Last reviewed 11 July 2012

Rob Bell looks at the problems associated with the air pollutant ozone and how best policy makers can address ground-level ozone levels across the EU.


Ozone is a particularly problematic air pollutant. Rather than being directly emitted by, for example, vehicle exhausts, it is formed at ground level through the reaction of other pollutants and sunlight.

The European Environment Agency (EEA) says: “Ozone is a 'secondary' pollutant, formed in the lower part of the atmosphere — the troposphere — from complex photochemical reactions following emissions of precursor gases such as NOx and volatile organic compounds (VOCs).

“Ozone, one of the air pollutants giving rise to the greatest concern in Europe, is a powerful oxidising agent.”

This means policy-makers lack a straightforward target for efforts to reduce ozone levels. To make matters even more complex, the presence of nitrogen oxides (NOx) suppresses the formation of ground-level ozone (O3), and so any success in reducing the amount of one pollutant (NOx) entering the atmosphere removes a hindrance to the production of higher levels of ozone. This means that one step forward on NOx is a step backwards on O3.

The EEA says: “Ozone levels become particularly high in regions where considerable ozone precursor emissions combine with stagnant meteorological conditions during the summer, when high insolation (a measure of the amount of solar radiation hitting the earth’s surface) and temperatures occur.”

Tackling ground-level ozone levels

Efforts to address ground-level ozone levels across the European Union have so far met with limited success, although monitoring in summer 2011 showed positive results. (Due to the importance of sunlight as a catalyst in ozone formation, summer is the most important part of the year when addressing the problem of ozone production.)

The EEA says: “Ground level ozone causes health problems, decreases crop yields and damages the environment. Ozone levels exceeding certain targets in Europe were less frequent in summer 2011 than in any year since monitoring started in 1997.

“However, the long-term objective was exceeded in all EU Member States and it is likely many of them will not meet the target value, applicable as of 2010.”

EEA Executive Director Jacqueline McGlade is keen to highlight the importance of ozone pollution and its negative impacts on human health, but also in areas such as agriculture.

She says: “In summer 2011, exceedances of ozone targets were lower than average, but it is still one of the most serious air pollutants in Europe. And air pollution affects people’s quality of life.”

The EEA publishes an annual report on summer ozone levels. The 2012 report — Air Pollution by Ozone across Europe during Summer 2011: Overview of Exceedances of EC [European Community] Ozone Threshold Values for April-September 2011 — is based on data from 2186 monitoring sites across Europe.

According to the EEA: “Ozone can cause respiratory problems and other severe health problems. Ground level ozone production depends on weather conditions such as solar intensity and temperature, and is a result of chemical reactions between other pollutants in the air.

“These include nitrogen oxides, carbon monoxide, methane (NH4) and non-methane volatile organic compounds (NMVOCs). These substances are emitted by industry, transport, agriculture and other sources.”

Last summer saw the “information threshold” and “long term objective” (LTO) for the protection of human health exceeded at the lowest proportion of air-monitoring stations since the start of comprehensive Europe-wide data reporting in 1997.

However, actions to reduce emissions of ozone-precursors from transport, industry, etc were not the main cause of this good news. The EEA says: “This reduction was mainly due to unusually low temperatures and increased rainfall during the summer months, although there have also been some reductions in the emissions of ozone-precursor pollutants.”

Exceeding the threshold

Sixteen Member States and four non-member countries experienced exceedences of the information threshold (a one-hour ozone concentration of 180 micrograms per cubic metre (µg/m3)), with around 18% of monitoring stations measuring higher than desirable ozone levels.

All Member States, however, exceeded the LTO, although this was in line with results from previous years. The EEA says: “Exceedences of the maximum daily eight-hour mean concentration of 120µg/m3 were registered at approximately 84% of all operational stations. This limit was exceeded on more than 25 days in a significant part of Europe.

“Some monitoring stations also measured ozone levels in excess of the alert threshold (a one-hour average ozone concentration of 240µg/m3). The threshold was exceeded 41 times, concentrations of 300µg/m3 or more were measured three times in 2011, in Bulgaria, Italy and Spain.”

For once, the UK was not among countries failing to meet pollution targets, and was one of the few across Europe that did not experience sufficiently high levels of ground-level ozone exceeding legislative demands.

The EEA report says: “No exceedances were reported from the Baltic states, Iceland, Ireland, Scandinavia (excluding Denmark) and the United Kingdom in summer 2011. Only northern Italy and several more isolated locations reported a substantial number of exceedences.”

The number of exceedences of EU ground-level ozone concentration standards for protecting human health remain at a “serious” level, the EEA says. According to its report: “In the summer of 2011, the threshold of 120ug/m3 maximum daily eight-hour mean was exceeded on more than 25 days in a significant part of Europe.”

This is the threshold that will be used to assess whether countries meet the target value (TV) for protecting human health as set by Directive 2008/50/EC, the Directive on Ambient Air Quality and Cleaner Air for Europe (more commonly known as the Air Quality Directive).

The report says: “Exceedances of the target value threshold occurred in 17 EU Member States (Austria, Belgium, Bulgaria, Cyprus, the Czech Republic, France, Germany, Greece, Hungary, Italy, Luxembourg, Malta, Poland, Portugal, Slovakia, Slovenia and Spain) and in five other countries (Croatia, Liechtenstein, the former Yugoslav Republic of Macedonia, Serbia and Switzerland). As in previous years, the most widespread concentrations occurred in the Mediterranean area.”

