shipping sector. Therefore, this study strongly recommends the revision of current NOx regulations, the implementation of additional NOx emission regulations and decisive and effective enforcement measures in the North Sea and Baltic Sea. Furthermore, the increased NO2 VCD in the Bay of Biscay, the Mediterranean Sea and Iberian coast are highly notable. These ?ndings strongly emphasize the importance of establishing a Mediterranean NECA in conjunction with the Mediterranean SECA, which is entering into force on May 1, 202442. The introduction of a North Atlantic NECA encompassing the waters of France, Spain, and Portugal is also recommended. Based on the analysis conducted on the existing European NECAs, it should be emphasized that the introduction of additional NECAs without enhanced emission regulations would not result in the anticipated improvements in air quality. The ?ndings from this study emphasize the need to review the current air quality models that include ship emissions31. These models should accurately incorporate both the real-world emis- sion factors re?ecting actual ship emissions and the observed compliance levels at sea, accounting for spatial variations within the ECAs. By updating these models, valuable insights could be gained on the health impact of shipping, which could serve as a driving force for policymakers to reconsider and improve the prevailing international regulatory framework. Methods Research area Bonn Agreement. The BA is a regional cooperation established in 1969 to prevent marine pollution from OGVs in the North Sea. Initially, this focused on oil pollution. The BA consists of 11 CPs including all North Sea coastal States, Spain, Ireland and the EU (represented by EMSA)43. Among others, through intensive cooperation, the BA recorded a signi?cantly reduced amount of oil pollution over the last two decades44–46. Over the years, the BA was expanded to include other harmful substances besides oil. From 2015, several BA CPs petitioned the BA to extend the scope of the BA to include air pollution from shipping. In October 2019, the Ministerial Meeting of the BA formally approved the exten- sion of the scope of the BA to include MARPOL Annex VI, recognizing the BA as the appropriate intergovernmental forum to roll out OGV emission monitoring activities43,47. North Sea and Baltic Sea emission control area. The establishment of the ECAs was included in the adoption of the MARPOL Annex VI regulations in 200815. After several years of negotiations, the North and Baltic Sea SECAs were established in 2011, in addition to ECAs in North America and the Caribbean Sea17. The SECA covers the North Sea, the English Channel, and the Baltic Sea. The European NECAs were established in 202118. The North and Baltic Sea NECAs cover the same area as the SECAs and are therefore both referred to as ECAs. The North and Baltic Sea ECAs cover a vast sea area, spanning from the English Channel to the Russian border, impacting air quality for over 280 million European citizens. Remote measurement data in the Bonn Agreement Remote monitoring locations. To obtain a comprehensive assess- ment of the impact of maritime SO2 emission regulations in the European SECAs, annual compliance monitoring data from ?ve ?xed monitoring sites and nine airborne monitoring operations from six BA CPs in the SECA were analyzed (Supplementary Table 2), encompassing a total of 115,274 OGV measurements. Some deployments have been terminated, others are still ongoing, data from operations that are still ongoing were obtained from: (1) MUMM operates the Belgian coastguard aircraft equipped with a sniffer sensor from 2015; (2) BSH (Hamburg, Germany) is in charge of the German network of ?xed sniffer stations from 2015, with stations in Hamburg, Kiel and Bremerhaven; (3) TNO (Delft, The Netherlands) operates a ?xed sniffer station at the port of Rotterdam in The Netherlands on behalf of the Human Environment and Transport Inspectorate (ILT) from 2015; (4) Explicit executes RPAS measurements with dual mini-sniffers on behalf of EMSA in Denmark, France, Germany, Lithuania and the Spain29,48–57 from 2019. Data from following terminated opera- tions was used: (1) Chalmers University (Goteborg Sweden) operated an airborne sniffer and a ?xed sniffer station at the Great Belt bridge in Denmark on behalf of the Danish Environ- mental Protection Agency (EPA) from 2015 to 2020; (2) Explicit (Virus, Denmark) conducted airborne measurements using heli- copters equipped with dual mini-sniffers, and uses single mini- sniffers on RPAS, in Denmark on behalf of the Danish EPA from 2017 to 2022; (3) Explicit, conducted dual-sniffers measurement with a helicopter in the Netherlands in cooperation with ILT in 2016. Out of the total measurements taken, the majority (101,464) were conducted using ?xed stations along frequently navigated shipping lanes. This was followed by airborne measurements carried out by aircraft (8210), and subsequently by RPAS and helicopter measurements using mini-sniffers (4732). The Belgian airborne dataset contributed to more than half of the total number of airborne measurements (6961). However, the number of measurements conducted by any of the ?xed sniffer sites greatly surpasses the number of measurements conducted by the airborne monitoring platforms. For determining average non- compliance rates within the SECA, a weighted average was calculated based on the number of measurements per station or deployment. Compliance cutoff levels. Three different cutoff levels were used to assess possible violations: 0.20% FSC, 0.15% and 0.13% FSC. These three cutoff levels were used to facilitate the evaluation of variations in non-compliance levels across different scales, as a direct comparison of average FSC levels was not feasible due to the unavailability of raw measurement data. In addition, although remote monitoring stations and platforms employ similar tech- niques, slight differences in measurement methodologies and uncertainties exist among the different stations. Nonetheless, the data used in this study remains suitable for conducting a com- parative analysis of general temporal and spatial compliance trends. Selection of data for temporal analysis. Several stations or plat- forms only measured a low number of vessels for certain years. A minimum of 100 operational measured OGVs per year was applied for the temporal compliance analysis. As a result, in total seven annual compliance results were omitted from four different locations. Spatial trend analysis. For the selection of the measurement sites for the spatial analysis, a minimum continued measurement period of two years and a total of 200 measured OGVs was applied. Locations that did not meet this requirement were either omitted or added to the location of the nearest other remote measurement location. The RPAS data from Lithuania (142 measurements in 2021) was therefore omitted. The data from the different airborne assets in Denmark were merged to one loca- tion. Furthermore, the airborne mini-sniffer measurements of the Netherlands in 2016 were added to the Belgian airborne measurements. For the spatial analysis, in addition to the SECA measurements, measurements conducted at the southern ECA border were used. ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7 12 COMMUNICATIONS EARTH & ENVIRONMENT | (2023) 4:391 | https://doi.org/10.1038/s43247-023-01050-7 | www.nature.com/commsenv