Investigation of maritime sector contribution to environmental pollution in the surrounding seas of Türkiye with satellite retrievals

Abstract

Maritime transport is a fundamental component of the global economy, carrying approximately 90% of international trade by volume. As a cost-effective mode for transporting large quantities of goods across continents, the shipping industry significantly contributes to economic growth and the international trade capacity of nations. Although shipping plays a vital role in global trade, it also poses significant environmental challenges, particularly in terms of air and marine pollution. Ships emit significant quantities of primary air pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), and volatile organic compounds (VOCs), which also contribute to acid rain and secondary aerosol formation. In addition, wastewater, ballast water, and bilge water might introduce pollutants into marine ecosystems, leading to biodiversity loss and deterioration of water quality. Moreover, the shipping sector is a notable contributor to climate change with significant greenhouse gas emissions globally. Regulating shipping-related pollution is rather complex due to the international operations of maritime transport, which often occur beyond the jurisdiction of any single nation. This transboundary nature requires a globally coordinated approach, primarily governed by the International Maritime Organization (IMO) through the International Convention for the Prevention of Pollution from Ships (MARPOL) Convention. While MARPOL sets the legal framework for controlling emissions and discharges, its effectiveness relies heavily on the enforcement efforts of flag and port states. This thesis focuses on the impacts of ship-related pollution on atmosphere and marine environment, utilizing recent state-of-the-art high-resolution remote sensing satellite retrievals. In this thesis, ship-related air pollution was assessed using Tropospheric Monitoring Instrument (TROPOMI) NO2 and SO2 satellite retrievals along with route density across the surrounding seas of Türkiye. Additionally, chlorophyll-a (Chl-a) concentrations derived from Global Change Observation Mission (GCOM) satellite retrievals were evaluated to investigate potential impacts of shipping activity, wildfire events, and dust transport. The first research chapter examines the effect of maritime activity, wildfires, and dust storms on phytoplankton growth and presents a comprehensive five-year spatio-temporal assessment of Chl-a levels across the study area covering the Sea of Marmara, Aegean Sea, and Eastern Mediterranean, including coastal and open sea areas. Investigations utilizing GCOM retrievals revealed areas and intervals of increased Chl-a levels. A novel quantitative method was developed to categorize open sea areas based on shipping intensity, demonstrating a significant association between high shipping activity and elevated Chl-a concentrations for the first time in the literature. The association was particularly remarkable during times of limited phytoplankton growth in the Eastern Mediterranean. In addition, three specific episodes focusing on one anthropogenic (livestock transport) and two natural sources (wildfire and dust) were selected for the investigation of high Chl-a concentrations in open sea. The dust episode exhibited the most widespread and intense Chl-a increase, followed by the wildfire episode with noticeable but lower Chl-a increase. On the other hand, livestock episode indicated more localized, but intense increases. These natural and anthropogenic factors elevated Chl-a levels compared to pre- and post-event periods. These findings underline the crucial role of overlooked shipping activities on phytoplankton dynamics in open seas. Natural events such as wildfires and dust storms are anticipated to occur more frequently and severely in the Mediterranean due to climate change, highlighting the need for efforts to quantify and mitigate shipping activities that might contribute to algal blooms. The second research chapter focuses on the Russia-Ukraine war, which began on February 24, 2022 and affected maritime transport in the Black Sea, leading to significant changes in air pollution levels in addition to other social and environmental impacts. This chapter examines the impact of reduced maritime activity on air pollution levels in the Black Sea by analyzing TROPOMI NO2 and SO2 as well as COBRA algorithm SO2 retrievals from two years: 2019 (pre-war) and 2022 (war) along with 2019-2022 interval. To determine spatio-temporal variations, retrievals were spatially and temporally matched with EMODnet route density data. The pollution levels over major shipping routes and ports in the study region were examined. The results indicated that maritime traffic declined sharply around Ukrainian ports (Odessa and Yuzhny), while it increased in the eastern Black Sea, particularly near Russian ports (Novorossiysk and Tuapse). This shift in shipping patterns influenced NO2 concentrations, with significant increases in the eastern regions where maritime activity intensified and decreases in the north-western regions. In