Which of the following provides the strongest evidence that HCFCs are present in the stratosphere?

• A. Detection of HCFCs in tropospheric rainwater
• B. Measurement of increased HCFC concentrations in urban air near ground level
• C. Detection of HCFC degradation products, such as ClO radicals, in stratospheric air samples
• D. Observation of HCFCs in ocean water samples

Answer: (C)

Verified step by step guidance

1. Understand the context: HCFCs (hydrochlorofluorocarbons) are compounds that can reach the stratosphere and contribute to ozone depletion through their breakdown products.
2. Recognize that direct detection of HCFCs in the stratosphere is difficult because they are present in very low concentrations and may degrade before measurement.
3. Identify that the strongest evidence for HCFC presence in the stratosphere comes from detecting their degradation products, such as chlorine monoxide (ClO) radicals, which are formed when HCFCs break down under UV radiation.
4. Note that detecting HCFCs in tropospheric rainwater, urban air near ground level, or ocean water samples only shows their presence in lower atmospheric or surface environments, not necessarily in the stratosphere.
5. Conclude that measuring ClO radicals in stratospheric air samples directly links HCFCs to stratospheric chemistry and ozone depletion, providing the strongest evidence of their presence there.