Emissions generated by shipping include sulfur oxides (SOx), nitrogen oxides (NOx) and carbon dioxide (CO2). Emissions to air are regulated by Annex VI in MARPOL, the International Convention for the Prevention of Pollution from Ships. The Regulations for the Prevention of Air Pollution from Ships (Annex VI) seek to minimize airborne emissions and their contribution to global air pollution and environmental problems.

Annex VI, which was added in 1997, limits the air pollutants in marine fuels and prohibits deliberate emissions of ozone depleting substances. The revision of Annex VI entered into force in 2010. Work is now being done to drastically reduce emissions of SOx, NOx and particulate matter. Special "Emission Control Areas" have been introduced to reduce emissions of air pollutants in designated areas.

Fuel efficiency

IMO regulations on fuel efficiency will soon be in force for vessels trading exclusively in national waters. This gives shipowners the opportunity to save on fuel consumption.

IMO regulations on fuel efficiency for ships

MARPOL Annex VI now includes a chapter with regulations for fuel efficiency in ships. The goal is to improve fuel efficiency in both new and existing ships. This will lead to reductions in all fuel induced emissions to air, for exampe CO2, NOx, SOx, PM and so forth. IMO regulations are in force for all vessels over 400 tonnes in international waters from January 1st 2013.

CO2 - Carbon dioxide

Carbon dioxide (CO2) is a colourless, odourless gas that is produced when carbon is burned in an excess of oxygen. The main removal processes of CO2 from the atmosphere are absorption by seawater, and uptake (in photosynthesis) by plankton and plants.

In ship operations the main source of CO2 emission is combustion of fossil fuels in main engines, auxiliary engines, boilers and incinerators. CO2 can also be used in fire fighting systems.

SOx - sulphur oxides

The emissions of sulphur oxides from ships originate from the combustion of fuels containing sulphur. Sulphur dioxide (SO2), the main sulphur oxide gas, is a colourless, non-flammable gas with a penetrating odour that irritates the eyes and air passages. It reacts on the surface of a variety of airborne solid particles, is soluble in water and can be oxidised within airborne water droplets.

Marine fuels usually have a high sulphur content compared to fuels used on land. In Europe shipping constitutes approximately 20% of the SOx emitted. This share is expected to grow in the years to come as land based sources reduce their SOx emissions more relative to shipping.

IMO has been monitoring the worldwide average sulphur content of residual fuel supplied for use on board ships since 1999 following the adoption of resolution MEPC.82(43) Guidelines for monitoring the world wide average sulphur content of residual fuel supplied for use on board ships.

VOC - Volatile Organic Compounds

Volatile Organic Compounds or VOCs are organic chemicals that easily vaporize at room temperature. They are called organic because they contain carbon in their molecular structures. VOCs have no colour, smell, or taste. VOCs include a very wide range of individual substances, such as hydrocarbons (for example benzene and toluene), halocarbons and oxygenates. Hydrocarbon VOCs are usually grouped into methane (CH4) and other non-methane VOCs (NMVOC).

VOC is generated during combustion, and handling of oil products, whereas the latter is the most significant emission source related to shipping. The petroleum sector is the most important European source of emissions of VOCs due to loading of crude oil onto tankers generating large quantities of VOCs.

Following the loading, the ship commences the loaded voyage. During the voyage, additional gas may be released from the cargo. Gas release may contribute to tank pressure exceeding the pressure relief valves limit and tank gas containing VOC is emitted.

During unloading, inert gas is added to the cargo tanks. There will normally be very limited or no emissions during unloading. However, the adding of inert gas will affect the amount of VOC in the tanks after unloading completion. Most of the remaining VOC will be emitted during the loading operation.

NOx - Nitrogen oxides

Nitrogen oxides (NOx) are toxic gases which are formed by high-temperature combustion of fuels like oil, diesel and gas. Emission of NOx contributes to acid rain and the formation of ground-level ozone, which can harm plants and wildlife. It can also lead to serious health problems in humans - primarily damage to the lungs and respiratory system.
The major sources of man-made NOx emissions in Norway are energy production, road traffic and shipping. Domestic shipping and fishing account for about a third of the total emissions. However, it is not only Norwegian emissions that threaten our nature; this kind of air pollution can be carried over long distances and across national borders.
Through the Gothenburg Protocol, Norway has undertaken to limit NOx emissions to a maximum of 156.000 tonnes per year from 2010. Shipping has a great potential for reducing its NOx emissions, by cleaner fuel or installation of treatment systems.
On 1 January 2007 a tax on emissions of NOx was introduced by the Norwegian government as an incentive to reduce NOx emissions. The tax applies to the offshore and shipping industry as well as large-scale land-based industry. In 2013 the tax is 17.01 NOK/kg NOx emitted. Companies can get tax exemption by entering into an environmental agreement with the government on measures to reduce NOx.