Spatial analysis. For the spatial analysis, average SO2 and NO2 levels were compared between different areas. The North Sea ECA was divided into three zones: (1) the Northern part of the ECA; (2) the BAQPZJR and; (3) the English Channel. In addition, a fourth zone was added outside the SECA in the Bay of Biscay (Supplementary Fig. 14). The Baltic Sea was not included in the spatial satellite data analysis due to the substantial in?uence of land-based sources and the lack of satellite coverage for the winter months in the northern Baltic Sea. Temporal analysis SO2. Several studies provided scienti?c evi- dence of the impact of the global COVID-19 pandemic on ambient SO2 levels, although this mainly concerned inland SO2 pollution, pollution levels over sea were less impacted63,67. Additionally, the implementation of the global sulfur cap in 2020 marked a turning point for SO2. Consequently, the temporal data analysis for SO2 excluded the year 2020 to ensure unbiased comparability. For the temporal analysis of SO2, ?rst the SO2 VCD maps per month (i) for the period before (2018–2019) and after (2021–2022) the global sulfur cap came into effect were combined in a monthly mean SO2 VCD map. As the year 2020 was omitted, the period before and the period after the global sulfur cap implementation composed the same amount of months (21). SO2Period VCD? ? ? ? 21 i SO2i 21 VCD? ? ?2? This data demonstrated substantial seasonal variability in pollution levels (Supplementary Fig. 7). The years 2018 and 2022 could not be used as they did not contain data for the full year. Therefore, the annual SO2 VCD maps of 2019 and 2021 were calculated. In the second step, the proportional difference between these two maps was calculated to create the proportional difference between the annual SO2 VCD. Diff SO2 %? ? ? SO22021  SO22019 SO22019 ?3? Temporal analysis NO2. In 2022, Ward Van Roy et al. used TROPOMI data to evaluate the impact of the implementation of the NECA28. That analysis was limited to the determination of the absolute difference in NO2 VCD between 2020 and 2021. This indicated a potential decrease of NO2 in the northern part of the North Sea SECA, but an increase in the southern part. However, by looking at the absolute difference, areas with high pollution levels are more prone to be highlighted. In addition, the analysis was limited to the years 2020 and 2021, which were impacted by the global COVID-19 pandemic28,67,68. Riess et al. reported a reduction of observed NO2 concentrations in shipping lanes, between 10 and 20% as a result of the global COVID-19 pandemic69. For these reasons, a wider analysis was required that incorporated the relative impact of the NOx regulations on the overall NO2 pollution levels. The NOx temporal analysis in this study focused on two analyses, an annual proportional difference and a monthly proportional difference. It must be acknowledged that, due to limited satellite coverage throughout the year the Baltic Sea ECA could not be fully assessed. For the annual proportional difference, a similar analysis was conducted as what was performed for SO2, while for NO2, data for the complete 2022 was obtained. With 2021 being the turning point year, with the introduction of the NECA, the years 2019 and 2022 were compared. DiffNO2 %? ? ? NO22022  NO22019 NO22019 ?4? As initially a potential seasonal effect was observed (Supplemen- tary Fig. 10), an average monthly proportional difference was also calculated. First, monthly mean VCD maps were created for the period before (BFNC) and after (AFNC) the NECA came into effect. The BFNC period was composed of the months fromMay 2018 until December 2020 (32 months), and the AFNC period was composed of the months from January 2021 until December 2022 (24 months). Thus, for every month (i) two or three years (j) were available. NO2i period VCD? ? ? ? years j NO2j years VCD? ? ?5? This provided 12 NO2 maps before (BFNC) and 12 NO2 maps after (AFNC) the NECA came into force. In the second step, the proportional difference between these maps was calculated per month. DiffNO2i %? ? ? NO2i AFNC  NO2i BFNC NO2i BFNC ?6? In the ?nal step, the average monthly proportional difference was calculated DiffNO2 %? ? ? ? 12 i DiffNOi 12 %? ? ?7? To provide an analysis throughout the entire 2018–2022 period, the years 2020 and 2021 were not omitted. As NO2 levels in 2021 were lower compared to 2020 (Supplementary Fig. 13), this almost certainly creates a negative bias for the period after the NECA came into force. It must also be acknowledged that local concentrations of NO2 are affected by the lifetime through background levels of NO2 itself, O3, and available sunlight70. As these vary in time, this will in?uence uncertainty in temporal comparisons. This dynamic is investigated in detail by Riess et al., showing that this will in?uence the calculated changes in emissions69, not VCD levels used for this study. Meteorological conditions, driving dispersion, will also show temporal variability in?uencing the extent to which the study areas will be impacted by land-based sources or inversely drive ship emissions outside of the study areas. Such effects are not accounted for in this study but are expected to be minor given the year-to-year comparison. Data availability The anonymized full Belgian airborne monitoring dataset is available on the repository: https://doi.org/10.24417/bmdc.be:dataset:2687. The raw remote measurement data of the other Bonn Agreement countries is not provided as the data is legally owned by the authorities of the relevant countries and can therefore not be distributed. The port inspection results were provided by EMSA; however, the original data can not be made available due to legal concerns. The annual results of the remote monitoring efforts of the BA CPs, the inspection results of the EU MS and the sanctions and violations observed by the BA CPs are available on the repository: https://surv.naturalsciences.be/d/ 76719a8375fd409ebe5f/. The TROPOMI satellite data and geo-data are available on the repository: https://surv.naturalsciences.be/d/04c1441989684255b6ed/. Code availability The code used for the satellite analysis is available on the repository: https://surv. naturalsciences.be/d/3b8de56010584c39ac4b/. Received: 28 June 2023; Accepted: 9 October 2023; ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7 14 COMMUNICATIONS EARTH & ENVIRONMENT | (2023) 4:391 | https://doi.org/10.1038/s43247-023-01050-7 | www.nature.com/commsenv