A Clean Alternative or a New Environmental Challenge? A Perspective on Downstream Emissions and Environmental Trade-Offs of Ammonia-Diesel as a Low-Carbon Future Fuel

The use of ammonia (NH 3 ) as a carbon-free fuel in non-road engines, e.g. for maritime traffic or agricultural machines, emerges as an option to reduce carbon dioxide (CO 2 ) emissions. However, the environmental consequences of large-scale NH 3 combustion extend beyond CO 2 mitigation and remain insufficiently explored. This perspective briefly discusses the benefits and drawbacks of different NH 3 combustion technologies and highlights key challenges, including combustion byproduct formation, incomplete NH 3 conversion, secondary aerosol formation, and potential toxicological impacts for NH 3 with diesel pilot fuel injection. NH 3 combustion may introduce reactive nitrogen species and organic byproducts that affect atmospheric chemistry, air quality, and public health. The interaction of NH 3 -derived emissions with biogenic and anthropogenic pollutants adds complexity to their environmental fate. Given these uncertainties, early stage assessments of emissions, atmospheric transformations, and their downstream effects are mandatory. By integrating combustion science, atmospheric chemistry, and toxicology, a proactive approach is suggested to identify and mitigate unintended consequences before NH 3 becomes a dominant energy carrier.