1. Introduction
99% of the world’s population is exposed to air containing high levels of pollutants that exceed the limits of the World Health Organization (WHO) guidelines [
1]. The negative health effects of air pollution are well documented, with an estimated 9 million annual deaths worldwide attributed to exposure to ambient air pollution [
2]. Despite significant reductions in emissions for many air pollutants over the past two decades, concentrations of air pollutants in the European Union remain too high. In 2020, the European Environment Agency (EEA) reported that 96% of city residents were exposed to harmful concentrations of particulate matter (PM) [
3]. For the Belgian region of Flanders it was estimated that PM is responsible for 300-600 premature deaths per year [
4]. Not only PM is a reason for concern, despite a decrease of more than 50% in NO
x emissions in the EU since 1990, several European countries still record values exceeding the annual EU limits and all 35 EU member states (MS) record values exceeding WHO guidelines for NO
x [
3,
5].
Ocean Going Vessels (OGVs) emit a range of air pollutants such as SO
x, NO
x, and PM [
6,
7,
8,
9], contributing to 10-15% of global anthropogenic SO
2 emissions and 15-20% of NO
x emission [
10,
11]. At the EU level, OGVs are estimated to be responsible for 24% of SO
2 and NO
x emissions [
12]. These emissions have severe health and environmental impacts [
13,
14,
15,
16,
17]. While land-based sources of air pollution have been regulated for years thus leading to a reduction in their contribution to air pollution [
5,
18], shipping has long been excluded from regulation. As a result, in 2008, regulations were introduced under the Marine Pollution Convention (MARPOL) of the International Maritime Organization (IMO) to decrease emissions from OGVs, in particular SO
x and NO
x [
19,
20]. In addition, MARPOL Annex VI introduced Emission Control Areas (ECAs) with stricter emission limits. [
21,
22,
23,
24,
25].
As part of MARPOL Annex VI, Regulation 14 sets limits on SO
x emissions from OGVs. In 2008, the North Sea and Baltic Sea were declared as Sulfur Emission Control Areas (SECAs) [
20,
24,
25] (Figure S.1A). In the SECA OGVs are required to use compliant fuels or use an exhaust gas cleaning system (EGCS) [
26,
27,
28,
29]. Outside the SECA, sulfur limits have been tightened in 2020 by the so-called “Global Sulfur Cap” [
30] [
30] (
Figure S1B) and “Carriage Ban” [
31]. SO
x emission regulations have been implemented in both EU and Belgian legislation [
32,
33,
34,
35]. The EU SOx directive led to the implementation of mandatory inspection numbers and the creation of Thetis EU – the port inspection database managed by the European Maritime Safety Agency (EMSA) for the exchange of inspection and monitoring results [
36].
Regulation 13 of MARPOL Annex VI introduces the NO
x emission limits [
24,
37,
38]. In 2021 a NO
x Emission Control Area (NECAs) came into force in the North Sea and Baltic Sea [
20,
22] (
Figure S1A). The NO
x emission limits are expressed as the weighted amount of NO
x emission (g) per Brake Horse Power (BHP) on the crankshaft (kWh). Based on the Keel Laying Date (KLD), the merchant fleet is divided in 4 tiers. The emission limit per tier is furthermore based on the Engine Rated Speed (ERS or n). Certification is done before and after installation of the engines on board, based on test procedures described in the NO
x Technical Code [
39]. These procedures are based on a weighted averages of 5 different test cycles with 4 to 5 engine loads and corresponding weighting factors (
Table S1) [
39,
40].
3. Results and Discussion
From 2015 onwards, the Royal Belgian Institute of Natural Sciences (RBINS) has been conducting airborne surveillance operations for the monitoring of Regulation 14 of MARPOL Annex VI concerning sulphur emissions from OGVs[
47,
50,
74]. The RBINS expanded its monitoring efforts in 2020 to include Regulation 13 of MARPOL [
47,
56]. The collected dataset contains 6954 FSC measurements and 2353 NO
x measurements and therefore comprises the largest set of airborne OGV emission measurements to date.
