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Environmental fate & pathways

Phototransformation in air

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Endpoint:
phototransformation in air
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
GLP compliance:
not specified
Light source:
not specified
Reference substance:
not specified
Key result
DT50:
9.4 h
Remarks on result:
other: data published in OECD SIDS dossier.
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 s-1
Transformation products:
not specified

The OECD SIDS (1994) dossier reported an OH radical reaction rate constant of 1.2e-11 cm3/molecule.second (reported to be equal to a DT50 of 9.4 hours) was calculated based on an estimated environmental OH radical concentration of 1.7e-6 radicals/cm3. This data was taken from a review paper by Atikinson and Lloyds (1984).

Validity criteria fulfilled:
yes
Conclusions:
The OECD SIDS (1994) dossier reported an OH radical reaction rate constant of 1.2e-11 cm3/molecule.second (reported to be equal to a DT50 of 9.4 hours) was calculated based on an estimated environmental OH radical concentration of 1.7e-6 radicals/cm3.
Executive summary:

Data on the indirect photolysis (i.e. reaction with photochemically produced OH radicals) of benzaldehyde was published in the 1994 OECD SIDS Dossier for benzaldehyde. This data is originally from a paper published by Atkinson and Lloyds in 1984 in in which they reviewed evidence from many sources on the photochemical degradation of components of smog. No further information is known as the paper is not accessible. The OECD SIDS (1994) dossier reported an OH radical reaction rate constant of 1.2e-11 cm3/molecule.second (reported to be equal to a DT50 of 9.4 hours) was calculated based on an estimated environmental OH radical concentration of 1.7e-6 radicals/cm3.

Endpoint:
phototransformation in air
Type of information:
other: Publication/review of reported data
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Peer reviewed pubication with well documented methods and data analysis.
Qualifier:
no guideline required
Principles of method if other than guideline:
The literature kinetic and mechanistic data for the gas-phase reactions of the OH radical with organic compounds (through 1988) have been tabulated, reviewed and evaluated over the entire temperature ranges for which data are available.
Estimation method (if used):
Two general experimental approaches, namely absolute and relative rate constant measurement methods, have been used to determine rate constants for the reactions of the OH radical with organic compounds. In the absolute technique, for the bimolecular reaction, either the psuedo-first order decay of one species is measured in the presence of a known excess concentration of the other reactant or the concentrations of both species are measured and the rate constant k is derived by calculation.

In the relative rate method, the rate constant of interest is determined relative to that for another reaction, normally a reaction of the OH radical with a second, or reference, species. Generally, the decay rates of two or more compounds are monitored in the presence of OH radicals, with other loss processes (chemical or physical) of these reactants being either quantitatively known or minimized.
Key result
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 s-1

Atkinson (1989) identified two room temperature rate constants and upon analysis found they were in general agreement. Therefore, the authors suggested that a unit-weighted average of these data should be used. This resulted in an overall OH rate constant = 1.29E-11 cu cm/molecule-sec at 25 ℃.

Validity criteria fulfilled:
not applicable
Conclusions:
The rate constant for the vapor-phase reaction of the test substance with photochemically-produced hydroxyl radicals has been reported to be 1.29E-11 cu cm/molecule-sec at 25 °C.
Executive summary:

The literature kinetic and mechanistic data for the gas-phase reactions of the OH radical with organic compounds (through 1988) have been tabulated, reviewed and evaluated over the entire temperature ranges for which data are available. Atkinson (1989) identified two room temperature rate constants for benzaldehyde during this exercise and upon analysis found they were in general agreement. Therefore, the authors suggested that a unit-weighted average of these data should be used. This resulted in an overall OH rate constant for benzaldehyde of 1.29E-11 cu cm/molecule-sec at 25 ℃.

Endpoint:
phototransformation in air
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
no guideline followed
Principles of method if other than guideline:
The photochemistry of the test substance has been studied by determining the quantum yields of benzene and carbon monoxide formation, the test substance consumption and phosphorescence emission, and by following the phosphorescence lifetime over a range of pressure and excitation wavelengths.
GLP compliance:
no
Light source:
other: UV light
Light spectrum: wavelength in nm:
265 - 380
Details on light source:
Most photolyses were carried out using cylindrical quartz optical cells 100 mm long and 22 mm in diameter. Photolyses were effected with a high pressure mercury are in conjunction with a monochromator set so that the spectral bandwidth was 1.6 nm. For emission and lifetime measurements rectangular 10-mm path length flouresence cells were used.
Details on test conditions:
The experiment was conducted at room temperature, 22±2 °C.
Reference substance:
not specified
Test performance:
No information provided
Quantum yield (for direct photolysis):
0.4
Transformation products:
yes
No.:
#1
No.:
#2
Validity criteria fulfilled:
yes
Conclusions:
Although the absorption is relatively weak, the quantum yield (which is approximately 0.34-0.41) is quite high and it suggests that the test substance may directly photolyze in the environment.
Executive summary:

The photochemistry of the test substance has been studied by determining the quantum yields of benzene and carbon monoxide formation, the test substance consumption and phosphorescence emission, and by following the phosphorescence lifetime over a range of pressure and excitation wavelengths. Although the absorption is relatively weak, the quantum yield (which is approximately 0.34-0.41) is quite high and it suggests that the test substance may directly photolyze in the environment.

Description of key information

A DT50 of 9.4 h has been predicted for the indirect photolysis of benzaldehyde in air caused by reaction with photochemically produced hydroxyl radicals.

A DT50 of 16.4 days has been predicted for the indirect photolysis of benzaldehyde in air caused by reaction with photochemically produced nitrate radicals.

Direct photolysis of benzaldehyde in air is possible but may be slow and benzene and carbon monoxide and a yellowish polymer have been identified as transformation products.

Key value for chemical safety assessment

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

Additional information

Indirect photolysis in air by OH radicals

Data on the indirect photolysis (i.e. reaction with photochemically produced hydroxyl (OH) radicals) of benzaldehyde in air have been reported in two separate collation and review papers. Atkinson (1989) reported two room temperature rate constants for benzaldehyde and suggested that a unit-weighted average of these two data points was valid. This resulted in an overall OH rate constant for benzaldehyde of 1.29E-11 cm3/molecule/second at 25. Atkinson and Lloyds (1984) also reported an OH radical reaction rate constant of 1.2e-11 cm3/molecule/second, no further information is known as the paper is not accessible. In the 1994 OECD SIDS Dossier for benzaldehyde, the Atkinson and Lloyds (1984) value was quoted and reported to be equal to a DT50 of 9.4 hours (based on an estimated environmental OH radical concentration of 1.7e-6 radicals/cm3). As these two OH radicle reaction rates are comparable and the OECD is considered an authoritative resource, DT50 of benzaldehyde in the atmosphere following indirect photolysis by OH radicals is considered to be 9.4 days. This is further supported by the QSAR produced by the Episuite program which predicted a DT50 of 7.18 hours.

Atkinson et al. (1987) reported a reaction rate for indirect photolysis by photochemically produced nitrate (NO3) radicals in air of 2.0E-15 cu cm/molecule-sec at 25 °C, which corresponds to a DT50 of 16.7 days based on an atmospheric concentration of 2.4E+8 NO3 radicals/cm3 (as quoted by HSDB).

Direct photolysis in air

The available studies show that direct photolysis of benzaldehyde in air is possible but may be slow. Benzene and carbon monoxide and a yellowish polymer have been identified as transformation products following direct photolysis of benzaldehyde in air.