contrast, the results of the inquiry on NOx enforcement and legal follow-up within the BA were disappointing (Fig. 4B). Only two BA CPs have reported NOx infringements and only one BA CP has imposed a penalty for a NOx violation. Most of the other BA CPs are currently not enforcing NOx regulations nor collecting data on the results of the NOx inspections at the time of publication. This demonstrates that enforcement of NOx regula- tions by BA CPs is currently lacking. Upon examining the limited available NOx inspection data, it becomes evident there has not been a decrease in violations since the NECA was implemented, but rather, an increase. However, the scarcity of data does not allow statistical analysis or strong conclusions to be drawn about compliance rates within the BA. Results within the EU. Upon examining the data on sulfur inspections and non-compliance rates within the EU, similar patterns were observed within the Baltic Sea and North Sea ECA as within the BA (Fig. 5A). In the wider SECA, in total 110,657 documentary inspections were conducted. The annual amount showed a slight increase since entering into force in 2015, with a relatively stable trend over the entire period, except for a small decline in 2020. This increase was mainly a result of the increased number of inspections by the North Sea ECA countries, while the Baltic Sea countries had a more stable number of conducted inspections throughout the entire period. The non-compliance rate based on documentary inspections followed a similar trend as the number of infringements in the BA. However, it is important to note that this pattern is largely in?uenced by a noteworthy reduction in non-compliance in the North Sea, while the reduction in the Baltic Sea is less pronounced. Also, when looking at the compliance results outside the SECA, the reduction was less pronounced. The overall non-compliance rate in the Fig. 5 Overall non-compliance results for the EU. Non-compliance for EU MS in the North Sea and Baltic Sea based on Thetis-EU data on documentary inspections (A) and fuel sample analysis (B). ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7 6 COMMUNICATIONS EARTH & ENVIRONMENT | (2023) 4:391 | https://doi.org/10.1038/s43247-023-01050-7 | www.nature.com/commsenv