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Environmental fate & pathways

Phototransformation in air

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Description of key information

Atmospheric half-life of HHCB is 3.7 hours according to a study by Aschmann et al. (2001). This half-life is < 2 days being the cut off for potential long-range transport (

HHCB does not have an ozone depletion potential because it does not contain halogens and does not have the potential to reach the stratosphere.

Key value for chemical safety assessment

Additional information

The photodegradation of HHCB was studied by Aschmann et al. (2001) under laboratory conditions using blacklamps for irradiation (l > 300 nm) at 25 °C and 740 mm Hg (0.986 bar) total pressure of purified air at ~5% relative humidity. The rate constant was measured for the gas phase reaction of OH radicals. The measured rate constant (in cm3molecule-1s-1) is 2.6 ± 0.6.10 -11. Combined with estimated ambient atmospheric concentrations of OH radicals, an atmospheric lifetime of 5.3 hours is calculated (t½ = 3.7 h) based on a 12 -hour daylight average atmospheric concentration of 2.0E6 molecule cm-3, assuming a daylight period of 12 h and 1.5E6OH cm-3, t½ based on the empirical rate is 4.96 h. This data indicates that the atmospheric lifetime of HHCB is sufficiently short that it will not undergo long-range transport to any significant extent (Aschmann et al. 2001).

Similar results were observed using computer modelling. In a study by Li (2018), it was calculated that at 298 K, the total rate constant of OH-initiated degradation of HHCB is 2.71E−11 cm3 molecule−1 s−1. The atmospheric lifetime of HHCB by OH-initiated reactions was determined to be 10.09 h, which is in favor of the phenomenon of medium-range transport for HHCB in the atmosphere. 

In the QSAR model AopWin, the DT50-value after reaction with hydroxyl radicals is determined to be 3.397 hours (rate constant is 37.7872 E-12 cm3/molecule-sec). The reaction with ozone could not be calculated. This data indicated that the atmospheric lifetime of the substance is sufficiently short that it will not undergo long-range transport to any significant extent.

Overall, it is concluded that photodegradation is an important degradation pathway in the environment.

Reference list:

Li et al. (2018), Theoretical and Kinetic Properties of OH Radical-Initiated Oxidation of Galaxolide in the Atmosphere, J. Phys. Chem. A, 122, 9151−9159