Annex II. Phenomenology of ozone concentrations

Annex II

Phenomenology of ozone concentrations

For a better understanding of the report, some of the main characteristics of ambient ozone are summarized here. For more advanced information on ozone and its photochemical formation the reader is referred to information documents provided by DGXI (Peter Borrell and Dick van den Hout, "Tropospheric ozone: a review of current understanding", Doc.Ref XI/363/95; Dick Derwent and Dick van den Hout, "Computer modelling of ozone formation in Europe", Doc.Ref. XI/364/95) and to reports prepared in the framework of UN-ECE Convention on long-range transport of air pollution and EUROTRAC.

Ozone is a secondary air pollutant formed in the atmosphere under the influence of sunlight. Ozone formation occurs at all levels in the atmosphere, from ground level up to the stratosphere. Here the discussion is limited to ozone at ground level. It has been shown that under the present conditions in Europe the ozone exposure of humans, vegetation and materials leads to adverse effects.

The precursors of the ozone formation are Volatile Organic Compounds (VOC), carbon monoxide (CO) and nitrogen oxides (NOx). VOC and CO act as "fuels" as they are oxidized in the process; the nitrogen oxides play an important role as "catalysts": they are not 'consumed' in the formation process but are essential for the continuation of the process. However, nitrogen oxides are consumed in side reactions by which they are further oxidized to nitric acid or nitrates. For the continuation of the photochemical oxidation process a continuous injection of nitrogen oxides is therefore necessary.

The ozone formation takes place on various time and spatial scales: on the local scale as in urban areas as Athens or Milan, on the regional scale as is demonstrated by the photochemical episodes in Central and Northwest Europe and on the hemispheric/global scale. Highly reactive VOCs are important precursors on the local and regional scale whereas the less reactive, relatively long-lived VOCs such as methane contribute to ozone formation on the global/hemispheric scale.

The role of nitrogen oxides is complex and may be different at various distances from the source. In heavily populated areas the ozone concentrations may be lower than the regional concentrations due to chemical scavenging by local nitrogen oxide emissions. This scavenging is presented by the chemical reaction:

O₃ + NO NO₂ + O₂ (R1)

Nitrogen dioxide (NO₂) formed in this reaction can be seen as 'potential ozone' as in the photolysis of NO₂, nitrogen monoxide, NO, and ozone are produced:

NO₂ (+ O₂) O₃ + NO (R2)

As both the time scales of the NO-scavenging reaction (R1) and of the NO₂ photolysis are relatively short, an equilibrium between the three components will be established:

O₃ + NO NO₂ + O₂ (E1)

Note that the sum of O₃ and NO₂ (frequently indicated as oxidant or Ox) is constant. Knowledge on simultaneously measured NOx concentrations in general and NO₂ concentration in particular is therefore important for interpretation of ozone levels. The oxidant levels are spatially less variable than the ozone levels. A mapping procedure based on oxidant levels is therefore preferred.

The time scales of photochemical ozone formation are generally longer than the time scale associated with the above reaction R1 and R2 and close to NOx sources a decrease in ozone concentrations may be observed. In Figure II.1 the ozone concentration in an air parcel passing a NOx source is schematically presented.

One of the consequences of this interaction with NOx is that the representativeness of ozone monitoring stations depends strongly to what extent the station is influenced by local NOx sources: concentrations measured at a station in a traffic situation will be representative for its direct surroundings only (e.g. less than a few 100 m) whereas a background station may measure concentrations representative for an area of several tens of kilometers. The information requested for in Article 4 of the Directive should form the basis on which a representative area for each of the monitoring stations could be defined.

At ground level ozone concentrations generally show a strong diurnal variation. At night concentrations are low, both caused by removal by dry deposition and by titration by NO-emissions. In the morning concentrations increase, caused not only by the sun-light induced photochemical formation but also by the downward mixing of higher, ozone-rich layers. In the afternoon both processes will become less important and concentrations will decrease again when the loss processes dominate. The maximum concentration is frequently found in the late afternoon, around 16.00. For the eight hour periods reported in the framework of the Ozone Directive, the highest value will generally be observed between 12.00 and 20.00.

Figure III.1 Schematic presentation of ozone concentrations downwind of a large NOx source where ozone scavenging will take place.