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Photochemical air pollution in the Netherlands: sources and policy

Photochemical air pollution (smog) is formed as a result of the effect of the sun and consists of, among other things, ozone (O3) and particles. Too much ozone at ground level harms public health and nature. The policy for preventing summer smog targets both air quality and emissions.

How is ozone formed?


Ozone is naturally present in the air. Ozone is also formed at ground level as a result of the influence of sunlight on various substances:
  • volatile organic compounds (VOCs);
  • carbon monoxide (CO);
  • methane (CH4).
In addition to these substances, oxides of nitrogen (NOx) are needed as catalysts for ozone formation. At very low concentrations of oxides of nitrogen, ozone is not formed and it may even be broken down. However, at ground level in most of continental Europe, there are enough oxides of nitrogen for ozone to be formed.
Currently, ozone-forming substances enter the atmosphere mainly as a result of human activities. In populated areas, the emission of volatile organic compounds and oxides of nitrogen by, for example, traffic, industry and consumers results in extra ozone formation. Natural processes are also involved, such as the emission of ozone-forming substances by coniferous trees.

What is summer smog?


Summer smog is formed by chemical reactions in the air that are influenced by sunshine and measured on the basis of the concentrations of ozone (O3) in the air. The chemical reactions form, among other things, ozone. In conditions that favour the formation of ozone - lots of sunlight, high temperatures and little wind - the ozone concentration can easily rise above the European standard of 120 µg/m3.

Where is summer smog most common in the Netherlands?


In the Netherlands, the highest annual averages for ozone levels are found in a band from the south-west to the south-east of the country. The highest concentrations are mainly found there because of the proximity of densely-populated and highly-industrialised areas, such as the Randstad urban agglomeration, and of the relatively high levels of ozone and ozone-forming substances that are already formed outside the country in Germany (the Ruhr area, for example) and Belgium. The transboundary dimension of the problems results from the long atmospheric life of ozone and the ozone-forming substances. This can be as long as a few days, even extending to more than a week.

Impact of summer smog


At the ground level, ozone is a polluting substance with harmful effects on people, ecosystems and materials. During a summer smog period, the inhalation of air containing ozone can result in the following effects on health:
  • transitory respiratory symptoms such as: dry throat, chest pain, coughing, tightness of the chest and pain when breathing deeply. Headaches, general malaise, nausea and dizziness are also possible.
  • temporary impairment of lung function. This can be accompanied by damage to lung tissue and, as a result, impaired performance. There can also be a reduction in the speed with which our bodies eliminate particles, bacteria and viruses from the lungs.
The effects of a single period of summer smog are transitory. Repeated exposure to high concentrations of ozone can however result in the permanent impairment of lung function. The effect of ozone on health can vary considerably from person to person.

Ozone peaks on the decrease, background level higher


Although the ozone peaks appear to be on the decrease throughout Europe, the trend for ozone concentrations that are relevant for the directive (EU, 2002) went down only slightly, if at all, in the period 1996-2000 (EEA, 2003). In the Netherlands also, peak concentrations of ozone at ground level have declined since 1992. There are indications that the fall is the result of the European emissions policy for ozone-forming substances, volatile organic compounds and oxides of nitrogen (Roemer 2001, De Leeuw 2000).
On the other hand, the background concentrations of ozone throughout Europe have very probably increased. Current information indicates that the typical annual average ozone concentration in the Northern Hemisphere is 70 µg/m³, approximately twice as high as the natural background level. The increase in the concentrations of methane (CH4) and carbon monoxide (CO) in the Northern Hemisphere may possibly (=33-66%; probably=66-90%; very probably: 90-99%) contribute to the increase in the background level for ozone.

The dual role of ozone


Ozone plays a dual role in the environment. In the higher levels of the atmosphere, the stratosphere, it makes up the ozone layer that provides protection against harmful UV radiation. Lower down, at ground level, ozone is a polluting substance with harmful effects on people, ecosystems and materials.

Policy


The policy is intended to prevent, alleviate or reduce the harmful effects of air pollution on the health of people and the environment. It therefore targets both air quality and emissions.
  • Air quality
    Given the harmful impact of photochemical air pollution at ground level, the European Commission adopted the ozone directive 92/72/EEC as early as 1992. This directive has now been amended and tightened up, resulting in the third European daughter directive for ozone, which was adopted with effect from 9 March 2002 (EU, 2002). This directive has been in force in the Netherlands since 9 September 2003. The European and national (VROM, 2001) ozone standards for public health and vegetation correspond (National air quality: overview of standards).
  • Emissions
    For the ozone-forming substances oxides of nitrogen (NOx) and volatile organic compounds (VOC), emission objectives have formulated in the context of the European directive for national emission ceilings (NEC directive; EU, 2001) and the Gothenburg protocol (UNECE, 1999). In the context of the National Environmental Policy Plan 4 (NMP4, VROM 2001), the commitments formulated for emissions of the same substances are stricter than the NEC directive. The emissions policy is established using a variety of instruments and technical measures.

References


Relevant information outside of the Environmental Data Compendium
This page was last changed on November 18, 2005  (version 01).