Editing Ground-level ozone (section)

==Formation==
The majority of ground-level ozone formation occurs when [[nitrogen oxides]] (NOx), [[carbon monoxide]] (CO), and [[volatile organic compounds]] (VOCs), react in the atmosphere in the presence of sunlight, specifically the UV spectrum. NOx, CO, and VOCs are considered ozone precursors.<ref name=":12"/><ref name=":42"/> Motor vehicle exhaust, industrial emissions, and chemical solvents are the major anthropogenic sources of these ozone precursors.<ref name=":42"/> Although the ozone 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 volatile organic compound]] (NMVOC) is often used.

Indoors ozone is produced by certain [[high voltage|high-voltage]] electric devices (such as [[air ionizer]]s) and as a by-product of other types of pollution.<ref>{{cite journal |last=Weschler |first=Charles J. |title=Ozone in Indoor Environments: Concentration and Chemistry: Ozone in Indoor Environments |date=December 2000 |journal=Indoor Air |language=en |volume=10 |issue=4 |pages=269–288 |pmid=11089331 |doi=10.1034/j.1600-0668.2000.010004269.x |url=https://onlinelibrary.wiley.com/doi/full/10.1034/j.1600-0668.2000.010004269.x |access-date=April 11, 2024 |url-status=live |archive-url=https://web.archive.org/web/20240415110851/https://onlinelibrary.wiley.com/doi/full/10.1034/j.1600-0668.2000.010004269.x |archive-date=April 15, 2024|url-access=subscription }}</ref> Outdoor air used for ventilation may have sufficient ozone to react with common [[indoor air quality|indoor pollutants]] as well as skin oils and other common indoor air chemicals or surfaces. Particular concern is warranted when using "green" cleaning products based on citrus or [[terpene]] extracts, because these chemicals react very quickly with ozone to form toxic and irritating chemicals<ref name="pmid38258874">{{cite journal |last1=Harding-Smith |first1=Ellen |last2=Shaw |first2=David R. |last3=Shaw |first3=Marvin |last4=Dillon |first4=Terry J. |last5=Carslaw |first5=Nicola |title=Does green mean clean? Volatile organic emissions from regular versus green cleaning products |date=January 23, 2024 |journal=Environmental Science: Processes & Impacts |language=en |volume=26 |issue=2 |pages=436–450 |issn=2050-7887 |pmid=38258874 |doi=10.1039/D3EM00439B |doi-access=free |url=https://pubs.rsc.org/en/content/articlelanding/2024/em/d3em00439b}}</ref> as well as [[atmospheric particulate matter|fine]] and [[ultrafine particles]].

The chemical reactions involved in ground-level 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]] (<sup>•</sup>OH).<ref name=":22"/> The radical intermediate formed by this reacts rapidly with oxygen to give a [[hydroperoxyl|peroxy radical]] {{chem2|HO2^{•}|}}

An outline of the chain reaction that occurs in oxidation of CO, producing {{O3}}:<ref name=":32"/><ref name=":22"/>

The reaction begins with the oxidation of CO by the [[hydroxyl radical]] (<sup>•</sup>OH). The radical [[adduct]] (•HOCO) is unstable and reacts rapidly with oxygen to give a [[hydroperoxyl|peroxy radical]], {{chem2|HO2^{•}|}}:
:{{chem2|•OH + CO -> •HOCO}}
:{{chem2|•HOCO + O2 -> HO2• + CO2}}

Peroxy-radicals then go on to react with NO to produce {{chem2|NO2}}, which is [[photolysis|photolysed]] by UV-A radiation to give a [[ground state|ground-state]] atomic oxygen, which then reacts with molecular oxygen to form ozone.<ref name=":6"/>
:{{chem2|HO2^{•} + NO -> ^{•}OH + NO2}}
:{{chem2|NO2 + hν -> NO + O(^{3}P)}}, λ<400 nm
:{{chem2|O(^{3}P) + O2 -> O3}}
: ''note that these three reactions are what forms the ozone molecule, and will occur the same way in the oxidation of CO or VOCs case.''

The net reaction in this case is then:
:{{chem2|CO + 2O2 -> CO2 + O3}}

The amount of ozone produced through these reactions in ambient air can be estimated using a modified [[Leighton relationship]]. The limit on these interrelated cycles producing ozone is the reaction of •OH with {{chem2|NO2}} to form [[nitric acid]] at high [[NOx]] levels. If nitrogen monoxide (NO) is instead present at very low levels in the atmosphere (less than 10 approximately ppt), the peroxy radicals ({{chem2|HO2•}}) formed from the oxidation will instead react with themselves to form [[peroxide]]s, and not produce ozone.<ref name=":6"/>