Ozone concentrations in the Netherlands tested against the standard for vegetation, 1992-2002
The fall in the ozone concentrations since 1992 has kept the Netherlands well within the ozone standard (AOT40) for vegetation. The European emissions policy would appear to be bearing fruit.
Ozone concentrations continue to fall
The ozone concentration (expressed as AOT40) fell in 2002 to an average of 4700 (µg/m3.hour) in the Netherlands as a whole. The highest concentrations were found in Limburg and the east of the Netherlands. The maximum measured AOT40 value in 2002 was 7200 (µg/m3.hour). The standard of 18,000 (µg/m3.hour) for 2010 has not been exceeded for a long time now.
The fall in the AOT40 value confirms the indications that high ozone concentrations here and elsewhere in Europe are on the decrease. The most probable cause is the reduction in the emissions in Europe of substances from which ozone is formed (see text box, ozone formation).
The effect of the weather on the ozone concentration
The fluctuation in ozone concentrations is primarily caused by variations in the weather from year to year. More exceedances are measured during years with several fine, sunny days (such as 1994 and 1995) than in years with less sunny weather (such as 1998, which was a relatively wet year). Taking an average for five years reduces the effect of the weather on the ozone concentrations. This makes the effect of the European emissions reduction policy clearer.
What is the AOT40?
The AOT40 (Accumulated Ozone exposure over a Threshold of 40 ppb) is a unit for expressing ozone concentrations that is relevant for nature. The AOT40 takes into account both the level of exceedance of the threshold value of 80 µg/m3 (= 40 ppb) and the duration of the exceedance. The AOT 40 is calculated only during the three summer months May - July, from 08.00 to 20.00 (Central European Time).
Standard for vegetation
The EU (2002) has set a target value for the protection of nature, expressed as an AOT40 of 18,000 (µg/m3·hour), averaged out over 5 years. The calculation of the average is based on the year in question and the four previous years. The target value must be met by 2010. An objective has also been adopted for the long term of 6,000 (µg/m3·hour), averaged out over one year.
Link between ozone standard and emissions policy
The EU has linked interim standards for 2010 for ozone concentrations in air to a maximum permitted emission per EU country of the 'ozone-forming' substances VOC and NOx (see text box). If it emerges that the target values and the objectives for the long term are not being achieved, the EU may reduce the permitted maximum for the emissions of VOC and NOx.
| Ozone formation | |
| Ozone is a natural component of air. It is formed from volatile organic compounds (VOC), carbon monoxide (CO), methane (CH4) and nitrogen oxides (NO and NO2, collectively referred to as NOx) under the catalytic action of sunlight. | |
| Ozone is formed at different levels in space and time. Far from densely-populated areas, ozone is mainly formed by the reaction of CH4 and CO. The increase in the amounts of these substances in our atmosphere in recent years may be contributing to the rise in the background level of ozone in the Northern Hemisphere. | |
| In densely-populated areas, excessive emissions of NOx and VOC are the main contributors to ozone formation. Under certain atmospheric conditions (sunlight, high temperature, little wind) this process is responsible for photochemical smog (summer smog). In these circumstances, average hourly ozone concentrations may rise to over 200 µg/m3. | |
| On average, the highest concentrations of ozone in the Netherlands are found in the south and east. The highest concentrations are found there because of the proximity of densely-populated and highly-industrialised areas with high emissions of VOC and NOx. These are the Randstad urban agglomeration and areas in Germany (such as the Ruhr) and Belgium. Ozone from other countries also makes a major contribution to ozone concentrations in the Netherlands. |
