Tropospheric Ozone - Formation

Formation

The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs), such as xylene, react in the atmosphere in the presence of sunlight. NOx, CO, and VOCs are called ozone precursors. Motor vehicle exhaust, industrial emissions, and chemical solvents are the major anthropogenic sources of these chemicals. Another source is windshield washer fluid. Although these precursors often originate in urban areas, winds can carry NOx hundreds of kilometers, causing ozone formation to occur in less populated regions as well. Methane, a VOC whose atmospheric concentration has increased tremendously during the last century, contributes to ozone formation but on a global scale rather than in local or regional photochemical smog episodes. In situations where this exclusion of methane from the VOC group of substances is not obvious, the term Non-Methane VOC (NMVOC) is often used.

The chemical reactions involved in tropospheric ozone formation are a series of complex cycles in which carbon monoxide and VOCs are oxidised to water vapour and carbon dioxide. The reactions involved in this process are illustrated here with CO but similar reactions occur for VOC as well. The oxidation begins with the reaction of CO with the hydroxyl radical (•OH). The radical intermediate formed by this reacts rapidly with oxygen to give a peroxy radical HO2•

•OH + CO → •HOCO
•HOCO + O2 → HO2• + CO2

Peroxy radicals then go on to react with NO to give NO2 which is photolysed to give atomic oxygen and through reaction with oxygen a molecule of ozone:

HO2• + NO → •OH + NO2
NO2 + hν → NO + O(3P)
O(3P) + O2 → O3

The balance of this sequence of chemical reactions is:

CO + 2O2 + hν → CO2 + O3

The amount of ozone produced through these reactions can be calculated using the Leighton relationship.

This cycle involving HOx and NOx is terminated by the reaction of OH with NO2 to form nitric acid or by the reaction of peroxy radicals with each other to form peroxides. The chemistry involving VOCs is much more complex but the same reaction of peroxy radicals oxidizing NO to NO2 is the critical step leading to ozone formation.

Read more about this topic:  Tropospheric Ozone

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