Which gas has a dual effect, being beneficial in the upper atmosphere while harmful in the lower atmosphere?

Ozone is a gas that has a unique dual role in the atmosphere, which is why it is the correct answer. In the upper atmosphere, particularly in the stratosphere, ozone plays a crucial role in protecting life on Earth by absorbing the majority of the sun's harmful ultraviolet (UV) radiation. Without this layer of ozone, increased UV radiation would lead to higher rates of skin cancer, cataracts, and other health issues, as well as detrimental effects on ecosystems.

Conversely, in the lower atmosphere, or troposphere, ozone is considered a harmful pollutant. It is formed by chemical reactions between volatile organic compounds (VOCs) and nitrogen oxides in the presence of sunlight. Ground-level ozone is a key component of smog and can cause respiratory problems, exacerbate asthma, and reduce lung function, making it hazardous to human health and the environment.

This duality of ozone highlights its complex nature as a molecule that can be both protective and harmful depending on its location in the atmosphere. The other gases listed do not exhibit this beneficial versus harmful dynamic in the same way, which solidifies ozone's unique status in atmospheric chemistry.