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Environmental fate & pathways

Phototransformation in air

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phototransformation in air
Type of information:
experimental study
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
Method: other (measured)
GLP compliance:
Details on test conditions:
Sensitiser (for indirect photolysis): OH
Sensitiser concentration: up to 100000000 molecule/cm³

Test condition: Reaction chamber 420 l, at atmospheric pressure and 298 +/-2 degree K. Irradiation by 24 Philips TL/05 40W lamps (k photolysis = 0.0062/sec, similar to natural sunlight) and OH radicals generated by photolysis of methyl nitrite in air at wavelengths >300 nm, in presence of nitric oxide to prevent formation of ozone and nitrate radicals. Concentration-time profiles of test substance were determined using in-situ long-path FT-IR absorption spectroscopy. A competitive rate technique, using toluene as reference substrate, obviated the need for precise knowledge of substrate concentrations.

Test medium: Air containing <40ppm of water
Positive Controls: Toluene

Concentrations of test substances were 1-5 ppm (0.005-0.040 mg/l) for TDA and MDA.
25 °C
6.4 h
Transformation products:

Rate constant (for indirect photolysis): = 0.00000000003 cm³/(molecule*sec)

It was concluded that no accumulation of atmospheric MDA would be predicted from an emission of MDA.

Description of key information

A tropospheric half-life time of 6.4 hours has been determined based on a tropospheric hydroxyl radical concentration of 1 x 10(6)/ml. A rapid photodegradation is expected when exposed to air.

Key value for chemical safety assessment

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

Additional information

The study provides a rate constant of 3 x 10-11cm3/mole*sec for the indirect photolysis of MDA. A tropospheric half-life time of 6 hour was calculated assuming a tropospheric hydroxyl radical concentration of 1 x 106/ml (Becker al, 1988). These results are consistent with those obtained by QSAR (Pemberton et al., 2008)