The potential future contribution of shipping to acidification of the Baltic Sea
Abstract
International regulation of the emission of acidic sulphur and nitrogen oxides from commercial shipping has focused on the risks to human health, with little attention paid to the consequences for the marine environment. The introduction of stricter regulations in northern Europe has led to substantial investment in scrubbers that absorb the sulphur oxides in a counterflow of seawater. This paper examines the consequences of smokestack and scrubber release of acidic oxides in the Baltic Sea according to a range of scenarios for the coming decades. While shipping is projected to become a major source of strong acid deposition to the Baltic Sea by 2050, the long-term effect on the pH and alkalinity is projected to be significantly smaller than estimated from previous scoping studies. A significant contribution to this difference is the efficient export of surface water acidification to the North Sea on a timescale of 15–20 years.
Keywords
Acidification Baltic Sea Biogeochemical modelling Shipping ScrubbersIntroduction
Ocean acidification
It is now well-established that the increasing concentration of carbon dioxide in the atmosphere is causing a continuing reduction in the pH of surface ocean water at a current rate close to 0.002 pH units per year (Rhein et al. 2013); the accumulated decrease since the onset of industrialisation is estimated to be 0.1 pH units. Carbon dioxide is, however, not the only anthropogenic acid that has the potential to reduce oceanic pH. High temperature combustion leads to the formation of nitrogen oxides (NOX), while combustion of sulphur-rich fuels releases a mixture of sulphur oxides (SOX) to the atmosphere. In the atmosphere, these oxides are converted to nitric and sulphuric acids, respectively.
Deposition of strong acid also causes a change in the relative proportions of the inorganic carbon species but, in contrast to CO2 uptake, is not reversible without the addition of a strong base:
The research agenda in respect of these anthropogenic gas emissions has focused first on the atmosphere, and only later on the consequences for seawater. For many years, studies of the large-scale emission of anthropogenic CO2 had a clear focus on the greenhouse effect and climate change; the ocean featured only as a sink for a significant proportion of the anthropogenic CO2 (Solomon et al. 2007). In due course, the realisation that this oceanic uptake of CO2 causes a reduction in pH led to Ocean Acidification being dubbed “The Other CO2 Problem” (Doney et al. 2009); it is now included in the IPCC assessments of climate change (Rhein et al. 2013).
Acidification by sulphur and nitrogen oxides
While the consequences of CO2 emissions are long-term and global, the consequences of SOX and NOX emissions have a more direct impact on air quality and human health. They are also more local since these oxides have a much shorter residence time than CO2, of the order of days (Rodhe et al. 2002), so that the greater part of the resulting acidic aerosols is deposited within tens or hundreds of kilometres of the source. Since the effects of SOX and NOX emissions are relatively local, and have consequences for human health, the first priority was to reduce the emissions of these gases from terrestrial activities, a process that continues to this day.
There have been few studies of ocean acidification by sulphuric and nitric acids derived from anthropogenic emissions of SOX and NOX. Doney et al. (2007) carried out a global assessment using data from the 1990’s, which give a deposition flux equivalent to 4 Tmol protons per year after nitrification of deposited ammonia, compared with a CO2 uptake of 138 Tmol per year. These authors concluded that the resulting changes in alkalinity and dissolved inorganic carbon served to minimise the resulting decrease in surface water pH to less than 0.0001 pH units per year over most of the ocean, compared with a decrease of 0.002 pH units per year due to CO2 uptake (Orr 2011).
