Reactions involved in the formation of tropospheric ozone, meteorological conditions favourable to ozone formation and the role of ozone in climate change

Photochemical (or photo-oxidant) pollution involves a series of complex phenomena leading to the formation of ozone (O3) and other oxidising compounds (such as hydrogen peroxide, aldehydes, peroxyacetyl nitrate or PAN) from primary pollutants, called precursors: nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOC), carbon monoxide (CO) and methane (CH4) and the energy from solar ultra-violet (UV) radiation. This type of ozone-laden atmospheric pollution, often called "smog", occurs in the lower layer of the atmosphere, called the troposphere, 0 to 8-10 km above the Earth's surface. The estimated lifetime of ozone is about 22 days, but it is much shorter – only 1 or 2 days – in the layer 0 to 2 km above the Earth's surface).

Ozone and photochemical oxidants are secondary pollutants.

The chemical reactions involved are complex but can be represented fairly simply. In a polluted atmosphere, they involve NO2 and NMVOCs as primary pollutants or precursors.

UV radiation dissociates NO2:

NO2 + h ν→ NO + O [a] at wavelengths λ < 430 nm

The atomic oxygen thus formed reacts with molecular oxygen to form ozone, which is a molecule comprising 3 atoms of oxygen:

O + O2 → O3 [b]

Ozone reacts with nitrogen monoxide to re-form NO2:

O3 + NO → NO2 + O2 [c]

The NO is described as an "ozone sink", because it reduces ozone concentrations by consuming ozone.

Ozone concentrations depend on the ratio between NO2 and NO concentrations.

If no VOCs are present, tropospheric ozone concentrations are low.

When VOCs are present, a complex series of reactions occurs that causes ozone to accumulate. These reactions increase the NO2 sink in the atmosphere by consuming NO [through the reaction f described below], which then ceases to function as an ozone sink [reaction c]. NO2 thus forms with no ozone destruction. The decomposition of VOCs is triggered by the OH radical (hydroxyl radical), which is highly reactive and naturally present in the atmosphere.

RH + OH → R + H2O [d] (RH is a simplified representation of a VOC, R represents an organic molecule associated with hydrogen)

R + O2 → RO2 [e]

RO2 + NO → NO2 + RO [f]

These reactions generate a great many organic gaseous species, especially organic nitrogen compounds such as peroxyacetyl nitrate (PAN).

The diagram below shows the chemical processes involved in tropospheric ozone formation. The quantity of ozone present in the troposphere is estimated at 4 500 Tg/year (one Teragramme (Tg) = 1 million tonnes). Ozone input from the stratosphere is estimated at 540 Tg/year. Ozone is destroyed by chemical processes and dry deposition.

Figure 1 : Key chemical processes involved in tropospheric ozone formation and ozone budgets in the troposphere

Source [RSS, 2008]



Meteorological conditions that are favourable to high ozone concentrations include, in particular:

    • high air temperatures,
    • a low air moisture content,
    • long hours of sunlight,
    • high irradiation,
    • low synoptic wind speeds (large-scale winds as opposed to local winds).

Photochemical pollution is a characteristic phenomenon during anticyclone conditions in the summer.


Concentrations measured and trends

An important characteristic of atmospheric chemistry is its non-linear nature. This means that ozone formation is not proportional to concentrations of its precursors. The reactions involving the different compounds will either form or destroy ozone, depending on their relative abundance.

This is what accounts for the surprising fact that ozone concentrations measured at a distance from the source of ozone precursors (an urban area for example) are higher than those measured near the actual sources of emission. This means that peak ozone concentrations affect suburban and rural areas more than cities. Over a city, for example, NO emissions (especially from traffic) are high. Any ozone likely to form is quickly destroyed by the high NO concentrations present. If the cloud of pollutants over the city moves away over the countryside, where NO emissions are lower, the concentration of ozone will increase as it is no longer consumed by NO.

Figure 2 below, taken from the French Environment Ministry's 2012 air quality assessment [DGEC, 2013], shows average annual ozone concentrations observed in France from 2000 to 2012 in rural and urban areas. The difference between urban and rural areas ranges from about 10 to 15 µg/m3. Average annual concentrations over French cities vary from 40 to 60 µg/m3. In suburban and rural areas, they range from about 60 to 70 µg/m3. In summer, concentrations are measured and reported that are well above the 120 µg/m3 target value for the protection of human health (maximum daily 8 hour mean) established by EU Directive 2008/50/EC (see below).

Figure 2 : Average annual O3 concentrations observed in France from 2000 to 2012 in urban and rural areas

Source [DGEC, 2013]


In general, although emissions of the two precursors NOx and NMVOC dropped in France by 45% and 61% respectively from 2000 to 2014 [CITEPA, 2016], average ozone concentrations in France increased by 8% over the 2000-2011 period. France is not an exceptional case, as average ozone concentrations have also increased globally, and more so in the Northern than the Southern hemisphere.

