Highly light-absorbing particle emissions from low-sulfur marine fuels

Why ship exhaust still matters for a warming world

As the world has tightened rules on sulfur pollution from ships, many hoped that cleaner marine fuels would sharply cut the climate impact of ocean traffic. This study shows that the story is more complicated: even fuels that meet today’s strict sulfur limits can emit dark particles that warm the atmosphere, and that warming effect largely survives as the exhaust mixes and reacts in air. Understanding this hidden side of “cleaner” fuels is crucial as Arctic shipping expands and global trade continues to rely on large diesel engines at sea.

Cleaner fuels, but not cleaner skies

International rules now severely limit sulfur in marine fuels to protect air quality and reduce acid rain. To comply, ship operators have turned to low-sulfur heavy fuel oil and marine gas oil instead of the dirtier, high-sulfur residual fuels used in the past. Sulfur cuts do reduce the emission of sulfate particles that scatter sunlight and help form bright clouds, which used to produce a small cooling effect. But ships also emit dark carbon particles—especially black carbon, or soot, and some brown carbon—that absorb sunlight and warm the air. The key question this paper addresses is whether low-sulfur fuels also reduce this heat-trapping component, and how the exhaust changes as it travels through the atmosphere.

Peering into ship exhaust particles

The researchers ran a medium-size marine research engine on two real-world fuels that meet current rules: a low-sulfur heavy fuel oil and a low-sulfur marine gas oil. They measured the tiny particles in the exhaust using a suite of instruments, including electron microscopes and advanced optical sensors. They also passed part of the diluted exhaust through a special reactor that simulates several days of exposure to sunlight and atmospheric chemicals, mimicking what happens as plumes drift away from the ship. This allowed them to compare “fresh” particles leaving the stack with “aged” particles that had undergone photochemical processing.

Dark soot dominates, then gets a shiny coat

At the point of emission, particles from both fuels were dominated by black carbon soot. Under the microscope, these appeared as irregular, lacy clumps, and measurements showed that they absorbed light much like bare soot does. The low-sulfur heavy fuel oil produced roughly three times as much black carbon per unit of engine power as the marine gas oil, especially at low engine loads. After simulated aging, the particles became more varied: many soot aggregates shrank and compacted while acquiring coatings of organic material and sulfate. For the marine gas oil, unusual rod- and needle-like particles also appeared, while the low-sulfur heavy fuel oil produced more spherical, sulfate-rich particles. These coatings increased the mass of non-black-carbon material by more than an order of magnitude.

How aging changes their impact on sunlight

These structural changes altered how the exhaust interacted with light. Coated soot acts like a dark core inside a clear lens: the shell bends extra light into the core, boosting absorption. The team found that after aging, each gram of black carbon absorbed about 20–60% more light than when freshly emitted. At the same time, added sulfate and organic material increased scattering, nudging the particles’ behavior slightly toward reflection. Even so, the particles remained strongly absorbing, with absorption outweighing scattering across realistic conditions. When the authors folded these measurements into a simple metric of “forcing efficiency”—how much the particles warm or cool per unit of engine energy—they found that emissions from both fuels generally produced net warming, especially over bright surfaces like snow and ice. Low-sulfur heavy fuel oil had 2–3.5 times higher warming efficiency than marine gas oil.

Why this matters for climate and policy

To a layperson, the main message is that cutting sulfur from ship fuels, while vital for health and the environment, does not automatically make shipping climate-friendly. The new fuels can still release large amounts of powerful warming particles, and sunlight-driven chemical aging does little to erase that warming potential. In some cases, low-sulfur heavy fuel oil can even be more climate-warming than older, high-sulfur fuels because it sheds much of the cooling sulfate while keeping or increasing soot. As Arctic routes open and global shipping grows, these findings suggest that future regulations will need to address black carbon directly and encourage truly low-soot alternatives—such as cleaner engines, different fuels, and new propulsion technologies—if we are to reduce the climate footprint of ships on the world’s oceans.

Citation: Kokkola, T., Sipkens, T.A., Paul, A. et al. Highly light-absorbing particle emissions from low-sulfur marine fuels. npj Clim Atmos Sci 9, 108 (2026). https://doi.org/10.1038/s41612-026-01369-w

Keywords: ship emissions, black carbon, marine fuel, aerosol aging, Arctic warming