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Environmental fate & pathways

Phototransformation in air

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Endpoint:
phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
US EPA document. Cited in EU RAR
Principles of method if other than guideline:
Estimated for troposhere based on OH and ozone reaction rate constants.
DT50:
42 d
Test condition:
EPA estimate
DT50:
321 d
Test condition:
EU estimate, for average tropospheric conditions

An overall half-life for acetonitrile in the troposhere of about 42 days has been estimated by US EPA (1987), considering both the OH radical and ozone reaction rate constants quoted by Harris (1981). The EU RAR notes that this value should be about 10 times higher for average troposheric conditions, which will give a half-life of about one year (321 days).

Executive summary:

An overall half-life for acetonitrile in the troposhere of about 42 days has been estimated by US EPA (1987), considering both the OH radical and ozone reaction rate constants quoted by Harris (1981). The EU RAR notes that this value should be about 10 times higher for average troposheric conditions, which will give a half-life of about one year (321 days).

Endpoint:
phototransformation in air
Type of information:
(Q)SAR
Remarks:
Estimated via US EPA AOPWIN software (v1.92)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Remarks:
Data source cited in R.7 guidance
Principles of method if other than guideline:
AOPWIN v1.92 model
Estimation method (if used):
PHOTOCHEMICAL REACTION WITH OH RADICALS
- Concentration of OH radicals: (12-hr; 1.5 E6 OH/cm3)
- Degradation rate constant: 0.0258 E-12 cm3/molecule-sec





DT50:
414 d
Executive summary:

US EPA AOPWIN software (v1.92) estimated an Overall Hydroxyl Radical Reaction Rate Constant of 0.0258 E-12 cm3/molecule-sec, and degradation half-life of 414 days (12 -hr day; 1.5 E6 OH/cm3), for acetonitrile.

Endpoint:
phototransformation in air
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
4 (not assignable)
Rationale for reliability incl. deficiencies:
documentation insufficient for assessment
Remarks:
Limited information available. Not a widely recognized source.
Principles of method if other than guideline:
Measured phtotolysis under black lights and sun lamps.
Light source:
sunlight
Initial conc. measured:
0.007 mg/L
Test condition:
Exposure of an atmospheric acetonitrile concentration of 4 ppm to 22 black lights and 7 sun lamps did not result in photolysis.

Exposure of an atmospheric concentration of 4 ppm (0.00684 mg/l; 1ppm = 1.706 mg/m3) to 22 black lights and 7 sun lamps did not result in photolysis. It is not clear from this citation whether ozone was also involved.

Executive summary:

Exposure of an atmospheric concentration of 4 ppm (0.00684 mg/l; 1ppm = 1.706 mg/m3) to 22 black lights and 7 sun lamps did not result in photolysis (Dimitriades, 1977; as cited in Envirofate, 1994). It is not clear from this citation whether ozone was also involved.

Endpoint:
phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Data source cited in R.7 guidance
Principles of method if other than guideline:
Estimated for atmosphere based on OH reaction rate constant.
Estimation method (if used):
PHOTOCHEMICAL REACTION WITH OH RADICALS
- Concentration of OH radicals: 5 x 10E5 hydroxyl radicals per cm3
- Degradation rate constant: 2.63 x 10E-14 cm3/molecule-sec at 25 deg C
Details on test conditions:
Sensitiser (for indirect photolysis): OH
DT50:
620 d

The rate constant for the gas-phase reaction of acetonitrile with photochemically-produced hydroxyl radicals is 2.63 x 10E-14 cm3/molecule-sec at 25 deg C. This corresponds to an atmospheric half-life of about 620 days at an atmospheric concentration of 5 x 10E5 hydroxyl radicals per cm3 (Atkinson, 1994).

Executive summary:

The rate constant for the gas-phase reaction of acetonitrile with photochemically-produced hydroxyl radicals is 2.63 x 10E-14 cm3/molecule-sec at 25 deg C. This corresponds to an atmospheric half-life of about 620 days at an atmospheric concentration of 5 x 10E5 hydroxyl radicals per cm3 (Atkinson, 1994, cited in HSDB 2009).

Endpoint:
phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Data source cited in R.7 guidance.
Principles of method if other than guideline:
Estimated photochemical lifetime of acetonitrile in the troposphere.
GLP compliance:
not specified
DT50:
500 d
Test condition:
Estimated lifetime of acetonitrile in the lower troposphere; relatively longer in the upper troposhere.

