Ship air pollution standards are contained in the International Convention for the Prevention of Pollution from Ships (MARPOL) and regulated by the International Maritime Organization, a UN Agency. Specifically, Annex VI of MARPOL sets NOx limits for marine engines and sulfur limits for marine fuels to reduce SOx and PM. Compliance is ensured by periodic inspections and surveys as well as flag state and port state control.
Emissions limits for NOx and ozone depleting substances, as well as fuel sulfur requirements
International Maritime Organization (IMO)
The International Maritime Organization (IMO) is an agency of the United Nations that regulates the international maritime shipping industry on safety and environmental issues. It was formally established by an international conference in Geneva in 1948 and became active in 1958 when the IMO Convention entered into force. The original name was the Inter-Governmental Maritime Consultative Organization (IMCO) but the name was changed in 1982 to IMO. IMO currently consists of over 170 Member States and 3 Associate Members, as well as 80 international non-governmental organizations with consultative status and 63 intergovernmental organizations that have signed agreements of cooperation with IMO.
|Name of treaty||Key events|
|International Maritime Organization||1948: Convention establishes IMO|
|1958: Ratified, enters into force|
|International Convention on Prevention of Pollution from Ships (MARPOL 73/78)||1973: Codified|
|1978: Amended and codified|
|United Nations Convention on the Law of the Sea (UNCLOS)||1973: First meeting of the United Nations Convention|
|1994: Ratified, enters into force|
|MARPOL Annex VI (Regulations for the Prevention of Air Pollution from Ships)||1997: Codified|
|2005: Ratified, enters into force|
IMO ship pollution rules are contained in the International Convention on the Prevention of Pollution from Ships, known as MARPOL 73/78. In 1997, the MARPOL Convention was amended by the “1997 Protocol,” which added Annex VI: Regulations for the Prevention of Air Pollution from Ships. MARPOL Annex VI sets limits on NOx and SOx emissions (through fuel sulfur content regulations) from ship exhaust, and prohibits deliberate emissions of ozone depleting substances.
The IMO emission standards are commonly referred to in tiers. The Tier I standards were defined in the 1997 version of Annex VI, while the Tier II and III standards were introduced by Annex VI amendments adopted in 2008.
- 1997 Protocol (Tier I): The “1997 Protocol” to MARPOL, which includes Annex VI, became effective 12 months after being accepted by 15 States with not less than 50% of world merchant shipping tonnage. In 2004, Samoa deposited its ratification as the 15th State (joining Bahamas, Bangladesh, Barbados, Denmark, Germany, Greece, Liberia, Marshall Islands, Norway, Panama, Singapore, Spain, Sweden, and Vanuatu). At that date, Annex VI was ratified by States with 54.57% of world merchant shipping tonnage.
Accordingly, Annex VI entered into force in 2005. It applies retroactively to new engines greater than 130 kW installed on vessels constructed starting in 2000, or engines which undergo a major conversion starting in 2000. The regulation also applies to fixed and floating rigs and to drilling platforms (except for emissions associated directly with exploration and/or handling of sea-bed minerals). In anticipation of the Annex VI ratification, most marine engine manufacturers began building engines compliant with the above standards in 2000.
- 2008 Amendments (Tier II and III): Annex VI amendments adopted in 2008 introduced (1) new fuel quality requirements beginning in 2010, (2) Tier II and III NOx emission standards for new engines, and (3) Tier I NOx requirements for existing pre-2000 engines.
- The revised Annex VI entered into force in 2010. By 2008, Annex VI was ratified by 53 countries (including the United States), representing 81.88% of tonnage.
Emission Control Areas
Two sets of emission and fuel quality requirements are defined by Annex VI: (1) global requirements, and (2) more stringent requirements applicable to ships in Emission Control Areas (ECA). An ECA can be designated for SOx and PM, NOx, or all three types of emissions from ships, subject to a proposal from a Party to Annex VI. In accordance with MARPOL Annex VI, an ECA may be proposed if there is need to prevent, reduce, and control air pollution from ships, demonstrated by eight specific criteria.
The table below shows the existing ECAs worldwide and the pollutants that are controlled in those areas. China is considering applying for an ECA as of 2021.
The table below shows the existing ECAs worldwide and the pollutants that are controlled in those areas.
|Adopted||Entered into force|
|Baltic Sea||SOx/PM & NOx (ships built from 2021+)||1997 (SOx/PM) & 2017 (NOx)||2005/2021|
|North Sea||SOx/PM & NOx (ships built from 2021+)||2005 (SOx/PM) & 2017 (NOx)||2005/2021|
|North American ECA, including most of US and Canadian coast||SOx/PM & NOx (ships built from 2016+)||2010||2012|
|US Caribbean ECA, including Puerto Rico and the US Virgin Islands||SOx/PM & NOx (ships built from 2016+)||2011||2014|
Greenhouse Gas Emissions
Amendments to MARPOL Annex VI in 2011 introduced mandatory measures to reduce emissions of greenhouse gases (GHG). The Amendments added a new Chapter 4 to Annex VI on “Regulations on energy efficiency for ships.” For more information, see the International Marine GHG emissions page.
NOx Emission Standards
NOx emissions from diesel engines are controlled through specific requirements under the Engine International Air Pollution Prevention (EIAPP) Certificate program, as well as compliance with in-service requirements under regulations 13.8 and 5.3.2 of the NOx Technical Code 2008 (resolution MEPC.177(58)).
