Anthony Y. H. Wong, Noelle Eckley Selin , Sebastian David Eastham, Christine Mounaïm-Rouselle, Yiqi Zhang, Florian Allroggen

As carbon-free fuel, ammonia has been proposed as an alternative fuel to facilitate maritime decarbonization. Deployment of ammonia-powered ships is proposed as soon as 2024. However, emissions of NOx, NH3 and N2O resulting from ammonia combustion could cause impacts on air quality and climate. In this study, we assess whether and under what conditions switching to ammonia fuel might affect climate and air quality. We use a bottom–up approach combining ammonia engine experiment results and ship track data to estimate global tailpipe NOx, NH3 and N2O emissions from ammonia-powered ships with two possible engine technologies (NH3–H2 vs pure NH3 combustion) under three emission regulation scenarios (with corresponding assumptions in emission control technologies). We then use the GEOS–Chem High Performance global chemical transport model to simulate the air quality impacts of switching to ammonia-powered ships. We find that the tailpipe N2O emissions from ammonia-powered ships have climate impacts equivalent to 5.8% of current shipping CO2 emissions. Globally, switching to NH3–H2 engines avoids 33,100 (18900 to 47300, 95% confidence interval) mortalities annually, while the unburnt NH3 emissions (82.0 Tg NH3 yr-1) from pure NH3 engines could lead to 595,100 additional mortalities annually under current legislation. Requiring NH3 scrubbing within current Emission Control Areas leads to smaller improvements in public health outcomes (38,000 avoided mortalities for NH3–H2 and 554,200 additional mortalities for pure NH3 annually, respectively), while extending both Tier III NOx standard and NH3 scrubbing requirements globally leads to larger improvement in public health outcomes associated with a switch to ammonia-powered ships (79,100 and 21,100 avoided mortalities for NH3–H2 and pure NH3 annually, respectively). Our findings suggest that while switching to ammonia fuel would reduce tailpipe greenhouse gas emissions from shipping, stringent ammonia emission control is required to mitigate the potential adverse effects on air quality.