contrast, SO2 levels showed a more complex response due to additional influences and uncertainties on SO2 retrievals. These results highlighted the importance of satellite-based measurements in evaluating air quality impacts at offshore maritime regions with no ground-based monitoring. The study showed the complex effects of the Russia-Ukraine war on regional air pollution, demonstrating that while reduced maritime traffic led to lower pollutant concentrations in some areas, alternative shipping routes, military activities, and other factors contributed to increased pollution in others. Third research chapter examines implementation of the IMO 2020 Sulfur Cap regulation, which came into effect on January 1, 2020, has expected significantly influencing maritime transport concentrations by mandating a reduction in the sulfur content of marine fuels. This study investigates the impact of this regulation on air pollution levels in the Aegean Sea and Eastern Mediterranean by analyzing operational TROPOMI SO2 and NO2, as well as COBRA algorithm SO2 retrievals, for the years of 2019 (pre-regulation) and 2020-2023 (post-regulation) time interval. To assess spatio-temporal variations, the satellite retrievals were spatially and temporally matched with EMODnet route density data. Pollution levels in the study region were examined with a focus on major shipping routes and ports, enabling an evaluation of the effectiveness of the IMO 2020 regulation in reducing ship-related air pollutants in the Aegean Sea and Eastern Mediterranean. The results indicated a heterogeneous response to the regulation across the study area. In the Aegean Sea, reductions in SO2 were detected in both satellite and ground-based data, particularly around ports such as Canakkale, Izmir, and Aliaga, highlighting the effectiveness of the regulation in these areas. In contrast, the Eastern Mediterranean displayed a different pattern. Ports such as Mersin, Tasucu, Iskenderun, and the Suez Canal routes exhibited increases in SO2 concentrations after 2020. Both satellite retrievals and AQMS measurements confirmed these findings. The persistence of these hotspots suggested that the benefits of the Sulfur Cap were reduced by two main factors: the steady increase in shipping traffic through this region, and likely cases of non-compliance with the regulation. A route-based analysis revealed additional findings. When SO2 concentrations were normalized by the intensity of shipping activity, declining trends became more apparent. These per-unit reductions were around 15% in the Aegean Sea and about 10% in the Eastern Mediterranean. This showed that the regulation did reduce emissions per ship, but the absolute reductions were smaller than expected because overall shipping volumes continued to grow. The results for NO2 showed a different pattern. Compared to SO2, TROPOMI NO2 retrievals showed stronger correlations with AQMS measurements. Around some ports, including Beirut, Ashdod, Haifa, and Souda, NO2 levels decreased. However, in the open sea and along the busiest shipping routes, increases were observed. Possible reason was the increased use of low-sulfur marine fuels such as marine gas oil, which may produce slightly higher NOx emissions due to combustion characteristics. IMO 2020 Sulfur Cap had brought slightly improvements in air quality, but these improvements were not uniform. Importantly, the observed reductions were smaller than what would be expected from the seven-fold decrease in the sulfur content of the fuel. These findings highlighted those global regulations alone cannot guarantee success. Their effectiveness depends on how well they are enforced, how much compliance is achieved, and how regional differences are addressed. The designation of the Mediterranean Sea as an Emission Control Area (ECA), from 1 May of 2025, represents an important milestone for improving regional air quality. Maritime transport in the Mediterranean is steadily increasing, and with it, the associated emissions of pollutants. At the same time, the ongoing geopolitical instability in the Black Sea is reshaping shipping routes and traffic intensity, creating new pressures on surrounding seas. In addition, climate change is expected to increase both the frequency and intensity of dust transport events and wildfires, which can inject large amounts of particulate matter and nutrients into the atmosphere and marine environment. These overlapping pressures mean that the air and marine water quality of the surrounding seas of Türkiye will require attention in the coming decades. To sum up, this thesis showed that ship-based pollution plays a major role in shaping the environmental conditions of the seas surrounding Türkiye. By using satellite observations, the study demonstrated that maritime transport emissions contribute to both air and water pollution, not only near ports but also along busy shipping routes and in open-sea areas. In addition to shipping, natural events such as wildfires and dust storms were found to influence phytoplankton growth in open seas. These findings point to the need for stricter controls on ship-related emissions and stronger regional cooperation.