Shipping as a source of ocean acidification
Hunter et al. (2011) modelled strong acid acidification in three sea areas on an annual basis, giving decreases of 0.00056, 0.00010 and 0.00027 pH units per year for the North Sea, Baltic Sea and South China Sea, respectively. Hassellöv et al. (2013) modelled the shipping-derived pH decreases worldwide on a seasonal basis, indicating that sea areas with heavy shipping traffic and seasonal stratification can be subject to larger pH decreases on a seasonal basis, although over a full year, the results were broadly compatible with the earlier work of Doney et al. (2007) and Hunter et al. (2011). Hagens et al. (2014) showed that the modelled pH decreases can be substantially modified by taking account of the rates of nitrification and of air-sea exchange of CO2. Omstedt et al. (2015) applied a process-oriented Baltic Sea model indicating that acidification due to the atmospheric deposition of acids peaked around 1980, with a cumulative pH decrease of approximately 0.01 in surface waters. The acid contribution of shipping was estimated to one order of magnitude less than that of land emissions. More recently, Stips et al. (2016) have used a spatially-resolved model to examine the potential impact of scrubber operation on acidification of the North Sea over a 1-year period. These authors conclude that the largest effects are confined to near-coastal areas, most particularly in the vicinity of major ports, where the acidifying effect due to SOX can equal or exceed that due to CO2.
Regulation of acid emissions from shipping
Since commercial shipping also acts as a source of these acidic oxides, emission regulations for shipping are under development, albeit at a much slower pace. A brief summary is given here; a more detailed account can be found in Turner et al. (2017). Environmental regulation of shipping activities worldwide is a responsibility of the International Maritime Organisation (IMO), an agency of the United Nations. Regulations are adopted in the framework of the International Convention for the Prevention of Pollution from Ships (MARPOL), where Annex VI is concerned with air pollution.
For SOX, this Annex limits the maximum sulphur content of marine fuel to 3.5%, due to be reduced to 0.5% in 2020. However, separate regulations apply in Sulphur Environmental Control Areas (SECA), where the limit is 0.1% from January 2015. These restrictions are relatively modest compared with those applying to terrestrial fuels: in the European Union, for example, the maximum sulphur content of fuels for terrestrial transport is 10 ppm (0.001%). At present the only SECA in European waters covers the Baltic Sea and the North Sea.
For NOX, the regulations are more complex since the maximum allowed emission depends on the age of the ship, in contrast to SOX regulations that apply to all ships irrespective of age. The most stringent regulations apply to newly built ships within Nitrogen Environmental Control Areas (NECA). There is at present only one NECA, along the east coast of North America. A second NECA in the Baltic Sea is planned to come into effect in 2021 (HELCOM 2016).
Scrubber technology
Although MARPOL Annex VI prescribes the maximum sulphur content of marine fuels in different areas, it explicitly allows the adoption of engineering solutions that achieve the same low flux of SOX to the atmosphere. This has led to a strong interest in scrubber technology as a cost-effective method to meet the stricter emission regulations applying in SECA areas from 1 January 2015. The recent decision of IMO to reduce the maximum sulphur content of marine fuel from 2.5 to 0.5% worldwide from 2020 rather than 2025 can be expected to generate a more widespread interest in scrubber technology. Interestingly, while the regulations for atmospheric emission of SOX are mandatory, there are no mandatory regulations concerning the properties of the scrubber effluents. Instead, IMO has issued guidelines that are suggested as a basis for national discharge regulations. The guidelines state that the pH at a distance of 4 m from the discharge point should not be lower than 6.5, and that the scrubber should not take up more than 12% of the emitted NOX. Limits are also proposed for turbidity and polycyclic aromatic hydrocarbons in phenanthrene equivalents.
The simplest (and cheapest) option is an open-loop scrubber, which discharges the resulting acidified water back to the sea surface. An alternative approach uses closed-loop scrubbers that limit the acidic discharge by neutralising the absorbed acid and recycling part of the treated water. The description “closed-loop” is somewhat misleading since these scrubbers discharge recycled neutralised water back to the sea surface, but in much smaller volumes that open-loop scrubbers.
The aim of the work reported here is to assess the potential consequences for the Baltic Sea of extensive use of scrubber technology in the coming decades.
