NOx

Nitrogen oxides - Formation and Relevance

Nitrogen oxides play an important role in atmospheric processes. How are they formed, why are they important?

 

1. Traffic - still an important source of nitrogen oxides.(c) FreeFoto.com

Where do nitrogen oxides come from?

The most important nitrogen oxides are nitrogen monoxide NO and nitrogen dioxide NO₂. Both together are called NO_x. The nitrogen molecules (N₂) in the air are very stable and it is not easy to oxidise them. A few bacteria have developed special mechanisms to crack the N-N triple bond and to form oxidised compounds. But by far more relevant are processes where the bonding is cracked by heat. This can only happen under extreme conditions. One example is during the combustion of fuel in a car engine. Most anthropogenic (= human made) NO_x comes from this source. It can also happen during other very hot reactions, e.g. in the hottest parts of biomass burning flames. Finally lightning is a major source. In the flash channel temperatures reach up to 30,000 degrees Celsius and easily crack nitrogen bonds.

2. right: Lightning is another important source of nitrogen oxides.
picture by Bernhard Mühr  Karlsruher Wolkenatlas

 Where are they involved?

We find NO_x (= NO + NO₂) and other nitrogen oxides nearly everywhere in atmospheric chemistry. During the night, nitrate radicals NO₃ are formed and are the most active oxidants. Radicals are chemical species, which are very instable and usually react extremely fast.
If N₂O₅ is formed in polluted areas, it can react on droplets or wet surfaces with water and nitric acid HNO₃ is formed. HNO₃ contributes to the acid character of the rain. Nitric acid, which can also be formed during the day by oxidation of NO₂, is the main way how nitrogen oxides are removed again from the atmosphere, either by dry or by wet deposition (wash out by rain).
Nitric acid is also part of polar stratospheric clouds. Nitric acid tryhydrate forms the particles on which the ozone hole developed (details in 'Higher atmosphere - More - Unit 2').

Where do nitrogen oxides come from? The most important nitrogen oxides are nitrogen monoxide NO and nitrogen dioxide NO2. Both together are called NOx. The nitrogen molecules (N2) in the air are very stable and it is not easy to oxidise them. A few bacteria have developed special mechanisms to crack the N-N triple bond and to form oxidised compounds. But by far more relevant are processes where the bonding is cracked by heat. This can only happen under extreme conditions. One example is during the combustion of fuel in a car engine. Most anthropogenic (= human made) NOx comes from this source. It can also happen during other very hot reactions, e.g. in the hottest parts of biomass burning flames. Finally lightning is a major source. In the flash channel temperatures reach up to 30,000 degrees Celsius and easily crack nitrogen bonds. 2. right: Lightning is another important source of nitrogen oxides. picture by Bernhard Mühr  Karlsruher Wolkenatlas

3. Where are nitrogen oxides involved in atmospheric processes? The scheme gives a little (not complete) overview of important processes in atmospheric chemistry. Please click the scheme to enlarge! (100 K) by Elmar Uherek for ESPERE

Where are they involved? We find NOx (= NO + NO2) and other nitrogen oxides nearly everywhere in atmospheric chemistry. During the night, nitrate radicals NO3 are formed and are the most active oxidants. Radicals are chemical species, which are very instable and usually react extremely fast. If N2O5 is formed in polluted areas, it can react on droplets or wet surfaces with water and nitric acid HNO3 is formed. HNO3 contributes to the acid character of the rain. Nitric acid, which can also be formed during the day by oxidation of NO2, is the main way how nitrogen oxides are removed again from the atmosphere, either by dry or by wet deposition (wash out by rain). Nitric acid is also part of polar stratospheric clouds. Nitric acid tryhydrate forms the particles on which the ozone hole developed (details in 'Higher atmosphere - More - Unit 2').

Names of nitrogen compounds: Formula Systematic Name Common Name NO nitrogen monoxide nitric oxide N2O dinitrogen monoxide nitrous oxide NO2 nitrogen dioxide nitrogen peroxide N2O5 dinitrogen pentoxide nitric anhydride N2O3 dinitrogen trioxide nitrous anhydride HNO3  - nitric acid NH3  - ammonia

Nitrogen oxides as gases are very important for the formation and degradation of tropospheric ozone, because they are involved in catalytic cycles. This is mainly, because NO₂ can be photolysed by the sunlight. It forms NO and this NO is oxidised again to NO₂. Ozone as well as organic peroxi-radicals (instable oxidised compounds) can be involved in this cycle as we see in detail in the next text.
The main reason for the invention of the catalytic converter for cars was, to avoid strong emissions of nitrogen oxides. We emit too much of them in combustion processes, especially in cars, and disturb the equilibrium in the air.
Nitrous oxide N₂O is formed for example by baceteria in so called degradation processes. The microbiology of such tiny living organism plays a big role in the nitrogen cycle. But N₂O does not react in the troposphere. It goes directly to the stratosphere, where it is split by sunlight (photolysed).
Ammonia NH₃ is the most important basic gas in the atmosphere. Coming e.g. from livestock farming and fertilization, but also from microbiological degradation (bacteria), it can form salt particles of NH₄NO₃ together with nitric acid.

Nitrogen oxides at the crossways of atmospheric chemistry

Even if we do not have a closer look on nitrogen oxide chemistry, we can keep in mind, that these compounds are a little bit the heart of atmospheric chemistry. A major part of the chemical compounds, which are oxidised and removed from the atmosphere or transformed into other compounds come into touch directly or indirectly with NO or NO₂.

4. Nitrogen oxides - in the centre of atmospheric chemistry
image: Elmar Uherek

 

Related pages:
Nitrate radicals play a special role at night. Read more about it in:
Lower atmosphere - More - Unit 1 - Night and nitrate
You find further information about the chemistry of nitrogen oxides in the ozone hole at:
Higher atmosphere - More - Unit 2 - Chlorine chemistry