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Environmental fate & pathways

Phototransformation in air

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Description of key information

Phototransformation in air:

Rate constant for reaction with OH radicals:

Cyrene™ (submission substance): 2.5E-11 cm3/molecule.sec (half-life 0.6 days)

Gem Diol: 3.9E-11 cm3/molecule.sec (half-life 0.4 days)

Key value for chemical safety assessment

Half-life in air:
0.6 d
Degradation rate constant with OH radicals:
0 cm³ molecule-1 s-1

Additional information

No measured phototransformation in air data are available for the submission substance.

The substance contains no chromophores that would absorb visible or UV radiation, so direct photolysis is not likely to be significant. Indirect photolysis resulting from gas-phase reaction with photochemically-produced hydroxyl radicals may occur.

The AOPWIN program (v1.92, EPA 2010) has been used to obtain values of the rate constant kOH for reaction of the submission substance with hydroxyl radicals.

The overall half-life in air under default conditions of hydroxyl radical concentration was calculated using the following expressions:


kdegair (d-1) = kOH (cm3/molecule.sec) x OH Concair (molecules/cm3) x 24 x 3600


DT50 (d) = ln 2/ kdegair (d-1)



kdegair = total rate constant for degradation in air

kOH = rate constant for reaction with hydroxyl radicals

OH Concair = concentration of hydroxyl radicals in air = 5E+05 OH molecules/ cm3

DT50 = half-life


The concentration of hydroxyl radicals in air of 5E+05 OH molecules/cm3, and the 24 hour photoperiod, are the values specified in ECHA Guidance on Information requirements and chemical safety assessment, Part R.16 Environmental exposure estimation (ECHA, 2016).


The results are given in the table below:

Table 4.1.1: Results of photodegradation in air calculations


Result (Cyrene™)

  Result (Gem Diol)

kOH (cm3/ molecule.sec)



kdegair (d-1)



DT50 (days)




EPA, 2010. US Environmental Protection Agency. AOPWIN program v1.92a (September, 2010)

ECHA (2016). REACH Guidance on Information Requirements and Chemical Safety Assessment Chapter R16: Environmental Exposure Assessment Version: 3.0. February 2016. R.16-3.2.2 Photochemical reactions in the atmosphere