Materials and Methods
Emission scenarios
Scenarios investigated for the sulphur content of marine fuels and for the use of wet scrubbers
Scenario no. | Shipping not using wet scrubbers | Shipping using wet scrubbersb | ||
---|---|---|---|---|
% of total | % sulphur in fuela | % of total | % sulphur in fuel | |
1 | 100 | 1.0 | 0 | |
2 | 100 | 0.5 | 0 | |
3 | 100 | 0.1 | 0 | |
4 | 50 | 0.1 | 50 | 2.7 |
5 | 0 | 100 | 2.7 |
For each scenario, three model runs were carried out. The first was a control run without emissions of SOX and NOX, although the nitrate content of the surface water was increased to correspond to the amount of anthropogenic NOX deposition: this was done to ensure that the effects of acidification could be examined in isolation. The second run included SOX and NOX deposition from shipping alone; and the third run included deposition from both shipping and terrestrial sources.
Atmospheric modelling
The spatial distribution of atmospheric deposition of SOX and NOX from the ships is estimated by the atmospheric chemical transport model EMEP (Simpson et al. 2012). The model was run for the meteorological years 2009–2011 and the deposition fields from these 3 years were scaled with ship emission scenarios covering other periods. Details of the method used to determine the atmospheric deposition are described in Claremar et al. (2013) and Omstedt et al. (2015). Historical data were produced by a combination of emission databases and deposition from the EMEP model (Omstedt et al. 2015). Atmospheric background concentrations for future scenarios follow the RCP 4.5 emissions scenarios from 2010 (Lamarque et al. 2010) and deposition simulations (Engardt and Langner 2013) using the MATCH model (Robertson et al. 1999). For the ship contributions to atmospheric deposition, the RCP 4.5 information (Eyring et al. 2010) is replaced by the different traffic and scrubber scenarios described in the previous section and in Table 1.
Biogeochemical modelling
Map of the Baltic Sea showing the 13 sub-basins modelled in this work
Results
Projected pH and alkalinity changes due to shipping
In all figures, the projected pH and alkalinity changes are plotted as the difference between the individual scenarios and the corresponding control run where no emissions of SOX or NOX are included. In this way, the effect on the water chemistry of the emissions in an individual scenario can be examined.
Transects from Kattegat to the Bothnian Bay for the five emission scenarios (Table 1), showing the pH and alkalinity changes due shipping alone
Modelled future changes in pH and alkalinity due to shipping alone, plotted as differences from the relevant control runs, in surface waters of the Arkona Basin, East Gotland Basin and the Bothnian Bay, according to the five scenarios described in Table 1
Modelled future changes in pH and alkalinity, plotted as differences from the relevant control runs, in surface waters of the Arkona Basin, East Gotland Basin and the Bothnian Bay, assuming that all ships use open-loop scrubbers (scenario 5). Total deposition includes both terrestrial and shipping sources. The grey vertical lines in the left panels show the annual range of pH changes
The proportion of strong acid deposition to the Baltic Sea due to shipping. The coloured lines show projections for the five scenarios presented in Table 1
Acid deposition reduces CO2 uptake
While the oceans act as a sink for anthropogenic carbon dioxide, deposition of strong acids can be expected to result in a reduced CO2 uptake due to the lowering of both pH and alkalinity. We have used our model results to examine this effect, and find a clear linear relationship between strong acid input and reduced CO2 uptake on an annual basis. The molar ratio between reduced CO2 uptake and strong acid addition is 0.812 ± 0.001 for total emissions and 0.826 ± 0.006 for shipping emissions. This indicates that shipping in the Baltic Sea has an additional, indirect “carbon footprint” amounting to ca. 82% of strong acid emissions.
To what extent do the chemical changes accumulate in the Baltic Sea?
Decreases in the alkalinity content of the entire Baltic Sea due to strong acid deposition, together with the resulting calculated changes in dissolved inorganic carbon (DIC) compared to the relevant control runs. Both parameters are normalised to their maximum decrease
Discussion
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Firstly, shipping activities are not evenly distributed over the surface water of the different basins, but in many cases confined to specified shipping routes. This means that discharges of scrubber effluent will be concentrated along the shipping routes before spreading to the remainder of the basin by lateral advection. This effect could not be explored in the basin-oriented model described here. Stips et al. (2016), using a 10 km horizontal resolution, modelled the effects of SOX deposition in the North Sea over a 1-year period, concluded that the acidification effects are largely confined to coastal areas close to major ports. Examining the consequences of scrubber operation along the major shipping routes would, however, demand a model with a significantly smaller horizontal grid size.
