The exhaust gases from ships’ engines are rich in sulphur and nitrogen oxides (SOX and NOX), which are converted to sulphuric and nitric acids in the atmosphere, resulting in acidified rain. SOX production is directly dependent on the sulphur content of the fuel, which content is subject to international regulation. Stricter regulations apply in Emission Controlled Areas (ECA), which are judged to be more sensitive to SOX release. The Baltic Sea is part of an ECA where the maximum sulphur content of fuel will be further reduced in 2015. This new limit is still 100 times higher than that allowed for land transport within the EU. The introduction of a limit for NOX release in the Baltic Sea is at present still under negotiation. The development of these regulations has been based on the need for good air quality: the consequences for the marine environment of SOX and NOX release have not been considered.

Shipping companies can meet the new ECA limit either by switching to a (much more expensive) low sulphur fuel, or by stripping the SOX from the exhaust gas using a device called a scrubber. The simplest form of scrubber uses seawater, which due to its alkalinity, or buffer capacity, absorbs SOX very effectively. If the acidified seawater is then pumped directly overboard, the net result will be a very effective transfer of acids from the exhaust gases direct to the surface water.

It has generally been assumed that the natural buffer capacity of seawater is able to cope with SOX and NOX output from shipping. This conclusion is based on the total annual deposition of shipping-derived acids, averaged over the surface ocean. However, our preliminary results show that heavily trafficked coastal surface waters in the northern hemisphere, that develop a strong stratification in the summer months, can suffer significant acidification. Although this acidification will be diluted to deeper water during the autumn and winter, the marine ecosystem may have been affected.

This project ”SHIpH” follows up these preliminary results with a study of the Baltic Sea system, with the aim of developing a scientific basis for future regulations in the Baltic including the use of scrubbers. The release and subsequent deposition of SOX and NOX is modelled using data from EMEP, a European monitoring programme for atmospheric pollution. The consequences of this deposition are then assessed using an oceanographic model of the Baltic Sea, which will include an improved description of the acid-base chemistry. The new chemical model will include the contribution of dissolved organic matter to the alkalinity (buffer capacity): recent measurements show that this contribution can be significant in the Baltic Sea. The modelling explores potential scenarios both for climate change and for shipping. The climate change scenarios are based on those developed by the Intergovernmental Panel on Climate Change (IPCC). Discussions with a reference group, including representatives from government and the shipping industry, contribute to the development of shipping scenarios; including for example the potential development of scrubber use. This reference group is already established as a project group in the Zero Vision Tool, an initiative whose aims include reducing the environmental impact of shipping in close cooperation with the industry. Shipping traffic in the Baltic Sea is expected to double in the next twenty years, which underlines the importance of assessing whether large scale release of untreated scrubber water is acceptable.

Even if the modelling results indicate that acidic scrubber water should not be released without neutralising the acid, it is important to determine whether the scrubber water contains other constituents that can affect the plants and animals in the marine food chain. The project therefore includes an experimental component that assesses the effect of scrubber water on the natural microbiological community from different parts of the Baltic Sea, in different seasons. The scrubber water is generated at the marine motor laboratory at Chalmers, which is equipped with a research scrubber.

The overall aim of the project, through modelling and experiment and through dialogue with the reference group, is to arrive at an optimal approach for meeting the current and future regulations, with the least possible effect on the marine ecosystem.

In order to determine whether the model projections are realistic, it will be necessary to follow the project up with an appropriate monitoring programme. A proposal for a future monitoring programme will be developed in the final phase of the project.