locations since there are no other NECA countries reporting on NOx non-compliance in an operational setup. However, it is worth noting that numerous other agencies conduct measure- ments of NOx levels in addition to monitoring SO2 concentra- tions in OGV exhaust plumes. While a direct comparison of NOx non-compliance data may not be possible yet, these additional measurements provide valuable insights into the overall emissions pro?le and environmental impact of OGVs. The examination of the Belgian data reveals that the mean NOx emissions are not decreasing as anticipated with the implementa- tion of stricter emission limits. On the contrary, the data indicates that average NOx emissions are increasing24,28. Furthermore, non-compliance levels for NOx emissions are also rising24,28. This trend can be attributed to the higher emission levels reported for Tier II OGVs compared to Tier I OGVs24,28,29. Based on the Belgian data, the observed increase in average NOx emission coincides with an increase in the amount of measured Tier II vessels (Fig. 3). These ?ndings have important implications for the parameterization of atmospheric emission models—such as the Steam Model30,31—which are fundamental sources for global emission inventories for shipping. By incorporating the correct NOx emission factors based on the real-world emission factors per IMO tier, more accurate global assessments of NOx emissions from OGVs can be achieved, thereby improving the under- standing of their environmental and human health impact. The Danish company Explicit took a different approach to the Belgian one by using modeling. They estimated main engine power and fuel consumption as input for the calculation of NOx emission factors in grams of NOx per kilowatt-hour (g NOx/ kWh)29. Explicit used this approach for reassessing the Danish historic NOx measurement data. The ?ndings of this study align with the results of the empirical approach of Belgium, revealing Fig. 2 Non-compliance function of distance to port/SECA border. Non-compliance ?tting based on the mean non-compliance of the different remote measurement locations in function of the distance to the SECA border, for all remote measurements (A) and for when only the airborne measurements are considered (B). Non-compliance ?tting in function of the distance to port (C). Difference in compliance rates between Baltic Sea and North Sea SECA, with the number of measurements per SECA (D). Fig. 3 Annual average NOx emission factor in function of the proportion of Tier II OGVs. NOx emission factors expressed in g/kWh in function of the proportion (%) of Tier II OGVs, error bars visualize standard error (based on the Belgian airborne dataset collected between 2020 and 2022)24. The Y-axis range shows the minimum and maximum emission limits for respectively Tier II and Tier I. ARTICLE COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7 4 COMMUNICATIONS EARTH & ENVIRONMENT | (2023) 4:391 | https://doi.org/10.1038/s43247-023-01050-7 | www.nature.com/commsenv