NOx Tiers

MARPOL Annex VI Regulation 13 sets mandatory limits on oxides of nitrogen (NOx — mainly NO and NO₂) emitted by marine diesel engines. NOx contributes to acid rain, ground-level ozone formation, and fine particulate matter (PM₂.₅) — a particular concern near major shipping routes and in enclosed semi-enclosed seas with dense traffic. The regulation applies to diesel engines with a power output exceeding 130 kW installed on ships subject to MARPOL Annex VI, other than emergency engines and lifeboat engines.

Three progressive tiers define the applicable limit based on the engine's installation date. Limits are expressed as grams of NOx per kilowatt-hour (g/kWh) and depend on the engine's rated speed (n, in rpm) — slower engines have higher absolute NOx limits because slow-speed diesels run hotter and longer per cycle, but the formula accounts for the different thermal regime. The tiered approach allows existing fleets to operate to their original standard while requiring progressively cleaner engines in new installations.

Tier I, II, and III limits

Tier IEngines installed 1 January 2000 – 31 December 2010
17.0 g/kWh at n < 130 rpm; 45 · n⁻⁰·² at 130–2000 rpm; 9.8 g/kWh at n ≥ 2000 rpm
Applies globally. No geographic restriction.
Established the baseline NOx limit for modern marine diesel engines. Most slow-speed two-stroke main engines built before 2011 are Tier I. Many are still in service and will not be required to retrofit unless operating in a NECA.
Tier IIEngines installed on or after 1 January 2011
14.4 g/kWh at n < 130 rpm; 44 · n⁻⁰·²³ at 130–2000 rpm; 7.7 g/kWh at n ≥ 2000 rpm
Applies globally. Approximately 15–20% reduction from Tier I.
Achieved primarily through engine tuning: modified fuel injection timing, higher compression ratios, and optimised turbocharging. No after-treatment device required for most engines. Tier II is the current baseline standard outside NECAs.
Tier IIIEngines installed on or after 1 January 2016 (when operating in a NECA)
3.4 g/kWh at n < 130 rpm; 9 · n⁻⁰·² at 130–2000 rpm; 2.0 g/kWh at n ≥ 2000 rpm
Only in designated NOx Emission Control Areas (NECAs). Approximately 80% reduction from Tier II.
Cannot be achieved by engine optimisation alone. Requires after-treatment technology — either Selective Catalytic Reduction (SCR) or advanced Exhaust Gas Recirculation (EGR). A ship must be capable of switching into Tier III compliance mode when entering a NECA. Outside NECAs, the engine operates in Tier II mode.

NOx Emission Control Areas (NECAs)

Tier III applies only within designated NECAs. A NECA is established by IMO following a proposal from one or more Parties demonstrating need (air quality impacts, health effects) and technical feasibility. Currently three NECAs are in force:

North American NECAIn force: 1 January 2016
US and Canadian Atlantic coast, Pacific coast (including Alaska), and the Caribbean Sea (Hawaii not included in Caribbean NECA; separate Pacific NECA). Extends 200 nm from the baseline.
First NECA to enter into force. Applies to ships constructed on or after 1 January 2016 when operating within the area. Ships constructed before 2016 are exempt from Tier III in this NECA, operating Tier I or Tier II as applicable.
Baltic Sea NECAIn force: 1 January 2021
The Baltic Sea as defined in MARPOL Annex I (roughly the sea area enclosed by Denmark, Sweden, Finland, Estonia, Latvia, Lithuania, Russia, Poland, and Germany).
The Baltic is also a SECA (sulphur ECA, 0.10% since 2015). Combined with Tier III, new ships operating in the Baltic must meet both low-sulphur fuel and Tier III NOx requirements simultaneously — operationally demanding for ships with SCR systems that have narrower operating windows.
North Sea NECAIn force: 1 January 2021
The North Sea including the English Channel and the Skagerrak.
Also a SECA. Major European ferry, RoRo, and short-sea shipping routes transiting the North Sea were among the first significantly affected by simultaneous sulphur ECA and Tier III requirements on new-build vessels.

