Impact of Sulfur Cap on Pollution Levels over the Aegean and the Eastern Mediterranean

Here, IMO 2020 Sulfur Cap regulation impacts on air quality were comprehensively investigated over the Aegean and the Eastern Mediterranean by integrating satellite-based SO₂ and NO₂ retrievals, and shipping route density analysis for the period of 2019–2023. This approach provided insights into how international maritime fuel regulations affect SO₂ and NO₂ levels, and heavily trafficked seas. TROPOMI SO₂ and NO₂ retrievals were evaluated over major shipping routes, open sea areas and important ports. The findings demonstrated that IMO 2020 regulation improvements were regionally and seasonally heterogeneous. In the Aegean Sea, satellite data indicated reductions in SO₂ levels, particularly during the summer and autumn. Ports such as Canakkale, Izmir and Aliaga exhibited declines in SO₂ concentrations confirming the measurable impact of the Sulfur Cap on air quality around the coastal regions. In contrast, the Eastern Mediterranean presented a more complex picture. Certain ports and routes, notably around Mersin, Taşucu, and Iskenderun, exhibited either no change or even increases in SO₂ levels during the post-2020 period. At the regional scale, TROPOMI retrievals showed elevated SO₂ over high-traffic corridors, including the Suez Canal approaches and Levantine Basin, despite the Sulfur Cap. Several factors may account for this variability, with potential non-compliance and weaker inspection regime with the Sulfur Cap regulation and the growing intensity of maritime traffic in critical transit corridors such as the Suez Canal. All of these impacts can diminish the anticipated reductions in ship-related SO₂ emissions, and thereby weaken the overall improvement in regional air quality.

Route-based analysis showed a slowly declining trend emerged along high-intensity corridors when SO₂ concentrations were normalized by shipping activity. These decreases, ranging from 14–16% in the Aegean, and approximately 10% in parts of the Eastern Mediterranean, suggested that the regulation achieved per-unit emission reductions, even though absolute SO₂ levels did not decline as strongly due to growing shipping traffic volumes. The analysis of NO₂ revealed a different regulatory outcome. While localized reductions were evident around ports such as Beirut, Ashdod, Haifa and Souda, broad-area increases were observed in open-sea regions, particularly along the main transit corridors.

In conclusion, IMO 2020 Sulfur Cap has yielded positive, but irregular air quality improvements across the Aegean and the Eastern Mediterranean. The benefits were pronounced in the Aegean Sea and around major Turkish ports, while the Eastern Mediterranean exhibited mixed outcomes shaped by maritime traffic growth and possible regulatory non-compliance. The observed reductions were generally smaller than the expected emission decreases in fuel sulfur content. These findings showed the significant contribution of shipping to regional air pollution and the necessity for stricter control measures, especially on open seas where enforcement is limited. The observed spatial heterogeneity emphasizes the critical role of localized monitoring and regional governance complementing global maritime policies to achieve cleaner air.

Keywords: Eastern Mediterranean; Shipping; IMO Sulfur Cap; SO₂; NO₂