NOx emission limits (Regulation 13) are set for diesel engines depending on the engine’s rated speed (n, rpm). Tier I and Tier II limits are global, while the Tier III standards apply only in NOx ECAs. See table below.
|Tier||Ship construction date on or after||NOx Limit, g/kWh|
|na < 130||130 ≤ n < 2000||n ≥ 2000|
|I||1 Jan 2000||17.0||45 · n-0.2||9.8|
|II||1 Jan 2011||14.4||44 · n-0.23||7.7|
|III†||1 Jan 2016 (North American and US Caribbean ECAs) or 1 Jan 2021 (Baltic and North Sea ECAs)||3.4||9 · n-0.2||2.0|
|a Engine’s rated speed (rpm)|
Tier II standards are met by optimizing combustion by modifying fuel injection timing, pressure, and rate (rate shaping); fuel nozzle flow area; exhaust valve timing; and cylinder compression volume.
Tier III standards usually require dedicated NOx emission control technologies such as exhaust gas recirculation or selective catalytic reduction, or the use of liquefied natural gas in low-pressure injection (Otto-cycle) engines.
Under the 2008 Annex VI amendments, Tier I standards become applicable to existing engines installed on ships built between 1990 and 1999, with a displacement ≥ 90 liters per cylinder and rated output ≥ 5000 kW, subject to availability of approved engine upgrade kit.
Engine emissions are tested on various ISO 8178 cycles (E2, E3 cycles for various types of propulsion engines, D2 for constant speed auxiliary engines, C1 for variable speed and load auxiliary engines).
Engines are tested using distillate diesel fuels, even though residual fuels are usually used in real life operation.
Further technical details pertaining to NOx emissions, such as emission control methods, are included in the mandatory NOx Technical Code, adopted under “Resolution 2.”
Sulfur Content of Fuel
Annex VI regulations include caps on sulfur content of fuel oil (Regulation 14) as a measure to control SOx emissions and, indirectly, PM emissions (there are no explicit PM emission limits). Special fuel quality provisions exist for SOx Emission Control Areas (SOx ECA, or SECA).
|Date||Sulfur limit in fuel (% m/m)|
Most ships that operate both inside and outside SECAs will carry multiple fuels onboard and switch between higher sulfur and lower sulfur fuels when entering and exiting the ECAs. Before entering the SECA, the vessel must switch to using the ECA-compliant oil, as well as possess written procedures on the process. Switching back to non-SECA compliant fuel should not occur until the vessel has left the SECA. Quantities of each fuel, date, time, and position of the ship must be recorded in the ship’s logbook at each entry or exit. Alternatively, ships are allowed to use Exhaust Gas Cleaning Systems, also called “scrubbers,” to remove an equivalent amount of sulfur from the exhaust to achieve SOx emissions equivalent to ECA-compliant fuels.
Fuel control measures
One control is in regards to the sulfur content of the fuel oils. The sulfur value should be recorded by the supplier at the time of delivery, per Regulation 18. It is the ship’s responsibility to ensure that the proper fuel is being used both inside and outside the SECA. Heavy fuel oil (HFO) is allowed provided it meets the applicable sulfur limit (i.e., there is no mandate to use distillate fuels).
Alternative control measures
Scrubbers can be used as an equivalent compliance option. However, most scrubbers installed on ships are open-loop, meaning contaminated washwater is discharged overboard. Some ships use closed-loop scrubbers that also discharge washwater, known as bleed-off water, which is highly concentrated and contaminated. Therefore, several ports, states, and countries prohibit the use of any scrubbers in their waters and instead require ships to use lower sulfur fuels.
Ozone Depleting Substances
Annex VI prohibits deliberate emissions of ozone-depleting substances, which include halons and chlorofluorocarbons (CFCs). New installations containing ozone-depleting substances are prohibited on all ships. But new installations containing hydro-chlorofluorocarbons (HCFCs) were permitted until 1 January 2020.
Annex VI also prohibits the incineration on board ships of certain products, such as contaminated packaging materials and polychlorinated biphenyls (PCBs).
Compliance with the provisions of Annex VI is determined by periodic inspections and surveys. Upon passing the surveys, the ship is issued an “International Air Pollution Prevention Certificate”, which is valid for up to five years. Under the NOx Technical Code (Resolution MEPC.177(58)), the ship operator (not the engine manufacturer) is responsible for in-use compliance.
The IMO is currently assessing how to reduce the impact on the Arctic from black carbon emitted by international shipping. IMO may regulate black carbon in the future.
International Convention on the Prevention of Pollution from Ships, MARPOL 73/78
Resolution MEPC.177(58), NOx Technical Code 2008 amended by Resolution MEPC.251(66)
Regulation 13 – Nitrogen Oxides
Regulation 14 – Sulphur oxides (SOx)
Revised MARPOL Annex VI (adopted 2008)
ICCT working paper on feasibility of IMO Annex VI Tier III implementation using Selective Catalytic Reduction (2014)
ICCT publication on global scrubber washwater discharges under IMO’s 2020 fuel sulfur limit (April 2021)
Workshop summary from sixth ICCT workshop on marine black carbon emissions (2019)
ICCT report on Air Pollution and Greenhouse Gas Emissions from Ocean-going Ships: Impacts, Mitigation Options and Opportunities for Managing Growth (2007)