Thermodynamic evaluation of contrail formation from a conventional jet fuel and an ammonia-based aviation propulsion system

Condensation trail (contrail) formation in an airplane’s wake requires thermodynamics supersaturation and ice nucleation to form visible ice crystals. Here, using a thermodynamic analysis, we evaluate the potential for forming contrails in a carbon-free, ammonia-powered propulsion system compared to conventional planes powered by jet fuel. The analysis calculates the moisture released by fuel into the atmosphere for each one-degree increase in air temperature due to exhaust gas. It then determines if this moisture can saturate the initially undersaturated atmosphere, maintain saturation as temperature rises, and result in supersaturation with respect to ice while leaving enough moisture for a visible cloud to form. With ammonia increases the critical temperature required for supersaturation. Although ammonia does not generate soot particles in the exhaust gas, various aerosols exist in the atmosphere through other sources that can facilitate heterogeneous ice nucleation. Hence, while ammonia’s contrails might not be as dense, they can form at lower altitudes where the air is warmer and endure longer due to the increased water content, which preserves supersaturation for longer as fresh air dilutes the contrail. Trevor Cannon and colleagues evaluate the impact of a carbon-free, ammonia-powered propulsion system on contrail formation during flight. The report suggests that there are benefits compared to the use of conventional jet fuel from reduced soot formation. However, the increased critical temperatures caused by burning ammonia result could lead to increased volumes of more enduring contrails at lower altitudes.