3.1.1. Belgian Airborne Monitoring Dataset
During the monitoring period from July 2015 to November 2022, the RBINS conducted 414 flights and 645 operational flight hours, making approximately 10446 low passes in the exhaust plumes of 7536 OGVs, measuring 6954 OGVs’ FSC and 2353 OGVs' NO
x emission [
47,
50,
56,
74]
The predominant type of measured OGVs were container OGVs (31%), followed by tanker OGVs (29%), general cargo and bulk carrier OGVs (20%) and passenger and Roll On Roll Off (RORO) OGVs (15%;). Regarding NOx monitoring, the majority of monitored OGVs were Tier I OGVs (52.6%); followed by Tier II OGVs (37.5%); Tier 0 OGVs (9.6%) and Tier III OGVs (0.3%).
The standard operating procedures (SOPs) specified a minimum ship length of 100 m, although smaller OGVs were occasionally included. The average length of the monitored OGVs was 214 m (Figure S.4) and only OGVs that were on-route were measured, with an average speed of 12.7 knots (Figure S.5). The majority of the monitored OGVs had a port of destination in Belgium (30%), Netherlands (24%), the UK (10%), and Germany (7%), with the main ports of Antwerp (22%), Rotterdam (17%), and Zeebrugge (5%) (Figure S.6). The most frequently observed flag states were Liberia (11%), Panama (13%), and Marshal Island (10%) (Figure S.7), which corresponds to the global fleet distribution [
75].
For the data collected between 2015 and 2019, no correction for the NO cross sensitivity was to be made. Therefore the data between 2015 and 2019 was corrected to allow for a long-term analysis together with to the measurement results from 2020. For this purpose, the FSC values without NO correction (
FSC) and the FSC values with NO correction (
FSC) for the period 2020-2022 were used to conduct a linear regression (
Figure 2). The regression values (
a = 0.93;
b = 0.02. and
r = 0.98) were consequently used to correct the 2015-2019 FSC measurement data (
Figure 2).
3.1.2. Average FSC and Compliance Trends
The improved measurement quality and reduced measurement uncertainty in 2020 allowed for more accurate measurements of the FSC [
47]. To enable long-term trend analysis, the measurements for the 2015-2019 period were first corrected for their systematic bias. Based on the corrected FSC data, it was found that throughout the monitoring period 2015-2022, the average FSC of the measured OGVs remained relatively consistent (
Figure 3A). However when examining temporal trends, it was discovered that the average FSC significantly decreased from 0.083% to 0.068% after the implementation of the Global Cap in 2020 (
P < 0.01). The improved accuracy in 2020 reduced the OGV compliance threshold from 0.15% FSC to 0.13% FSC. Therefore, the FSC measurements from 2015 to 2019 were re-categorized according to the 2020 thresholds. While these thresholds are not suitable for individual OGV compliance re-assessments for 2015-2019, they can be used for long-term compliance analysis. The overall compliance rate spanning from 2015 to 2022 was 92.8%, increasing from 83.3% in 2015 to over 95.1% in 2019 (
Figure 3B). After the introduction of the Global Cap in 2020, compliance rates continued to rise, reaching a maximum of 98.0% in 2021, but then decreased to 95.1% in 2022. Comparing the non-compliance rates of 2018-2019 (6.1%) with 2020-2022 (3.1%) showed a significant decrease (
P < 0.001), indicating the effectiveness of the Global Cap. Non-compliance rates per flag state (OGV’s nationality) were proportionate to the number of observations per flag state, although in contrary to what was expected it was found that flag states like BE, UK, and HK had higher non-compliance rates than the “flags of convenience”, i.e. flags of convenience are countries with favorable regulations and lower taxes, allowing OGV owners to avoid stricter regulations and labor standards in their home countries (Figure S.7).
