# IMO Tier III & MARPOL Annex VI: The NOx Rules Explained

MARPOL Annex VI Regulation 13 + the IMO NOx Technical Code 2008 limit NOx from marine diesel engines over 130 kW. Limits are tiered by ship construction date and engine rated speed (n, rpm).

## The three tiers

| 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 | Constructed on/after 1 Jan 2011 (global) | 14.4 g/kWh | 7.7 g/kWh |
| Tier III | Inside NOx ECAs, constructed on/after ECA effective date | 3.4 g/kWh | 1.96 g/kWh |

Tier III is an ~76% cut versus Tier II, achievable only with after-treatment (usually SCR running AUS 40), EGR, or gas-mode dual-fuel engines.

## Where Tier III applies (NOx Emission Control Areas)

- **North American ECA**: ships constructed on/after 1 Jan 2016
- **US Caribbean Sea ECA**: 1 Jan 2016
- **Baltic Sea & North Sea ECAs**: 1 Jan 2021
- Further designations progress through IMO MEPC (Mediterranean became a SOx ECA in 2025)

Outside ECAs, Tier III ships may run in Tier II mode, but SCR and AUS 40 supply must be ready at the ECA boundary.

## Compliance evidence

EIAPP certificate, IAPP certificate, approved Technical File, onboard records, including reductant consumption logs and urea bunkering receipts with Certificates of Analysis. Port state control can detain and fine non-compliant vessels; off-spec or substitute fluid (e.g. AUS 32/AdBlue) can void certification.

## FAQ

- **Tier II vs Tier III?** Tier II is global (2011+); Tier III applies only inside ECAs to ships built after the ECA effective date, at ~76% lower NOx.
- **How do ships comply?** SCR with AUS 40 (up to 95% conversion, the dominant route), EGR, or LNG dual-fuel.

Related: [What is AUS 40](https://aus40.in/what-is-aus-40-marine-urea.html) · [How marine SCR works](https://aus40.in/how-marine-scr-systems-work-urea-dosing.html) · [Indian port supply](https://aus40.in/marine-urea-supply-ports-india.html)

PROTICKS: ISO 18611 & IS 17661 certified AUS 40, Jamnagar, India. info@proticks.com · +91-74909-17799