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Environmental fate & pathways

Phototransformation in air

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Reference
Endpoint:
phototransformation in air
Type of information:
experimental study
Adequacy of study:
key study
Study period:
Not specified
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Taken from publically available database, and is considered accurate based on the registrants experience of the substance.
Qualifier:
no guideline followed
Principles of method if other than guideline:
The study created a database of all available rate constants for photochemical degradation with the reactive molecules OH, O3 and NO3. In addition, quantum efficiences for gas phase reactions are reported for some substances.
GLP compliance:
not specified
Estimation method (if used):
Not specified.
Light source:
not specified
Details on light source:
Not specified.
Details on test conditions:
The direct photochemical transformation was quantified by the quantum efficiency and the UV/Vis absorption spectrum.
Reference substance:
not specified
Preliminary study:
Not specified.
Test performance:
Not specified.
Reaction with:
other: OH, O3, NO3
Rate constant:
< 0 cm³ molecule-1 s-1
Transformation products:
not measured
Results with reference substance:
Not specified.

The substance is reported as having a rate constant of <7 E-18 when reacting with molecules of OH, O3 and NO3 under atmospheric conditions.

Validity criteria fulfilled:
yes
Conclusions:
The substance is reported as having a rate constant of <7 E-18 when reacting with molecules of OH, O3 and NO3 under atmospheric conditions.
Executive summary:

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.

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.

Information from the WMO report (2014)

Total atmospheric lifetime reported : 102 years

Stratospheric partial lifetime: 95.5 years

Tropospheric UV photolysis partial lifetime: 11600 years

[https://csl.noaa.gov/assessments/ozone/2014/report/chapter1_2014OzoneAssessment.pdf]