Indoors, using photocopiers and laser printers produces relatively large amounts of tropospheric ozone, hence the need to properly ventilate rooms where these machines are kept.

Outdoors, however, ozone is not a pollutant caused directly by human activity. It is thus a "secondary" pollutant which forms under quite specific conditions, based on certain pollutants known as "ozone precursors".

These ozone precursors are:
·        nitrogen oxides (NOx), more specifically nitrogen dioxide (NO2) and nitrogen monoxide (NO);
·        volatile organic compounds (VOCs).

Nitrogen oxides and volatile organic compounds are a consequence of road traffic (which alone is responsible for almost half of NOx emissions), as well as industrial furnaces, electricity power stations, use and manufacture of products containing solvents (paints, inks, varnishes, glues, cleaning products, solvents, etc.), industrial processes, petroleum product refining and distribution and, to a minor extent, the residential sector (via combustion in central heating systems).





The various components

The reactions that form and destroy tropospheric ozone are quite complex. What are the main ones?

Volatile organic compounds are a big family of chemical compounds (VOCs). They each contain at least one carbon and one hydrogen atom, whilst having a vapour tension or a boiling point that allows them to be present in the atmosphere at ambient temperatures in the form of a gas. Carbon dioxide (CO2) and carbon monoxide do not therefore fit this definition.

Some VOCs present in the atmosphere, such as terpenes and pinenes, come from natural sources (vegetation) that are much more significant for VOC generation than human activity. These substances contribute to the formation of a residual concentration of tropospheric ozone.

Besides the presence of precursor pollutants, UV rays and the sun's heat are essential for the reactions that form ozone.

Ozone concentration in the air varies during the day: it is minimal in the morning and gradually increases, peaking between mid-afternoon and early evening, after which it gradually decreases again during the night.

In fact the pollutants that "form" ozone during the day are also those that destroy it at night.

Nitrogen monoxide (NO) emitted in large quantities by motor vehicles is a major contributor to all this. This is the main reason why, during peak ozone episodes, we find that ozone concentrations are often higher in the country than in the city. Because there is less road traffic in rural areas, the air is less "polluted" with nitrogen monoxide...., so it cannot help to destroy the ozone that has formed.

How can precursor gas emissions be reduced?

Reducing precursor gas emissions is not easy: the complex chemistry of ozone and the atmosphere is such that (for several years) initial measures to reduce nitrogen oxide emissions are likely to increase ozone concentrations. Reducing VOCs, on the other hand, brings about a reduction in ozone concentrations.

Moreover in many cases, and given current concentrations of precursor pollutants in the atmosphere, adoption of short-term measures when there is an ozone peak has little effect on ozone concentrations. At best, these concentrations are reduced by a few percent, or they might even increase.

The only solution is a long-term, Europe-wide policy aimed at sharply reducing precursor pollutants, accompanied by appropriate short-term measures during ozone peaks. See International and European actions (HTML) and What does Belgium do? (HTML)