The contribution of ship emissions to global air pollution is becoming increasingly important as ship traffic is growing rapidly (annual growth rate in total seaborne trade was 5.2% per year from 2002 to 2007), and ship emissions tend to be less limited by regulations than land-based emissions. Ship emissions appear to have a strong impact of concentrations of pollutants over the oceans, but as about 70% of all ship traffic takes place within 400km from land, ship emissions also have a relevant impact on air pollution on land. Ship emissions of SO₂ are particularly important, but studies have also shown an impact of ships on NO_x and black carbon levels. The contribution to global anthropogenic CO₂ emissions was 3.3% in 2007.

An international agreement, the MARPOL convention, will in the coming years reduce ship emissions of SO₂ and NO_x; the limit for sulfur content of ship fuels is planned to be reduced from the present 4.5% by weight to 3.5% in 2012 and then to 0.5% in 2020. In sulfur emission control areas (e.g. the Baltic Sea and the North Sea), the limit will be as low as 0.1%. Also NO_x emission limits will be progressively reduced.

Experimental techniques for measurements of ship emissions have (at least) two principal scopes:

– provide reliable and comprehensive data on ship emissions in order to improve emission inventories;
– identify ships that do not comply with legal emission limits.

The special issue of AMT will present and discuss available techniques for observation of ship emissions, together with some information about the results they have provided. These techniques range from optical methods such as the UV camera, DOAS and lidar, to point measurements, where the so-called "sniffer" method is applied, and on-board measurements with sampling from the stack of a ship. With the "sniffer" method, air pollutants like SO₂ or NO_x are measured along with CO₂ at a point within the plume of the stack emissions from a ship; knowing (approximately) the carbon content of the ship fuel, the emission factor for SO₂, for example, can be estimated from the ratio between SO₂ and CO₂, when their background concentrations have been subtracted.

Another issue regarding measurements of ship emissions is the type of platform that is applied for the instruments. In the papers that are expected for this special issue, both shore-based and several types of mobile platforms, airborne as well as ship-borne, have been tested. Contributions from studies using these and other platforms, including satellites, are very welcome.