When can negative buoyancy of an airborne COTA exist?

Negative buoyancy of an airborne Chemical Ozone Transport Assessment (COTA) can occur under specific conditions related to temperature and molecular weight. When a substance is cooler than the surrounding air (lower temperature than ambient), it becomes denser and tends to sink, resulting in negative buoyancy. Additionally, if the molecular weight of the airborne substance is greater than that of air, it also contributes to negative buoyancy, as heavier substances will tend to fall rather than rise.

Therefore, both having a lower temperature than the ambient air and having a greater molecular weight than air contribute to creating conditions for negative buoyancy. When these two factors are present, they can combine to cause the airborne substance to decrease in altitude or remain suspended in the lower layers of the atmosphere instead of rising.