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Environmental fate & pathways

Phototransformation in air

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Reference
Endpoint:
phototransformation in air
Type of information:
other: secondary source
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Studies reviewed in EU RAR (2004)
Qualifier:
no guideline followed
Version / remarks:
The EU RAR (2004) reviews several studies which investigated the atmospheric degradation of acrylonitrile.
Principles of method if other than guideline:
The EU RAR refers to studies and theoretical modelling in the discussion of atmospheric degredation of acrylonitrile
GLP compliance:
not specified
Estimation method (if used):
The photooxidation of acrylonitrile by ozone and hydroxyl radicals under simulated atmospheric conditions
Light source:
not specified
Details on light source:
No information available
Details on test conditions:
No information available
Preliminary study:
No information available
Test performance:
No information available
DT50:
84 d
Test condition:
Reaction with ozone
Remarks on result:
other: Munshi et al (1989)
DT50:
> 115 d
Test condition:
Reaction with ozone
Remarks on result:
other: Atkinson et al (1982)
DT50:
131
Test condition:
Reaction with ozone
Remarks on result:
other: Hoechst (1994)
DT50:
5
Test condition:
Reaction with OH radicals
Remarks on result:
other: Hansen et al (1982); Edney et al (1982)
DT50:
4.1 d
Test condition:
Reaction with OH radicals
Remarks on result:
other: Hoechst (1994)
Reaction with:
ozone
Rate constant:
0 cm³ molecule-1 s-1
Remarks on result:
other: Munshi et al (1989)
Reaction with:
ozone
Rate constant:
< 0 cm³ molecule-1 s-1
Remarks on result:
other: Atkinson et al (1982)
Reaction with:
ozone
Rate constant:
0 cm³ molecule-1 s-1
Remarks on result:
other: Hoechst (1994)
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 s-1
Remarks on result:
other: Hansen et al (1982); Edney et al (1982)
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 s-1
Remarks on result:
other: Hoechst (1994)

Table 1. Rate constants for reaction of acrylonitrile with hydroxyl radical and ozone and dervied tropospheric lifetimes.

Species

Rate constant cm³/mol/s

Trophospheric lifetime/half-life (days)

Reference

O3

1.38 x 10-19

Lifetime 84 *

Munshi et al (1989)

O3

< 1.0 x 10-19

Lifetime > 115 *

Atkinson et al (1982)

O3

0.87 x 10-19

Half-life 131 **

Hoechst (1994)

OH.

3.2 x 10-12****

Half-life 5 ***

Hansen et al (1982)

Edney et al (1982)

OH.

3.95x 10-12

Half-life 4.1 ***

Hoechst (1994)

*        Value assumes an O3concentration of 1 x 1012mol/cm³

**      Value assumes an O3concentration of 0.7 x 1012mol/cm³

***    Value assumes an O3concentration of 5 x105mol/cm³

**** Value used in EUSES

Validity criteria fulfilled:
not applicable
Conclusions:
The estimated half-life for reaction with OH. is sufficiently long to allow redistribution of acrylonitrile to the aqueous compartment and to soil, with associated exposure of populations in the vicinity of the emission source, but is unlikely to be long enough to allow redistribution to the stratosphere.
Executive summary:

Acrylonitrile is labile in the atmosphere, due to photodegradation processes. Studies of photooxidation of acrylonitrile by ozone and hydroxyl radicals (OH.) by several groups (Atkinson et al., 1982; Hansen et al.; 1982, Edney et al., 1982; Munshi, 1989) under simulated atmospheric conditions indicate that reaction with OH.is the major loss process in the troposphere for acrylonitrile. The reaction with ozone is slow and is not likely to constitute a major route of degradation. Munshi (1989) determined a rate constant of 1.38.10-19 cm³/mol/s for the reaction of acrylonitrile with O3, giving a trophospheric lifetime of 84 days, while Atkinson et al. (1982) determined a rate constant of < 1.0 x 10-19cm³/mol/s at an O3concentration of < 2.4 x 1013mol/cm³. Both Hansen et al. (1982) and Edney et al. (1982) have published a rate constant of 3.2 x10-12cm³/mol/s for the reaction of acrylonitrile with OH., giving an estimated half-life of 5 days in the troposphere, based on an estimated hydroxyl radical concentration of 5 x105mol/cm³.

Theoretical estimation of the photo-oxidation of acrylonitrile using the AOPWINvl.55a Atmospheric Modelling Programme (Hoechst, 1994) has provided similar results, giving an overall predicted rate constant of 3.945 x10-12cm³/mol/s for the reaction of acrylonitrile with OH., and a half-life of 4.067 days, based on a hydroxyl radical concentration of 5.105 mol/cm³. The rate constant for the reaction of acrylonitrile with O3is predicted as 0.87 x 10-19cm³/mol/s, giving a half-life of 130.971 days at an O3concentration of 7.1011 mol/cm³. Both experimental results (Hashimoto et al., 1984) and theoretical modelling (Hoechst, 1994) suggest that addition of OH.to the olefinic double bond represents the initial degradation reaction. Harris and co-workers (1981) showed that the reaction of acrylonitrile with OH.was independent of temperature in the range studied but showed a small increase with increasing pressure. Formaldehyde has been demonstrated as a primary reaction product following reaction of acrylonitrile with OH.in the presence of NO by Edney at al. (1982), Hashimoto et al. (1984) and Spicer et al. (1985). CO, HCN, formyl cyanide (HCOCN) and formic acid have also been reported as degradation products (Edney et al., 1982; Hashimoto et al., 1984).

The estimated half-life for reaction with OH.is sufficiently long to allow redistribution of acrylonitrile to the aqueous compartment and to soil, with associated exposure of populations in the vicinity of the emission source, but is unlikely to be long enough to allow redistribution to the stratosphere.

Description of key information

Acrylonitrile is labile in the atmosphere, due to rapid photodegradation.

Key value for chemical safety assessment

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

Additional information

Acrylonitrile is labile in the atmosphere, due to rapid photodegradation. The EU RAR reports a rate constant of 3.2 x 10e-12 for the reaction of acrylonitrile with hydroxyl radicals, giving an estimated half life of 5 days in the troposphere. Theoretical assessment gives similar results and a predicted half life of 4 days.