What does opacity measure in relation to stack plume?

Opacity measures the reduction of light transmission through a stack plume, meaning it quantifies how much light can pass through the emitted gases and particulates. When emissions are released from a stack, they can contain a mixture of pollutants, including particulate matter and gas, which scatter and absorb light. A higher opacity indicates that more light is blocked, suggesting a greater concentration of suspended particles or pollutants within the plume. This measurement is critical in environmental monitoring because it can indicate the level of pollution being emitted into the atmosphere, helping authorities determine compliance with air quality standards.

The other options, while they may relate to attributes of emissions or stack operation, do not directly connect to the specific role that opacity plays in assessing the visibility and purity of a plume. Temperature relates to the thermal characteristics of the gases, moisture content pertains to the level of water vapor present, and pressure levels are indicative of the physical dynamics of the gases being emitted but do not capture the concept of light reduction affecting visibility.