NOx Measurement by Chemiluminescence

How does the chemiluminescence method work?

Chemiluminescence is based on measuring the light released during a chemical reaction. In nitrogen oxide measurement, nitric oxide (NO) in the sample reacts with ozone (O₃) generated inside the analyzer. The reaction produces NO₂ molecules in an excited state, and these molecules emit weak light in the near-infrared region as they return to the ground state.

The intensity of the emitted light is proportional to the NO concentration in the sample; the light is detected by a photomultiplier tube. NO₂ in the sample does not emit light directly. The sample is therefore first passed through a heated converter, where NO₂ is chemically reduced to NO. The difference between measurements with and without the converter gives the NO₂ concentration, and the measurement with the converter gives total NOx.

What chemiluminescence measures and where it is used

The method is used to measure NO, NO₂ and NOx. Depending on the instrument configuration, measurement ranges can cover a wide span, from ppb levels in ambient air to hundreds of ppm in process and emission applications. Typical applications are:

• Air quality monitoring stations in urban and industrial areas
• Flue gas and process emission measurements, extractive CEMS
• Calibration laboratories and reference measurements
• Periodic emission measurements with portable flue gas analyzers

Where chemiluminescence works well and where it struggles

Because chemiluminescence offers high sensitivity at low concentrations and a fast response, it is a widely used reference method for NOx measurement. Its response stays linear over a wide range, and measurement can be continuous.

There are also limitations: the analyzer needs a dry air or oxygen source to generate ozone, moisture condensation in the sample line can compromise the measurement, and because the converter ages over time, its conversion efficiency must be checked periodically. Some compounds such as ammonia can convert to NO in the converter and interfere with the NOx value, so sample composition and converter type should be chosen to suit the application.

Related standards

The chemiluminescence method for measuring NO and NO₂ in ambient air is described in EN 14211. On the emissions side, the NOx measurement method used by portable analyzers relates to EN 14792. Type approval of instruments is assessed under the EN 15267 certification system.

Choosing an instrument for chemiluminescence

When selecting, the measurement range (trace-level ambient or high-level emission), converter type, sample moisture and the air source needed for ozone generation should be assessed together. To determine the right configuration for the application, Tetra also takes sample line, filter and calibration gas needs into account.

Tetra and chemiluminescence analyzers

In Türkiye, Tetra carries out the supply, installation, commissioning, calibration and periodic maintenance of chemiluminescence-based analyzers. The product range includes Thermo Scientific 42i and 42iQ NO-NO₂-NOₓ analyzers and the Mega System Thyris portable flue gas analyzer. PAC NSure measures total nitrogen by chemiluminescence after burning the sample. These instruments can also be integrated into air quality station cabinets and CEMS systems prepared by Tetra.