During the monitoring period from 2015 to 2022, there was a significant increase in the number of OGVs equipped with Exhaust Gas Cleaning Systems (EGCS). At the start of the monitoring in 2015, less than 1% of OGVs had an EGCS. However, as a result of the implementation of the Global Cap, by the end of 2022, approximately 30% of the global fleet was equipped with an EGCS [
76]. The effect of this trend was examined by comparing the average FSC and non-compliance rates for EGCS and non-EGCS OGVs. It was found that EGCS OGVs had significantly higher FSC levels (0.097% FSC) compared to non-EGCS OGVs (0.076% FSC) (
P < 0.001). Also the non-compliance rate was found to be significantly higher for EGCS OGVS (9.4%) compared to non-EGCS OGVs (6.9%) (
P = 0.0302). Furthermore, the analysis revealed both a very high relative contribution and very high absolute emission values for non-compliant EGCS OGVs. In the period 2015-2019, 11 out of 102 red flags were related to EGCS OGVs (11%), whereas from 2020 onward, out of the 20 observed red flags, 16 were related to EGCS OGVs (80%). This indicates that EGCS OGVs not only result in higher amounts of non-compliance, but of equal concern was that they were found to emit substantially higher levels of SO
2 once identified as non-compliant. This trend can be attributed to certain international regulations, with MARPOL Annex VI Regulation 13 that allowed the use of EGCS systems in the first place. The introduction of the Global Cap, resulted subsequently in more EGCS OGVs operating outside ECAs. While the average FSC and non-compliance decreased with the introduction of the Global Cap, it also lead to a wider use of EGCS OGVs and which, based on this analyses, resulted in higher SO
2 emissions and non-compliance rates. It should be noted that this adverse effect on air quality is in addition to other environmental concerns arising from the discharge of washwater from EGCS [
28,
77,
78,
79].
3.1.3. Real World NOx Emissions and Compliance Trends
The airborne monitoring of NO
x emissions from OGVs began a year before the implementation of the NECA in 2021, this allowed to assess the impact of the new regulations on real-world NO
x emissions and compliance to regulation 13 of MARPOL Annex VI. It was found that neither the average NO
x emission level nor the non-compliance rate were reduced after the NECA was introduced. In fact, there was an observed significant increase in the average NO
x emissions from OGVs from 12.6 to 13.5 g NO
x/kWh (
P < 0.001) (
Figure 6A). Also the non-compliance rated increased from 3.7% to 3.9%, although this difference was not found to be significant (
P = 0.8).
The previously discovered trend that Tier II OGVs had a higher average NO
x emission and non-compliance rate compared to Tier I [
56] was re-confirmed by including the data from 2022 (
Figure 6B,C). This is a result of the validation method for engine certification in the NO
x Technical Code which defines two engine cycles for main engines based on four different engine states for the calculation of the weighted average (Table S.1). This means that an engine is considered compliant as long as the weighted average is below the limit, even if emissions at certain engine states exceed the limit. Tier II engines were found to have higher NO
x emissions in the lower engine states due to fuel optimization, while Tier I engines have more constant emission levels with engine load[
56]. As lower engine states are often used in coastal shipping lanes with heavy traffic density, this results in Tier II OGVs emitting more NO
x in areas most prone to significant environmental and health impacts. Furthermore, currently for main engine states below 25% there are no emission limits in place and 23% of the observed non-compliant OGVs had an engine load below 25% and therefore were not regulated (
Figure 4).
Tier III engines have a not-to-exceed (NTE) limit for all engine states, set at 150% of the emission limit, making it easier to assess compliance as it doesn’t require an assessment of the weighted average. However, only 7 Tier III OGVs (0.3%) have been monitored so far. This is concerning as many OGVs that were built after 2021, when the Tier III limits came into force, but also those that are planned still have a KLD before 2021, which means that they may still follow the Tier II limits. It was discovered that only 21% of the merchant OGVs larger than 5000GT, built in 2021 and 2022 had a KLD in or after 2021 and were certified as Tier III.
Figure 5.
Difference between Built Year and Keel Laying Date over time.
Figure 5.
Difference between Built Year and Keel Laying Date over time.
