Regionwide analysis of the remote monitoring data
Non-compliance data from all remote measurement stations and deployments was collected based on three different cut-off levels. This allows the assessment of the severity of the non-compliance behavior in addition to a temporal and spatial non-compliance trend analysis. For the main results, the 0.15% FSC cut-off level was used.
Temporal sulfur compliance trends
A decreasing trend in FSC non-compliance rates was observed across all measurement locations within the European Sulfur Emission Control Area (SECA) regions. The non-compliance rate decreased from 7.1–0.7%, with an average non-compliance rate of 1.5% when a 0.15% FSC cut-off level is used (Fig. 1). The pattern is similar for the other cut-off levels (figure S.3A&B). Following the implementation of the global sulfur cap in 2020, the non-compliance rates reached their lowest point, with an average non-compliance rate of 0.6%. It is important to acknowledge that the implementation of the sulfur cap in 2020 coincided with the global COVID-19 pandemic, which led to reduced fuel prices25,26. Additionally, several monitoring operations observed a slight increase in non-compliance, starting from 2022. This increase can be attributed to the rise in marine fuel prices resulting from the Russian invasion in Ukraine and the subsequent global price inflation27.
Among the different remote measurement operations applied by the SECA countries, the French measurements with the remote piloted airborne systems (RPAS) exhibited the highest non-compliance rates. The average non-compliance rate was 9.4% and therefore substantially higher than the non-compliance observed by the other remote measurement operations, which varied between 0.1% and 3.7% for the same period. When considering the remote monitoring locations that conducted measurements throughout the entire 2015–2022 period, the Belgian airborne measurements recorded the highest non-compliance rate for the 0.15% FSC cut-off level (5.2%). However, the Danish helicopter measurements displayed the highest non-compliance rate for the 0.13% FSC cut-off level (8.5%). This distinction is noteworthy as it illustrates that while the OGVs in Danish water exhibited higher absolute non-compliance rates, the level of the FSC exceedances was higher for the OGVs in Belgian waters.
Spatial sulfur compliance trends
The temporal analyses reveal notable disparities in non-compliance rates between fixed stations (1.0%), typically situated near ports, and airborne measurements (5.3%) (Fig. 1). Similar patterns were observed for the other cut-off levels (figure S.3A&B). These differences in non-compliance rates between airborne measurements and fixed stations were statistically significant for all cut-off levels (P < 0.001).
Although based on the same methodology (supplementary material, Measurement techniques), fixed and airborne measurements use different operational methods, which can partly explain the differing non-compliance rates. Airborne platforms try to avoid redundant measurements of the same OGVs, while fixed stations take a non-selective approach and measure all passing OGVs. This may therefore lead to a slight underestimation of the determined non-compliance rate by fixed stations if compliant OGVs like compliant ro-ro ferries are overrepresented in these datasets. Furthermore, aerial remote measurements may tend to focus on OGVs with a higher risk profile, and, to some extent, avoid OGVs operating only in the SECA, or smaller coasters, i.e. small to medium-sized cargo OGVs designed for transportation along coastlines or in relatively calm waters. This may overestimate the overall non-compliance rate by airborne measurements. Nevertheless, these findings indicate a clear pattern of adaptive non-compliant behavior among OGVs.
Comparison of non-compliance trends between the various measurement campaigns revealed a high consistency ( table S.3&4). It was observed that locations in closer proximity to the SECA border have higher non-compliance rates. Measurements taken at the border by the MUMM and Chalmers University28 demonstrated an average non-compliance rate of approximately 30%. When plotting the non-compliance data against the distance from the border, it followed an exponential decreasing curve, with a high goodness of fit (Fig. 2A). In order to mitigate the influence of the high compliance rate in ports, the combined airborne data from RPAS, helicopter, and aircraft was utilized (Fig. 2B). In this case, an excellent goodness of fit was also observed. Given the significant disparity between non-compliance rates observed in ports compared to those at sea, the relationship between compliance and the distance from port was determined (Fig. 2C). Similar patterns were observed for the other FSC cut-off levels (figure S.4A-C). The fitting constants and correlation factors (R²) of the curve fittings for all cut-off levels are provided in table S.5&6.
This spatial analysis provided valuable insights into the distribution of non-compliance risks along the SECA border. Notably, the analysis revealed that the highest risk for non-compliance was observed within the first 500 nm from the SECA border. The results indicate that compliance rates at sea, beyond a distance of 500 nm, varied between 2–3% for the 0.15% FSC cut-off level. Furthermore, these findings indicate that non-compliance begins to notably increase at approximately 50–70 nm from port. At a distance of 100 nm from the port, the proximity to the port stops influencing non-compliance behavior. It must be acknowledged that the number of points for these fittings were, in particular for the non-compliance in function of the distance from port, very low. To obtain a better understanding of these relationships it is recommended that a dedicated more in-depth analysis based on the raw measurement data is conducted.
