氨船用燃料价值链中直接和间接N2O排放的气候影响

Oi3−), and5,9r reducecarbon emissions,22itdoes not guarantee a low-emissions value chain. Combusting ammonia releases N2O,NOx, and unburned fuel (NH3slip), with emissions varyingbasedon engine configuration,fuel mixture,combustiontemperatureand efficiency,and exhaust gas treatmenttechnology.23−25Additionally,total Nremissionsare notlimited to direct emissions during the combustion stage butalso include indirect pathways that release N2O (N2Oindirect).26For example, NH3engine slip and other NH3releases to theenvironment throughout the value chain, such as leaks, spills,and vented boil-off gas (BOG), are naturally removed by waterbodies and atmospheric moisture and deposited on terrestrialandaquatic surfaces,where they undergo chemical andbiologicaltransformations into N2O(and other Nrspe-cies).5,8,27−31Similarly, NOxspecies can be converted intoN2O in soil and water bodies.32The Nrto N2Oindirect(N-to-N2Oi) conversion across variousenvironments is still relatively poorly understood,33especiallyin marine and estuary ecosystems. Key parameters determiningthis conversion include Nrspecies concentration, microbialcommunity composition, and environmental conditions.34−37Current data on N-to-N2Oiconversion, primarily derived fromagricultural studies, estimate that 0.1−7.3% (depending oncontext) of the nitrogen in NH3-based fertilizers is convertedintoN2O,32,38−40withsome measured rates as high as21%.32,40−42Additionally, volatile NH3emissions that enterthe atmosphere can react with OH radicals, converting∼1% ofthe nitrogen in ammonia into N2O, driving∼8% of globalanthropogenic N2O emissions.3,6Overall, given that ammoniafuel can still produce GHGs, it is important to consider a rangeof possible conversion factors to understand the scenarios inwhich ammonia could be a climate-beneficial alternative fuel.Recent studies addressing direct and indirect N2O emissionsindicate that if 0.4% of the nitrogen in ammonia marine fuel(AMF) were to become N2O (directly and/or indirectly), theclimate benefits of switching from current fuels to low- andhttps://doi.org/10.1021/acs.est.4c13135Environ. Sci. Technol.2025, 59, 9037−9048 shipping fuel to support international climate goals, as it is aGHG 273 times more potent than CO2and persists in theatmosphere for over a century.10TheHaber−Bosch(H−B)process is the conventionalmethod to manufacture ammonia by combining molecularhydrogen (H2) with atmospheric nitrogen. H2productionprimarily relies on natural gas (NG) or coal, generating largeamountsof CO2emissionsand other climate and airpollutants. Consequently, conventional ammonia productionusing fossil-based hydrogen (termed gray ammonia) generates1.5−2.6 kg CO2-equivalents (CO2e) per kg ofNH3andaccounts for >1.2% of global anthropogenic GHG emissions,based on a 100 year global warming potential (GWP).11−14Recent evidence suggests that methane (CH4) emissions fromthe NG supply chain and the H−B process may be significantlyunderreported, which would increase the upper range of CO2eemissions considerably.15Efforts are underway to develop amore efficient, lower-emission H−B process to decrease theclimate impacts of ammonia production (due to direct andindirect GHGs).10,16,17These efforts involve integrating H2production from fossil fuels with carbon capture and storage(CCS) (termed blue H2) or from water electrolysis poweredby surplus or additional renewable energy (termed green H2),along with other renewable-based alternatives (e.g., electro-chemical ammonia synthesis).2,18Using additional renewableelectricity ensures that existing sources are not diverted fromother uses, preventing increased reliance on fossil fuels andassociatedemissions.19,20Moreover,for blue ammonia�produced from blue H2�to be low-emissions, both CH4andH2(an indirect GHG) emissions across their supply andutilization chains must be minimized, while CCS must ensurehigh CO2capture efficiency and long-term storage.20,21Whileproducing low-emissions ammonia(e.g.,blueammonia with mitigated CH4, H2, and CO2emissions) ornear-zero-emissions ammonia produced entirely with renew-able energy (termed electro- or e-ammonia) can significantly near-zero-emissionsammonia would be offset entirely.3,43While these initial assessments, together with well-to-wake(wtw) climate impact estimates for ammonia (from productiontocombustion),44,45providevaluable context,they eitheraddress indirect N2O emissions broadly3,43or omit them,44,45underscoringthe need for a more granular investigationassessing both direct and indirect N2O contributions across thevalue chain. Such an investigation should provide insight intokey stages that have a disproportionate impact on the overallimpact and identify the conditions under which ammoniaappears to have a climate advantage relative to fossil fuels.Here, we developed a functional model to estimate theclimate-relevant N2O impact of AMF by incorporating directand indirect N2O emissions in the ammonia fuel supply andutilization chain. We used documented N2