These measurements were conducted by Chalmers University (114 in 2016) and the Belgian coastguard aircraft (23 in 2022). As suf?cient data was not available for temporal analysis and as these measurements were conducted with the same sensor technology, the data at the SECA border was combined for the spatial analysis. The emission data was ?tted on an S-curve in function of the distance to the SECA border (db) and the distance to port (dp). Non compliance %? ? ? ?k  p? 1? eo?dxm? ? p ?1? with: k ? highNon compliance rate ?%? p ? lowNon compliance rate ?%? o ? Non compliance increase=decrease rate ?%? m ? midpoint distance ?km? dx ? distance to SECAborder=port The weighted average non-compliance rates were used for the factors k and p, the least square method was applied for the determination of the factors o and m. Inspection results of port inspection authorities in the Bonn Agreement. In accordance with the EU Sulphur Directive and the Commission Implementing Decision, the port state authorities of the EU MS conduct sulfur inspections35,36. These are done by either documentary inspection or by analyzing the fuel in accordance with the fuel inspection guidelines from IMO58. Within the scope of this research and the work conducted under the BA, inspection and sanctioning results from port State authorities from 9 out of 10 BA CPs were obtained. This data is part of the annual inspection results that are reported to the EC. The most CPs apply a 0.15% threshold for reporting infringe- ments to the EC. Of all BA CPs, two are located outside the ECA (Spain and Ireland) and two are currently not reporting to the EC (Norway and the United Kingdom). In addition to the sulfur inspection results, NOx inspection results were also collected. As NOx is currently not regulated through an EU directive, inspec- tion results are currently not being shared with the EC. EU inspection results and Thetis-EU. The EU Sulphur Directive led to the creation of Thetis-EU, an online database used for exchanging inspection results. EMSA manages and hosts the database. Thetis-EU is accessible to inspectors across all EU MS, including Norway and Iceland. However, due to Brexit, the UK no longer has access to the database36,59. Keel laying date. Information on the KLD is required for the determination of the tier level when assessing NOx compliance. For the Belgian NOx remote monitoring assessment, this KLD data was acquired based on merging two database sources: (1) the Global Integrated Shipping Information System (GISIS) of the IMO60 and; (2) Thetis-EU of EMSA. The GISIS database was ?rst used to gather information on OGVs larger than 75 meters with construction year. In the second step, EMSA provided the accu- rate KLD28. For the Danish analysis, a ship database was acquired from IHS Markit. Inspection results. Under the EU Member States’ obligations to report inspection outcomes to the EC as stipulated by the EU Sulphur Directive and Commission Implementing Decision35,36, data on sulfur inspections conducted by the EU MS were obtained from EMSA59. As most, but not all, BA CPs are part of the EU, there is a certain overlap with the BA data. The EU data contains more States and includes all EU MS outside the SECAs. The EMSA data therefore gives a broader overview of the inspection results throughout the EU. However, the EU data does not contain results on penalties. Moreover, due to the lack of EU regulation regarding NOx emissions from OGVs, the EMSA data does not contain NOx inspection results. The EMSA website publicly displays the amount of port inspections and compliance levels resulting from documentary inspections conducted by EU MS59. Its purpose is to enable EMSA to deliver thorough reports to the EC to assess the implementation of the EU Sulphur Directive by the EU MS35,36. Additionally, the EMSA website includes records of non- compliance detected through fuel sample analysis. However, the actual number of fuel samples themselves is not available on the website. A request for this information was made to EMSA to facilitate the analysis of temporal trends in fuel sample results. Statistical analysis of remote monitoring data and port inspection results. Previous studies using the Belgian airborne data have already shown that emission measurements deviate from a normal distribution24,26,28. When the distribution of remote measurement data is compiled, they initially appear to follow a normal pattern, with the emission limit as the central point. However, although small negative values are occasionally observed, there are no highly negative values, while very high FSC values are possible. As a result, this inevitability renders the distribution non-normal. Pearson chi-square tests were used to assess the difference in compliance rate between two locations/ deployments, with statistical signi?cance de?ned as P < 0.0561. Satellite analysis. A spatiotemporal analysis was conducted using satellite data from the Tropospheric Monitoring Instrument (TROPOMI) Sentinel 5 to investigate the distribution and changes in SO2 andNO2 levels over European waters. The temporal analysis of SO2 focused on the evaluation of the impact of the global sulfur cap, which entered into force in 202062, while the temporal analysis of NO2 focused on the implementation of the European NECAs in 202118. Additionally, a spatial analysis compared SO2 and NO2 pollution levels between different areas within the ECAs and dif- ferences between the ECAs and regions outside the ECA. TROPOMI data. Data was gathered from TROPOMI on board the Copernicus Sentinel-5 Precursor satellite, which is operated by the European Space Agency (ESA). The satellite data contains measurements of the SO2 and NO2 VCD in the lower atmo- spheric layer (up to 80 km). For SO2, the retrievals from the scienti?c COBRA V01 scheme processed by BIRA-IASB were used63 for the period May 2018 until September 2022. For NO2, the satellite operational data product (Level 2 data) was collected from the Copernicus Open Access Hub64 for the period May 2018 until December 2022, using the PAL v2.3.1 retrieval algo- rithm. With the NO emissions converted to NO2, factors such as ambient meteorological conditions, O3, and solar radiation in?uence the conversion speed. However, conversion is con- sidered to be in the time span of seconds to tens of minutes during daytime65,66. Therefore, the NO2 satellite analysis gives a good representation of NOx pollution levels. TROPOMI retrievals for SO2 and NO2 have been ?ltered based on their quality assurance (QA) value. Only pixels with a QA value equal to or larger than 0.75 were selected, removing cloudy pixels (cloud radiance fraction > 0.5) and erroneous retrievals64. Subsequently, they were averaged to generate monthly VCD products. The monthly average VCD products were further compiled using ArcGIS and Qgis to generate VCD maps for spatial and temporal analysis. 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 13