NOx analyser

The importance of measuring NOx pollutants

The term nitrogen oxides (NOx) describes a mixture of nitric oxide (NO) and nitrogen dioxide (NO2) that forms when fossil fuels — diesel, coal, oil or gas — are burned at high temperatures. As a significant contributor to particle pollution and chemical reactions that affect ozone, monitoring NOx levels is a critical part of any effective air quality management program.

Sources of NOx emissions

The majority of NOemissions come from land based vehicles like diesel-powered cars, trucks and buses as well as ships and other marine craft.

Power plants, heavy construction equipment, kilns, turbines, engines and industrial boilers are also key sources. In poorly ventilated indoor areas, domestic appliances such as gas stoves and gas or wood heaters can produce nitrogen oxides.

There are also natural sources, like biological decay processes and lightning, that contribute to airborne NOx.

Health & environmental implications of NOx emissions

Exposure to NOx, in particular NO2, can significantly impact the human respiratory system. In addition to potential lung damage, research has also linked NO2 to cardiovascular problems, lower newborn birth weights, asthma in children, greater reactions to allergens and increased risk of premature death. 

In urban areas, high NO2 concentrations can lead to photochemical ‘smog’ formation and decrease visibility due to secondary aerosol formation.

It can also affect human senses, for example, reducing a person’s ability to smell odours.

High levels of nitrogen oxides are harmful to vegetation—damaging foliage, decreasing growth or lowering crop yields. They can even fade and discolour fabrics and react with other indoor surfaces. 

Nitrogen oxides (NOx) are regulated as criteria pollutants by environmental protection agencies around the world.

Technologies used for NOx monitoring

A variety of methods exist for measuring NO and NO2, ranging from relatively inexpensive off-the-shelf commercial analysers to research grade, highly sensitive techniques.

  • Chemiluminescence is currently the most commonly used method of measuring NOx (both NO and NO2). It involves reducing NO2 to NO followed by a reaction with ozone and chemiluminescence the emission of electromagnetic radiation during the course of chemical reactions, where the radiation, whether ultraviolet, visible, or infrared, is most commonly generated by oxidation. The concentration of NO2 is determined from the difference between NOx (the sum of NO and NO2) and NO measured directly.
  • Cavity Attenuated Phase Shift (CAPS) is a technique closely related to Cavity Ringdown Laser Absorption spectroscopy (CRDS). It uses a wideband, incoherent light source (a 430-nm LED) which is well-matched to the broad absorption band of NO2 in the blue region of the spectrum.In the CAPS technique, photons are injected into a low-loss optical cavity defined by two or more mirrors. If the mirrors are highly reflective, the photons will make many passes within the mirror cavity, slowly leaking out and traveling to the detector. The resultant signal is detected using demodulation techniques to produce a phase shift that is proportional to the absorbance of the light by the presence of nitrogen dioxide. CAPS reliably measures NO2 concentrations of well under 1 ppb up to 1000 ppb and  is widely considered the most effective technology for directly measuring NO2.

NOx analyser applications

Traffic emissions — roadside, tunnels
Outdoor & indoor air quality
Mobile & fixed monitoring stations
Industrial fence-line
Stack emissions
Research studies
Medical gases
Process control

Find out more about our Acoem NO & NO2 analysers

Find out more about Acoem NOx analysers

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