Recent global regulations have significantly reduced sulfur emissions from ships, helping to improve air quality in coastal regions – confirmed by a new international study led by researchers at the National Centre for Atmospheric Science.
The research, published in Environmental Science: Atmospheres, used aircraft and ground-based instruments to measure sulfur dioxide and nitrogen oxides emitted by ships in the North-East Atlantic and European coastal waters between 2019 and 2023.
The team found that the average sulfur content in ship fuel dropped nearly tenfold in open ocean areas following the International Maritime Organization’s 2020 regulation, which capped sulfur content in marine fuel at 0.5%.
Before the change, many ships exceeded the previous 3.5% limit. After 2020, only a small number of ships were found to breach the new standard.
In European Sulfur Emission Control Areas (SECAs), such as the English Channel and the Port of Tyne, sulfur levels were even lower – well below the stricter 0.1% limit. Interestingly, ports outside these zones, like Valencia in Spain, also showed low sulfur levels, likely due to EU rules requiring cleaner fuel when ships are docked for extended periods.
This is the first study to use aircraft-based measurements and predictions from the Ship Traffic Emission Assessment Model (STEAM3) to assess ship emissions outside of sulfur control zones since the 2020 regulation came into effect. The findings support the widely held view that ships now emit around seven times less sulfur than before the rule change – an important step toward cleaner air and healthier coastal environments.
Professor James Lee and Professor Hugh Coe spoke to us about the study:
What motivated this research? Was there a specific concern or gap in knowledge you wanted to address?
Large amounts of sulfur in the atmosphere can be detrimental for people’s health and the environment. It’s important to understand where sulfur emissions come from and how they behave, because it helps us make better predictions about air quality and our changing climate. Shipping is the most important human source of sulfur in marine environments, largely found as sulfur dioxide, and is estimated to be responsible for around 13% of global sulfur emissions.
– Professor Hugh Coe, atmospheric composition research scientist at NCAS and the University of Manchester
“There was a large reduction in the maximum allowed sulfur emission from ships in 2020, so we wanted to measure emissions before (in 2019) and after the change (in 2021 and 2022) to see what the effect was.” – Professor James Lee, an atmospheric chemist at NCAS and the University of York.
Why use aircraft and land-based platforms for measuring ship emissions? What advantages did this approach offer?
"Regulations are different in ports and inland waterways compared to the open ocean so using an aircraft was the only way to get the open ocean measurements. To our knowledge this is the only study of its type."– Professor James Lee
“Because of their location, often a long distance from land, emissions from ships are difficult to study. Compared to ports, inland waterways, or special sulfur control zones, ships actually spend most of their time in the open ocean. This is also where they produce most of their emissions.” – Professor Hugh Coe
How significant is the reduction in sulfur emissions since the 2020 IMO regulation change? Can you describe what this means for air quality and marine ecosystems?
“It’s a big reduction, largely in line with the change in regulations. It will undoubtedly improve air quality in regions close to shipping, although the reduction in sulfur might have an unintended global cooling effect due to the change in ship plumes interacting with clouds.” – Professor James Lee
How do these findings compare with expectations or predictions made before the regulation took effect? And what might explain why some ships still exceed the sulfur limits?
It’s worth noting that, in the open ocean, 5 ships out of 78 breached the 0.5% sulfur content limit in 2021 and 2022. That’s quite a substantial proportion of the observed fleet. While it’s reasonable to assume the fleet’s average is below the 0.5% limit, there could be a significant number of outlier ships breaching that limit, potentially having a large effect on local air quality.
"The limited sample size makes it difficult to draw any conclusions about what might cause a ship to breach the limit e.g. its age or its tonnage, so more measurements would be needed to investigate the cause of these outliers." – Professor Hugh Coe
“I think it was generally expected that the regulations would be adhered to and this research shows evidence that this is the case. I’m not sure why some ships still exceed the limits – it could be that exhaust scrubbing systems are not working very well or that high sulfur fuel is still being used in some cases.” – Professor James Lee
Can you explain in simple terms what the NOx/CO₂ ratio tells us about ship emissions? Why do emissions vary between locations like the open ocean, SECA zones, and ports?
The ratio tells us how efficiently ship engines are removing NOx compared to other vehicles. The lower ratio in ports is probably because ships are using auxiliary engines rather than their main engines, and these probably run on different and cleaner fuel. There are also different regulations in the different regions. In general SECA zones and ports have more stringent regulations, which the ships were largely keeping to. – Professor James Lee
“This study was limited to the Eastern North Atlantic Ocean, English Channel and two European ports, so measurements in other parts of the world, especially in the open ocean, would be required to see if this result is representative of the global shipping industry.” – Professor Hugh Coe
What’s the biggest takeaway from the research, and what can policymakers do to support cleaner shipping practices?
We found that there is fairly good compliance with the new sulphur regulations, both in SECA zones and the open ocean. This is the first experimental evidence of this in the open ocean. Air quality in ports and coastal regions is going to become more of an issue going forward as emissions from road transport reduce. There is no obvious way to move away from combustion for shipping, although alternative fuels are being thought of (such as liquefied natural gas and ammonia).
Policymakers need to keep a careful eye on this to make sure that any change that might be beneficial from a carbon reduction point of view does not have unexpected and adverse air quality implications. – Professor James Lee
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