Modeling atmospheric dispersion of ammonia over water and implications for in-water fate and trajectory

Ammonia is increasingly being considered as an alternative to fuel oils for powering vessels and as a carrier for hydrogen. A deeper understanding of the environmental and human health risks of unplanned ammonia releases will further inform this promising contribution to efforts to decarbonize shipping. Integral plume models, including Phast, have been previously applied to evaluate ammonia dispersion in air. However, most studies modeled terrestrial releases. In this study, Phast was used to model volatilization, transport, and atmospheric exposure of ammonia hypothetically released from a bunkering hose over water at a representative location along the Gulf Coast of the United States. The results indicated that fate and exposure estimated by Phast are dependent on how the release is configured in the model, with a resulting change in affected area of 0.1 km² on average between model modes (∼16% difference of total affect area). Overall, Phast estimated that 71-90% of the ammonia mass would dissolve into the water rather than volatilize into the air. Phast results indicated there would be >1% probability of 0.016-0.083 km² exceeding human health hazard thresholds from a hypothetical 5-min release over water, representative of typical unplanned releases. For a larger, catastrophic release, Phast estimated 0.6 to 7.8 km² would be affected. There are insufficient water-side field studies with which to validate Phast model results, particularly the estimations of air-sea partitioning. Such atmospheric modeling is needed for human health risk assessment and to provide input to in-water dispersion models that estimate fate and effects of spilled ammonia.