Lower Atmosphere

Read more

! significantly changed !
2004-05-07

Oxidation in the Atmosphere

Numerous chemical compounds are emitted to the atmosphere and would accumulate if they would not be removed again. Removal can occur via dry deposition or wash out by rain (wet deposition). In particular for gaseous organic compounds removal from the atmosphere is easier if they are transferred into a less volatile, water soluble oxidised state ...

Oxidation in a chemical sense does not necessarily mean a reaction with oxygen containing compounds. But in the air in most cases oxygen is involved. There are three major oxidants which govern such processes in the atmosphere:

the hydroxyl radical OH
the nitrate radical NO₃
the ozone molecule O₃
Also HO₂ radicals are important and sometimes the sum of OH and HO₂ is called HO_x. The most important oxidant however is the hydroxyl radical OH. It is extremely reactive and able to oxidise most of the chemicals formed in the troposphere.
Therefore OH is called the 'detergent of the atmosphere'.

Only a few compounds like chlorofluorocarbons CFCs (for example CF₂Cl₂), nitrous oxide N₂O or carbon dioxide CO₂ are very stable and do not react at all or react very slowly with OH. Also the reaction rate of methane CH₄ is 100-1000 times lower compared to other organic compounds. This explains why methane concentrations (about 1.7 ppm = 1.7 µmol/mol) can be rather high in the atmosphere, while most of the organic trace gas concentrations range below 1 ppb (= 1nmol/mol)*.

How is OH formed?

OH governs atmospheric chemistry during the day, because its formation primarily depends on the radiation from the sun. The initial reaction (above) is the cracking of ozone by sunlight (photolysis) taking place at wavelenghts less than 310 nm, followed by the reaction of the formed O atom with water. This is why a basic amount of ozone in the troposphere is essential for its chemistry, although too much is not healthy. 
Other sources of OH are the photolysis of nitrous acid HONO, hydrogen peroxyde H₂O₂ and peroxy methane CH₃OOH, the reaction of NO with the hydroperoxy radical HO₂ or the reaction of alkenes with ozone. The scheme on the left shows, in which way OH is intertwined with the daytime reaction cycles of nitrogen oxides.

How much OH is formed?

Since OH is an extremely reactive radical it reacts as soon as it is formed. It's lifetime is about a second or less. This means the concentration is extremely low, in the range of 1x10⁵ to 2x10⁷ molecules cm^-3. For sea level pressure this is a mixing ratio of 0.01 - 1 ppt (pmol/mol).
Since the formation depends on the availability of water vapour, the concentration of OH tends to decrease with increasing altitude (cooler and dryer air). 

But it decreases in particular with the latitude because not only the water vapour concentration decreases but also the intensity and the duration of sunlight towards the poles.

How does OH react?

The image on the right shows an interesting effect over the tropical rainforest. The OH concentration decreases near the ground. What is the reason? Many organic compounds, first of all isoprene, are emitted by the forest and react with OH. Therefore a strong OH removal occurs close to the ground. It is consumed in chemical reactions. OH has a strong tendency to abstract an hydrogen atom from organic compounds RH whenever possible and to form water H₂O. In the next step the radical R reacts with oxygen O₂ and forms organic peroxides which are for example essential for the ozone formation cycle.

However, worldwide OH does not react first of all with organic compounds from forests. Organic gases contribute with 30% to the removal of OH plus another 15% for methane, the most important and smallest of the organic molecules. The main gas which reacts with OH is carbon monoxide (40%) and the remaining 15% react with ozone O₃, hydroperoxy radicals HO₂ and hydrogen H₂.

Reacting with small alkenes, a special class of organic compounds, OH tends to add to the double bond as long as the saturated rest is not much bigger and H abstraction statistically favoured. Also here a formation of peroxides occurs.
OH is able to oxidise carbon monoxide CO to carbon dioxide CO₂. As we saw CO and methane CH₄ are the main sinks of OH. Other reactive organic compounds are only available in traces of some ppt, while CO reaches average levels of 120 ppb in the northern hemisphere (more combustion processes) and 60 ppb in the southern hemisphere.

Although OH is the most important oxidant in the atmosphere, its night time concentration is close to zero because radiation is needed for the formation. This is why during dark periods and during the night, the chemistry of nitrate NO₃ and also ozone O₃ becomes more important.

* The mixing ratio ppb or ppm (= 1 molecule among 1 billion molecules, or 1 molecule among 1 million molecules) is often used in scientific publications as well as in other literature on atmospheric and climate science. We also use it here in the Climate Encyclopaedia. However, more correct is the unit 1 nmol/mol (= 1 ppb) or 1 µmol/mol (= 1 ppm). Because the quantity of molecules n is measured in the unit mol.

About this page:

author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz
scientific reviewer: Dr. Mark Lawrence - Max Planck Institute for Chemistry, Mainz 2004-05-05
educational proofreading: Michael Seesing - Uni Duisburg - 2003-07-02
revised and last published: 2004-05-07