How does a NOx monitor work?

User:JXCTUpload time:Aug 22 2023


NOx (nitrogen oxides) is a group of highly reactive gases that are formed during combustion processes. These gases have detrimental effects on both human health and the environment. To mitigate the impact of NOx emissions, it is crucial to monitor and control their levels. This article aims to explain the working mechanism of a NOx monitor, which plays a vital role in measuring and analyzing NOx concentrations in various settings.

Principles of NOx Monitoring:

1.1 Chemiluminescence: Chemiluminescence is a widely used technique in NOx monitoring. It involves the reaction of nitrogen dioxide (NO2) with ozone (O3) to produce excited-state nitrogen dioxide (NO2*) molecules. These excited molecules subsequently emit light when they return to their ground state. The intensity of the emitted light is directly proportional to the concentration of NOx in the sample, allowing for accurate measurement.

1.2 Electrochemical Analysis: Electrochemical analysis relies on the electrochemical reaction between NOx gases and specific electrodes. In this technique, the NOx gas is passed over a sensing electrode, which undergoes oxidation or reduction reactions depending on the type of NOx gas present. The current generated during the reaction is proportional to the NOx concentration and is measured to determine the gas levels.

1.3 Infrared Absorption: Infrared absorption is another commonly used method for NOx monitoring. It utilizes the unique absorption characteristics of NOx gases in the infrared region. The NOx monitor emits infrared light at specific wavelengths, and the gas sample is passed through an absorption cell. The amount of light absorbed by the NOx gases is measured, and the concentration is determined based on the absorption intensity.

Components of a NOx Monitor:

2.1 Sample Collection System: The sample collection system is responsible for extracting a representative gas sample from the environment. It usually includes a probe or an inlet that draws the gas into the monitor. The sample is then conditioned to remove any contaminants or moisture that may interfere with the measurement process.

2.2 Gas Analyzer: The gas analyzer is the core component of the NOx monitor. It utilizes one of the aforementioned principles (chemiluminescence, electrochemical analysis, or infrared absorption) to measure the concentration of NOx gases in the sample. The analyzer converts the physical or chemical changes caused by the NOx gases into an electrical signal that can be further processed.

2.3 Signal Processor: The signal processor receives the electrical signal from the gas analyzer and processes it to obtain accurate NOx concentration readings. It may include amplifiers, filters, and other electronic components to enhance the signal quality and remove any noise or interference.

2.4 Display and Data Logger: The display unit of the NOx monitor presents the measured NOx concentration in a user-friendly format, such as numerical values or graphical representations. Additionally, a data logger is often included to record the measurements over time, allowing for trend analysis and data retrieval.

Calibration and Maintenance:

To ensure the accuracy and reliability of NOx monitors, regular calibration and maintenance are essential. Calibration involves comparing the monitor's readings against known reference standards and adjusting the instrument accordingly. This process corrects any potential drift or deviation in the measurement system. Maintenance includes routine checks, sensor replacements (if applicable), and cleaning procedures to keep the monitor in optimal condition.


NOx monitors play a critical role in monitoring and controlling NOx emissions, which have adverse effects on human health and the environment. By utilizing techniques such as chemiluminescence, electrochemical analysis, and infrared absorption, these monitors accurately measure NOx concentrations. Understanding the working mechanism of a NOx monitor and its components helps ensure accurate and reliable measurements, allowing for effective pollution control and mitigation strategies.