Registration Dossier

Environmental fate & pathways

Phototransformation in air

Currently viewing:

Administrative data

Link to relevant study record(s)

Description of key information

Phototransformation in air.

Key value for chemical safety assessment

Additional information

The substance is reported as having a rate constant of <7 E-18 when reacting with molecules of OH, O3 and NO3. However, it is understood that under the protective ozone shield, CFCs are stable chemicals that are capable of acting as affective absorbers of infrared radiation, which makes them important greenhouse gases. The half-life of CFCs in the troposphere is reported within the public domain as 10-50 years. However, in the stratosphere, CFCs found above the protective ozone shield, undergo photodegradation from exposure to UVR. Thus, the absence of short wavelength UVR prevents the photodegradation of CFCs, allowing them to remain longer in the troposphere and diffuse to the stratosphere. During photodegradation, chlorine atoms are released and react with stratospheric ozone, forming chlorine monoxide. This monoxide does not absorb short wavelength UVR. Thus, chlorine monoxide does not compensate for the loss in UVR's absorbing capability of ozone. This depletion in stratospheric ozone concentrations results in more short wavelength UVR (UV-B,280-320 nm and UV-C,<280 nm) penetrating the protective ozone shield, striking the earth's surface and increasing human exposure. Prolonged human exposure to short wavelength UVR has been associated with increased incidences of non-melanoma skin cancers, cataracts and possible adverse effects on immune function.

Due to the high vapor pressure of dichlorodifluoromethane, volatilization to the atmosphere is quite rapid. The compound is tropospheri­cally stable; it does not react readily with hydroxyl radicals, nor does it photodissociate in the troposphere since it exhibits no absorption of light greater than 200 nm. Public domain information suggests a tropospheric residence time of 30 years for dichlorodifluoromethane before diffusion to the stratosphere.

In the stratosphere, dichlorodifluoromethane is broken down by the absorption of higher energy, shorter wavelength ultraviolet light. Thus the photodissociation of dichlorodifluoromethane results in the release of two chlorine atoms since less energy is required for the cleavage of the C-Cl bond than for the cleavage of the C-F bond. According to information available in the public domain, the photolysis of dichlorodifluoromethane in the presence of O2 at 213.9 nm and 25°C leads to the production of CF2O and Cl2 and, potentially, chlorine atoms. Chlorine atoms, released by reactions such as these, are theorized to be catalysts in the destruction of the stratospheric ozone layer.