The chemical used as a measure of the oxidant level of the atmosphere at any given time is:

The chemical that serves as a measure of the oxidant level of the atmosphere is ozone. Ozone is a triatomic molecule consisting of three oxygen atoms and is known for its role in both the stratosphere and the troposphere. In the stratosphere, ozone forms a layer that protects living organisms from the sun’s harmful ultraviolet radiation. However, at ground level in the troposphere, ozone is considered a pollutant and a significant component of smog.

Ozone is formed through chemical reactions involving precursor pollutants, such as volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. This formation process makes ozone a key indicator of atmospheric oxidant levels, as its concentration reflects the combined influence of these pollutants and environmental conditions. As such, measuring ozone levels gives insight into the oxidative capacity of the atmosphere and the extent of photochemical reactions occurring within it.

This context is important when considering the other options. Nitrogen dioxide and sulfur dioxide are both significant air pollutants, but they do not serve as direct measures of the overall oxidant capacity of the atmosphere in the same way that ozone does. Likewise, carbon dioxide, while a critical greenhouse gas, is not indicative of atmospheric oxidant levels.