COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7

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Fig. 9 Spatiotemporal analysis—impact NECA, annual approach. Annual proportional difference of NO» VCD levels between 2019 and 2022.

and 2022 was determined (Fig. 9). This map shows varying results
on the proportional difference in NO, 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 NO, pollution
trends.
The comparison of the monthly NO, 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 NO, 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 NO, VCD levels.
Subsequently, the proportional difference maps per month were
combined to create an average monthly proportional NO,
difference map (Fig. 10A). This map demonstrates a slightly
different picture compared to the annual proportional difference.
Also here a NO, 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
influence 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 NO, 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
‚mpacted by the COVID-19 pandemic (Supplementary Fig. 13A).
Additionally, significant increases were observed in the
Mediterranean Sea, and Atlantic Ocean, with increases in the
average monthly NO, VCD levels of up to 20%. Due to a lack of
data for the winter months in the north of the Baltic Sea, a fullyear
 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 confirm that
che ambient NO, 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

COMMUNICATIONS EARTH & ENVIRONMENT | (2023)4:391 | https: //dol.org /15. 1038/s43247-023-01050-7 | www.nature.com/commsenv