IMO Tier III & MARPOL Annex VI
NOx Rules, Explained

MARPOL Annex VI is the air-pollution chapter of the International Convention for the Prevention of Pollution from Ships, administered by the International Maritime Organization (IMO). Its Regulation 13, together with the NOx Technical Code 2008, limits nitrogen oxide emissions from marine diesel engines over 130 kW output.

The limits are tiered by ship construction date and expressed as a curve against rated engine speed (n, in rpm). Slower engines are allowed more NOx per kWh because of their combustion characteristics:

Tier	Applies To	NOx Limit (n < 130 rpm)	NOx Limit (n ≥ 2000 rpm)
Tier I	Ships constructed 2000–2010	17.0 g/kWh	9.8 g/kWh
Tier II	Ships constructed on/after 1 Jan 2011 (global)	14.4 g/kWh	7.7 g/kWh
Tier III	Inside NOx ECAs, ships constructed on/after ECA effective date	3.4 g/kWh	1.96 g/kWh

The step from Tier II to Tier III is the one that changed ship design: an ~76% cut in allowable NOx that in-engine tuning alone cannot reach. It requires after-treatment, which in most newbuilds means SCR running on AUS 40 marine urea.

Tier III is not a global limit. It switches on inside designated NOx Emission Control Areas (ECAs):

• North American ECA: most of the US and Canadian coastline. Tier III applies to ships constructed on or after 1 January 2016.
• United States Caribbean Sea ECA: Puerto Rico and the US Virgin Islands. Also effective for ships constructed on or after 1 January 2016.
• Baltic Sea ECA and North Sea ECA: Tier III applies to ships constructed on or after 1 January 2021.

The list is growing. The Mediterranean Sea became a SOx ECA in 2025, and NOx designations for further waters, including regions relevant to Indian Ocean trade, continue to be discussed at IMO's Marine Environment Protection Committee (MEPC). A vessel built Tier III-capable trades freely through every current and future ECA; one that isn't faces routing constraints for its entire service life.

Outside ECAs, a Tier III ship may legally operate in Tier II mode, but the SCR system, the urea tanks and the reductant supply chain must all be ready the moment the vessel crosses an ECA boundary. That is a fluid-logistics problem as much as an engineering one, and it is why reliable AUS 40 supply at Indian ports matters to operators on Europe- and US-bound rotations.

Three technology routes dominate:

• Selective Catalytic Reduction (SCR). The market leader. Injects AUS 40 into the exhaust; ammonia reduces NOx over a catalyst with up to 95% conversion. Works with conventional fuels, retrofittable, proven across two-stroke and four-stroke installations. See how marine SCR systems work.
• Exhaust Gas Recirculation (EGR). Recirculates cooled exhaust into the cylinder to suppress NOx formation. No reductant needed, but adds complexity inside the engine and is largely a newbuild, two-stroke solution.
• LNG / dual-fuel engines. Gas combustion (Otto cycle) produces inherently low NOx, so many dual-fuel engines meet Tier III in gas mode without after-treatment, though they may still need SCR or EGR in diesel mode.

Enforcement in practice

Compliance is evidenced by the engine's EIAPP certificate, the ship's IAPP certificate, the approved Technical File and onboard records, including reductant consumption logs that should reconcile with engine running hours inside ECAs. Port state control officers increasingly cross-check urea bunkering receipts and Certificates of Analysis, so keep supplier documentation with the compliance file.