Abstract
Ammonia (NH(3)) emerges as a near-zero-carbon shipping fuel, but its value chain can emit noncarbon greenhouse gases (GHGs) like nitrous oxide (N(2)O) and pollutants, such as NH(3) and nitrogen oxides (NO(x)). N(2)O, with 273 times the global warming potential of carbon dioxide (CO(2)), is also indirectly emitted through transformations of reactive nitrogen (N(r)) species, like NH(3) and NO(x), 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 N(2)O emissions. The model incorporates N(r) emissions─NH(3), NO(x), and N(2)O─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 N(2)O (N-to-N(2)O(i)) conversion pathways. In a low emissions scenario, its CO(2)-equivalent intensity is 68-80% lower than fuel oil, assuming 1-10% N-to-N(2)O(i) conversion. In a high emissions scenario, this benefit drops to 11-23% for 1-2% N-to-N(2)O(i) conversion, and at 5-10% N-to-N(2)O(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-N(2)O(i) conversion. While more data on emissions and N-to-N(2)O(i) conversion are needed, minimizing NH(3), NO(x), and N(2)O emissions is crucial to maximizing ammonia's climate benefits.