Poschl (2001) specified the photochemical lifetime of acetonitrile to be approximately 500 days in the lower troposphere and relatively longer in the upper troposphere (HSDB, 2009).

Executive summary:

Poschl (2001) specified the photochemical lifetime of acetonitrile to be approximately 500 days in the lower troposphere and relatively longer in the upper troposphere (HSDB, 2009).

Endpoint:
phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
supporting study
Reliability:
4 (not assignable)
Rationale for reliability incl. deficiencies:
documentation insufficient for assessment
Remarks:
Limited information available. Not a widely recognized data source.
Principles of method if other than guideline:
Estimated for atmosphere based on reaction rate constant for singlet oxygen.
GLP compliance:
not specified
Estimation method (if used):
PHOTOCHEMICAL REACTION WITH singlet oxygen
- Degradation rate constant: 2.400 x 10-16 cm3. molec-1. sec-1
- Temperature for which rate constant was calculated: 25 deg C
Details on test conditions:
Sensitiser (for indirect photolysis): other: O
DT50:
5 501 yr
Test condition:
Singlet oxygen reaction rate = 2.400 x 10-16 cm3. molec-1. sec-1; 25 deg C

Rate constant (for indirect photolysis): = 0.00000000000000024 cm3/ (molecule*sec).

Executive summary:

A half-life of 5501 years has been calculated for the reaction of acetonitrile with the reactant O in the atmosphere, at a temperature of 25 degrees C; the reaction rate was 2.400 x 10-16 cm3. molec-1. sec-1 (Graedel, 1978; as cited in IPCS, 1992).

Endpoint:
phototransformation in air
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
4 (not assignable)
Rationale for reliability incl. deficiencies:
documentation insufficient for assessment
Remarks:
Limited information available. Not a widely recognized data source.
Principles of method if other than guideline:
Measured degradation under mercury lamp.
GLP compliance:
not specified
Light source:
other: other
Initial conc. measured:
3.412 mg/L
% Degr.:
0
Test condition:
An atmospheric acetonitrile concentration of 300 -2000 ppm in air saturated with water was exposed to a mercury lamp. No degradation occurred.

An atmospheric concentration of 300 -2000 ppm (0.512 -3.412 mg/l; 1 ppm = 1.706 mg/m3), in air saturated with water, was exposed to a mercury lamp. No degradation occurred. No further details were given in the citation.

Executive summary:

An atmospheric concentration of 300 -2000 ppm (0.512 -3.412 mg/l; 1 ppm = 1.706 mg/m3), in air saturated with water, was exposed to a mercury lamp. No degradation occurred (Kagiya, 1975; as cited in IPCS, 1992). No further details were given in the citation.

Endpoint:
phototransformation in air
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Remarks:
Cited in EU RAR
Principles of method if other than guideline:
Estimated for atmosphere based on OH reaction rate constant.
GLP compliance:
not specified
Estimation method (if used):
PHOTOCHEMICAL REACTION WITH OH RADICALS
- Concentration of OH radicals: 5 x 10E5 cm3
DT50:
> 160 d
Test condition:
OH radical concentration = 5 x 10E5 cm3

In conditions of an average tropospheric OH radical concentration, [OH] = 5 x 10E5 cm3, Klopffer (1988) obtained a half-life of >160 days for acetonitrile.

Executive summary:

In conditions of an average tropospheric OH radical concentration, [OH] = 5 x 10E5 cm3, Klopffer (1988) obtained a half-life of >160 days for acetonitrile (EU RAR).

Endpoint:
phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Data source (HSDB) cited in R.7 guidance
Principles of method if other than guideline:
Based on UV absorption characteristics
Test condition:
Direct photolysis is not expected because cetonitrile absorbs light only in the far UV region.

Acetonitrile absorbs light only in the far UV region (Arijs, 1983) and therefore is not expected to be susceptible to direct photolysis by sunlight (HSDB, 2009).

The maximum absorption for acetonitrile in the UV range is lower than 160 nm, therefore direct photolysis of acetonitrile in the atmosphere is not expected to be an important fate process (Silverstein and Bassler, 1967; Howard, 1991).

Executive summary:

Acetonitrile absorbs light only in the far UV region, therefore, direct photolysis of acetonitrile in the atmosphere is not expected to be an important fate process (Silverstein and Bassler, 1967; Howard, 1991), (Arijs, 1983; HSDB, 2009).

