Mixing State, Morphology, and Chemical Composition of Ship-Emitted Soot Revealed by UAV-Based Plume Observations

Ship-emitted soot exerts substantial climate forcing, yet its effects remain uncertain because models often oversimplify particle mixing state and structure. Key microphysical properties of ship-emitted soot under real-world operating conditions remain poorly constrained. Here we deployed an unmanned aerial vehicle to intercept ship exhaust plumes and applied electron microscopy to characterize the mixing state, size, and fractal dimension (D_f) of 5714 individual soot particles. Fresh ship-emitted soot was dominated by partly-coated soot (62.4%), followed by bare-like (29.3%) and embedded soot (8.3%). Vanadium and nickel were detected mainly in the coatings of internally mixed soot. Partly-coated soot showed the largest modal diameter (443 nm) and a lower compactness (D_f = 1.85), whereas embedded soot was the smallest (278 nm) and most compact (D_f = 2.34). This overall size-morphology pattern remained broadly similar across operational modes. Higher engine loads were associated with larger and less compact soot aggregates, and lower-sulfur fuels appeared to suppress particle growth. These results provide the first in situ constraints for improving representation of ship-emitted soot in climate models and support targeted emission controls to mitigate climate effects.