Why is there such a fuss about NOx?


What actually is NOx, and is this gas a climate-change sinner? And where does ozone come into the picture? SINTEF- scientist Mario Ditaranto, an expert on combustion, provides all the answers on flue gases.

“What is NOx?”

“NOx is actually a collective expression that refers to two distinct gases that are subject to quite strict emission limits: NO or nitric oxide, and NO2 or nitrogen dioxide. Both of these gases are formed when practically anything that is combustible is burned. Nitrous oxide (N2O), better known as laughing gas, is also occasionally included in the NOx concept,” explains Ditaranto.

In fact, almost 80 per cent of the air that we breathe, and which is consumed in a fire, is nitrogen. The nitrogen molecule, N2, which is oxidised to NOx during the combustion process, may come from fuels such as wood, coal, biofuel, etc., but also from the air itself.

NOx have long been known as significant pollutants in many sectors, including transport, electricity generation and industry. A number of measures have been introduced to reduce NOx emissions.

“Why are NOx pollutants?”

“NOx are toxic to human beings when they are inhaled, even at relatively low concentrations (tens of parts per million).”

“NOx are present both in vehicle exhausts and industrial chimney smoke.”

“They (especially NO2) are also responsible for smog and the typical brown sky that often covers large cities and reduces air quality. NOx are also the most important source of acid rain. These gases are therefore not only sources of atmospheric pollution, but also contribute to the general acidification of soil and water,” points out Ditaranto.

“Are NOx from vehicles a greenhouse gas?”

“Well, the answer is both yes and no. As I have just mentioned, NOx refers to two different molecules whose relative concentration in exhaust gases depends on the type of combustion involved, that is, whether it is petrol, diesel oil, or wood that is being burned. Diesel cars produce more NOx than petrol vehicles, due to the higher combustion pressures and temperatures in diesel engines.”

Although the most important component of vehicles exhausts is NO, this is rapidly oxidised to NO2, which is not a greenhouse gas itself. That is to say, if we fill a bottle with NO2 and leave it out in the sun, the temperature of the gas will not rise. However, NO2 is responsible for the formation of ozone, which is a greenhouse gas, via secondary reactions with other gases.

Although ozone itself has a short lifetime, measured in hours to days, it influences cloud formation and the absorption of carbon by plants, which in turn has an effect on global warming.

The lifetime of NOx in the atmosphere is therefore influenced by how rapidly it is transformed into other directly dangerous components (acids and ozone), a process that can take place in the course of a few hours or days. The conclusion is that although NOx themselves are not greenhouse gases, emissions of these gases do lead to global warming, points out the SINTEF scientist.

“What is the problem with ozone and solar radiation?”

“Ozone in the troposphere, which is the lowest layer of the Earth’s atmosphere, and in which we live, is a danger to life, and also leads to global warming via the interactions that we have just mentioned. However, ozone is also essential higher up in the stratosphere, because it protects us against ultraviolet radiation from the Sun. Ozone also has a cooling effect in the stratosphere. Unfortunately, however, human activity is leading to a breakdown of stratospheric ozone and an increase in ozone levels closer to ground level.”

“What about the really wicked climate gas; nitrous oxide?”

“N2O, or laughing gas, is also produced during combustion, especially from biomass or coal in certain types of combustion process, but to a lesser extent by vehicles. Unfortunately, N2O reduces stratospheric ozone, which is regarded as “good” ozone.”

“The most important source of N2O emissions is the use of artificial fertilisers. Unlike the other NOx, N2O is a powerful greenhouse gas, 300 times as strong as CO2 in a hundred-year perspective. Furthermore, this gas has a lifetime of about 110 years in the atmosphere. It is responsible for about six per cent of the global greenhouse gas effect, according to the Intergovernmental Panel on Climate Change (IPCC).”

“Which is worse; diesel or petrol, as far as N2O are concerned?”

“As far as vehicles are concerned, there is little to choose between diesel and petrol in terms of N2O emissions. Ironically, laughing gas is largely produced by the catalysts that are used to reduce NOx emissions from cars!

“The global warming effect of N2O is estimated to be in the order of one to ten per cent of total lifetime geenhouse gas emissions from a vehicle”, says SINTEF combustion expert Mario Ditaranto.

So now you know all you need to know about NOx.


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