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Secondly, the strong acids are not the only significant chemical component of scrubber effluent: toxic substances in the form of heavy metals, organic compounds and particulate matter have also been shown to be present in scrubber effluents. These components can be expected to have different chemistries for wet scrubber input and atmospheric deposition, since in the latter case, the material will be subject to chemical transformation in the atmosphere before deposition (Russell et al. 1999).
-
Thirdly, smokestack gases and scrubber effluents also contain plant nutrients, notably nitrogen and iron. Further work is needed on the consequences for the marine environment of the toxic and nutrient components of smokestack gases and scrubber effluent. This is particularly important in the case of scrubber effluent, since the technology is developing rapidly, and the currently available analytical data on scrubber effluents are restricted to first generation scrubbers that may not be representative of the current generation.
In contrast to other estimates, the changes in pH on a basin scale are relatively small, being largest in the Arkona Basin. However, basin scale modelling does not allow us to see the smaller scale effects e.g., close to harbours, which are highlighted in the higher-resolution model of Stips et al. (2016). Figure 3 shows pH reductions due to the most extreme scenario of shipping emissions (red lines) ranging between 0.001 and 0.003 over a period of approximately 30 years, giving a maximum change of the order of 0.0001 pH unit per year. This is the annual rate estimated by Hunter et al. (2011) for the Baltic Sea on a “business as usual” basis, corresponding to the blue lines in Fig. 3, where the annual rate of reduction is an order of magnitude lower than the extreme scenario (red lines). Our new estimates are therefore significantly lower, which may in part be due to the ability of the Baltic Sea to export these chemical changes to the North Sea (Fig. 6). This export arises since, in contrast to most other contaminants, strong acids are not exported to deep water via the marine food chain and thus do not accumulate there to a large extent. Thus the deep water alkalinity changes in Fig. 2 are relatively modest in comparison with the surface waters of the Bornholm and Arkona Basins and the Belt Seas that contribute to the surface outflow from the Baltic Sea to the Kattegat, Skagerak and North Sea. However, the other contaminants associated with smokestack emissions and scrubber water discharge (organic compounds, trace metals and small particles) can be expected to enter the food chain and accumulate in the Baltic deep waters, thus affecting Baltic Sea chemistry over much longer time periods that the strong acid input. There is thus a need for further research that focuses on quantifying the emissions of these contaminants from smokestacks and scrubbers, and examining their fate in the Baltic Sea.
The results presented here focus on the effects of SOX and NOX ship emissions in relation to a control scenario where no such emissions occur. The control scenario is by no means constant, but reflects the climate-driven changes expected according to the A1B storyline and changes in deposition of emissions from other sources following RCP4.5. Müller et al. (2016) have recently reviewed the temporal development of alkalinity in the Baltic Sea, and have shown that the alkalinity is currently increasing. This may be connected to the observed increase in organic carbon concentrations (Fleming-Lehtinen et al. 2015), since it has been shown that organic matter contributes to the total alkalinity in the Baltic Sea (Kulinski et al. 2014). The alkalinity reduction due to the deposition of acidic oxides thus occurs against the background of increasing alkalinity that is included in our control runs.
Conclusions
Basin scale modelling projections of the Baltic Sea through to 2050 indicate that shipping will become the major source of strong acid addition to surface waters, most particularly if there is widespread use of wet scrubber systems. These strong acid additions result in a reduced uptake of atmospheric carbon dioxide of approximately 82% on a molar basis. The effects on the chemistry of the Baltic Sea are projected to be transient with a timescale of 15–20 years. The overall consequences for the alkalinity and pH of the Baltic Sea are small on a basin scale.
Notes
Acknowledgements
This work was supported by the Swedish Research Council Formas (Contract No. 210-2012-2120).
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