COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-023-01050-7 
ARTICLE 
a potential rise in non-compliance in recent years. Despite certain 
differences in measurement uncertainty and methodology, a 
strong correlation was observed between the results obtained 
from fixed sniffers, sniffer sensors utilized on fixed wing aircraft 
and mini-sniffers employed on RPAS and rotary wing aircraft. 
This indicates a considerable level of agreement between the 
different measurement methods and sensors, further supporting 
the reliability of the findings and suggesting a consistent pattern 
of compliance trends. 
A spatiotemporal analysis conducted on the remote measure- 
ments revealed relatively high compliance within the SECA 
region, while also highlighting specific spatial compliance pat- 
terns in relation to the distance from ports, distance to the SECA 
border, and differences between the Baltic Sea and North Sea 
areas. It should be noted that this spatial analysis provides only an 
initial broad-scaled assessment. To obtain a more comprehensive 
understanding of the spatial distribution of non-compliance 
average FSC levels, it is necessary to conduct a thorough analysis 
of the raw measurement data. Conducting such an analysis would 
enable a deeper understanding of the specific factors influencing 
non-compliance risks within the SECA region. To compare 
average FSC levels, an intercomparison study first needs to be 
conducted. In light of this intercomparison, it should be noted 
that the Belgian monitoring data was adjusted for its measure- 
ment bias by utilizing plume simulation gas mixtures with specific 
concentrations of CO, and SO„‚26. These plume simulation mix- 
tures could be employed as a “round robin” mechanism to rectify 
measurement discrepancies between different sensors, enabling a 
comprehensive comparison of average FSC levels. This in turn 
would enable a more refined spatial analysis of compliance pat- 
terns, thus providing a higher resolution assessment. The insights 
gained from such an analysis can be used to identify areas with 
high risk of non-compliance, guiding the strategic deployment of 
remote monitoring operations. By focusing on these high-risk 
areas, enforcement efforts can be optimized to better address 
non-compliance and ensure better enforcement of international 
emission regulations. 
In contrast to the relatively good FSC compliance following the 
global sulfur cap implementation, the introduction of the NECA 
in 2021 did not lead to any substantial impact on the remotely 
measured NO, concentrations in European waters, nor did it 
result in an effective enforcement and sanctioning mechanism. 
On the contrary, it was found that NO, emission factors and 
potential NO, non-compliance are increasing. This outcome was 
somewhat expected, considering that the stricter emission reg- 
ulations within the ECA primarily apply to Tier III vessels. Given 
the limited number of Tier III vessels currently in operation, the 
NECA implementation has not yet made a measurable impact in 
reducing average NO, emission factors. Furthermore, previous 
studies already indicated increased NO, emission factors for Tier 
II vessels compared to Tier I vessels?124,28,32,38,39. Consequently, 
the outcomes of this study align closely with the conclusions 
drawn from other research studies. 
The Parties of the BA, and by extension the EU MS, have 
successfully implemented a comprehensive and efficient system 
for conducting on-board port inspections of OGVs with regard to 
the sulfur content of marine fuels. Based on the results of more 
than 110,000 documentary inspections and more than 26,000 fuel 
samplings, the study indicates that non-compliance with FSC 
regulations in ports in both European SECAs exhibit a similar 
declining pattern. Nevertheless, by implementing remote mon- 
itoring to identify non-compliant OGVs and establishing an 
enforcement mechanism that considers the highest measured 
emission levels, the enforcement capacity can potentially be fur- 
ther improved. Conversely, the study revealed that the enforce- 
ment of the international NO, emission regulations for OGVs in 
port is currently highly ineffective due to insufficient inspection 
and sanctioning procedures outlined in international regulations. 
The absence of a comprehensive EU regulatory framework spe- 
cifically addressing NO, emissions from shipping with inspection 
<ules can be considered a critical factor in this context. In light of 
‘his, Canada recently submitted a document for the July 2023 
meeting of the Marine Environment Protection Committee 
‚MEPC80), a subsidiary body of the IMO, to address some of 
chese Tier III-related issues*9. It is worth noting that as early as 
2007, the US already had submitted a document to the IMO Sub- 
committee on Bulk Liquid and Gases, foreseeing and addressing 
some of the very challenges that are currently being observed in 
che certification and testing processes of Tier III OGVs*, 
In order to evaluate the influence of international regulations 
on air quality within the European SECA, this study analyzed 
TROPOMI Sentinel 5 data from 2018 to 2022. The analysis of the 
spatiotemporal patterns in SO, satellite data demonstrates a 
‚imited decrease in SO, pollution levels in the North Sea SECA 
and a status quo or even a slight enhancement outside the SECA. 
The subsequent international and EU regulations on SO, have 
iherefore had a substantial, positive impact on public health and 
the environment in the European SECAs. It is important to 
acknowledge that data for 2021 may have been affected by the 
global COVID-19 pandemic. Unfortunately, a complete annual 
dataset for 2022 could not be obtained. Future analysis will need 
:o determine if the recent increase in FSC non-compliance, 
observed by the remote monitoring operations, and the recovery 
of economic activity after the global COVID-19 pandemic, will 
'mpact overall SO, pollution levels. Additionally, it should be 
aoted that as shipping activities increase, an overall rise in SO, 
emissions is anticipated?4, However, it should be acknowledged 
that the sensitivity of the TROPOMI data for SO, does not seem 
to be optimal for detailed analysis outside densely 
navigated areas. 
In contrast, the TROPOMI NO, products proved substantially 
more useful for performing a meaningful analysis. The spatio- 
temporal analysis of the NO, satellite data reaffirmed the findings 
from the remote measurements and port inspections, indicating 
‘hat the implementation of the NECA has not resulted in a 
substantial reduction of the NO, pollution pressure from the 
shipping sector. On the contrary, it appears that NO, pollution 
‚evels only decreased substantially in the most polluted area 
BAQPZIR), in other areas of the NECA, signs of an increase 
were observed. Strong decreases were observed over land, in 
particular above the cities of Paris, Brussels, Londen, etc. This also 
‚nevitably impacted the temporal analysis, particularly within the 
BAQPZIR region, explaining the observed NO, reduction in that 
area. Although this reduced input from land-based pollution was 
not accounted for in this study, it is safe to assume that if this 
ıand-based pollution were to be incorporated, an overall increase 
across all areas would be observed. 
The findings derived from this study provide substantial evi- 
dence of the effectiveness of international regulations and the 
enforcement measures taken to address FSC non-compliance in 
‘he shipping sector. Notably, the implementation of inspection 
:egimes in EU ports and the deployment of remote monitoring 
operations at sea have functioned as a considerable deterrent 
effect. These measures have proven instrumental in discouraging 
FSC non-compliant behavior within the shipping industry, 
emphasizing the pivotal role of well-defined and stringent reg- 
alations and effective enforcement measures. However, the results 
‚.ndicate that compliance is lower at sea compared to the port 
areas. In addition, the comprehensive analyses of the remote NO, 
measurements and NO, pollution levels at sea solidify the con- 
clusion that the international regulations on NO, emissions from 
ships were ineffective in reducing NO, pollution from the 
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