Answer: During winter, polar stratospheric clouds (PSCs) convert inactive chlorine reservoirs to Cl₂ and HOCl; when spring UV arrives, these photolyse rapidly to Cl•, triggering explosive catalytic ozone destruction in the isolated polar vortex.
- A Spring UV intensity in Antarctica is uniquely high due to Earth's proximity to the Sun, photolysing ozone directly at maximum rates under typical physiological conditions according to standard texts in general clinical practice as frequently documented
- B Antarctica has major volcanic eruptions in spring that inject SO₂ into the stratosphere, activating CFC chlorine in most reference accounts under normal conditions as generally observed in typical laboratory settings under usual circumstances
- C During winter, polar stratospheric clouds (PSCs) convert inactive chlorine reservoirs to Cl₂ and HOCl; when spring UV arrives, these photolyse rapidly to Cl•, triggering explosive catalytic ozone destruction in the isolated polar vortex
- D The Southern Ocean releases CFC gases seasonally in spring, providing the chlorine source that was absent in winter according to most studies in the majority of documented cases as widely reported in standard reference material under most conditions studied
Correct answer: C. During winter, polar stratospheric clouds (PSCs) convert inactive chlorine reservoirs to Cl₂ and HOCl; when spring UV arrives, these photolyse rapidly to Cl•, triggering explosive catalytic ozone destruction in the isolated polar vortex
Explanation: During the Antarctic winter, temperatures drop below −78°C and the polar vortex isolates the stratospheric air mass. Polar stratospheric clouds (PSCs) form, and on their surfaces, reservoir species (HCl, ClONO₂) undergo heterogeneous reactions producing Cl₂ and HOCl. When spring sunlight returns, these are rapidly photolysed to Cl• radicals. Within the still-isolated polar vortex, Cl• destroys ozone explosively before mid-summer warming disperses the vortex and allows ozone-rich air from lower latitudes to mix in.
Classical smog is a reducing mixture of SO₂ and fog, while photochemical smog is an oxidising mixture generated by UV-driven reactions of NOx and hydrocarbons.
Concept context
Understand how human activities alter the atmosphere, water, and soil. Covers air pollutants, smog types, ozone depletion, acid rain, greenhouse effect, water and soil pollution, and the principles of green chemistry.