higher emission factors and a greater non-compliance rate for Tier II OGVs compared to Tier I OGVs. The Danish study also con?rmed that OGVs emit more NOx when operating at lower engine loads. Additionally, the results demonstrated that larger engines generate higher emission factors, which is in line with the Belgian measurements, albeit with a weak correlation. An increasing trend was observed across all Tier levels, excluding Tier III due to limited measurements (Supple- mentary Fig. 5). Non-compliance with NOx standards has also recently been investigated within the SCIPPER project. A particular emphasis was placed on Tier III OGVs. The advantage of the enforcement of Tier III OGVs is that a not-to-exceed limit is de?ned for all four engine load points, set at 50% of the applicable emission limit (Appendix II, MARPOL Annex VI)15. However, because the keel laying date (KLD) is de?ned in the MARPOL Annex VI regulations to determine Tier III classi?cation, the large majority (73%) of the recently constructed OGVs are registered with a KLD prior to 2021. Consequently, they are subjected to the Tier II emission limits instead of the stricter Tier III emission limits24. In total 65 Tier III OGVs were monitored by the SCIPPER partners. The ?ndings indicated that approximately half of the observed Tier III OGVs did not comply with the maximum NOx emission limits for Tier III; ca 20% of the observed Tier III OGVs did not even meet Tier II emission limits32. This observation aligns with the limited Tier III non-compliance results reported by Belgium, where a non-compliance rate of 43% was observed. Various other studies have also highlighted concerns regarding elevated levels of NOx emissions from Tier III OGVs33,34. Port inspections on sulfur and NOx infringements Results within the Bonn Agreement. The results of the sulfur infringements from most BA CPs follow an increasing trend between 2015 and 2020 (Supplementary Fig. 6A). The primary reason for this is that not all CPs immediately implemented inspection protocols; needed to gain experience; and had initially only limited information available to single out suspicious OGVs for inspection. As a result, not all CPs have inspection results for 2015. From 2016, all BA CPs were actively conducting inspections within their ports. During this time, remote monitoring opera- tions and the exchange of alerts via Thetis-EU began to gain momentum, leading to the discovery of a higher number of infringements and de?ciencies. Due to a high number of observed sulfur infringements by one BA CP, the total observed number of infringements in the years 2015 and 2016 still provided the highest number of observed infringements (243 and 223) (Fig. 4A). The year 2018 provided the third highest number of recorded infringements (178). However, following that year, the number of identi?ed infringe- ments began to decline. It is important to note that the EU Sulphur Directive mandates Member States (MS) to provide port inspection data by June, as a result, at the time of publication, not all CPs were able to submit data for the year 2022. The EU-Commission Implementing Decision played an important role in maintaining a consistent number of inspections conducted on OGVs throughout the entire time period. Although there was a decrease in inspections in 2020 due to the global pandemic, the majority of CPs were still able to ful?ll the mandatory inspection requirements. It is worth noting that in this context, numerous CPs utilized the exemption outlined in the Implementing Decision to reduce the number of inspections by implementing remote monitoring (Art 3.3(a))35. Regarding the reported penalties on sulfur, an upward trend was observed between the years 2015 and 2017, reaching a peak of 126 cases in 2017 (Supplementary Fig. 6B). Subsequently, the number of penalties declined. It should be noted that there is a time lag in the reporting of penalties, as often the reported penalties correspond to infringements observed in the previous year. Therefore, the peak in penalties in 2019 aligns with the peak of infringements in 2018. To address this time lag, it is necessary to analyze the data from the original cases and assign them to the year of observation. However, this analysis was not feasible due to the sensitive nature of the legal cases involved. When looking at the mean number of sulfur de?ciencies and infringements observed by the BA CPs’ port inspection authorities, a substantial decrease was observed after the global sulfur cap came into effect (Fig. 4A). Over the total period 2015–2022, 996 infringements were observed of which 544 were penalized. In the period 2015–2020, before the global sulfur cap came into effect, 885 infringements were observed by the port inspection authorities with a mean of 21.6 cases per year, 442 penalties were executed in the same period or on average 10.8 penalties per year, corresponding to 56% of the infringements. In the period 2020–2022, after the global sulfur cap came into force, a total of 111 infringements were observed. The mean annual number of observed de?ciencies per BA CP decreased therefore signi?cantly to 4.8 cases per year (P < 0.001). In total, 102 penalties were handed out after the global sulfur cap came into effect (91%). The mean number of penalties per BA CP per year therefore decreased signi?cantly to 4.3 penalties (P < 0.05), which is just below the mean number of observed infringements, indicating that as of today there is a good legal follow-up of possible infringements within the BA. There is a notable disparity between sulfur and NOx. The result of the inquiry with the BA CPs provided proof of the successful enforcement and legal follow-up for sulfur infringements. In Fig. 4 Number of observed infringements and penalties in the Bonn Agreement. Infringements observed by port inspection authorities for SO2 (A) and NOx (B). Note that for 2022 not all BA CPs were yet able to provide data. 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 5