Negative buoyancy of an airborne COTA can exist when:

Negative buoyancy of an airborne COTA can exist when:

• A. The COTA cloud is denser than air
• B. NOT just COTA cloud denser than air
• C. The surrounding air is hotter
• D. The COTA cloud is lighter than air

Answer: (B)

Negative buoyancy means the plume’s density relative to the surrounding air is higher, causing it to sink. The density of a COTA cloud isn’t just about the gas phase; the total mixture density includes particles and any entrained air. So you can get sinking even if the gas in the cloud isn’t denser than ambient air, because heavy particle loading increases the overall density of the plume. Likewise, if the surrounding air is hotter, its density drops, which lowers ρ_air and makes it easier for the plume to be denser than its environment and thus negatively buoyant. In short, negative buoyancy can arise from more than just the cloud being denser than air, which is why the statement that it isn’t limited to that scenario is the best fit.

Negative buoyancy means the plume’s density relative to the surrounding air is higher, causing it to sink. The density of a COTA cloud isn’t just about the gas phase; the total mixture density includes particles and any entrained air. So you can get sinking even if the gas in the cloud isn’t denser than ambient air, because heavy particle loading increases the overall density of the plume. Likewise, if the surrounding air is hotter, its density drops, which lowers ρ_air and makes it easier for the plume to be denser than its environment and thus negatively buoyant. In short, negative buoyancy can arise from more than just the cloud being denser than air, which is why the statement that it isn’t limited to that scenario is the best fit.