In fact, the importance of weather conditions in ground-level ozone formation means the southerly parts of Europe face the greatest challenges in meeting ozone targets. The report says: “The highest number of exceedances occurs frequently in the Mediterranean region, the lowest in northern Europe. The number of occurrences in southern Europe was lower between 1999 and 2002 than in the extreme summer of 2003, which saw a very large number of occurrences. This was also the case in more northern parts of Europe.

“Ozone levels decreased in 2004 and 2005 to previous levels, but in 2006 there was a further increase, not only in southern Europe. According to several indicators, ozone levels during the summers of 2007, 2008 and 2009 rank among the lowest in the past decade.

“After a slight increase in the number of exceedances in 2010, there was a decline in 2011. In the summer of 2011, the occurrence of the exceedances of the information threshold and the LTO for the protection of human health was the lowest since comprehensive Europe-wide data reporting commenced in 1997. This was principally due to unusually low temperatures and increased rainfall during the summer months.”

Meeting the TV — which came into force as of 2010 — will likely remain a “significant challenge for many EU Member States in view of the maintained high surface ozone levels in Europe in the last four years”, the EEA says.

However, there was some positive news. According to the EEA: “Europe-wide, summer 2011 was characterised by short regional ozone episodes of only two to three days followed by days with no or only a few exceedences — there were no widespread multi-day episodes.

Human activities also have a part to play. The EEA says: “In Europe, ozone concentrations in a particular country are also influenced by emissions in other Northern Hemisphere countries and by sectors such as international shipping and aviation. Thus, ozone pollution is not only a local air quality issue but also a hemispheric and global problem.”

Unfortunately, efforts to reduce emissions of ozone precursors have not resulted in significant impacts on ground-level ozone formation. The EEA report says: “Independent of the episodic nature of ozone pollution and the strong influence of meteorological conditions, emissions of ozone precursor gases are sustaining a baseline number of exceedances of the information and alert thresholds, the TV threshold and the LTO.

“Decreased anthropogenic emissions of some ozone precursors (NOx, carbon monoxide (CO) and some VOCs) in the past two decades did not manifest in significant reductions in the number of such exceedances. The ozone pollution problem therefore requires further mitigation efforts.”

Reducing the levels

Quite what actions should be taken, however, are far from clear. A number of regulations are already in place that impact on the precursors that lead to ozone formation. Ozone pollution as a global or hemispheric problem is addressed by the Task Force on Hemispheric Transport of Air Pollution (TF HTAP) under the United Nations Economic Commission for Europe (UNECE) Convention on Long-range Transboundary Air Pollution (LRTAP) (UNECE, 2010).

The Gothenburg Protocol to the LRTAP Convention contains emission ceilings for NOx, NMVOCs, sulphur oxides (SOx) and ammonia (NH3), which parties to the protocol were to meet by 2010.

In addition to ceilings for individual countries, the Protocol also specifies ceilings for the EU, itself a party. Member States report data on emissions of air pollutants annually to the European Commission (with copies to EEA) under the Directive on National Emission Ceilings for Certain Atmospheric Pollutants (the NEC Directive).

The NEC Directive contains national emission ceilings that, for EU Member States, are either equal to or slightly more ambitious than those in the Gothenburg Protocol, and the ceiling applies to the EU-15 grouping of Member States that constituted the European Community at the time the Gothenburg Protocol was agreed.

The EEA says: “Emissions of three air pollutants primarily responsible for the formation of harmful ground-level ozone in the atmosphere fell significantly in the period from 1990 to 2009: carbon monoxide (62% reduction), NMVOCs (55% reduction) and NOx (44% reduction).

“Emission reductions have been achieved from the road transport sector for all three pollutants, primarily through legislative measures requiring abatement of vehicle tailpipe emissions. On the other hand, for another ozone precursor, CH4, concentrations increased continuously during the 20th century, before growth slowed after 1990 and eventually stabilised between 1999 and 2007.

“Since 2007, however, measurements suggest concentrations have started to rise again. CH4 is a well-mixed pollutant globally, carried across long distances. Isolated local and regional abatement of emissions may therefore have limited impact on local ozone concentrations. Formation of tropospheric ozone from increased concentrations of CH4 may contribute to the sustained ozone levels in Europe. “European countries have significantly reduced anthropogenic emissions of ozone precursor gases since 1990, albeit briefly. In general, however, ambient air measurements in urban and rural areas of Europe do not show any downward trends in ground-level ozone.”

According to the EEA, there are a number of factors that may be responsible for these discrepancies, including: inefficient reductions of the ozone precursor emissions (for instance, under certain conditions, reductions of NOx would lead to increased ozone concentrations); increasing intercontinental transport of ozone and its precursors in the northern hemisphere; climate change/variability; and tropospheric ozone from increased concentrations of CH4.

The report says: “Other factors counteracting the possible positive effects of European measures to reduce ozone precursor emissions from anthropogenic sources are biogenic NMVOC emissions and fire plumes from forest and other biomass fires.”

However, the Agency cannot suggest a magic bullet to solve the ozone problem. In fact, despite the failure of Europe-wide efforts to address the issue, the best the report can come up with is: “There is a need for more research to ascertain and quantify these possible contributions. At the time of writing this report, this quantification is lacking.”

For the foreseeable future, therefore, ground-level ozone, with its impacts on human health and damage to agriculture, as well as other environmental impacts, is here to stay.