MARPOL Emission Control Areas (ECA), NOx Tier III & Scrubber Compliance Guide

Under MARPOL Annex VI Regulation 14 (Sulphur Oxides and Particulate Matter) and Regulation 13 (Nitrogen Oxides), the International Maritime Organization (IMO) designates sea areas with stringent emission caps known as Emission Control Areas (ECAs). Inside any designated SOx ECA, the sulphur content of fuel oil used on board must not exceed 0.10% m/m (compared to the global limit of 0.50% m/m), unless the vessel operates an approved equivalent arrangement such as an Exhaust Gas Cleaning System (EGCS / scrubber).

Furthermore, in designated NOx Tier III ECAs, marine diesel engines with a power output greater than 130 kW installed on ships constructed on or after the specified keel-laying date must achieve an 80% reduction in NOx emissions compared to Tier I baselines, typically requiring Selective Catalytic Reduction (SCR) or Exhaust Gas Recirculation (EGR).

With the entry into force of the Mediterranean Sea SOx ECA on 1 May 2025 (Resolution MEPC.361(79)) and the adoption of the Canadian Arctic ECA (Resolution MEPC.392(82)) and Norwegian Sea ECA (Resolution MEPC.393(82)) taking effect on 1 March 2026, the global network of ECAs now encompasses seven distinct maritime zones. Ship masters and chief engineers must ensure full compliance with both fuel sulphur and engine NOx standards across these areas.

Under MARPOL Annex VI Regulation 13 and the NOx Technical Code 2008 (Resolution MEPC.177(58) as amended), NOx emission limits apply to every marine diesel engine with an output greater than 130 kW, except for emergency generators, lifeboat engines, and devices solely used in emergencies. The maximum allowable total weighted NOx emissions depend on the engine's rated speed n (crankshaft revolutions per minute, rpm):

• Tier I Baseline: Applies to marine diesel engines installed on ships constructed between 1 January 2000 and 31 December 2010.
• Tier II Global Fleet Standard: Applies to all engines installed on ships constructed on or after 1 January 2011 worldwide. Reduces NOx by approximately 15% to 22% relative to Tier I through internal engine modifications (such as high-pressure common rail injection, Miller timing, and optimised turbocharging).
• Tier III ECA Standard: Enforces an aggressive 80% reduction below Tier I levels. Applicable only when the vessel operates inside a designated NOx Tier III ECA. Outside the ECA, the engine is permitted to operate in its global Tier II certified state.
• Engine International Air Pollution Prevention (EIAPP) Certificate: Each engine must carry an individual EIAPP Certificate and an Administration-approved Technical File detailing components, settings, and operating values that influence NOx emissions, alongside an on-board Record Book of Engine Parameters.

For vessels operating dual-certified engines (capable of switching between Tier II and Tier III mode via SCR or EGR), MARPOL Annex VI Regulation 13.5.3 imposes strict operational logging requirements upon entering or leaving a NOx Tier III ECA:

• Pre-Entry Preparation & SCR Warm-Up: Selective Catalytic Reduction systems require the catalyst bed to reach its minimum operating temperature (typically 300°C–320°C) before urea injection can begin. Injecting urea below this temperature causes the formation of solid ammonium bisulphate and ammonium sulphate deposits, leading to catalyst fouling, backpressure rise, and ammonia slip. The changeover procedure must be initiated in advance of crossing the ECA boundary.
• Urea Consumption Verification: Port State Control Officers regularly cross-check the hours logged in Tier III mode against the volume of aqueous urea solution (AUS 40 per ISO 22241 / ISO 18611) consumed from the urea storage tanks. Typical consumption rates range from 5% to 8% of fuel volume consumed. A recorded Tier III status with zero urea consumption constitutes prime evidence of an inoperative SCR or deliberate bypass.
• Exhaust Gas Recirculation (EGR) Washwater Discharge: On engines utilizing EGR (IMO Resolution MEPC.307(73)), scrubber bleed-off water must be routed through an approved Water Treatment System (WTS) capable of maintaining effluent oil content at or below 15 ppm. Overboard discharge must be monitored continuously with an approved 15 ppm bilge alarm and automatic three-way valve, or retained in an EGR bleed-off holding tank for shore disposal.

