At what ozone concentration level are CRJ operations prohibited above flight level 320?

The correct choice is based on the regulations and operational guidelines concerning aircraft operations in certain atmospheric conditions. For CRJ operations above flight level 320, the limitation is specifically related to ozone concentrations, which can negatively affect both aircraft performance and crew health.

Ozone at high altitudes can lead to increased respiratory issues for crew members and may also impact the aircraft's systems. The restriction at the 0.25 parts per million level is established as a safety measure to ensure the well-being of the crew and passengers, as well as to maintain optimal aircraft performance.

This threshold represents a balance between operational efficiency and safety, ensuring that pilots are not exposed to levels of ozone that could impair their ability to operate the aircraft effectively or impact the aircraft’s systems. Therefore, operational limits are precisely defined to avoid any potential risks related to atmospheric conditions, particularly in terms of ozone.

Other concentration levels listed either do not conform to the safety guidelines or represent thresholds that would allow operations that could introduce risk factors related to ozone exposure.