TABLE 9-1 Physical and Chemical Data on Ozone


Triatomic oxygen

CAS registry number


Molecular formula


Molecular weight


Boiling point


Melting point


Flash point


Explosive limits


Specific gravity

2.144 g/L at 0°C

Vapor pressure



49 mL/100 mL water at 0°C; soluble in alkaline solvents and oils

Conversion factors

1 ppm = 1.96 mg/m3; 1 mg/m3 = 0.51 ppm

Abbreviations: NA, not available or not applicable.

Sources: Solubility data from HSDB 2005; all other data from Budavari et al. 1989.

and proteins (Wojtowicz 1996). It is used to treat drinking water, industrial process streams, and municipal wastewater effluents and to treat water in cooling towers, swimming pools, and spas. It is also used for pulp delignification and bleaching and in the production of specialty organic chemicals and intermediates.

Ozone occurs naturally in the stratosphere at concentrations of 1-10 ppm and shields Earth from biologically damaging ultraviolet (UV) radiation (Wojtowicz 1996). In the stratosphere, short-wave UV radiation directly splits molecular oxygen (O2) into atomic oxygen (O·) that rapidly combines with O2 to form ozone. In the troposphere, “ground-level” ozone is generated predominantly by a series of complex reactions involving nitrogen oxides, oxygen, and sunlight. Nitrogen dioxide (NO2) absorbs longer-wavelength UV radiation, and this results in the generation of O· and nitric oxide (NO). O· then combines with O2 to form ground-level ozone. NO2 is regenerated by the reaction of NO with the newly formed ozone. In the absence of volatile organic compounds (VOCs), that reaction would approach a steady state with no buildup of ozone. However, atmospheric VOCs react with O· to produce oxidized compounds and free radicals that react with NO to form more NO2. Consequently, the NO scavenging of ozone is upset, and this results in increased ozone concentrations.

In urban areas—such as Los Angeles, California—with high motor-vehicle traffic that emits large amounts of VOC-containing exhaust and with intense midday sunlight, complex atmospheric reactions are common place and result in what is termed photochemical smog. Ozone, the principal oxidant pollutant in photochemical smog, is considered both an environmental and a public-

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