Anthropogenic air pollution, a leading cause of premature death globally, is significantly contributed to by emissions from ocean-going vessels (OGVs). SO2 and NOx are major pollutants from OGVs, contributing to the formation of secondary PM2.5 and having direct adverse health and environmental effects. International regulations under the MARPOL Convention aim to reduce these emissions, including the establishment of ECAs with stricter standards. While the International Maritime Organization (IMO) reported a decrease in SO2 emissions following the implementation of ECAs and stricter limits in 2015, NOx emissions increased slightly during the same period. Although projections suggested significant health benefits from implementing stricter regulations, a research gap existed regarding the effectiveness of these regulations in reducing real-world emissions within ECAs. This study addresses this gap by examining the impact of European ECAs and other international maritime regulations on OGVs' emissions in the North and Baltic Seas, utilizing a three-step approach combining remote OGV emission measurements, inspection results, and satellite data.

Existing literature highlights the substantial health and environmental impacts of air pollution from shipping, particularly from SO2 and NOx. Studies like Sofiev et al. (2018) quantified the premature deaths attributable to OGV emissions, emphasizing the importance of emission reduction strategies. The IMO's own reports showed initial success in reducing SO2 emissions with the establishment of ECAs and stricter limits. However, the lack of commensurate reductions in NOx emissions, despite regulations, prompted further research. Previous studies, such as Van Roy et al. (2023), demonstrated varying success rates of international regulations in improving air quality depending on location, underscoring the need for a more comprehensive regional assessment like the one undertaken in this paper.

This research employed a three-pronged methodology. First, it analyzed compliance rates based on over 100,000 remote OGV emission measurements collected by the Bonn Agreement (BA) contracting parties using in-situ air quality sensors. These measurements were categorized into fixed stations (near ports) and airborne measurements (in territorial and exclusive economic zones). Three compliance cutoff levels (0.20%, 0.15%, and 0.13% FSC) were used to assess the severity of non-compliance. Second, the study examined data on emission violations and penalties from the BA, along with overall port inspection results from the EU. Third, satellite data (2018-2022) from the TROPOMI instrument on Sentinel-5 was utilized to assess changes in atmospheric concentrations of SO2 and NO2 within the European ECAs. Statistical analyses, including Pearson chi-square tests, were conducted to compare compliance rates across different locations and time periods. The spatial analysis involved fitting non-compliance data to S-curves based on distance from ports and the SECA border. For satellite data, both spatial (comparing SO2 and NO2 levels across different regions) and temporal (analyzing changes before and after the implementation of the global sulfur cap and the NOx ECAs) analyses were performed. The impact of the COVID-19 pandemic was considered when interpreting temporal trends.

The study found that compliance with fuel sulfur content (FSC) regulations was higher near ports than in open waters, showing a clear pattern of adaptive non-compliant behavior. While the global sulfur cap implemented in 2020 significantly reduced SO2 non-compliance, a potential recent rise in non-compliance was observed, possibly due to increased fuel prices. Spatial analysis showed higher non-compliance rates closer to the SECA border and in the Baltic Sea compared to the North Sea. Port inspections showed a significant decline in sulfur infringements after the global sulfur cap, indicating effective enforcement. In contrast, NOx emission regulations proved less effective. Remote measurements showed increasing NOx emissions, and port inspections revealed a lack of enforcement for NOx regulations within the BA. Satellite data analysis confirmed a substantial decrease in atmospheric SO2 concentrations within the SECAs following the global sulfur cap, supporting findings from remote measurements and port inspections. However, the analysis of NO2 satellite data indicated that the implementation of the European NOx ECAs (NECAs) in 2021 did not lead to a significant reduction in NOx pollution levels at sea; increases were observed in some areas. The COVID-19 pandemic also affected NO2 levels, particularly in areas with high pollution. The overall trend showed an increase in NOx pollution in many regions within the ECAs.

The findings highlight the success of international regulations and enforcement measures in reducing SO2 emissions from OGVs, particularly through port inspections and remote monitoring. However, compliance remains lower at sea. The ineffectiveness of current NOx regulations is a significant concern. The increase in NOx emissions, despite the introduction of NECAs, necessitates a review of current regulations and enforcement mechanisms. The study's results align with other research showing increased NOx emission factors for Tier II vessels compared to Tier I. The limited number of Tier III vessels currently in operation means the NECA implementation hasn't yet had a substantial impact on reducing average NOx emission factors. The disparity between SO2 and NOx compliance highlights the need for a more comprehensive and effectively enforced regulatory framework for NOx emissions. The satellite data analysis supports the remote measurement and port inspection results, but also reveals regional variations in NO2 trends. The impact of the COVID-19 pandemic on air quality needs further investigation.

This study demonstrates the effectiveness of international regulations and enforcement in curbing SO2 emissions from OGVs, but reveals the urgent need for improved NOx emission regulations and enforcement. The increasing NOx pollution levels, despite the introduction of NECAs, call for a revision of current regulations and the implementation of more stringent measures, including the potential establishment of additional NECAs in the Mediterranean and North Atlantic regions. The findings also underscore the importance of updating air quality models to reflect real-world emission factors and compliance levels to accurately assess the health impacts of shipping.

While the study analyzed a large dataset, the lack of access to raw measurement data from all BA countries limited the ability to directly compare average FSC levels across different measurement methods and locations. The analysis of satellite data was influenced by factors such as seasonal variations, the impact of the COVID-19 pandemic, and the potential influence of land-based pollution sources. The limited availability of NOx inspection data from the BA also restricted the depth of the analysis of NOx compliance.