NOx and SOx Emissions Explained

The terms "NOx" and "SOx" are often seen in emissions measurement specifications, but many people are unsure of their accurate definitions. The terms come from the study of atmospheric chemistry, and each refers to a family of compounds.

Oxides of Nitrogen (NOx)
NOx refers to the total concentration of the most important oxides of nitrogen that are emitted by combustion sources: nitric oxide (NO) and nitrogen dioxide (NO₂). The principal sources of NOx in a combustion process are fuel NOx and thermal NOx. As you might guess from the name, fuel NOx results from the oxidation of nitrogen atoms bound in the fuel. Thermal NOx occurs when nitrogen molecules in the air react with oxygen in the high temperature of the combustion zone.

It is appropriate to treat the two species together because most of the NOx emitted by a typical combustion source such as a power plant is in the form of NO. However, this species oxidizes readily, so it is quickly converted to NO₂ once it is in the atmosphere. When the concentrations of the two species are expressed as parts-per-million (ppm), the NOx concentration is calculated as:

ppm NOx = ppm NO + ppm NO₂

There are two common sources of confusion when discussing NOx. These are the presence of other nitrogen oxides and the calculation of NOx concentration in mass units of mg/m3. Although their concentrations in flue gases are very small, other oxides of nitrogen are important in atmospheric chemistry. Therefore, the term NOy is used to describe all oxidized nitrogen species, including N₂O and N₂O₂, as well as NO and NO₂.




















Calculating the mass concentration of NOx is complicated because a molecule of NO₂ weighs more than a molecule of NO. Converting a concentration in ppm to a concentration in mg/m³ involves a factor that depends on the molecular weight of the gas in question.

mg/m³ NO = ppm NO x 1.34

mg/m³ NO₂ = ppm NO₂ x 2.05

Because all NOx ends up as NO₂ in the atmosphere, it is usual to apply the conversion factor for NO₂.

mg/m³ NOx = ppm NOx x 2.05

Because of this factor, we have the rather surprising fact that: 

mg/m³ NOx ≠ mg/m³ NO + mg/m³ NO₂

Oxides of Sulphur (SOx)
As with nitrogen, many oxides of sulphur are important in atmospheric chemistry, but only two are commonly present in flue gases. These are sulphur dioxide (SO₂) and sulphur trioxide (SO₃). The SO₂ results from the oxidation of sulphur in fuels such as coal and oil, which occurs in the combustion zone. A small fraction, typically around 1%, of the SO₂ is further oxidized to SO₃. By analogy with NOx, atmospheric chemists use the term SOx to refer to all oxides of sulphur in a gas mixture. 

There are many sensitive and reliable methods to measure SO₂ in flue gases, but SO₃ measurements are much more difficult. Given the low concentration of SO₃ in typical flue gases relative to SO₂, most emissions regulations do not prescribe an emission limit value. Therefore, there is little practical need to make a true measurement of SOx, and so most flue gas measuring equipment measures SO₂ alone.

FREQUENTLY ASKED QUESTIONS

1. What are the full forms of NOx and SOx?

NOx stands for nitrogen oxides—primarily NO (nitric oxide) and NO₂ (nitrogen dioxide)—while SOx stands for sulphur oxides, chiefly SO₂ (sulphur dioxide) and SO₃ (sulphur trioxide). Both are classified as major regulated combustion pollutants under environmental legislation worldwide, including US EPA regulations, EU Industrial Emissions Directive and national standards. Understanding and controlling these emissions is essential for industrial compliance.

2. What is the difference between NOx and SOx?

The fundamental difference is their origin: NOx forms through thermal oxidation when atmospheric nitrogen and oxygen combine at high combustion temperatures (above ~1,300°C), whereas SOx originates from sulphur compounds already present in the fuel. This distinction is critical because it dictates different control strategies—NOx requires combustion optimization, while SOx control focuses on fuel selection or flue gas desulphurization.

3. Why are NOx and SOx harmful?

Both pollutants pose significant health and environmental risks. NOx contributes to photochemical smog, ground-level ozone formation, acid rain and respiratory conditions. SOx causes acid rain, particulate formation and respiratory problems particularly affecting vulnerable populations. These impacts are why environmental agencies enforce strict emission limits with substantial penalties for non-compliance.

4. How are NOx and SOx monitored?

NOx and SOx are monitored using Continuous Emissions Monitoring Systems (CEMS) and dedicated gas analysers. LAND offers proven solutions including the FGA Series extractive gas analysers and WDG-1200/1210 in-situ analysers that measure concentrations directly in the flue gas stream in real time. This continuous data enables plants to optimize combustion, demonstrate compliance and respond quickly to excursions before they become regulatory violations.

5. Which industries emit the most NOx and SOx?

The highest NOx and SOx emitting industries are power generation (especially coal and oil-fired plants), cement manufacturing, petroleum refining, marine transportation and steel production—all characterized by high-temperature combustion of fossil fuels. LAND has over 75 years of experience providing measurement solutions for these demanding applications, helping operators balance process efficiency with environmental compliance.