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Chemistry in the Sunlight
 

 

Step 1. Sunlight splits ozone into an oxygen molecule and an oxygen atom.
O3 + sunlight yields O2 + O
An electronically excited oxygen atom reacts with water vapor to generate hydroxyl. (Oxygen and water molecules are abundant in the atmosphere, and so are available to take part in ozone-forming reactions.)
O + H2O yields OH + OH
Step 2. Hydroxyl reacts readily with other chemicals, and initiates another sequence of reactions. It combines with methane, producing water and a chemical called a methyl radical.
CH4 + OH yields CH3 + H2O
Step 3. The methyl radical combines with oxygen to produce methyl peroxy radical.
CH3 + O2 yields CH3O2
Step 4. The methyl peroxy radical combines with nitric oxide (from fossil fuel combustion) to produce a methyloxy radical and nitrogen dioxide.
CH3O2 + NO yields CH3O + NO2
Step 5. Sunlight splits the nitrogen dioxide into nitric oxide and atomic oxygen, which combines with molecular oxygen to yield ozone, as in Reactions 5 and 6 above.
NO2 plus sunlight yields NO + O
O + O2 yields O3
Some of the methyl oxy radical from Step 4 can participate in a different series of reactions that ultimately also make more nitrogen dioxide available for ozone formation. In this second series, the methyl oxy radical combines with oxygen to produce formaldehyde and a hyperoxy radical.
CH3O2 + O2 yields CH2O + H2O
The hyperoxy radical reacts with nitric oxide to yield hydroxyl and nitrogen dioxide.

Sunlight splits nitrogen dioxide into nitric oxide and atomic oxygen, which combines with molecular oxygen to yield ozone, as in Reactions 5 and 6 above (copied below).
NO2 plus sunlight yields NO + O
O + O2 yields O3
A third series of reactions, one involving formaldehyde produced in Reaction 12, can ultimately produce even more nitrogen dioxide that then becomes available for ozone formation. Sunlight splits formaldehyde into a formyl radical and atomic (single atom) hydrogen. Both of these species are extremely short lived and react almost instantaneously with molecular oxygen to form hyperoxy radicals.
CH2O + sunlight yields NCO + H
HCO + O2 yields CO + HO2
H + O2 yields H2O
The hyperoxy radical and nitric oxide then combine to form hydroxyl and nitrogen dioxide.
HO2 + NO yields OH + NO2
Reactions 5 and 6 then take place, resulting in more ozone.

Some Chemicals Involved in Ozone Formation:

Symbol Compound Name
CH2O formaldehyde
CH3 methyl radical
CH3O methyloxy radical
CH3 O2 methyl peroxy radical
CH4 methane
CO carbon monoxide
H hydrogen
HCO formyl radical
HO2 hyperoxy radical
H2O water
NO nitric oxide
NO2 nitrogen dioxide
NOx nitrogen oxides (NO & NO2)
N2O nitrous oxide
OH hydroxyl radical
O atomic oxygen
O2 molecular oxygen
O3 ozone

back: Chemistry of Ozone Formation

  

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