North Sea (7%) was found to be signi?cantly higher compared to the Baltic Sea (3%) (P < 0.001). The non-compliance rate within the SECA (5%) was markedly higher compared to the non- compliance rate outside the SECA (2%) (P < 0.001). In addition to the documentary inspections, in accordance with EU regulations35,36, fuel samples were collected by the EU MS (Fig. 5B). Besides a small reduction in the number of fuel samples collected in 2020 due to the global COVID-19 pandemic, the number of samples remained fairly consistent, with most EU MS providing a number above the mandatory requirement. When analyzing the inspection results from the fuel samples within the SECA, a signi?cant increase in non-compliance was observed in 2016 and 2017, followed by a drastic reduction toward 2020, which then stabilized. This trend was observed for both the North Sea and the Baltic Sea. However, there was a slight increase in non-compliance observed in the North Sea in 2022, aligning with the ?ndings from the remote monitoring operations in the BA. The North Sea non-compliance results of the fuel analysis (5%) were notably higher than the Baltic Sea (2%) (P < 0.001). The non-compliance trend of the fuel analysis outside the SECA also showed a substantial decrease by 2020, while the overall non- compliance rate (4%) was not found to be signi?cantly different from the overall non-compliance rate of the fuel analysis within the SECA (4%) (P= 0.9488). Spatiotemporal analysis of satellite data Spatial analysis of atmospheric SO2 data. Upon comparing the SO2 vertical column density (VCD)—expressed in molecules/ cm?—across the various regions (Fig. 6) for 2019 and 2021, notable ?ndings emerged. Speci?cally, the BA Quadripartite Zone of Joint Responsibility (BAQPZJR) exhibited the highest con- centrations of SO2 pollution within the ECA. Meanwhile, the Bay of Biscay displayed a much lower pollution pressure of SO2 (Supplementary Table 7). The implementation of the global sulfur cap is shown to have created a comparable reduction of SO2 pollution levels across the SECA. The region outside the SECA did not seem to be impacted. When looking at the period 2018–2022, for some areas an increase was observed (Supple- mentary Fig. 7). However, due to the absence of certain months in 2018 and 2022, this was attributed to seasonal effects. Temporal analysis of atmospheric SO2 data. From the start point of the satellite data in 2018, the overall emission levels of SO2 at sea were already relatively low, particularly in the SECA due to the implementation of the 0.1% FSC limit in 2015. Consequently, the SO2 VCD maps for 2019 and 2021, the respective years before and after the global sulfur cap came into effect, visualize widely dispersed concentration levels, although areas with high shipping activities can be, to some extent, identi?ed. (Supplementary Fig. 8). Accordingly, the proportional difference of SO2 pollution levels before and after the implementation of the global sulfur cap does not exhibit a distinct pattern (Fig. 7). When comparing the proportional difference in SO2 VCD after the implementation of the global sulfur cap amongst the different areas (Supplementary Fig. 9), the most substantial decrease was observed for the BAQPZJR (?22.5%), the northern part of the SECA (?15.9%) and the English Channel (?9.5%). The Bay of Biscay was less impacted by the global sulfur cap and even showed a negligent increase (+3.0%), most probably because this area already had a lower SO2 pollution pressure compared to the densely navigated waters of the SECA. However, there is also an indication that the sensitivity of the TROPOMI SO2 data might be insuf?cient to conduct a thorough analysis of SO2 pollution trends in areas with lower SO2 pollution levels. To conclude, the conducted spatiotemporal analysis indicated a positive in?uence of the global sulfur cap and other international and EU regulations on ambient SO2 concentrations in the European SECAs. The ?ndings are in line with the results obtained from the remote measurements and inspections conducted within the BA and the EU, therefore strengthening the validity and reliability of the ?ndings. However, it should be noted that when utilizing satellite images to assess air quality improvement for SO2 outside the ECAs, the analysis heavily relies on the shipping density and ambient SO2 pollution levels. Spatial analysis of atmospheric NO2 data. When comparing absolute NO2 VCD levels across different areas (Fig. 8), it was demonstrated that the NO2 VCD within the North Sea NECA is overall considerably higher compared to the areas outside the NECA. Particularly in the BAQPZJR and the English Channel, NO2 VCD levels are notably elevated, although there are some seasonal differences (Supplementary Fig. 10). However, it is important to acknowledge that the elevated NO2 VCD levels in these areas are likely to be in?uenced, to some degree, by industrial activities and other densely populated areas in the southern parts of the UK, northern parts of France, Flanders, and the Netherlands. On the other hand, Riess et al. provided evi- dence that the TROPOMI data primarily captures emissions within the ?rst 200 meters above sea37. In addition, despite possible other contributing factors, the monthly NO2 VCD Fig. 6 Impact of global sulfur cap on SO2. Box plot of annual SO2 VCD levels between different areas before (BFGC) and after (AFGC) the global sulfur cap entered into force in 2020, with minimum, 25% percentile, median, 75% percentile and maximum. The left plots include the maximum values, while the right plots give the 25–75% percentile range. COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7 ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | (2023) 4:391 | https://doi.org/10.1038/s43247-023-01050-7 | www.nature.com/commsenv 7