As a result from different onboard combustion and energy transformation processes, remarkably for propulsion and energy production, ships represent sources of different substances to the atmosphere. Sulphur Oxides (SOx), nitrogen oxides (NOx), particulate matter (PM) and carbon dioxide (CO2) are emitted to the atmosphere as a direct result. Collectively, ship generated emissions can be significant in areas subject to heavy marine traffic leading to concerns regarding air quality, bot at local level, in coastal areas, or in a more global level, regarding to CO2 emissions leading to Greenhouse Gas emissions and contributing to global warming. Many actions have however been undertaken in recent years to significantly reduce air emissions from ships. Most of these actions have been taken through Annex VI of MARPOL, an international instrument developed through the International Maritime Organization (IMO) that establishes legally-binding international standards to regulate specific emissions and discharges generated by ships.
Source: http://www.emsa.europa.eu/main/air-pollution.html