Upon comparing the Baltic Sea and the North Sea, noticeable differences in non-compliance rates were observed. In general, the Baltic Sea exhibited higher non-compliance rates, with an overall non-compliance rate of 1.3%, compared to 2.2% for the North Sea for the 0.15% FSC cut-off level (Fig. 2D). Similarly for the other cut-off levels the Baltic Sea demonstrated higher non-compliance rates. Importantly, for all cut-off levels, the differences were determined to be statistically significant (P < 0.001). When comparing the airborne results, for the North Sea a higher non-compliance rate is observed for the 0.15% FSC and the 0.20% FSC cut-off levels. However, the Baltic Sea showed a higher non-compliance rate for the 0.13% cut-off level. This indicates that non-compliant OGVs at sea in the North Sea tend to have higher absolute FSC levels compared to those in the Baltic Sea, whereas in the Baltic sea, low FSC exceedances appear to occur more often.
NOx Emission Control Area
For this study it was not feasible to compare the Belgian NOx non-compliance data with other locations since there are no other NECA countries reporting on NOx non-compliance in an operational setup. However, it is worth noting that numerous other agencies conduct measurements of NOx levels in addition to monitoring SO2 concentrations in OGV exhaust plumes. While a direct comparison of NOx non-compliance data may not be possible yet, these additional measurements provide valuable insights on the overall emissions profile and environmental impact of OGVs.
The examination of the Belgian data reveals that the mean NOx emissions are not decreasing as anticipated with the implementation of stricter emission limits. On the contrary, the data indicates that NOx emissions are increasing 24,29. Furthermore, non-compliance levels for NOx emissions are also rising 24,29. This trend can be attributed to the higher emission levels reported for Tier II OGVs compared to Tier I OGVs. Based on the Belgian data, a strong correlation (R² = 0.9993) was found between the annual mean NOx emissions and the proportion of Tier II OGVs (Fig. 3). Nevertheless, the weakness of this 3-point correlation aligns with the previous findings demonstrating substantially higher NOx emissions for Tier II OGVs24,29,30. These findings have significant implications for the parameterization of emission models, such as the Steam Model31,32, which are fundamental sources for global emission inventories for shipping. By incorporating the correct NOx emission factors based on the real-world emission factors per IMO tier, more accurate global assessments of NOx emissions from OGVs can be achieved, thereby improving the understanding of their environmental impact.
The Danish company Explicit took a different approach to the Belgian one by using modelling. They estimated main engine power and fuel consumption as input for the calculation of NOx emission factors in grams of NOx per kilowatt-hour (g NOx/kWh) 30. Explicit used this approach for reassessing the Danish historic NOx measurement data. The findings of this study align with the results of the empirical approach of Belgium, revealing higher emission factors and a greater non-compliance rate for Tier II OGVs compared to Tier I OGVs. The Danish study also confirmed that OGVs emit more NOx when operating at lower engine loads. Additionally, the results demonstrated that larger engines generate higher emission factors, which is in line with the Belgian measurements, albeit with a weak correlation. An increasing trend was observed across all Tier levels, excluding Tier III due to limited measurements (figure S.5).
Non-compliance with NOx standards has also recently been investigated within the SCIPPER project. A particular emphasis was placed on Tier III OGVs. The advantage for the enforcement of Tier III OGVs is that a not-to-exceed limit (NTE) is defined for all four engine load points, set at 50% of the applicable emission limit (Appendix II, MARPOL Annex VI)16. However, because the keel laying date (KLD) is defined in the MARPOL Annex VI regulations to determine Tier III classification, a significant majority (73%) of the recently constructed OGVs are registered with a KLD prior to 2021. Consequently, they are subjected to the Tier II emission limits instead of the stricter Tier III emission limits24.
In total 65 Tier III OGVs were monitored by the SCIPPER partners. The findings indicated that approximately half of the observed Tier III OGVs did not comply with the maximum NOx emission limits for Tier III; ca 20% of the observed tier III OGVs did not even meet Tier II emission limits 33. This observation aligns with the limited Tier III non-compliance results reported by Belgium, where a non-compliance rate of 43% was observed. Various other studies have also highlighted concerns regarding elevated levels of NOx emissions from Tier III OGVs 34,35.
