Explained: SOx(sulphur oxides) and NOx(nitrogen oxides) emissions from ships.

  • Air pollution is high on the global agenda.
  • More stringent measures have been adopted by the International Maritime Organisation (IMO) in relation to sulphur oxides(SOx) and nitrogen oxides(NOx).
  • Globally air pollution is regulated by IMO through its international convention for the prevention of pollution from the ships (MARPOL) and its Annex 6.

  • Marpol annexe 6, first adopted in 1997 limits the main air pollutants contained in ships exhaust gases, SOx and prohibits deliberate emission of ozone-depleting substances.
  • Marpol annexe 6 also regulates shipboard incineration and the emissions of volatile organic compounds from the tankers.
  • Annex 6 entered into force on 19 May 2005.
  • IMOs Marine environment protection committee (MEPC) agreed in 2005 to revise MARPOL Annex 6 with the aim of significantly strengthening the emission limits of technological improvements and implementation experience.

In October 2008 IMO adopted the amendments to annexe 6 which, among other things strengthened the requirements on the permitted sulphur levels in ships fuels.
The main changes to MARPOL Annex 6 are a progressive reduction globally in emissions of SOx, NOx and particulate matter.
The amendments entered into force on 1st July 2010.

Under the revised MARPOL Annexe 6, the global sulphur cap is reduced initially to 3.50% (from the current 4.50%), effective from 1st January 2012; then progressively to 0.50%, effective from 1st January 2020, subject to a feasibility review to be completed no later than 2018.

The limits applicable in the emission control zone (ECA's) for SOx and particulate matter were reduced to 1%, beginning on 1st July 2010 (from the original 1.50%); being further reduced to 0.10%, effective from 1 January 2015.

Progressive reductions in NOx emissions from marine diesel engines installed on ships are also included, with a "Tier 2" emission limit for engines installed on or after 1 January 2011; then with a more stringent "Tier 3" emission limit for engines installed on or after 1 January 2016 operating in ECAs.

Marine diesel engines installed on or after 1 January 1990 but prior to 1 January 2000 are required to comply with "tier 1" emission limits if an approved method for that engine has been certified by an administration.
The revised measures are expected to have a significant beneficial impact on the atmospheric environment and on human health, particularly for those people living in Port cities and coastal communities.

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