because it helps us make better predictions about air quality and our changing climate. Shipping is the most important human source of sulphur in marine environments, largely found as sulphur dioxide, and is estimated to be responsible for around 13% of global sulphur emissions,” said Professor Hugh Coe, atmospheric composition research scientist at NCAS and the University of Manchester.
Professor James Lee, an atmospheric chemist at NCAS and the University of York, commented, “There was a large reduction in the maximum allowed sulphur 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.”
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,” said Professor 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 sulphur control zones, ships actually spend most of their time in the open ocean. This is also where they produce most of their emissions,” added Professor Coe.
How significant is the reduction in sulphur emissions since the 2020 IMO regulation change? Can you describe what this means for air quality and marine ecosystems?
Professor Lee said, “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 sulphur might have an unintended global warming effect due to the change in ship plumes interacting with clouds.”
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 sulphur limits?
Commented Professor Coe, “It’s worth noting that, in the open ocean, 5 ships out of 78 breached the 0.5% sulphur 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.”
THE REPORT | MAR 2026 | ISSUE 115 | 147
Professor Lee added, “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 sulphur fuel is still being used in some cases.”
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,” said Professor 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,” said Professor Coe.
What’s the biggest takeaway from the research, and what can policymakers do to support cleaner shipping practices?
The final word goes to Professor Lee, who said, “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.”
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