Air quality and local pollutants – Part 2

What pollutes our air

Let’s continue the topic of air quality and local pollutants, this time talking about the different types associated with transport. In this article, you will learn the difference between the likes of NOx, PM, PN and other pollutants that perhaps you’ve heard of but don’t really know what they are and how they affect you daily.

The idea isn’t to go into great scientific detail but to provide a rather simplified overview of what can be considered as a complicated or technical topic by many. In this article, we will present to you the different kinds of pollutants associated with the natural gas engine.

If you’ve missed the first part of these series on health, emission standards and technology, you can read it here:

Air quality and local pollutants – Part 1

Local Pollutants – getting to know them

Chances are you’ve probably heard of some air pollutants already. In fact, there are several main types of pollutants that are relevant to the transport sector.

It all starts with the type of fuel used in your car, and because natural gas engines are normally developed starting from the gasoline base version, we will compare for you these two when comparing passenger car and light-duty applications.

Local pollutants

So, the different kind of pollutants are: CO (carbon monoxide), NOx (nitrogen oxide), THC (total hydrocarbon), NMHC (non-methane hydrocarbon), and PM (particulate matters).

Let’s take a closer look into each of these.

CO – Carbon Monoxide Emissions

The first local pollutant to talk about is CO. This is a product of incomplete combustion. Put simply, instead of forming the usual CO2 (carbon dioxide), this incomplete combustion leads to partially oxidized carbon (hence why it’s a mono[single]-xide and not di[double]-oxide).

Direct exposure to CO can be dangerous to human health.“Breathing CO can cause headache, dizziness, vomiting, and nausea. If CO levels are high enough, you may become unconscious or even die. Exposure to moderate to high levels of CO over long periods of time has also been linked to increased risk of heart disease. People who survive severe CO poisoning may suffer long-term health problems”, according to the Centers for Disease Control and Prevention.

When comparing CNG to gasoline, researchers have found that there is 70-80% less CO in mg/km in CNG (Bach et al., 2017). Furthermore, the results of CO emissions under real driving emissions measured with PEMS systems are even lower than the limit already fixed in current Euro6d limits on WLTC (Weber, C. 2014).

NOx – Nitrogen Oxide Emissions

NOx causes harmful effects on the lungs.

The formation of these local pollutants does not directly depend on fuel composition, but on the temperature and the oxygen in the combustion process. NOx emissions are produced when the fuel is combusted in the engine in the presence of air. 

NOx represents a group of two chemicals formed by the reaction of hydrogen – Nitric oxide (NO) and Nitrogen dioxide (NO2). The nitric oxide (NO) is not harmful to our health in the typical concentrations found in the atmosphere but long-term exposure to nitrogen dioxide can lead to a range of environmental and health problems. 

According to the American Lung Association, nitrogen dioxide causes a range of harmful effects on the lungs, such as increased inflammation of the airways, worsened cough, reduced lung function and increased asthma attacks.

NOx emissions are frequently high, for both gasoline and CNG fuel types, and the final result as exhaust emissions depends on the efficiency of the gas after-treatment system (catalytic converter). Natural gas operations normally lead to a higher conversion efficiency of NOx compared to gasoline.

THC – Total Hydrocarbon Emissions

THC are responsible for the smog in the atmosphere.

Similarly to Carbon Monoxide, hydrocarbons (HCs) are formed due to incomplete combustion. Total Hydrocarbon emissions (THC) include volatile organic compounds (VOCs), methane (CH4) and non-methane hydrocarbons (NMHC) with the last making up almost 90% of the total amount of THC when running with conventional fuel. These local pollutant compounds are responsible for what is termed smog in the atmosphere and ground-level ozone – toxic to human health and that of our pets. 

Thanks to its composition, THC from natural gas combustion, produce a very low amount of NMCH. This is also confirmed in the recent ADAC test where both Audi A4 Avant g-tron and VW Caddy 1.4 TGI  score with over 20 mg less THC per km towards the Euro 6d limit of 100 g/km.

Particulate Matter

Particulate matter (PM) can be both primary – when released directly after combustion or secondary – when formed after release through bounding with precursor gases (mainly Sulphur dioxide (SO2),  nitrogen oxides (NO x), ammonia (NH 3) and some VOCs). PM is among the most harmful pollutants for human health. In terms of PM emissions gas engine vehicles emit particularly low levels of it, nearly 100% less than the required Euro6d limits and this is primarily due to the chemical composition and gaseous nature of natural gas.

Have you had the chance to listen to our podcast yet? Find out the latest episodes of gmobility here.

The right approach on local pollutants

The evolution of the emission standards asks for clean solutions with a strong focus on Real Driving conditions. Clean fuel and reliable technologies such as natural gas ones are the key to ensure the lowest level of local pollutants under the wider operating range conditions.

  • Assessment of Euro6 passenger car technologies demonstrates that CNG provides better figures in terms of pollutant emissions (CO, NOx, NMHC, PM, PN) compared to gasoline.
  • Natural gas using SIS engines can be coupled with three-way catalysts  (TWCs) to reduce air pollutant emissions of nitrogen oxides (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). TWCs are effective at oxidizing any unburned methane and therefore lead to low-levels of methane slip from the tailpipe.
  • Furthermore, since the combustion of natural gas in a SIS engine produces relatively low levels of particulate matter (PM) and a particulate filter tends not to be required.


Bach, C., Soltic, P., Rojewski, J., Cabalzar, U., Teske,S., & Bütler, T. (2017). CNG mobility State-of-the-art technology. Swiss: Empa.

Weber, C. (2014). Gas-Only Internal Combustion. ERTRAC-EGVI.

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