espere ESPERE-ENC working area - preliminary unevaluated websitesEnvironmental Science Published for Everyobody Round the Earth
Printer friendly version of this page
[Master Home]    English Sitemap    [Master Sitemap]   
Lower Atmosphere
1. Extension and composition
2. Greenhouse, light & biosphere
3. Ozone and nitrogen oxides
- ozone
- NOx
- ozone smog
* Worksheet 1
* Worksheet 2

Lower Atmosphere


Nitrogen oxides - Formation and Relevance

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


traffic jam

1. Traffic - still an important source of nitrogen oxides.


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




nitrogen oxide cycle

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:


Systematic Name

Common Name


nitrogen monoxide

nitric oxide


dinitrogen monoxide

nitrous oxide


nitrogen dioxide

nitrogen peroxide


dinitrogen pentoxide

nitric anhydride


dinitrogen trioxide

nitrous anhydride



nitric acid





    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.


    NOx crossways

    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


    About this page:
    author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz
    scientific reviewing: Dr. Rolf von Kuhlmann, Max Planck Institute for Chemistry, Mainz
    educational proofreading: Michael Seesing - Uni Duisburg - 2003-07-02
    revised and last published: 2004-04-30




    last updated 09.07.2005 11:38:29 | © ESPERE-ENC 2003 - 2013