Ammonia marine fuel can reduce or increase reactive nitrogen pollution depending on emissions controls

Ammonia used as a marine fuel can lower carbon emissions from shipping, but may substantially increase reactive nitrogen pollution through releases of nitrous oxide, nitrogen oxides, and ammonia. Here, we provide first-order global estimates of reactive nitrogen losses across the ammonia marine fuel value chain under low, medium, and high emission scenarios. Total reactive nitrogen emissions could be approximately 66 percent lower or up to approximately 185 percent higher than those from current marine fuel oil for the same energy output, driven mainly by uncertainty in ammonia leakage during production, storage, and bunkering and by engine-dependent emission factors. In medium- and high-emission scenarios without strict controls, ammonia marine fuel could become a major reactive nitrogen source by mid-century, contributing to climate forcing, air quality deterioration, and ecological impacts near ports and shipping lanes. Rapid leak detection, capture of boil-off gases, and verified low-emission engine operation are essential. Coordinated governance is also required to secure climate benefits while limiting harm. Reactive nitrogen from ammonia marine fuel can fall by 66% or rise by 185% compared with fuel oil, depending on leakage during production, storage, and bunkering and on engine emissions, according to a global analysis of the fuel supply chain.