COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7
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Fig. 6 Impact of global sulfur cap on SO. Box plot of annual SO» VCD levels between different areas before (BFGC) and after (AFGC) the global sulfur cap
entered into force in 2020, with minimum, 25% percentile, median, 75% percentile and maximum The left plots include the maximum values, while the
right plots give the 25-75% percentile range.
North Sea (7%) was found to be significantly higher compared to
the Baltic Sea (3%) (P<0.001). The non-compliance rate within
the SECA (5%) was markedly higher compared to the non-
compliance rate outside the SECA (2%) (P < 0.001).
In addition to the documentary inspections, in accordance with
EU regulations?3, fuel samples were collected by the EU MS
(Fig. 5B). Besides a small reduction in the number of fuel samples
collected in 2020 due to the global COVID-19 pandemic, the
number of samples remained fairly consistent, with most EU MS
providing a number above the mandatory requirement. When
analyzing the inspection results from the fuel samples within the
SECA, a significant increase in non-compliance was observed in
2016 and 2017, followed by a drastic reduction toward 2020,
which then stabilized. This trend was observed for both the North
Sea and the Baltic Sea. However, there was a slight increase in
non-compliance observed in the North Sea in 2022, aligning with
che findings from the remote monitoring operations in the BA.
The North Sea non-compliance results of the fuel analysis (5%)
were notably higher than the Baltic Sea (2%) (P< 0.001). The
non-compliance trend of the fuel analysis outside the SECA also
showed a substantial decrease by 2020, while the overall non-
compliance rate (4%) was not found to be significantly different
from the overall non-compliance rate of the fuel analysis within
the SECA (4%) (P = 0.9488).
Spatiotemporal analysis of satellite data
Spatial analysis of atmospheric SO2 data. Upon comparing the
SO, vertical column density (VCD)—expressed in molecules/
cm”’— across the various regions (Fig. 6) for 2019 and 2021,
notable findings emerged. Specifically, the BA Quadripartite Zone
of Joint Responsibility (BAQPZJR) exhibited the highest con-
centrations of SO, pollution within the ECA. Meanwhile, the Bay
of Biscay displayed a much lower pollution pressure of SO,
(Supplementary Table 7). The implementation of the global sulfur
cap is shown to have created a comparable reduction of SO,
pollution levels across the SECA. The region outside the SECA
did not seem to be impacted. When looking at the period
2018-2022, for some areas an increase was observed (Supple-
mentary Fig. 7). However, due to the absence of certain months in
2018 and 2022, this was attributed to seasonal effects.
Temporal analysis of atmospheric SO, data. From the start point
of the satellite data in 2018, the overall emission levels of SO, at
sea were already relatively low, particularly in the SECA due to
the implementation of the 0.1% FSC limit in 2015. Consequently,
the SO, VCD maps for 2019 and 2021, the respective years before
and after the global sulfur cap came into effect, visualize widely
dispersed concentration levels, although areas with high shipping
activities can be, to some extent, identified. (Supplementary
Fig. 8). Accordingly, the proportional difference of SO, pollution
‚evels before and after the implementation of the global sulfur cap
does not exhibit a distinct pattern (Fig. 7).
When comparing the proportional difference in SO, VCD after
che implementation of the global sulfur cap amongst the different
areas (Supplementary Fig. 9), the most substantial decrease was
observed for the BAQPZJR (—22.5%), the northern part of the
SECA (—15.9%) and the English Channel (—9.5%). The Bay of
Biscay was less impacted by the global sulfur cap and even
showed a negligent increase (+3.0%), most probably because this
area already had a lower SO, pollution pressure compared to the
densely navigated waters of the SECA. However, there is also an
‚ndication that the sensitivity of the TROPOMI SO, data might
ze insufficient to conduct a thorough analysis of SO, pollution
irends in areas with lower SO, pollution levels.
To conclude, the conducted spatiotemporal analysis indicated a
positive influence of the global sulfur cap and other international
and EU regulations on ambient SO, concentrations in the
European SECAs. The findings are in line with the results
obtained from the remote measurements and inspections
conducted within the BA and the EU, therefore strengthening
‘he validity and reliability of the findings. However, it should be
aoted that when utilizing satellite images to assess air quality
‘mprovement for SO, outside the ECAs, the analysis heavily relies
on the shipping density and ambient SO, pollution levels.
Spatial analysis of atmospheric NO2 data. When comparing
absolute NO, VCD levels across different areas (Fig. 8), it was
demonstrated that the NO, VCD within the North Sea NECA is
overall considerably higher compared to the areas outside the
NECA. Particularly in the BAQPZJR and the English Channel,
NO, VCD levels are notably elevated, although there are some
seasonal differences (Supplementary Fig. 10). However, it is
important to acknowledge that the elevated NO, VCD levels in
ihese areas are likely to be influenced, to some degree, by
‘ndustrial activities and other densely populated areas in the
southern parts of the UK, northern parts of France, Flanders, and
(he Netherlands. On the other hand, Riess et al. provided evi-
dence that the TROPOMI data primarily captures emissions
within the first 200 meters above sea”. In addition, despite
possible other contributing factors, the monthly NO, VCD
ZOMMUNICATIONS EARTH & ENVIRONMENT | (2023)4:391 | https: //dol.org /1C.1038/s43247-023-01050-7 | www.nature.com/commsenv
