Which chemical in the upper stratosphere is an indicator of ozone layer depletion?

Chlorine monoxide is a key indicator of ozone layer depletion because it plays a significant role in the chemical processes that damage ozone molecules in the stratosphere. When chlorine compounds, particularly those found in chlorofluorocarbons (CFCs), are released into the atmosphere, they eventually reach the stratosphere. There, they are broken down by ultraviolet radiation, releasing chlorine atoms.

Once released, chlorine can react with ozone (O3), leading to the formation of chlorine monoxide (ClO) and molecular oxygen (O2). This reaction is part of a cycle that significantly reduces the concentration of ozone molecules, contributing to thinning of the ozone layer. The presence of chlorine monoxide in the atmosphere can be directly linked to the activities that release CFCs, making it a valuable indicator of potential ozone depletion.

Other substances listed, such as carbon dioxide and methane, do not directly indicate ozone layer health and are more associated with climate change and greenhouse gas effects. Ozone itself, while critical to understanding atmospheric chemistry, is a product of the reactions influenced by chlorine and is not an indicator of depletion in the same manner. Hence, chlorine monoxide serves as a specific marker of ozone-depleting processes occurring in the upper stratosphere