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, NO x , NH 3 and N 2 O from ammonia combustion could impact 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 NO x , NH 3 and N 2 O emissions from ammonia-powered ships with two possible engine technologies (NH 3 -H 2 (high NO x , low NH 3 emissions) vs pure NH 3 (low NO x , very high NH 3 emissions) combustion) under three emission regulation scenarios (with corresponding assumptions in emission control technologies), and simulate their air quality impacts using GEOS-Chem high performance global chemical transport model. We find that the tailpipe N 2 O emissions from ammonia-powered ships have climate impacts equivalent to 5.8% of current shipping CO 2 emissions. Globally, switching to NH 3 -H 2 engines avoids 16 900 mortalities from PM 2.5 and 16 200 mortalities from O 3 annually, while the unburnt NH 3 emissions (82.0 Tg NH 3 yr -1 ) from pure NH 3 engines could lead to 668 100 additional mortalities from PM 2.5 annually under current legislation. Requiring NH 3 scrubbing within current emission control areas leads to smaller improvements in PM 2.5 -related mortalities (22 100 avoided mortalities for NH 3 -H 2 and 623 900 additional mortalities for pure NH 3 annually), while extending both Tier III NO x standard and NH 3 scrubbing requirements globally leads to larger improvement in PM 2.5 -related mortalities associated with a switch to ammonia-powered ships (66 500 avoided mortalities for NH 3 -H 2 and 1200 additional mortalities for pure NH 3 annually). 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.