Port inspections on sulfur and NOx infringements
Results within the Bonn Agreement
The results of the sulfur infringements from most BA CPs follow an increasing trend between 2015 and 2020 (figure S.6A). The primary reason for this is that not all CPs immediately implemented inspection protocols; needed to gain experience; and had initially only limited information available to single out suspicious OGVs for inspection. As a result, not all CPs have inspection results for 2015. From 2016, all BA CPs were actively conducting inspections within their ports. During this time, remote monitoring operations and the exchange of alerts via Thetis-EU began to gain momentum, leading to the discovery of a higher number of infringements and deficiencies.
Due to a high number of observed sulfur infringements by one BA CP, the total observed number of infringements in the years 2015 and 2016 still provided the highest number of observed infringements (243 and 223) Fig. 4A). The year 2018 provided the third highest number of recorded infringements (178). However, following that year, the number of identified infringements began to decline. It is important to note that the EU Sulphur Directive mandates Member States (MS) to provide port inspection data by June, as a result, at the time of publication, not all CPs were able to submit data for the year 2022.
The EU-Commission Implementing Decision played a significant role in maintaining a consistent number of inspections conducted on OGVs throughout the entire time period. Although there was a decrease in inspections in 2020 due to the global pandemic, the majority of CPs were still able to fulfill the mandatory inspection requirements. It is worth noting that in this context, numerous CPs utilized the exemption outlined in the Implementing Decision to reduce the number of inspections by implementing remote monitoring (Art 3.3(a))36.
Regarding the reported penalties on sulfur, an upward trend was observed between the years 2015 and 2017, reaching a peak of 126 cases in 2017 (figure S.6B). Subsequently, the number of penalties declined. It should be noted that there is a time lag in the reporting of penalties, as often the reported penalties correspond to infringements observed in the previous year. Therefore, the peak in penalties in 2019 aligns with the peak of infringements in 2018. To address this time lag, it is necessary to analyze the data from the original cases and assign them to the year of observation. However, this analysis was not feasible due to the sensitive nature of the legal cases involved.
When looking at the mean number of sulfur deficiencies and infringements observed by the BA CPs’ port inspection authorities, a substantial decrease was observed after the global sulfur cap came into effect Fig. 4A). Over the total period 2015–2022, 996 infringements were observed of which 544 were penalized. In the period 2015–2020, before the global sulfur cap came into effect, 885 infringements were observed by the port inspection authorities with a mean of 21.6 cases per year, 442 penalties were executed in the same period or on average 10.8 penalties per year, corresponding to 56% of the infringements. In the period 2020–2022, after the global sulfur cap came into force, in total 111 infringements were observed. The mean annual number of observed deficiencies per BA CP decreased therefore significantly to 4.8 cases per year (P < 0.001). In total, 102 penalties were handed out after the global sulfur cap came into effect (91%). The mean number of penalties per BA CP per year therefore decreased significantly to 4.3 penalties (P < 0.05), which is just below the mean number of observed infringements, indicating that as of today there is a good legal follow up of possible infringements within the BA.
There is a notable disparity between sulfur and NOx. The result of the inquiry with the BA CP on sulfur provided an abundance of data regarding sulfur infringements and demonstrated the successful enforcement and legal follow up with penalties for sulfur infringements. In contrast, the results of the inquiry on NOx enforcement within the BA CPs on reported NOx violations and penalties was disappointing Fig. 4B). Only two BA CPs have reported NOx infringements and penalties. Most of the other BA CPs are currently not enforcing NOx regulations nor collecting data on the results of the NOx inspections at the time of publication. Currently, only one BA CP has imposed a penalty for a NOx violation. This demonstrates that enforcement of NOx regulations by BA CPs is currently lacking. Upon examining the limited available NOx inspection data, it becomes evident there has not been a decrease in violations since the NECA was implemented, but rather, an increase. However, the scarcity of data does not allow statistical analysis or strong conclusions to be drawn about compliance rates within the BA.
Results within the EU
Upon examining the data on sulfur inspections and non-compliance rates within the EU, similar patterns were observed within the Baltic Sea and North Sea ECA as within the BA Fig. 5A). In the wider SECA, in total 110,657 documentary inspections were conducted. The annual amount showed a slight increase since entering into force in 2015, with a relatively stable trend over the entire period, except for a small decline in 2020. This increase was mainly a result of the increased number of inspections by the North Sea ECA countries, while the Baltic Sea countries had a more stable number of conducted inspection throughout the entire period. The non-compliance rate based on documentary inspections followed a similar trend as the number of infringements in the BA. However, it is important to note that this pattern is largely influenced by a significant reduction in non-compliance in the North Sea, while the reduction in the Baltic Sea is less pronounced. Also, when looking at the compliance results outside the SECA, the reduction was less pronounced. The overall non-compliance rate in the 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 significantly higher compared to the non-compliance rate outside the SECA (2%) (P < 0.001).
