Which of the following is a common human-made source of chlorine-releasing compounds that affect the ozone layer?

• A. Water
• B. Refrigerants
• C. Oxygen
• D. Nitrogen gas

Answer: (B)

Chlorine-releasing compounds from human-made sources drive ozone depletion because chlorine atoms catalytically destroy ozone in the stratosphere. Refrigerants, such as chlorofluorocarbons used in air conditioners and refrigerators, are a classic source. When these compounds rise high enough, ultraviolet light breaks them apart, releasing chlorine atoms or chlorine-containing radicals. A single chlorine atom can destroy many ozone molecules through cycles like: chlorine reacts with ozone to form ClO and O2, then ClO reacts with atomic oxygen to regenerate chlorine and produce more O2. This repeating process leads to substantial ozone loss even from small amounts of chlorine.

Water, oxygen, and nitrogen gas don’t contain chlorine, so they aren’t sources of chlorine-releasing compounds that drive ozone depletion (though they participate in other atmospheric reactions).