Sulphur Oxides (SOx) in atmospheric ship emissions resulting from the burning of fuel with high sulphur content are known to be harmful to human and ecosystem health. Since January 1st 2020, the International Maritime Organisation (IMO) lowered the previous limit for sulphur content in ship fuel from 3.5% m/m (mass by mass) to 0.50%. In the emission control areas (SECAs), the limit for the sulphur content had been set to 1.0% in 2010 and is kept below 0.1% since 2015. To comply with these limits, ship operators and owners can switch to fuel oil with lower sulphur content (LSFO). Alternatively, they can continue to burn fuel with high sulphur content by using technical means such as exhaust gas cleaning systems (or scrubbers) that reduce the atmospheric SOx emissions to a level equivalent to the required fuel oil sulphur limit. Scrubbers use sea water as cleaning media to remove SOx from the air emissions. There are three main categories of scrubbers: (1) the open-loop scrubbers that continuously discharge their wash water effluent, (2) the closed-loop scrubbers that treat the wash water before it is discharged, and (3) the hybrid scrubbers that can switch from open to closed modes. Scrubbers transform the air pollution into direct marine discharge. As hybrid scrubbers are more likely to discharge their sulphur waste into sea water rather than using land infrastructures, they are hereafter taken as open-loop ones. The effect of SOx contribution from ship on sea water pH is assessed for the English Channel and the southern North Sea by means of a marine biogeochemical model that includes a detailed description of the carbonate chemistry. This model allows testing different scenarios of SOx contribution resulting from the maritime traffic. To this end, realistic scenarios with ship traffic density estimated for the years 2019, 2020 and 2030, assuming a year-to-year ship traffic increase of 3.5% and several SOx pollution reduction strategies have been tested. An additional model simulation with null SOx contribution from the shipping sector is used as a reference level to comparatively assess the impact of each scenario on the sea water pH. Model results show a pH decrease of 0.004 units over the whole domain in case of a 2019-like ship traffic density with 15% of the fleet (in Gross Tonnage) using open-loop and hybrid scrubber systems. For future scenarios, assuming that 35% of the fleet is equipped with open-loop and hybrid scrubbers, the pH is estimated to decrease by 0.008 to 0.010 units in average over the whole domain. The magnitude of pH changes is not evenly distributed through space. According to the model results, the largest pH changes would occur in areas of high traffic density, such as along the Belgian and Dutch coasts and in the vicinity of large harbours such as Rotterdam. Ocean acidification rate attributed to climate change is estimated at 0.0017-0.0027 pH units per year. In comparison, the total pH decrease owing to the use of open-loop scrubbers would be equivalent to 2 to 4 years of climate change acidification on average over the whole domain, and to 10 to 50 years, in more local areas. The cumulative impact of ocean acidification due to climate change and to maritime traffic should therefore be considered in ecosystem assessment studies.
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RBINS Staff Publications 2020
Reliable satellite estimates of chlorophyll-a concentration (Chl-a) are needed in coastal waters for applications such as eutrophication monitoring. However, because of the optical complexity of coastal waters, retrieving accurate Chl-a is still challenging. Many algorithms exist and give quite different performance for different optical conditions but there is no clear definition of the limits of applicability of each algorithm and no clear basis for deciding which algorithm to apply to any given image pixel (reflectance spectrum). Poor quality satellite Chl-a data can easily reach end-users. To remedy this and provide a clear decision on when a specific Chl-a algorithm can be used, we propose simple quality control tests, based on MERIS water leaving reflectance (ρw) bands, to determine on a pixel-by-pixel basis if any of three popular and complementary algorithms can be used. The algorithms being tested are: 1. the OC4 blue-green band ratio algorithm which was designed for open ocean waters; 2. the OC5 algorithm which is based on look-up tables and corrects OC4 overestimation in moderately turbid waters and 3. a near infrared-red (NIR-red) band ratio algorithm designed for eutrophic waters. Using a dataset of 348 in situ Chl-a / MERIS matchups, the conditions for reliable performance of each of the selected algorithms are determined. The approach proposed here looks for the best compromise between the minimization of the relative difference between In situ measurements and satellite estimations and the number of pixels processed. Conditions for a reliable application of OC4 and OC5 depend on ρw412/ρw443 and ρw560, used as proxies of coloured dissolved organic matter and suspended particulate matter (SPM), as compared to ρw560/ρw490, used as a proxy for Chl-a. Conditions for reliable application of the NIR-red band ratio algorithm depend on Chl-a and SPM. These conditions are translated into pixel-based quality control (QC) tests with appropriately chosen thresholds. Results show that by removing data which do not pass QC, the performance of the three selected algorithms is significantly improved. After combining these algorithms, 70\% of the dataset could be processed with a median absolute percent difference of 30.5\%. The QC tests and algorithm merging methodology were then tested on four MERIS images of European waters. The OC5 algorithm was found to be suitable for most pixels, except in very turbid and eutrophic waters along the coasts where the NIR-red band ratio algorithm helps to fill the gap. Finally, a test was performed on an OLCI-S3A image. Although some validations of water reflectance are still needed for the OLCI sensors, results show similar behavior to the MERIS applications which suggests that when applied to OLCI data the present methodology will help to accurately estimate Chl-a in coastal waters for the next decade.
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RBINS Staff Publications 2022