Figure 3 below shows average ozone concentrations measured in spring at several points that are representative of background levels in Europe, the United States (US) and Japan [UNECE, 2010]. The Arkona and Zingst sites are both located on the coast of the Baltic Sea. Mace Head is on the west coast of Ireland and Hohenpiessenberg is in southern Germany at an altitude of 1 000 m. The Zugspitze (3 000 m high) is also in southern Germany, and the Jungfraujoch (3 600 m) is in Switzerland. The diagram at the bottom shows background levels measured in the US and Japan. The US sites are at sea level and in the Lassen National Park at 1800 m above sea level. The Japanese sites are on the Happo Peak at 1 900 m a.s.l. and on the island of Rishiri, at sea level. The correlations found are either linear regressions or 4th degree polynomials. Experts have found that the increase in concentrations is slowing somewhat in some sites, although this needs to be confirmed by further measurements. Overall, the trend is towards an increase in concentrations.

Figure 3 : Average ozone concentrations in spring measured at several representative sites of background levels in Europe, the United States and Japan

Source [UNECE, 2010]


In the last 20 to 30 years, an increase of 1 to 2% a year has been observed. This highlights the complexity of photochemical pollution and the role of the CO and CH4 precursors, which have a longer lifetime.


Tropospheric ozone and the greenhouse effect

Tropospheric ozone plays a part in the greenhouse effect. It is the 3rd main greenhouse gas in terms of GWP after CO2 and CH4 according to the 4th IPCC assessment report [IPCC, 2007], due to its radiative forcing of 0.35 W.m2 [IPCC, 2007]. However, it is not yet covered by the greenhouse gas reduction commitments established at international level under the United Nations Framework Convention on Climate Change (UNFCCC), although scientists consider that the increase in background ozone concentrations warrants worldwide reduction measures. The temperature increases resulting from climate change are conducive to the reactions that build up tropospheric ozone. Because of the impact of ozone on vegetation (see below) and therefore on carbon sinks, increasing concentrations of tropospheric ozone could accentuate the greenhouse effect. Scientists therefore recommend stronger action than the hitherto adopted regional measures to reduce emissions of precursors, by introducing measures at global level to reduce emissions of CH4 and CO [UNECE, 2010].


Impact of the nature of VOCs on tropospheric ozone formation

NMVOCs in the atmosphere do not all react in the same way. The concept of "photochemical reactivity" was developed to indicate the potential for ozone formation of each NMVOC. The higher the potential, the greater the role of NMVOCs in the mechanisms at work, but the ratio is by no means linear.

The most recent method developed to characterise photochemical reactivity is based on estimations of the actual contribution of each NMVOC to ozone formation in a geographically defined zone, taking into account the characteristics of the environment where the reaction takes place (composition of ambient air, emission characteristics). This highly complex method relies on very sophisticated models of atmospheric chemistry.

The Photochemical Ozone Creation Potential (POCP) of an NMVOC was developed by the Air Quality Department of the UK Environment Ministry by R.G. Derwent [DERWENT 1998] using a photochemistry model. This was used to simulate three trajectories assumed to be representative of the general situation in Europe during episodes of photochemical pollution, but they do not represent any particular pollution episode. The contributions of each initial NMVOC to ozone formation are obtained by running the model for each trajectory (once with all compounds and as many times as there are VOCs to be studied with all NMVOCs except one, where emissions are considered to be nil). The result shows in particular that ethylene is a highly active compound in photochemical processes. It is one of the NMVOCs whose concentrations are well known and also among the highest, and is therefore used as the reference NMVOC. Its POCP index is set at 100.

The formula defining the POCP of a compound i is as follows:

IV.1.5.Formule PCOPI EN


High potential : PCOP > 80

Moderate potential : PCOP 40 à 60

Low potential : PCOP 10 à 35

Very low potential : PCOP <10

NMVOC reactivity according to their POCP is as follows:

Aromatics > Alkenes > Aldehydes > Alkanes.

However, it must be remembered that the POCP value of an NMVOC (or any other index) depends on the model used, the value of all the parameters used to run the model, the climatic conditions modelled, the place where ozone concentrations were reported and the time lapse between the emission and the point when the ozone concentration was observed. A POCP index is not directly useable. It must also be remembered that the index varies over time and space (the role of compounds with low initial reactivity can become preponderant once the most reactive NMVOCs have reacted).

In the United States, the MIR parameter (Maximum Increment Reactivity) is used to characterise O3 creation potential.


