Emission Monitoring Methods under the IMO

This article explores the three monitoring methods set out in the IMO’s legislation, namely MARPOL Annex VI and the NOx Technical Code.

The main air pollutants contained in ships exhaust gas, namely sulphur oxides (SOx) and nitrogen oxides (NOx) are limited by MARPOL Annex VI. The NOx Technical Code 2008 supplements it with requirements for control and monitoring of NOx emissions.

1. The Parameter Method

How does it work?

Under the Parameter Method, a parent engine is tested by running a series of engine loads and then measuring the emission levels. This method is based on the assumption that any engine within that family of engines will comply with the emission levels as long as it is fitted with the same components as specified by the engine maker. However, several other factors may affect emission levels, so this method is not always accurate. Each engine has to be operated in accordance with the parameters tested which can be found in its Engine Technical File (ETF). The ETF contains the permitted settings and records for the NOx affecting components, as identified by an IMO reference number provided by the Engine Builder. Classification societies can then check the ETF to make sure that the engine installed on the ship complies with the regulations.

What are the dis-/advantages?

On one side, this method simplifies the work of surveyors as they only have to take a look at the logbook that sets out all the recorded operating settings. Moreover, no additional costs are required for the installation of equipment. On the other side, this also means that the actual level of NOx emissions is not measured or recorded, which in turn compromises the aim of these compliance methods. Also, as it is a paper-driven method it may increase the administrative burden for shipowners, surveyors and port state control authorities.  

In practice, this method has been misinterpreted as prohibiting some manufacturers from stamping IMO reference numbers on their parts and entailing the exclusive use of OEM components, neither of which are mentioned in the text of the regulations. The disadvantage of this is that the competition in the market can potentially be distorted which goes against one of the Fundamental Principles for future GHG Regulations.

2. On-Board Continuous Emissions Monitoring

How does it work?

Contrary to the aforementioned method, on-board continuous emissions monitoring systems (CEMS) provide a continuous, real time monitoring of the content of exhaust gases. Important to note is that with this method a modification to the ETF is required. At the time of the inspection, the surveyor will have a separate flowchart in the ETF to allow them to determine compliance and calibration of the CEMs system installed.

What are the dis-/advantages?

One advantage is the accuracy of the data CEMS are capable of collecting. Additionally, they then allow the data to be loaded to the long-range identification and tracking (LRIT) transponder and the automatic identification system (AIS) transponder, reducing the administrative burden on the shipowner, surveyors and port state control. Most notably, CEMS allow the monitoring of various emissions at the same time, which could provide a cohesive solution to the current separation between the different types of emissions. Moreover, this method requires little manual intervention and has low maintenance requirements.

Nevertheless, this method is not that widely used or recognized yet. It also comes with additional costs for the fitting of extra components. While the fully installed costs can range anywhere between €30.000 and €75.000, it must be considered that this price was initially much higher and their availability much scarcer.

3. The Simplified On-Board Measuring system

How does it work?

Lastly, under the NOx regulations it is permitted to carry out an individual test of the equipment fitted on-board a vessel.  This is a cut-down version of the factory parent engine test. However, this means of compliance is hardly ever used due to the cost of such testing.

What are the dis-/advantages?

While this method is easy to use and does not entail any additional installation costs,  it requires trained personnel and calibrated equipment which increases the costs. Additionally, it is intended to be used during periodical and intermediate surveys, and the test results may deviate.


Finally, all three options under the legal framework of the IMO have their dis-/advantages. It is important to choose a method which benefits the environment while at the same time protecting free and fair competition and the wellbeing of stakeholders. With the advance of technology, perhaps even more effective methods will become available. However, the choice remains yours.