Ships using conventional low-sulphur fuel oils (such as Ultra-Low Sulphur Fuel Oil, ULSFO 0.10% S, or Marine Gas Oil, MGO DMA/DMZ 0.10% S) must execute a structured changeover procedure before entering any SOx ECA, following the Bunkering & Fuel Changeover Checklist:

• Written Changeover Procedure (Reg. 14.6): The vessel must carry an approved, ship-specific written fuel changeover procedure. The procedure must define calculated changeover times taking into account fuel consumption rates, service tank volumes, pipework capacity, and viscosity temperature ramps (not exceeding 2°C per minute to prevent thermal shock and fuel pump seizure).
• Boundary Entry Logbook Entries: Upon completing the changeover to 0.10% fuel (prior to crossing the ECA boundary line), the officer in charge must record in the deck logbook or engine logbook: (1) date and time, (2) ship position (latitude and longitude), and (3) the exact volume of low-sulphur fuel oil stored in each service and settling tank.
• Bunker Delivery Note (BDN) Mandatory Retention (Reg. 18.6): BDNs must be retained on board for at least three years from delivery. Each BDN must contain all details required by Appendix V of MARPOL Annex VI, including fuel density (ISO 3675 or ISO 12185), sulphur content (% m/m per ISO 8754), and the supplier's signed declaration. Under MSC.520(106) (effective 1 Jan 2026), the BDN must also verify a flashpoint of ≥ 60°C (tested to ISO 2719:2016) or state ≥ 70°C.
• MARPOL Fuel Sample Retention (Reg. 18.8.1): The sealed, supplier-signed MARPOL primary fuel sample must be retained on board in a dedicated, secure sample locker for a minimum of 12 months from the delivery date, or until the fuel is substantially consumed (whichever is longer).
• In-Use & On-Board Sampling Points (Reg. 14.10 & 14.11): Ships must have designated, clearly labelled fuel sampling points fitted in accordance with IMO Guidelines (MEPC.1/Circ.864/Rev.1 for in-use samples between service tank and engine, and MEPC.1/Circ.889 for onboard storage tank samples).

Under MARPOL Annex VI Regulation 4, an Exhaust Gas Cleaning System (EGCS) is accepted as an equivalent compliance mechanism provided it meets the performance and testing standards of the 2021 EGCS Guidelines (IMO Resolution MEPC.340(77), superseding MEPC.259(68) and MEPC.184(59)).

• Emission Ratio Equivalent: For operating inside a 0.10% m/m SOx ECA, the scrubber must reduce the SO2 (ppm) to CO2 (% v/v) ratio in the exhaust plume to not more than 4.3 SO2/CO2. Outside the ECA (global 0.50% S cap), the maximum allowable ratio is 21.7 SO2/CO2.
• Washwater Quality Discharge Standards: Before discharging washwater overboard, the EGCS monitoring system must continuously verify: (1) pH (minimum pH 6.5 measured 4 metres from the discharge point or pH ≥ 6.5 at the discharge manifold), (2) PAH (Polycyclic Aromatic Hydrocarbons) (continuous fluorescence monitoring calibrated to phenanthrene equivalent), (3) Turbidity (not exceeding 25 FNTU / NTU above inlet seawater turbidity), and (4) Nitrates (discharge limit ≤ 60 mg/L NO3).
• Open-Loop Scrubber Discharge Prohibitions: A growing number of coastal States and major ports enforce complete bans on open-loop scrubber washwater discharge within their internal waters, ports, and territorial seas. Vessels equipped with hybrid scrubbers must transition to closed-loop mode (holding bleed-off sludge in dedicated tanks), while vessels with open-loop-only scrubbers must change over to compliant 0.10% low-sulphur distillate fuels before entering these jurisdictions.
• EGCS Record Book & Monitoring Log: Malfunctions, sensor calibrations, reagent consumption (caustic soda NaOH in closed-loop mode), and washwater discharge parameters must be logged in an approved EGCS Record Book. A continuous system failure exceeding one hour must be reported immediately to the Flag State and the port State of arrival.

Port State Control Officers (PSCOs) from Paris MoU, Tokyo MOU, and USCG utilize portable XRF sulphur analysers, drone & bridge sniffers, and detailed logbook audits to detect MARPOL Annex VI non-compliance. Typical inspection targets include:

• Fuel Changeover Calculations & Timing: PSCOs verify that the changeover was completed prior to entering the ECA boundary coordinates. Calculating changeovers backwards from engine fuel consumption rates is a standard PSC verification technique. Premature entry while running on high-sulphur fuel results in detention (Action Code 30).
• SCR Operation & Urea Storage: PSCOs inspect SCR operating logs, catalyst differential pressure transmitters, urea tank level sensors, and NOx emissions analysers. Tampered temperature sensors, bypass dampers locked in open position, or exhaust temperatures below catalytic threshold without alarm logging will be flagged as major ISM deficiencies.
• Fuel Oil Non-Availability Report (FONAR): If compliant fuel cannot be procured despite best efforts, a FONAR must be submitted to the Flag Administration and competent authority of the port of arrival under Regulation 18.2. A FONAR is not an exemption; it is a declaration of non-availability, and inspectors will demand documentary proof of supplier inquiries.
• Deficiency Action Codes: Minor logging discrepancies warrant Action Code 17 (rectify before departure); failed washwater discharge limits or deliberate SCR bypassing result in Action Code 30 (ship detention) and mandatory Flag notification.