Impacts of tropospheric ozone on human health and plants

Tropospheric ozone has an impact on human health. It causes irritation of the respiratory tract and eyes, lowers physical performance and deteriorates the functions of the lungs. In Europe, the number of premature deaths due to ozone was estimated at 22 700 in 2000. The baseline scenario defined by the International Institute for Applied Systems Analysis (IIASA) during the review of the Gothenburg Protocol (based on EU legislation to reduce greenhouse gas emissions and full implementation of EU directives on pollutant emission reductions - see below) would bring this number down to 17 400 by 2020 [AMANN, 2011].

Ozone disrupts photosynthesis in plants and erodes their resistance. It attacks plants through their stomata. Plants absorb less ozone in dry weather than in wet weather: the stomata close up in dry weather to protect the plant from drought, thus protecting it from ozone as well. Depending on individual species, plants are more or less sensitive to ozone which causes visible damage to leaves (yellowing, for example) and stunts their growth. Study results show that crop yields decline in the presence of ozone. Figure 4 below shows wheat yield losses in 2000 and 2020 in a scenario where precursor emissions have in fact dropped, according to studies conducted by the Working Groups on Effects (WGE) under the Convention on Long-Range Transboundary Air Pollution (CLRTAP) adopted under the aegis of the United Nations Economic Commission for Europe (UNECE) [WGE, 2011].

Figure 4 : Wheat yield losses in 2000 and 2020 in a scenario with reduced emissions of ozone precursors in 2020

Source [WGE, 2011]


Ozone and photochemical pollutants also heighten the acidifying potential of sulphur and nitrogen oxides by accelerating their oxidation into sulphates and nitrates. Oxidising pollution and acid pollution act in combination on plants and contribute to forest degradation.

Ozone also reduces carbon storage capacity in plants [ICPVEG, 2012].


Ozone concentration standards to be complied with

French Ministerial Decree n° 2010-1250 of 21 October 2010 (OJ of 23 October 2010) transposes Directive 2008/50/EC on ambient air quality (the Air Quality Framework Directive) into French law. Its provisions replace Section 1 of Chapter 1 (air quality monitoring and public information), Heading II (Air and atmosphere), Tome II (Physical Environments) of the French Environment Code.

Table 1 below shows the air quality standards for ozone set out in Directive 2008/50/EC on air quality monitoring to ensure health and ecosystem protection.

Table 1: Air quality standards for ozone to ensure health and ecosystem protection

Type of threshold Threshold

µg / m3

Calculation method
Human health    
Long term objective 120 Maximum daily 8 hour mean for health protection
Information threshold 180 Hourly average
Alert threshold 1 240 Hourly average exceeded for 3 consecutive hours
Alert threshold 2 300 Hourly average exceeded for 3 consecutive hours
Alert threshold 3 360 Hourly average
Long term objective 6 000 µg/m3.h AOT 40a calculated from 1 hour values from May to July
Target 18 000 µg/m3.h AOT 40a calculated from 1 hour values from May to July (averaged over 5 years)

a AOT40 (Accumulated Ozone exposure over a threshold of 40 parts per billion, expressed in µg/m³ per hour): sum of the differences between hourly concentrations above 80 µg/m³ (= 40 parts per billion) and 80 µg/m³ over a given period using only the 1 hour values measured between 8 a.m. and 8 p.m. each day.


Regulations to reduce emissions of precursors

Regulations have been adopted at international, European and local levels.

At international level, the United Nations Convention on Long-Range Transboundary Air Pollution (CLRTAP) was the first to focus on transboundary air pollution. This Convention was signed in Geneva in 1979 by 33 member countries of the UN Economic Commission for Europe (UNECE). Although the Convention itself does not set out any emission reduction commitments, several Protocols adopted thereunder by the Parties do so. As regards NMVOCs and NOx, the Sofia Protocol on reducing NOx emissions and the Geneva Protocol on reducing VOC emissions were introduced in 1988 and 1991 respectively. They have since been superseded by the Gothenburg Protocol on reducing acidification, eutrophication and tropospheric ozone (or Multi-Pollutant, Multi-Effects Protocol), which covers SO2, NOx, VOCs and NH3, was signed on 1 December 1999. The principle guiding the definition of the commitments set out for the Parties to the Gothenburg Protocol is the reduction of the effects of the four pollutants on health and ecosystems. The Protocol establishes national emission ceilings for each of the Parties to the Convention, to be complied with by 2010. The ceilings for France are 1 100 kt for NMVOC emissions and 850 kt for NOx. For an analysis of France's compliance with these ceilings, please refer to the section on France and its emission reduction targets.