It cannot be overstated that this delay will have a substantial impact on the environment and public health [
13,
56]. Another remarkable discovery was that 43% of the limited number of observed Tier III OGVs were found to be non-compliant, this is in line with the observations made during the EU-funded Horizon 2020 project "Shipping Contributions to Inland Pollution Push for the Enforcement of Regulations" (SCIPPER) [
80]. The analysis provided clear evidence of the inefficacy and even counterproductive nature of the international maritime NO
x emission regulations in reducing actual emissions from OGVs in the ECA.
To evaluate how an EGCS affects NO
x emissions, an analysis was conducted on the average NO
x emission levels and compliance rate to NO
x emission standards between EGCS OGVs and non-EGCS OGVs. The findings revealed that EGCS-equipped OGVs had an average NO
x emission level of 14.4 g NO
x/kWh, which was significantly higher than the 13.1 g NO
x/kWh for non-EGCS OGVs (P < 0.001). Furthermore, the non-compliance rate for EGCS OGVs was significantly higher at 6%, compared to 2% for non-EGCS OGVs (P < 0.001) (
Figure 6D). These results were consistent across all monitoring years and for both Tier I and Tier II (Figure S.8). These results confirm previous research that indicated the (minor) effect of EGCS on NO
x emission levels [
77].
Figure 6.
Boxplot with median, 10, 25, 75, 90% percentiles, and average NOx emissions per year (A) and per tier level (B). NOx non-compliance per tier and per year (Tier III is not represented due to the absence of Tier III data in 2020 and 2021).
Figure 6.
Boxplot with median, 10, 25, 75, 90% percentiles, and average NOx emissions per year (A) and per tier level (B). NOx non-compliance per tier and per year (Tier III is not represented due to the absence of Tier III data in 2020 and 2021).
3.1.4. Impact Emissions from OGVs on Belgian Inland Air Quality
By analyzing the annual average SO
2 and NO
x concentrations conducted in air quality monitoring stations located in coastal or port areas compared to those not situated in such areas the relative importance of air pollution from OGVs was elaborated. It was of no surprise that both SO
2 and NO
x concentrations were substantially higher in coastal/port areas, although declining concentrations were generally observed for both SO
2 and NO
x at all stations (
Figure 7).
The difference in SO
2 concentration between coastal/port and non-coastal/port stations was found to slightly decrease between 2008 and 2016, but later started to re-increase slightly, (
Figure 7, left). Clear reductions were observed in 2010, 2013, 2015, and 2016, which can be attributed to the implementation of international SO
2 emission limits for OGVs. However, the reduction in 2013 can be neglected as it was linked to a short-term increase in pollution for non-coastal stations due to inland pollution sources [
70].
Regarding the difference in NO
x concentration between coastal/port and non-coastal/port stations, a very stable trend was observed over the last decade (
Figure 7, right). This indicates that emissions from OGVs have not decreased over time and were not substantially impacted by the introduction of the NECA in 2021. Consequently, emissions from OGVs are increasingly contributing to the total inland NO
x pollution.
The SO
2 analysis categorized by emission source clearly indicate the impact of the introduction of the SECA and subsequent reductions in maximum allowed FSC in 2010 and 2015 (
Figure 8A). However, the future trend analysis also suggests that the contribution of SO
2 emissions from OGVs to the total SO
2 emissions in the Flemish region is expected to increase to approximately 7% by 2030.
Regarding NO
x emission sources, the impact of international shipping regulations was found to be less profound. The contribution of NO
x emissions from OGVs to inland NO
x in the Flemish region shows an upward trend. The establishment of the NECA in 2021 has had no significant impact on this trend, it is even projected that by 2025 the NO
x emission contribution from OGVs will surpass any other source and contribute to 40% of all NO
x emissions for the Flemish region by 2030 (
Figure 8B). These results largely corroborate the findings of previous large-scale studies that modelled the shipping contribution to inland pollution [
30,
31,
81,
82,
83,
84,
85]