COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7 
ARTICLE 
These measurements were conducted by Chalmers University 
(114 in 2016) and the Belgian coastguard aircraft (23 in 2022). As 
sufficient data was not available for temporal analysis and as these 
measurements were conducted with the same sensor technology, 
the data at the SECA border was combined for the spatial 
analysis. 
The emission data was fitted on an S-curve in function of the 
distance to the SECA border (d,) and the distance to port (d,). 
Non_compliance (%) = DM +P nn 
inspection results throughout the EU. However, the EU data does 
not contain results on penalties. Moreover, due to the lack of EU 
cegulation regarding NO, emissions from OGVs, the EMSA data 
does not contain NO, inspection results. 
The EMSA website publicly displays the amount of port 
ınspections and compliance levels resulting from documentary 
inspections conducted by EU MS°°. Its purpose is to enable 
EMSA to deliver thorough reports to the EC to assess the 
implementation of the EU Sulphur Directive by the EU MS?536, 
Additionally, the EMSA website includes records of non- 
compliance detected through fuel sample analysis. However, the 
actual number of fuel samples themselves is not available on 
the website. A request for this information was made to EMSA 
to facilitate the analysis of temporal trends in fuel sample 
results. 
with: 
k = high Non_compliance rate (%) 
P = low Non_compliance rate (%) 
0 = Non_compliance increase/decrease rate (%) 
m = midpoint distance (km) 
d, = distance to SECA border /port 
The weighted average non-compliance rates were used for the 
factors k and p, the least square method was applied for the 
determination of the factors o and m. 
Statistical analysis of remote monitoring data and port 
inspection results. Previous studies using the Belgian airborne 
data have already shown that emission measurements deviate 
from a normal distribution?+2628 When the distribution of 
:emote measurement data is compiled, they initially appear to 
follow a normal pattern, with the emission limit as the central 
point. However, although small negative values are occasionally 
observed, there are no highly negative values, while very high FSC 
values are possible. As a result, this inevitability renders the 
distribution non-normal. Pearson chi-square tests were used to 
assess the difference in compliance rate between two locations/ 
deployments, with statistical significance defined as P < 0.05%, 
Inspection results of port inspection authorities in the Bonn 
Agreement. In accordance with the EU Sulphur Directive and the 
Commission Implementing Decision, the port state authorities of 
the EU MS conduct sulfur inspections?, These are done by 
either documentary inspection or by analyzing the fuel in 
accordance with the fuel inspection guidelines from IMO®B. 
Within the scope of this research and the work conducted under 
the BA, inspection and sanctioning results from port State 
authorities from 9 out of 10 BA CPs were obtained. This data is 
part of the annual inspection results that are reported to the EC. 
The most CPs apply a 0.15% threshold for reporting infringe 
ments to the EC. Of all BA CPs, two are located outside the ECA 
(Spain and Ireland) and two are currently not reporting to the EC 
Norway and the United Kingdom). In addition to the sulfur 
inspection results, NO, inspection results were also collected. As 
NO, is currently not regulated through an EU directive, inspec- 
tion results are currently not being shared with the EC. 
Satellite analysis. A spatiotemporal analysis was conducted using 
satellite data from the Tropospheric Monitoring Instrument 
(TROPOMI) Sentinel 5 to investigate the distribution and changes 
in SO, and NO, levels over European waters. The temporal analysis 
of SO, focused on the evaluation of the impact of the global sulfur 
cap, which entered into force in 2020°2, while the temporal analysis 
of NO, focused on the implementation of the European NECAs in 
202118 Additionally, a spatial analysis compared SO, and NO, 
pollution levels between different areas within the ECAs and dif- 
ferences between the ECAs and regions outside the ECA. 
EU inspection results and Thetis-EU. The EU Sulphur Directive 
led to the creation of Thetis-EU, an online database used for 
exchanging inspection results. EMSA manages and hosts the 
database. Thetis-EU is accessible to inspectors across all EU MS, 
including Norway and Iceland. However, due to Brexit, the UK 
no longer has access to the database?®59, 
TROPOMI data. Data was gathered from TROPOMI on board 
che Copernicus Sentinel-5 Precursor satellite, which is operated 
5y the European Space Agency (ESA). The satellite data contains 
measurements of the SO, and NO, VCD in the lower atmo- 
spheric layer (up to 80km). For SO»,, the retrievals from the 
scientific COBRA VO01 scheme processed by BIRA-IASB were 
used®3 for the period May 2018 until September 2022. For NO,, 
che satellite operational data product (Level 2 data) was collected 
from the Copernicus Open Access Hub®* for the period May 
2018 until December 2022, using the PAL v2.3.1 retrieval algo- 
rithm. With the NO emissions converted to NO,, factors such as 
ambient meteorological conditions, Os, and solar radiation 
influence the conversion speed. However, conversion is con- 
sidered to be in the time span of seconds to tens of minutes 
during daytime®>, Therefore, the NO», satellite analysis gives a 
good representation of NO, pollution levels. 
TROPOMI retrievals for SO, and NO, have been filtered based 
on their quality assurance (QA) value. Only pixels with a QA 
value equal to or larger than 0.75 were selected, removing cloudy 
pixels (cloud radiance fraction > 0.5) and erroneous retrievals®*, 
Subsequently, they were averaged to generate monthly VCD 
products. The monthly average VCD products were further 
compiled using ArcGIS and Qgis to generate VCD maps for 
spatial and temporal analysis. 
Keel laying date. Information on the KLD is required for the 
determination of the tier level when assessing NO, compliance. 
For the Belgian NO, remote monitoring assessment, this KLD 
data was acquired based on merging two database sources: (1) the 
Global Integrated Shipping Information System (GISIS) of the 
IMO® and; (2) Thetis-EU of EMSA. The GISIS database was first 
used to gather information on OGVs larger than 75 meters with 
construction year. In the second step, EMSA provided the accu- 
rate KLD28, For the Danish analysis, a ship database was acquired 
from IHS Markit. 
Inspection results. Under the EU Member States’ obligations to 
report inspection outcomes to the EC as stipulated by the EU 
Sulphur Directive and Commission Implementing Decision?, 
data on sulfur inspections conducted by the EU MS were 
obtained from EMSA®. As most, but not all, BA CPs are part of 
the EU, there is a certain overlap with the BA data. The EU data 
contains more States and includes all EU MS outside the SECAs. 
The EMSA data therefore gives a broader overview of the 
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