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Environmental fate & pathways

Phototransformation in air

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Description of key information

The kinetic rate constant for the reaction of members of the category with OH radicals is measured to be 2.632 E-11 to 66.6 E-12 cm3/mol/s, indicating that the atmospheric half-life in air (with OH radicals) ranges from 0.161 days (1,3-butadiene) to 4.064 days (butane). The kinetic rate constant for the reaction of members of the category with ozone is measured to be 0.81 E-17 to 20 E-17 cm3/mol/s, indicating that the atmospheric half-life in air (with ozone) ranges from 1.375 days (cis 2-butene) to 2.116 days (trans 2-butene).

Key value for chemical safety assessment

Half-life in air:
2.12 d

Additional information

As the members of this category are gases, the most dominant environmental fate process will be photooxidation. Representative substances in this category have been shown to react rapidly with hydroxyl radicals and ozone. The reaction with ozone is less rapid, and therefore less important than the reaction with hydroxyl radicals. However, at night, when the concentration of hydroxyl radicals falls to negligible levels, this removal mechanism will become more important. It is expected that the members of this category will be rapidly removed from the atmosphere by photooxidation.

Atkinson (1989) wrote a review article, with no data reported on the test methods or conditions of the original data source. Never-the-less, the data are taken from a published, peer-reviewed journal article and are considered to be suitable for this endpoint. The measured results provide a rate constant of the reaction of OH radicals with three members of category (1,3-butadiene: 66.6 E-12 cm3/mol/s, butane: 2.632 E-12 cm3/mol/s, 2-butene: 56.67 (cis), 64.27 (trans) E-12 cm3/mol/s). The results indicate that the category would have an atmospheric half-life of approximately 0.161 days (1,3-butadiene), 4.064 days (butane) and 2.265 days (cis), 1.997 days (trans) (2-butene) and therefore would not be persistent in the atmospheric compartment.

For the reaction with ozone in air the measured results provide a rate constant of the reaction of ozone with two members of the category (1,3-butadiene: 0.81 E-17 cm3/mol/s, 2-butene: 20.0 (cis), 13.0 (trans) E-17 cm3/mol/s). The results indicate that the category would have an atmospheric half-life of approximately 1.415 days (1,3-butadiene) and 1.375 days (cis), 2.116 days (trans) (2-butene) and therefore would not be persistent in the atmospheric compartment.

The mean of the atmospheric half-life values for the category in is 2.12 days with OH radicals and this has been used to complete the 'key value for chemical safety assessment'.