How to monitor NOx emissions

Although SOx and CO2 are arguably the “trendiest” emission to talk about, we want to shift the focus back to NOx with this article. There have not been any significant changes in the NOx regulations since 2008, especially when it comes to monitoring methods. 

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

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 following the parameters tested,  found in its Engine Technical File (ETF). The ETF contains the permitted settings and records of the NOx affecting components, as identified by an IMO reference number provided by the Engine Builder. Classification societies can then check the ETF to ensure that the engine installed on the ship complies with the regulations.

What are the dis-/advantages?

On one side, this method simplifies surveyors’ work as they only have to take a look at the logbook that sets out all the recorded operating settings. Moreover, it requires no additional costs 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. As a paper-driven method, it may also 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. Onboard Continuous Emissions Monitoring

How does it work?

Contrary to the Parameter Method, onboard continuous emissions monitoring systems (CEMS) provide real-time information about the content of exhaust gases. It is important to note 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 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 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 for the simultaneous monitoring of various emissions, 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 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 Onboard Measuring system

How does it work?

Lastly, under the NOx regulations, it is permitted to carry out an individual test of the equipment fitted onboard 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, its intended use is 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 crucial to choose a method that 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.