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Environmental fate & pathways

Phototransformation in air

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Description of key information

After evaporation iodine will be rapidly degraded either by photolysis or reaction with free atmospheric radicals (e.g. ozone, OH etc.) and enter into the natural geochemical cycle of iodine.

Key value for chemical safety assessment

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

Additional information

Saiz-Lopez determined the absolute cross-section as well as the photolysis rate of iodine in the gas phase in a laboratory set-up.

Quintuplicate experiments at five different irradiances in order to analyse the variation of the photolysis rate with the light intensity were performed. The mean photo-dissociation rate constant for an irradiance of 1350 Wm-2(1 solar constant) was determined with J(I2)= 0.14±0.04 s-1and a linear fit with a slope of (1.10±0.07) x 10 -4 s-1W-1m2could be observed. The findings of the study are in good agreement to estimations of an existing computational model on solar irradiance and photo-dissociation rates.

Since the study was conducted in accordance with generally accepted scientific principles, is well documented and the absence of a standard test guideline for photolysis in air, this study is considered to be reliable and adequate for the environmental risk assessment.

The presented degradation rate constants for the reactions of iodine with hydroxyl radicals and ozone support the very short lifetime of iodine (DeMore, 1997). As these peer-reviewed data are used for atmospheric modelling they can also be considered as reliable for the assessment of the atmospheric fate of iodine.

Based on the natural occurrence of iodine in all environmental compartments and the rapid degradation of gaseous iodine by photolysis and reaction with free radicals in the atmosphere it can be assumed that iodine does not play a relevant role in the destruction of the stratospheric ozone layer. Thus, it is not classified as hazardous to the ozone layer.