Climate Impact of Direct and Indirect N₂O Emissions from the Ammonia Marine Fuel Value Chain

Ammonia (NH₃) emerges as a near-zero-carbon shipping fuel, but its value chain can emit noncarbon greenhouse gases (GHGs) like nitrous oxide (N₂O) and pollutants, such as NH₃ and nitrogen oxides (NO). N₂O, with 273 times the global warming potential of carbon dioxide (CO₂), is also indirectly emitted through transformations of reactive nitrogen (N_r) species, like NH₃ and NO, in natural environments. Understanding ammonia's climate impact is crucial for evaluating its efficacy as a fossil fuel alternative. We present a functional model to quantify the climate impact of renewable-based (e-ammonia) and natural-gas-based (blue and gray) ammonia, focusing on direct and indirect N₂O emissions. The model incorporates N_r emissions─NH₃, NO, and N₂O─across the value chain under "low", "medium", and "high" emissions scenarios, along with production and pilot fuel GHG emissions. Results show that e-ammonia's climate benefit depends on well-to-wake N_r emissions and natural processes that control relevant nitrogen to indirect N₂O (N-to-N₂O_i) conversion pathways. In a low emissions scenario, its CO₂-equivalent intensity is 68-80% lower than fuel oil, assuming 1-10% N-to-N₂O_i conversion. In a high emissions scenario, this benefit drops to 11-23% for 1-2% N-to-N₂O_i conversion, and at 5-10% N-to-N₂O_i conversion, e-ammonia's impact exceeds fuel oil by >24%. Blue ammonia could bring ∼30% climate benefit compared to fuel oil, but only in a low emissions scenario with 1-2% N-to-N₂O_i conversion. While more data on emissions and N-to-N₂O_i conversion are needed, minimizing NH₃, NO, and N₂O emissions is crucial to maximizing ammonia's climate benefits.