SO2 and NOx emissions from ships in North-East Atlantic waters:in situ measurements and comparison with an emission model

Measurements of apparent fuel sulfur content in ship exhausts (aFSC) and NOx/CO2 ratios were made from an airborne and ground-based platforms in the open Atlantic Ocean and the European sulfur emission control area (SECA) during multiple field campaigns from 2019 to 2023. In the open ocean a nearly 10-fold decrease in the mean aFSC demonstrates the strong impact the International Maritime Organization regulation change in 2020 had on sulfur emissions from ships. In 2019, 8 ships out of 19 showed a measured aFSC higher than the 3.5% limit at the time and in 2021 and 2022, 5 ships out of 78 were observed to be higher than the new 0.5% limit. In the SECA in the English Channel, the average aFSC across both 2019 and 2021 measurements was 0.04 ± 0.01%, well below the more stringent 0.1% limit. In the port of Valencia, Spain, which is not in a SECA, observed aFSC was on average much lower than in the open ocean and close to the EU Sulfur directive of 0.1% fuel sulfur content in port areas if the ship stays more than 2 hours in port. In the Port of Tyne (within the European SECA), the aFSC is virtually identical to those measured in the English Channel, with no ships breaching the 0.1% limit. On average, measured aFSCs agree well with the estimates of the Ship Traffic Emission Assessment Model (STEAM3), although the model does not pick up outliers that breach limits. In the open ocean in 2019 the NOx/CO2 ratio was 0.021 ± 0.002, with ratios observed in port significantly lower (Port of Tyne 0.009 ± 0.001, Port of Valencia 0.011 ± 0.001), with a switch to auxiliary engines in ports a potential reason for this lower emission ratio. This work presents the first aircraft-based measurements of aFSC from ships outside of sulfur control zones since the change in sulfur emission regulations in 2020 and largely justifies the assumption that is often made that ships now emit around 7 times less sulfur than before 2020.