a) Significance

Emissions of gases and particles from ocean-going ships may have a major impact on both photochemistry of the marine boundary layer and the biogeochemistry of the surface ocean. Throughout large regions of the ocean, ship emissions of SO2 and sulfate aerosols greatly exceed the natural DMS-borne sulfur emissions. Ship emissions must, therefore, be taken into account in estimates of global climate forcing by sulfate aerosols (Activity 1.3 of the SOLAS Science and Implementation Plan). Ship emissions are also a significant source of atmospheric nitrogen, and the deposition of this nitrogen into marine ecosystems might affect marine productivity (Activity 1.5). Finally, ship emissions of gaseous hydrocarbons and carbonaceous aerosol particles may impact both gas phase photochemistry and aerosol radiative properties over the oceans. The effects of ship emissions on atmospheric photochemistry include the production of ozone in regions that are usually sinks for ozone and the pollution-induced release of reactive chlorine from sea salt (Activity 1.2). The photochemical and radiative impacts of ship emissions may persist considerably longer than the lifetime of the ship plume itself. Ship traffic has increased significantly in recent years and is projected to keep increasing. In the Arctic Ocean, which is a particularly sensitive region, new routes are becoming available for international shipping due to decreasing sea ice cover in summer. Open ocean ship emissions are currently largely unregulated and shipping often relies on very dirty fuels that cannot be used for land-based transport. Current (2000-2002) global ship traffic as a fraction of global traffic per cell; from Wang et al. (2008); see this publication for details