and 2022 was determined (Fig. 9). This map shows varying results on the proportional difference in NO2 VCD levels throughout the North Sea NECA, with an annual proportional increase of +14.4% in the Bay of Biscay; +4.1% in the English Channel; and +1.0% in the Northern part and of the NECA. A decrease of ?5.8% was observed in the highest polluted BAQPZJR area. This analysis does however not include seasonal differences, which could result in an over- or underestimation of the NO2 pollution trends. The comparison of the monthly NO2 VCD levels did indeed reveal a seasonal effect in certain areas, particularly in the BAQPZJR. Consequently, to mitigate the potential for a seasonal bias and to get a better understanding of the ambient NO2 VCD trends throughout the year, a monthly proportional difference analysis was conducted. For this monthly analysis, the propor- tional difference between the period before and after the implementation of the NECA was calculated for each month (Supplementary Fig. 12). This analysis yielded variable results. In the months of January, February, May, July, August, September, October, and December, there was a limited impact with localized variations with either an increase or decrease. On the other hand, March and June showed an overall increase, while April and November demonstrated an overall decrease in NO2 VCD levels. Subsequently, the proportional difference maps per month were combined to create an average monthly proportional NO2 difference map (Fig. 10A). This map demonstrates a slightly different picture compared to the annual proportional difference. Also here a NO2 increase, albeit slightly lower, is observed for the Bay of Biscay (+10.3%) and the English Channel (+4.0%). However, a small decrease was observed for the Northern NECA (?1.4%). For the BAQPZJR, the monthly analyses demonstrate a similar reduction (?5.0%) as for the annual analysis. The differences with the annual analysis can be attributed to the in?uence of seasonal variability and the inclusion of the years 2020 and 2021, which were affected by the global COVID-19 pandemic. The evolution of the average NO2 VCD over 2018–2022 clearly demonstrates the effect of the global COVID-19 pandemic. The impact was most substantial in the Northern NECA zone (?44%), the BAQPZJR (?19%), and the English Channel (?9%). The Bay of Biscay (+18%) was not impacted by the COVID-19 pandemic (Supplementary Fig. 13A). Additionally, signi?cant increases were observed in the Mediterranean Sea, and Atlantic Ocean, with increases in the average monthly NO2 VCD levels of up to 20%. Due to a lack of data for the winter months in the north of the Baltic Sea, a full- year assessment could not be made. Nevertheless, the data that is available for the months of January–December indicates a slight reduction (Fig. 10B). In conclusion, these analyses con?rm that the ambient NO2 levels throughout the year either increased after the NECA implementation or where they decreased, the decrease was less substantial at sea compared to inland. Discussion Through the analysis of over 110,000 remote measurements of OGV emissions spanning a duration of seven years, valuable insights were obtained regarding the FSC compliance behavior of OGVs in the European ECAs. The results indicate a consistent decline in FSC non-compliance rates across the SECAs. However, it is important to remain vigilant as remote measurements suggest Fig. 9 Spatiotemporal analysis—impact NECA, annual approach. Annual proportional difference of NO2 VCD levels between 2019 and 2022. 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 9