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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)
Remarks:
Migrated phrase: estimated by calculation
Adequacy of study:
key study
Study period:
2005-09-21 to 2005-09-25
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: The calculation follows the SETAC Europe guideline recommendation (1995) and is in accordance with EU Directive 95/36/EEC.
Principles of method if other than guideline:
Guideline: Not applicable
The calculation of the substance decay rate in air was performed using the Atmospheric Oxidation Program, AOPWIN, software package v1.8 (Syracuse Research Corporation). The calculation follows the SETAC Europe guideline recommendation (1995) and is in accordance with EU Directive 95/36/EEC.
GLP compliance:
no
DT50:
14.35 h

The reaction with hydroxyl radicals dominates the decay of MIT during daylight while at night the much slower decay due to ozone may dominate:

The half-life and rate constant for the photochemical oxidative degradation of MIT in air via the hydroxyl reaction was estimated to be 4.78 hours and 26.84 x 10E–12 cm3 molecule-1 s-1, respectively, based on 1.5 x 10E6 OH radicals per cm3 which is considered to be a representative value for daylight hours (12 hours per day). The MIT half-life of 4.78 hours is equivalent to 0.399 daylight periods (12 h day). During night-time, OH radical concentrations will be much lower and hence the decay rate will decrease correspondingly.

Similarly, the half life and rate constant for the photochemical oxidative degradation of MIT in air via the ozone reaction was estimated to be 157 hours and 0.175 x 10E17cm3 molecule-1 s-1, respectively, based on 7 x 10E11 mol ozone/cm3 (=55 µg ozone/m3) which is considered to be a representative value for ozone over a 24 hours period. The MIT half-life of 157 hours is equivalent to 6.549 days (24 h day).

The half-life of MIT for the reaction with OH-radicals taking into account the concentration of OH-radicals in atmosphere of 5 x 10E5 molecules cm-3 as recommended in the TGD (2203) is equivalent to 14.35 hours or 1.196 days (12-h day)

Validity criteria fulfilled:
not applicable
Remarks:
The calculation follows the SETAC Europe guideline recommendation (1995) and is in accordance with EU Directive 95/36/EEC.
Conclusions:
The model substance 2-methyl-2H-isothiazol-3-one (MIT) is predicted to be rather efficiently removed from air by OH radical reactions during daylight situations, whereas at night the decay will predominantly be determined by reactions with ozone, however, at a much lower rate.
Endpoint:
phototransformation in air
Type of information:
(Q)SAR
Adequacy of study:
key study
Study period:
30 Oct 2002 - 15 may 2003
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: The study was conducted according to standard SAR method and GLP was not necessary. The report contains sufficient data for interpretation of study results
Qualifier:
no guideline followed
Principles of method if other than guideline:
Structure Activity Relationship was used to estimate photodegradation
GLP compliance:
no
Remarks:
Not applicable
Specific details on test material used for the study:
Details on properties of test surrogate or analogue material (migrated information):
Not applicable
Estimation method (if used):
Structure-Activity Relationships (SAR) Method
OH radical reaction (Atkinson, 1987)
SAR utilizes the fact that a number of separate radical reaction processes occur and that they can be dealt with individually. These processes involve: a) hydrogen atom abstraction from C-H bonds in alkanes, carbonyls, and other saturated organics; b) radical addition of >C=C< and -C=C-(triple bond) unsaturated bonds; c) radical addition to aromatic rings and d) radical interaction with -NH2, >NH, >N-, -SH, and -S- groups, etc.

