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Environmental fate & pathways

Phototransformation in air

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Reference
Endpoint:
phototransformation in air
Type of information:
experimental study
Adequacy of study:
key study
Study period:
no data
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Experimental results on test substances published in peer-reviewed scientific journal.
Qualifier:
no guideline available
Principles of method if other than guideline:
The reaction rate of OTNE with gas phase OH and NO3 radicals and O3 was measured using blacklamps under normal atmospheric conditions. Rate constants were determined using relative disappearance rates of OTNE and a reference compound, whose OH radical, NO3 radical or O3 reaction rate is reliably known.
GLP compliance:
not specified
Light source:
other: two parallel banks of Sylvania F40/350BL blacklamps
Light spectrum: wavelength in nm:
> 300
Duration:
18 min
Temp.:
25 °C
Initial conc. measured:
24 000 000 000 000 000 mol/L
Reference substance:
yes
Remarks:
2,3-dimethyl-2-butene for OH radical reactions, 2-methyl-2-butene for NO3 radical reactions and propene for O3 reactions
DT50:
1 h
Test condition:
Reaction with OH radicals, estimated for 12-h daylight average OH radical concentration of 2 * 10^6 molecules/cm3
Key result
DT50:
1.3 h
Test condition:
Reaction with OH radicals, estimated for 12-h daylight average OH radical concentration of 1.5 * 10^6 mol/cm3
DT50:
1.4 min
Test condition:
Reaction with NO3 radicals, estimated for 12-h nighttime NO3 radical concentration of 5 * 10^8 mol/cm3
DT50:
5.5 d
Test condition:
Reaction with O3 radicals, estimated for 24-h average concentration of 7 * 10^11 mol/cm3
Key result
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 d-1
Key result
Reaction with:
other: NO3 radicals
Rate constant:
0 cm³ molecule-1 d-1

Reaction rate constant ratios for OTNE
OH radicals: k1/k2 = 0.895
±0.080 and k1 = 9.85±0.88 * 10-11(cm3. mol-1.s-1)
NO3 radicals: k1/k2 = 1.83
± 0.20 and k1 = 17.1 ± 1.9 * 10-12(cm3. mol-1.s-1)
O3: k1/k2 = 0.22
±0.05 and k1 = 2.1± 0.5 * 10-18(cm3. mol-1.s-1)

Validity criteria fulfilled:
not applicable
Conclusions:
Reaction rate constant ratio with OH radicals for OTNE:
OH radicals: k1/k2 = 0.895±0.080 and k1 = 9.85±0.88 * 10-11(cm3. mol-1.s-1)
NO3 radicals: k1/k2 = 1.83± 0.20 and k1 = 17.1 ± 1.9 * 10-12(cm3. mol-1.s-1)
O3: k1/k2 = 0.22±0.05 and k1 = 2.1± 0.5 * 10-18(cm3. mol-1.s-1)
Executive summary:

The photodegradation of OTNE 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. Rate constants were measured for the gas phase reactions of OH radicals, NO3radicals and ozone. For reaction with OH radicals, using 2,3-dimethyl-2-butene as a reference substance, k1= 9.85 ± 0.88.10-11cm3molecule-1s-1. For reaction with NO3radicals, using 2-methyl-2-butene as a reference substance, k1= 17.1 ± 1.9.10-12cm3molecule-1s-1. For reaction with O3, using propene as a reference substance, k1= 2.1 ± 0.5.10-18cm3molecule-1s-1.

Initial addition to the C=C bond is the only significant pathway for the O3and NO3radical reaction, and addition is estimated to account for >80% of the OH radical reactions. For reaction with OH-radicals, the rate constant estimated from the structure, 12.9.10-11cm3molecule-1s-1, agrees very well with the empirical value.

Combined with estimated ambient atmospheric concentrations of OH radicals (a 12-h daylight average concentration of 2.0.106molecule cm-3), an atmospheric lifetime of 1.4 hours is calculated (t½ = 1 h). Assuming a daylight period of 12 h and 1.5.106OH cm-3, t½ based on the empirical rate is 1.9 h. For reaction with NO3radicals, the atmospheric lifetime is 2 minutes (t½ = 1.4 min) assuming a 12-h nighttime average concentration of 5.108NOcm-3. For reaction with O3, the atmospheric lifetime is 8 days (t½ = 5.5 days) assuming a 24-h average concentration of 7.1011Ocm-3.These data indicate that the atmospheric lifetime of OTNE is sufficiently short that it will not undergo long-range transport to any significant extent (Aschmann et al. 2001, Pers. comm. Atkinson[1]).

 

[1] Atkinson:Lifetime is the time for the initial concentration to decrease by a factor of "e". The half-life is therefore 0.693 x lifetime. Hence for an OH radical reaction, lifetime = 1/(k[OH]) and half-life = 0.693/(k[OH]) where k is the rate constant for reaction of the organic with OH radicals and [OH] is the ambient OH radical concentration.

Description of key information

See below.

Key value for chemical safety assessment

Half-life in air:
1.3 h

Additional information

The photodegradation of OTNE 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. Rate constants were measured for the gas phase reactions of OH radicals, NO3 radicals and ozone. The measured rate constants (in cm3molecule-1s-1) are 9.85 ± 0.88.10-11 (OH radicals), 17.1 ± 1.9.10-12(NO3 radicals) and 2.1 ± 0.5.10-18(O3).

Combined with estimated ambient atmospheric concentrations of OH radicals, an atmospheric lifetime of 1.4 hours is calculated (t½ = 1 h) based on a 12-hour daylight average atmospheric concentration of 2.0.106 molecule cm-3, assuming a daylight period of 12 h and 1.5.106 OH cm-3, t½ based on the empirical rate is 1.9 h.

For reaction with NO3 radicals and O3, the atmospheric lifetime is 2 minutes (t½ = 1.4 min) and 8 days (t½ = 5.5 days) respectively. These data indicate that the atmospheric lifetime of OTNE is sufficiently short that it will not undergo long-range transport to any significant extent

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