In addition to the documentary inspections, in accordance to the EU regulations36,37 fuel samples were collected by the EU MS Fig. 5B). Besides a small reduction in the number of fuels samples collected in 2020 due to the global COVID-19 pandemic, the number of fuel samples remained fairly consistent, with most EU MS providing a number above the mandatory required fuel sampling. 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 towards 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 the 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 SO2 vertical column density (VCD) across the various regions Fig. 6) for 2019 and 2021, notable findings emerged. Specifically, the BAQPZJR exhibited the highest concentrations of SO2 pollution within the ECA. Meanwhile, the Bay of Biscay, displayed a much lower pollution pressure of SO2 ( table S.7). The implementation of the global sulfur cap is shown to have created a comparable reduction of SO2 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 (figure S.7). However due to absence of certain months in 2018 and 2022, this was attributed to seasonal effects.
Temporal analysis of atmospheric SO2 data
From the start point of the satellite data in 2018, the overall emission levels of SO2 at sea were already relatively low, in particular in the SECA due to the implementation of the 0.1% FSC limit in 2015. Consequently, the SO2 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. (figure S.8). Accordingly, the proportional difference of SO2 pollution levels before and after the implementation of the global sulfur cap do not exhibit a distinct pattern Fig. 7).
When comparing the proportional difference in SO2 VCD after the implementation of the global sulfur cap amongst the different areas, the most substantial decrease was observed for the BA Quadripartite Zone of Joint Responsibility (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 SO2 pollution pressure compared to the densely navigated waters of the SECA. However, there is also an indication that the sensitivity of the TROPOMI SO2 data might be insufficient to conduct a thorough analysis of SO2 pollution trends in areas with lower SO2 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 SO2 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 the validity and reliability of the findings. However, it must be noted that when utilizing satellite images to assess air quality improvement for SO2 outside the ECAs, the analysis heavily relies on the shipping density and ambient SO2 pollution levels.
Spatial analysis of atmospheric SO2 data
When comparing absolute NO2 VCD levels across different areas Fig. 8), it was demonstrated that the NO2 VCD within the North Sea NECA is on overall considerably higher compared to the areas outside the NECA. Particularly in the BAQPZJR and the English Channel, NO2 VCD levels are notably elevated, although there are some seasonal differences (figure S.10–11). However, it is important to acknowledge that the elevated NO2 VCD levels in these areas are likely to be influenced, to some degree, by industrial activities and other densely populated areas in the southern parts of the UK, northern parts of France, Flanders, and the Netherlands. On the other hand, Riess et al. provided evidence that the TROPOMI data primarily captures emissions within the first 200 meters above sea38. In addition, despite possible other contributing factors, the monthly NO2 VCD satellite data clearly reveals visible shipping patterns (figure S.13). Consequently, it can be stated that emissions from OGVs provide the dominant factor in the observed NO2 VCD data.
When looking at the average NO2 levels before and after the implementation of the NECA for the different areas, it was demonstrated that NO2 levels after the introduction of the NECA were impacted in different ways. The BAQPZJR area remained the most polluted area, with the English Channel following closely behind. However, since the implementation of the NECA, the Bay of Biscay became the third most polluted area, before the Northern NECA.
Temporal analysis of atmospheric NO2 data
For the temporal analysis of NO2, an annual approach and a monthly approach were used. For the annual approach, the proportional difference between 2019 and 2022 was determined Fig. 9). This map shows varying results on the proportional difference in NO2 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 NO2 pollution trends.
The comparison of the monthly NO2 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 NO2 VCD trends throughout the year, a monthly proportional difference analysis was conducted. For this monthly analysis, the proportional difference between the period before and after the implementation of the NECA was calculated for each month (figure S.14). 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 NO2 VCD levels. Subsequently, the proportional difference maps per month were combined to create an average monthly proportional NO2 difference map Fig. 10A). This map demonstrates a slightly different picture compared to the annual proportional difference. Also here a NO2 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 demonstrates 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 overage NO2 VCD over 2018–2022 clearly demonstrate 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 impacted by the COVID-19 pandemic (figure S.15A).
Additionally, significant increases were observed in the Mediterranean Sea, and Atlantic Ocean, with increases in the average monthly NO2 VCD levels of up to 20%. Due to lack of data for the winter months in the north of the Baltic Sea, a full-year assessment could not be made. Nevertheless, the data that is available for the months January-December indicates a slight reduction Fig. 10B). In conclusion, these analyses confirm that the ambient NO2 levels throughout the year either increased after the NECA implementation or where they decreased, the decrease was less substantial at sea compared to inland.