Environmental protection: Why have international agencies advised against the use of chlorofluorocarbon (CFC) refrigerants in HVAC and refrigeration systems?

Introduction / Context:
CFCs (chlorofluorocarbons) were once widely used as refrigerants because they are non-flammable, non-corrosive, and stable. However, environmental protection agencies worldwide, supported by international agreements such as the Montreal Protocol, moved to phase them out due to their severe impact on the ozone layer, which shields life from harmful ultraviolet-B radiation.

Given Data / Assumptions:

• CFC molecules are chemically stable in the lower atmosphere.
• Over years, they diffuse upward to the stratosphere.
• Under high-energy UV, they release chlorine radicals.
• Ozone depletion increases UV-B reaching Earth’s surface.

Concept / Approach:
The key mechanism is catalytic: a single chlorine radical can participate in repeated cycles converting ozone (O3) into oxygen (O2) without being consumed. This dramatically reduces ozone concentration, particularly in polar springtime, creating “ozone hole” conditions. The decision to eliminate CFCs stems from this long-lived, global-scale risk.

Step-by-Step Solution:

Verification / Alternative check:
Observed global declines in stratospheric ozone and recovery trends after the CFC phase-out corroborate the mechanism. Laboratory photochemistry and satellite measurements align with the catalytic depletion pathway.

Why Other Options Are Wrong:

Acid rain: primarily linked to SO2 and NOx, not CFCs.Greenhouse effect from plant reaction: greenhouse forcing is due to gas radiative properties, not plant chemistry; CFCs are potent greenhouse gases but the question focuses on ozone depletion.Reaction with oxygen: not a primary atmospheric pathway for CFCs.Surface smog: associated with NOx and volatile organics under sunlight, not stable CFCs condensing.

Common Pitfalls:
Confusing ozone depletion (stratospheric) with acid rain or urban smog (tropospheric). Also, assuming rapid breakdown of CFCs near the ground—CFCs are too stable there; the issue arises aloft.

Final Answer:

They reach the stratosphere and catalytically destroy ozone, thinning the ozone layer