Endpoint:
phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Data from widely recognized sources
Principles of method if other than guideline:
Estimated for atmosphere based on OH reaction rate constant.
Estimation method (if used):
PHOTOCHEMICAL REACTION WITH OH RADICALS
Details on test conditions:
Sensitiser (for indirect photolysis): OH
DT50:
0.6 d
Test condition:
OH reaction rate constant of 2.1 x 10 -14 cm3/molecule.sec; Average atmospheric OH concentration of 5 x 10 +5 mols/cm3; at 25 degree C
DT50:
>= 20 - <= 200 d
Test condition:
OH reaction rate constant of 1.9 - 4.94 x 10-14 cm3. moles-1. sec-1; Average atmospheric OH concentration of 10E6 to 10E7 mols/cm3; at 20 to 27 degree C
DT50:
535 d
Test condition:
Based on Arrhenius activation energy of 1500 cal. mole-1

A rate constant of 2.1 x 10 -14 cm3/molecule.sec and average atmospheric OH concentration of 5 x 10 +5 mols/cm3, at 25 degree C or 'room temperature' (Atkinson, 1989), has also been estimated to give a half-life for acetonitrile of 0.6 days (Envirofate, 1994).

The rate constant for the reaction of acetonitrile with hydroxyl radicals has been determined by several authors to be in the range from 1.9 - 4.94 x 10-14 cm3. moles-1. sec-1, for the temperature range 20 -27 degree C. For an average atmosphere containing 106 hydroxy radicals/cm3 the tropspheric half life has been calculated to be about 200 days. In a moderately polluted atmosphere containing 107 OH radicals/cm3 the half life would be about 20 days (Atkinson, 1985; Gusten et al., 1981; Gusten et al., 1984; Harris et al., 1981; Wallington et al., 1988, Lyman et al, 1982). The Arrhenius activation energy, as determined by Harris (1981) was 1500 cal. mole-1.

Using data presented in Atkinson (1985), Howard (1993) estimated the half life for this reaction would be 535 days.

Executive summary:

Rate constants for the reaction of acetonitrile with hydroxyl radicals have been determined to range from 1.9 -4.94 x 10 -14, for the temperature range 20 -27 degree C. For an average atmosphere containing 10E6 hydroxy radicals/cm3 the tropospheric half life has been calculated to be about 200 days. In a moderately polluted atmosphere containing 10E7 OH radicals/cm3 the half life would be about 20 days (Atkinson, 1985; Gusten et al., 1981; Gusten et al., 1984; Harris et al., 1981; Wallington et al., 1988). The Arrhenius activation energy, as determined by Harris (1981) was 1500 cal. mole-1.

Endpoint:
phototransformation in air
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Widely recognized sources. Cited in EU RAR
Principles of method if other than guideline:
Estimated for atmosphere based on ozone reaction rate constant.
Estimation method (if used):
PHOTOCHEMICAL REACTION WITH ozone
Details on test conditions:
Sensitiser (for indirect photolysis): O3
DT50:
54 d
Test condition:
Ozone reaction rate constant = 1.5 x 10E-19cm3 molec-1 sec-1; 10E12 molecules ozone/cm3
DT50:
76.4 d
Test condition:
Ozone reaction rate constant = 0.15 x 10E-18 cm3 molec-1sec-1; 7 x 10E11 molecules ozone/cm3; at 25 degree C
DT50:
860 d

A half-life of 54 days has also been calculated for the reaction between acetonitrile and ozone, using a rate constant, determined in laboratory studies, of 1.5 x 10E-19cm3 molec-1 sec-1 (Harris et al. 1981) and 10E12 molecules ozone/cm3, which was described as a typical atmospheric concentration (Munshi, 1989).

Using a reaction rate of 0.15 x 10E-18 cm3 molec-1sec-1 and an atmospheric ozone concentration of 7 x 10E11 molecules/cm3, at 25 degree C or at room temperature, the half-life has been estimated to be 76.4 days (Atkinson and Carter, 1984; Envirofate, 1994).

Using data from Atkinson (1984, 1985), Howard (1993) estimated the half-life for acetonitrile in the presence of ozone would be about 860 days.

Executive summary:

A half-life of 54 days has also been calculated for the reaction between acetonitrile and ozone, using a rate constant, determined in laboratory studies, of 1.5 x 10E-19cm3 molec-1 sec-1 (Harris et al. 1981) and 10E12 molecules ozone/cm3, which was described as a typical atmospheric concentration (Munshi, 1989).