The Mediterranean Sea became a SECA (sulphur ECA, 0.10%) from 1 May 2025, but it is not a NECA — there is no Tier III NOx requirement for Mediterranean operations. The distinction is important: the Mediterranean sulphur ECA imposes fuel sulphur limits only; engine NOx certification requirements in the Mediterranean remain Tier I or Tier II depending on engine installation date.

EIAPP Certificate and NOx Technical Code

Engine International Air Pollution Prevention (EIAPP) Certificate — issued to each engine (not the ship) by the flag state Administration confirming the engine type meets the applicable Tier I, II, or III limit. The EIAPP is accompanied by a Technical File specifying the engine settings and components that define the certified configuration. Engines must not be operated outside the parameters in the Technical File.
NOx Technical Code 2008 — the procedural document specifying how NOx emission measurements are performed. It provides four approved test cycles and four compliance methods:
- Pre-certification (type approval on test bed)
- On-board parameter check (most common at PSC)
- Simplified measurement (on-board, during normal operation)
- Direct measurement and monitoring (continuous EMS — required for Scheme B under Regulation 14)

Compliance technologies

Selective Catalytic ReductionSCR
Urea solution (AdBlue/marine urea, 40% aqueous solution) is injected into the exhaust gas upstream of a catalyst. Urea decomposes to ammonia (NH₃), which reacts with NOx over the catalyst surface (typically vanadium pentoxide on titanium dioxide substrate) to produce nitrogen (N₂) and water (H₂O).
Reduction: 80–95% NOx reduction — fully capable of meeting Tier III in NECAs from Tier II baseline.
Issues: Catalyst poisoning by sulphur and phosphorus compounds — high-sulphur fuel degrades catalyst life rapidly (some operators combine SCR with scrubbers, but the SCR must receive desulphurised exhaust). Urea quality (ISO 22241 standard) is critical; sub-standard urea causes catalyst fouling and incomplete reduction (ammonia slip). Low-load operation reduces exhaust temperature below the catalyst window (typically 300–400°C); SCR bypassed at low load in harbour or cold-ironing approaches. Urea storage and bunkering logistics in ports.
Exhaust Gas RecirculationEGR
A fraction of exhaust gas is cooled and fed back into the engine scavenging air. Recirculated exhaust is high in CO₂, which displaces oxygen and reduces combustion temperature, directly reducing thermal NOx formation. Wet scrubber is used to clean the recirculated gas before injection.
Reduction: 50–80% NOx reduction depending on recirculation rate. Some engine families achieve Tier III with high EGR rates; others need a combination with engine derating.
Issues: Increases soot/particulate matter and cylinder liner wear due to sulphur compounds in recirculated gas. Only effective at high load; NOx increases at low load if EGR rate drops. Internal scrubber washwater management mirrors open-loop scrubber discharge issues. Typically unsuitable for ECA areas where SCR is mandated unless engine builder certifies Tier III compliance via EGR alone.

In-service compliance and PSC

Port state control officers verify NOx compliance primarily through the on-board parameter check — inspecting whether the engine components (fuel injection equipment, turbocharger settings, valve timing) match the Technical File. Common failures include:

SCR catalyst poisoning — Engines burning high-sulphur fuel (especially if the scrubber is bypassed or malfunctions) can poison the vanadium-based SCR catalyst within weeks. Catalyst performance degradation is not immediately visible without emissions testing; EIAPP compliance is compromised.
Urea quality failures — Marine AdBlue must meet ISO 22241. Diluted or contaminated urea reduces NOx conversion efficiency. Ships should sample and retain urea delivery records.
Low-load SCR bypass — Some ships routinely bypass SCR at low load in harbour. If the NECA boundary is entered before SCR is activated and fully heated, there is a period of non-compliance. The EMS log will record the bypass event.
Technical File deviations — Unrecorded engine modifications (injector nozzle changes, turbocharger substitutions) that deviate from the certified Technical File are a violation even if actual emissions are within limits.