone depletion, especially since it’s released from long-lived human-made compounds like CFCs that reach the stratosphere and release chlorine radicals under UV light. While nitrogen oxides can also participate in ozone-destroying cycles, chlorine’s abundance and reactivity make it the dominant catalyst in this context. Oxygen and hydrogen don’t participate in the same persistent catalytic cycle for ozone breakdown in the stratosphere.

Chlorine acts as a catalyst in stratospheric ozone destruction because it participates in reactions that break ozone into oxygen while being regenerated to do it again and again. In sunlight, a chlorine atom reacts with ozone to form chlorine monoxide and oxygen: Cl + O3 → ClO + O2. The ClO then reacts with a free oxygen atom to release another O2 and restore the chlorine atom: ClO + O → Cl + O2. The net effect is O3 + O → 2 O2, and the chlorine atom is free to destroy many more ozone molecules. This regenerative cycle is what makes chlorine such an effective catalyst for ozone depletion, especially since it’s released from long-lived human-made compounds like CFCs that reach the stratosphere and release chlorine radicals under UV light. While nitrogen oxides can also participate in ozone-destroying cycles, chlorine’s abundance and reactivity make it the dominant catalyst in this context. Oxygen and hydrogen don’t participate in the same persistent catalytic cycle for ozone breakdown in the stratosphere.