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Environmental fate & pathways

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Ozone is emitted to the outside atmosphere locally in only very small quantities as detailed in the environmental exposure assessment. This locally emitted ozone will, after dispersion, not influence the level of ozone already (naturally) present at ground-level, and will be subject to the same removal processes as the ‘natural’ ozone. Annual average background ozone levels (natural ozone) in the northern hemisphere range between approximately 40–90 µg/m³ (20–45 ppb), depending on location, elevation and level of anthropogenic influence (Vingarzan, 2004). In some areas and at some points in time the concentration of ground level ozone can be considerably higher.

Since emission of industrially produced ozone constitutes only a negligible contribution to the overall level of atmospheric ozone, and since the latter has been well studied, most data available refer to knowledge and studies in the context of atmospheric ozone that is naturally present. One exception is the chamber study by McClurkin et al. (2013) which tested the stability of ozone in air within the context of disinfection of storage containers. The half-life of ozone as a function of air movement, temperature and humidity was determined. Half-life in still air at 24 °C and zero humidity was as high as 1524 min (25.4 h). As airflow, temperature and humidity increased, half-life decreased to 39 min. The self-decomposition of ozone in indoor air (in the absence of pollutants and light) is therefore strongly influenced by relative humidity, temperature and air flow.

Ozone in air can be decomposed via complex radical chain reactions in the presence of sunlight (phototransformation). The phototransformation of ozone in ambient air is well studied, and humidity was found to play an important role (also see above). Ozone phototransformation is mainly accomplished by radical-type chain reactions. Direct photolysis of ozone is much less important (Jans and Hoigne, 2000).

Especially under dry conditions, ozone is much more stable in air than in water. The half-life of ozone in ambient air as measured by the US EPA is in the order of 12 hours (Rice and Browning, 1980).

The half-life of ozone in water is much shorter compared to half-life values in air. Overall half-lives of ozone in water were reported to range between 8 and 80 minutes (see IUCLID section 5.6 for details).

The half-life of ozone decomposition in sandy soil was approximately 40 minutes (Choi et al., 2002).

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