Photochemical Box Model: ozone


Chemical formula: O3
Physical state: colorless gas
Molecular weight: 48
Melting point: -192.7 C
Boiling point: -111.9 C
Gas density (0 C, 1 bar): 4.14 g/l
Solubility in water (0 C, 1 bar): 49 ml/100ml water
1 ppm = 2000 mcg/m3  





EU Directive on Air pollution by ozone: 92/72/EEC

Values are expressing in mg 03/m3.
The volume must be standardized at 293 degree Kelvin and 101,3 kPa.

  • Health protection threshold:
    0.110 mg/m3 for the mean value over eight hours
  • Vegetation protection thresholds:
    0.200 mg/m3 for the mean value over one hour
    0.065 mg/m3 for the mean value over 24 hours
  • Population information threshold:
    0.180 mg/m3 for the mean value over one hour
  • Population warning threshold
    0.360 mg/m3 for the mean value over one hour



Ozone, a natural constituent of the stratosphere formed by the photolysis of molecular oxygen, can be transported by atmospheric circulation into the lower atmosphere. Natural hydrocarbons including terpenes from trees and vegetation are also subject to photochemical reactions producing oxidants. These two processes are the natural source of background ozone concentrations. Ozone is one of the strongest oxidizing agents. Although it is present in only small concentrations (a few parts per million), ozone is responsible for shielding the earth from ultraviolet radiation (UV-B) that is biologically harmful.

Formation of ozone occurs predominantly at altitudes above 30 km where solar UV radiation with wavelengths of less that 242 nm slowly dissociates molecular oxygen (O2) into oxygen atoms (O). These oxygen atoms rapidly combine with molecular oxygen to form ozone. Ozone strongly absorbs solar radiation in the wavelength region of 240-320 nm.

Although it is known that ozone is only one of the number of photochemical oxidants and that there are many other components of photochemical air pollution, it is the only substance for which a health protection guideline can be given, based on existing exposure effect data.

From controlled human and community studies, ozone concentrations at which the first adverse effects in man appear have been reported to be 0.200-0.500mg/m3 (0.1-0.25 ppm). It is agreed that a 1-h exposure to ozone of 0.100-0.200 mg/m3 (0.05-0.10 ppm) should be used as a guideline for the protection of public health.


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