Using a reaction rate of 0.15 x 10E-18 cm3 molec-1sec-1 and an atmospheric ozone concentration of 7 x 10E11 molecules/cm3, at 25 degree C or at room temperature, the half-life has been estimated to be 76.4 days (Atkinson and Carter, 1984; Envirofate, 1994).

Using data from Atkinson (1984, 1985), Howard (1993) estimated the half-life for acetonitrile in the presence of ozone would be about 860 days.

Description of key information

Acetonitrile is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals; the half-life of this reaction in air has been estimated to range from 20-620 days.  An overall half-life for acetonitrile in the troposphere of about 42 days has been estimated considering both the OH radical and ozone reaction rate constants (USEPA, 1987). However, based on average tropospheric conditions reported by Klopffer (1988), this number would be about one year (321 days) according to the 2000 EU Risk Assessment for acetonitrile.  Acetonitrile absorbs light only in the far UV regions, and therefore is not expected to be susceptible to direct photolysis by sunlight.

Key value for chemical safety assessment

Half-life in air:
321 d

Additional information

The maximum of absorption for acetonitrile in the UV range is lower than 160 nm, therefore direct photolysis of acetonitrile in the atmosphere is not expected to be an important fate process (Silverstein and Bassler, 1967; Howard, 1991), (Arijs, 1983; HSDB, 2009).

Acetonitrile is reactive to oxidising materials and compounds in the atmosphere, being the reaction with hydroxyl radicals quoted as one of the main mechanisms for its removal from the environment (Lobert et al., 1990). The rate constant for the reaction of acetonitrile with hydroxyl radicals has been determined by several authors to range from 1.9 - 4.94 x 10-14cm3. molec-1. sec-1for the temperature range 20-27ºC. In a moderately polluted atmosphere, with a mean concentration of 107hydroxyl radicals/cm3, the calculated half-life was about 20 days (Atkinson, 1985; Guesten et al., 1981; Güsten et al., 1984; Harris et al. 1981; Wallington et al., 1988, Lyman et al., 1982). The Arrhenius activation energy, as determined by Harris et al (1981), was 1500 cal. mole-1.  Howard, 1993 presented a much longer half life for this reaction of 535 days based on data in (Atkinson, 1985), and HSDB (2009) estimated the atmospheric half-life of acetonitrile to be 620 days based on a hydroxyl radical reaction rate constant of 2.63 x 10E-14 cm3/molecule-sec at 25 deg C (Atkinson, 1994).

In conditions of an average tropospheric OH radical concentration, [OH] = 5 x 105cm-3, Klöpffer et al. (1988) obtained a half-life of >160 days, or about 10 times longer than that reported above. Poschl (2001) specified the photochemical lifetime of acetonitrile to be approximately 500 days in the lower troposphere and relatively longer in the upper troposphere (HSDB, 2009).

The reaction of acetonitrile with ozone is slow, with a reported reaction rate constant of 1.5 x 10-19cm3. molec-1. sec-1(Harris et al., 1981), which will yield a half-life of 54 days, considering an average ozone abundance of 1 x 1012molec/cm3(Munshi et al., 1989). Using a reaction rate of 0.15 x 10-18 cm3/mol.sec and an atmospheric ozone concentration of 7 x 10 E11 mols/cm3, at 25 degree C or at room temperature, the half-life has been estimated to be 76.4 days (Atkinson and Carter, 1984; Envirofate, 1994). A much longer half life for acetonitrile in the presence of ozone of about 860 days has been reported based on data in (Atkinson and Carter, 1984; Atkinson, 1985) by Howard, 1993).

A reaction rate constant between acetonitrile and single oxygen of 2.4 x 10-16cm3. molec-1. sec-1 has been reported (Graedel, 1978), which yields an estimated atmospheric half-life of >5000 years. Acetonitrile reaction with chlorine radicals is not thought to be significant in relation to hydroxyl radical reaction (Arijs et al., 1983).

An overall half-life for acetonitrile in the troposphere of about 42 days has been estimated, considering both the OH radical and ozone reaction rate constants (USEPA, 1987), based on rate constants quoted by Harris et al (1981). Nevertheless, as indicated above, that value should be about 10 times higher for average tropospheric conditions as reported by Klopffer (1988), which would give a half live of about one year (321 days) according to the 2000 EU Risk Assessment of acetonitrile.

Due to the nonreactivity of acetonitrile in the atmosphere, transport of acetonitrile from troposphere to stratosphere is expected to occur, and acetonitrile has been detected in the stratosphere (Arijs et al, 1983)