A series of amendments to the Gothenburg Protocol was adopted on 4 May 2012. New emission reduction commitments were set for the four pollutants indicated above and for PM2,5, to be achieved by 2020. France's commitment is to reduce NOx emissions by 50% by 2020 compared to its 2005 emissions, and NMVOC emissions by 43%. The amended Protocol must be ratified by one third of the Parties before it can enter into force.

At EU level, Directive 2001/81/EC of 23 October 2001 sets out national emission ceilings (NEC) for the four atmospheric pollutants indicated above. The ceilings for France are 1 050 kt for NMVOC emissions and 810 kt for NOx emissions. These ceilings are slightly stricter than those set under the 1999 Gothenburg Protocol. The European Commission is now reviewing this Directive and plans to present a proposal for a new Directive in 2013, based on the same approach as for the Gothenburg Protocol, i.e. ceilings to be set according to the targets to be reached to reduce effects on health.

The Protocol and the EU Directive thus set out targets for emissions from all sources combined (ceilings).

Directive 2010/75/EU of 24 November 2010 (OJEU L 334 of 17 December 2010) on industrial emissions (known as the IED Directive) recasts Directive 2008/1/EC on integrated pollution prevention and control (the IPPC Directive). The new IED Directive establishes rules on integrated prevention and control of pollution from the industrial activities covered, in order to prevent, or if prevention is impossible, to reduce emissions into air, water and land, and to prevent the generation of waste, in order to achieve a high level of overall environmental protection. The Directive thus gives priority to prevention, and, failing this, to the reduction of industrial pollution. CITEPA has produced an executive summary of the Directive, in Synthèses Document'Air n° 178 published in April 2011 [Tuddenham, 2011].

The IED Directive applies to the pollution-generating industrial activities covered in its Annex I and Chapters II to IV, i.e.:

    • facilities covered by the previous IPPC Directive (2008/1/EC) (cf. Annex I),
    • combustion plants covered by the previous LCP Directive on large combustion plants (2001/80/EC) (cf. Annex V),
    • waste incineration and co-incineration plants covered by the previous Directive on incineration (2000/76/EC) (cf. Annex VI),
    • facilities and activities using organic solvents covered by Directive 1999/13/EC on VOC emissions due to the use of solvents in certain activities (cf. Annex VII),
    • facilities producing titanium dioxide covered by the three previous Directives on this subject (78/176/EEC, 82/833/EEC, 92/112/EEC) (cf. Annex VIII).

The conclusions on Best Available Techniques (BAT) are to be used as a reference to for setting the conditions for delivering the operating permits required for the facilities covered by the Directive. The competent authority may set stricter permit conditions than those aimed for by BATs (as described in the BAT conclusions). For this purpose, Member States may establish rules allowing the competent authority to set stricter conditions.

At national level, a series of regulatory texts establishes the conditions of the authorisation or declaration of facilities. The IED directive has been transposed into French law. There are a large number of regulatory texts on reducing VOC and NOx emissions. Among these texts, the decree 2013-375 of the 2nd of may 2013 modifying the list of the industries classified for environmental protection and corresponding to activities covered by the European directive for industrial emissions (IED).


Action to reduce concentrations in the long term

In order to reduce average concentrations of tropospheric ozone and the frequency of concentration peaks, emissions of ozone precursors have to be reduced. However, it must be borne in mind that photochemical phenomena are not linear. Depending on the relative abundance of the various compounds, ozone concentrations may be higher or lower and action to reduce one precursor or several at once may produce the opposite effect to what is sought.

The choice of an effective strategy can only be made on the basis of in-depth scientific studies. Where ozone is concerned, it is not enough to require the application of the best available and most economically affordable techniques.

Emissions of precursors such as NOx and NMVOCs have dropped, bringing down peak concentrations in Europe, but background levels have continued to rise.

As ozone is the 3rd most important greenhouse gas after CO2 and CH4 [IPCC 2007], a global approach to reduce concentrations has become all the more necessary as climate change is conducive to its formation (higher temperatures, higher emissions of CH4 and higher emissions of naturally occurring NMVOCs, for example [UNECE, 2010].



  • [UNECE, 2010] UNECE - Hemispheric transport of air pollution - Part a: ozone and particulate matter – Air pollution studies no. 17– 2010
  • [DGEC, 2013] General Directorate for Air and Climate (DGEC)/French Environment Ministry – Air quality assessment 2012. 2013.
  • [RSS, 2008] The Royal Society of Sciences - Ground-level ozone in the 21st century: future trends, impacts and policy implications. 2008
  • [AMANN, 2011] Markus Amann et al - An updated set of scenarios of cost-effective emission reductions for the revision of the Gothenburg Protocol - Background paper for the 49th session of the working group on strategies and review - Geneva, September 12-15, 2011
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