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Why are ozone concentrations higher in rural areas than in cities?

Ozone is a secondary pollutant, which means it is not directly emitted by traffic, industry,.. but it is formed on warm summer days by the influence of solar radiation on a cocktail of airborne pollutants. These ozone precursors are nitrogen oxides (NOx) and volatile organic compounds (VOC). Traffic is the main source (>50%) of ozone precursors.

It may sounds curious, but it is a fact that most of the times ozone levels are higher in rural areas than in cities. This is sometimes called the 'ozone paradox'. Ozone can be degraded by the compounds (NOx) by which it is also formed. This degradation occurs more often in cities than in rural areas, because there is more NO in cities (see further).

This does not mean there is less air pollution in cities (other pollutants are present, with higher concentrations than at the countryside), yet there is less ozone. As such, there is no linear connection between the amount of ozone precursors and the amount of ozone itself. A diminution of ozon precursors can even lead to an increase of ozone concentrations. This is what is observed during weekends: although less traffic is present, more ozone is observed (the formed ozone gets degraded less, because there is less pollution). In the scientific world, this is called the 'weekend effect'. The whole process of ozone formation clearly is a complicated story.

To explain why most of the time there is more ozone at the countryside than in cities, some explanation regarding the chemical base reaction concerning formation of ozone is needed. This reaction can be presented as follows (in a simplified manner):

NO2 + O2 (+ solar UV-light, + heat) --> NO + O3

In human language: nitrogen dioxide and oxygen react, which results in nitrogen monoxide and ozone. This reaction is speeded up in warmer conditions and with more UV-light.

This is an equilibrium reaction, which means the reaction also works in the other direction (whereby ozone gets degraded again): NO + O3 --> NO2 + O2.

In chemistry there is a law that describes an equilibrium will try to re-establish itself after a disruption. This is where VOC enter the scene: they react with NO, with formation of NO2 as a result. The NO concentration decreases (because it reacts with VOC). The equilibrium will try to repair this, thus the reaction will form more NO, according to the law mentioned above: the equilibrium is shifted to the side of NO. Trying to form more NO however also means an increase of ozone. In addition, more NO2 is formed, which can form even more ozone, leading to even higher ozone concentrations.

If you still remember that a lot of NO can be found in the exhaust gases from cars and NO has a very short lifespan (a couple of minutes) and is immediately converted (oxidised) into NO2, which has a longer lifespan of hours to even days, which in turn allows it to be transported over larger distances by wind, from the city to more rural areas, you will understand that there is less ozone in cities than at the countryside.

So summarizing:

- in cities, high levels of NO due to traffic reacting with ozone, forming NO2

- NO2-based ozone formation at the countryside (in warm and sunny conditions), but much less NO (because of less traffic) and thus less ozone degradation.

The above also explains why short-term measures (such as stopping traffic) can have an adverse effect on ozone pollution. Less traffic means less NO and thus less ozone degradation. Taking traffic measures at the moment of ozone peaks is rather useless. Such measures would have to be taken for several days (4-5 days) before the effect would be noticeable. Predicting ozone episodes in advance is however not easy. Making a good ozone forecast depends on correct weather predictions, which aren't always as accurate. Severely limiting traffic, with a chance of the weather not being warm and sunny 5 days later (and thus chance of ozone pollution), is far from evident.

Even then the chance of success is not guaranteed. A large part of the emissions responsible for ozone formation in Belgium is transported from abroad.According to a study from VITO, temporal measures during 3 summer months, with the aim to reduce the number of exceedances of the hourly ozone concentration above 240 µg/m³, will only decrease the ozone peak concentrations with only 5% at maximum.

Short term measures however have a sensitizing character (they make a car driver feel responsible for ozone pollution) and will improve the general air quality, but do not have a direct effect on ozone concentration.

Ozone forecast maps for today, tomorrow and the day after tomorrow can be found at our website under ozone forecasts.

The forecast for today has a chance of approximately 75% of being correct, those for tomorrow and the day after have a lower chance.

Complex computer models teach us ozone pollution can only decrease if:

1. emissions of ozone precursors are drastically reduced (by 60-70%)!
2. this happens not only in Belgium but the whole of Europe!
3. measures are not only taken during ozone smog episodes, but throughout the whole year!

A whole series of measures have been taken at the European level and a lot more are in the pipelines, yet it will take a while (several years) before these measures will result in a decreased ozone pollution. An important measure in relation with this is thee European NEC directive from 2001, which dictates every EU country how many kilotons of NOx and VOC they are allowed to emit from 2010 onwards. This directive has already led to a sensible amelioration of the ozone problem. The number of ozone peak and the severity and duration thereof, is lower now in comparison with the 1990's, under similar meteorological conditions. At the moment, negotiations are taking place at the EU level, which will result in a new NEC directive with sharper emission limits by 2020, 2025 and 2030.