How is Tropospheric Ozone Formed?

Ozone is formed as ultraviolet radition from the sun breaks down oxygen in the atmosphere, O2, into atomic oxygen with a single atom.

O2 + sunlight --> O + O

The excited free oxygen atom can bond with O2 to form ozone.

O + O2 --> O3

In the stratosphere, the destruction of ozone occurs as quickly as ozone is formed. Sunlight in the upper atmosphere can easily split ozone into an oxygen atom and an oxygen molecule. Also, an excited oxygen atom can react with an ozone molecule to create two oxygen molecules. Under natural conditions there is a balance between the creation and destruction of ozone moleclues in the stratosphere.

O3 + sunlight --> O2 + O

O + O3 --> O2 + O2

In the troposphere, ozone is also formed by the splitting of molecules by sunlight. But in the lower atmosphere sunlight also splits nitrogen dioxide (NO2) into nitric oxide (NO) and an oxygen atom. Therefore nitrogen dioxide provides the molecular oxygen needed for ozone formation.

NO2 + sunlight --> NO + O

O + O2 --> O3

Then ozone reacts with nitric oxide to create nitrogen dioxide and oxygen. This process occurs naturally and does not result in a net gain of ozone.

NO + O3 --> NO2 + O2

However, the human induced production of ozone precursors, NO, NO2, and volatile organic compounds (VOC) has altered the atmospheric chemistry which contributes to elevated levels of ozone in the lower atmosphere.

NOx + VOC + sunlight --> O3 (and other products)

(Allen 2002)

Reactions with NOx and VOC

Nitric oxide (NO) is emitted through combustion or fuel burning processes, generally in fossil fuel fired boilers, automobiles, and trucks. The NO is quickly converted to nitrogen dioxide (NO2). As the reactions continue, nitrogen dioxide reacts to form nitrate. As the nitric oxide concentration drops, the levels of ozone rise rapidly. Emissions of NO and hydrocarbons are often high in the early morning due to rush hour traffic. This early morning emission of the pollutants that contribute to ozone formation is combined with increasing solar radiation which drives the reaction.

Volatile organic carbons also contribute to the formation of ozone. Volatile organic compounds are formed through industrial and commercial processes, motor vehicle emissions, and use of solvents. Vegetation is also a natural source of VOCs.

(Finlayson-Pitts and Pitts, 1993)

Sensitivity of NOx and VOC

The ratio of NOx and VOC in the atmosphere can alter the efficiency of ozone production. Higher NOx emissions can result in less efficient production of ozone. The areas to the right of the centermost line are characteristic of suburban or rural areas; concentrations of VOC are high in comparison to concentrations of NOx. The areas to the left of the line are representative of urban areas where there is a large amount of NOx production from combustion in motor vehicles and industrical processes.

Additionally, envrironmental variables such as temperature and humidity can influence the production of ozone.


Where is ozone a problem?
Reducing tropospheric ozone pollution