Chemistry Of Tropospheric Ozone Formation

Photodissociation of N02 by (visible) sunlight is the only significant anthropogenic source of 03 in the troposphere

immediately followed by

where the M in reaction (2) represents any nonreactive molecule that absorbs some of the excess energy of the intermediate product formed in the reaction (2).


The atmospheric oxidation of a hydrocarbon, RH, is initiated by reaction with the hydroxyl radical (OH):

where RH can be any molecule containing a hydrogen and a carbon, including methane, CH4, or any nonmethane hydrocarbon consisting of more than one carbon atom. The product is another radical, denoted R, and water vapor. The radical quickly combines with an oxygen molecule in a three-body reaction:

to form another oxygenated radical, R02, called a peroxy radical. The peroxy radicals are the key for converting NO to N02:

In addition, RO attaches to an oxygen molecule to form another peroxy radical:

where R'CHO is an aldehyde (and noting that R' is a shorter chained carbon radical than R). The H02 likewise reacts with NO to form another N02 molecule:

where the two N02 molecules photolyze and eventually produce ozone:


Additional ozone molecules can also be produced through the oxidation of R'CHO. In this reaction sequence, it is important to note that the nitrogen oxide emitted as a pollutant is still available to make more ozone. If NO were not present in the atmosphere, ozone would not be formed. In fact, the presence of many nonmethane hydrocarbons, by themselves, would result in a destruction of ozone since they, or some of their daughters of the oxidation process, could react with any ozone present in the atmosphere.

On the other hand, if only nitrogen oxides and ozone were present in the atmosphere, an equilibrium would quickly be established since 03 reacts quickly with NO:

and the ratio among NO, N02, and 03 is quickly established by the rates of the reactions among these species:

where the brackets denote the concentration of a particular species, j\ is the rate of photolysis of N02, and ks is the rate of reaction (8); the relationship among these three gases defined by this ratio is often referred to as the photostationary state and has had an important implication for understanding the formation of ozone near urban areas and subsequent strategies developed for the reduction of ozone concentrations.

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