The total radical reaction rate constant is then given by:

k(total) = k (hydrogen atom abstraction from C-H bonds) + k(radical addition to >C=C< and -C=C- (triple bonds) bonds) + k(radical addition to aromatic rings) + k(radical interaction with -NH2, >NH, >N-, -SH, and -S- groups)....................................................................................................eq. (1)

NO3 radical reaction (Sabljic and Gusten, 1990)
Unlike OH radical reaction, little is known about the reaction mechanism of NO3 radicals with organic compounds. Since no database with NO3 radical reactions is available, the rate constants for reactions of organic compounds with NO3 radicals are mostly estimated by correlations between kNO3 and kOH. Sabljic and Gusten conducted a survey of the literature which yielded 58 gas phase reaction rate constants for the reaction of the NO3 radical with organic compounds. From the good linear correlation, they concluded that both radicals react in a very analogous manner with organic compounds and the rate constants can be described by a correlation:

-log kNO3 = -18.86 + 3.05 x (-log kOH).................................................................................................................................................................................eq. (2)

Global average concentration of OH and NO3 radicals (Crutzen, 1982; Finlayson-Pitts and Pitts, 1982)
In the troposphere, the important direct sources of OH radicals are from the reaction of O atoms, formed from the photodissociation of O3, with water (Crutzen):

O3 + hv (lambda O(1D) +O2

O(1D) + H2O ->2OH

and from the photodissociation of HONO:

HONO + hv(lambda OH +NO

On the other hand, the important sources of NO3 radicals are from the following reactions (Finlayson-Pitts and Pitts):

HNO3 + hv(lambda O + HNO2 -> H + NO3, and

N2O5<-->NO3 + NO2

Phototransformation half-lives (Sabljic and Gusten, 1990)

By assuming a pseudo first-order behavior of radical reaction in the troposphere, the tropospheric half-file of a chemical compound can be calculated as:

t1/2 = ln 2/(k[C])............................................................................................................................................................................................................................eq. (3)

Where [C] is the concentration of the radicals in the troposphere such as OH, NO3, etc.

References:
Atkinson, R. A structure-activity relationship for the estimation of rate constants for the gas-phase reactions of OH radicals with organic compounds. International Journal of Chemical Kinetics, vol 19, 799-828, 1987

Crutzen, P.J. The global distribution of hydroxyl. Atmospheric Chemistry, ed., E.D. Goldberg, pp. 313-328, 1982

Finlayson-Pitts, B.J. and Pitts, Jr., J.N. Atmospheric Chemistry: fundamentals and experimental techniques, John Willy & Sons, 1982.

Sabljic, A. and Gusten, H. Predicting the night-time NO3 radical reactivity in the troposphere. Atmospheric Environment, vol. 24A, No.1, pp 73-78, 1990

DT50:
ca. 13 h
Results with reference substance:
Not applicable

RH-573

Due to relatively low vapor pressure and high water solubility, the concentration of isothiazolone compounds in the trophosphere is expected to be low. This ensures that the photodegradation of radicals with these compounds obeys first-order kinetics.

Phototransformation rate constants related to individual bonds of RH-573 are listed in Table 1 for OH radical reaction (Atkinson, 1987). The largest values belong to those that are related to nitrogen and sulfur type of bonds.


According to Atkinson(1986), the gas phase reactions of OH radical with the thiols (RSH), sulfides (RSR'), and amines (RNH2, RR'NH, and RR'R"N) proceed via initial OH radical addition to the S- and N- atoms. For the thiols and sulfides, the OH radical reaction proceeds via OH radical addition to the S-atom. For the amines, the majority of the initial OH radical reaction proceeds via OH radical addition to the N-atom, followed by a number of decomposition reaction of the adduct leading to products.

Table 1 OH rate constant of different type of reactions (10 -13 cm3/molecule/sec

 Bond Type  kOH  Number of Bonds, RH-573
 C-H  0.14  5
 >C=C<  11.0  1
 >C=O  0.31  2
 >N-  60.2  3
 -S-  20.0  2

Phototransformation rate constants of OH and NO3 radical reactions of isothiazolone compounds are calculated using equation (1) and equation (2), as shown in Table 2. kOH is about 1000 fold of kNO3.

Table 2. Phototransformation rate constants of OH and NO3 radicals (10 -13 cm3/molecule/sec)

 Rate constant  RH-573
 kOH  232.6
 kNO3  0.268

Table 3 shows the global average OH and NO3 radical concentrations in daylight and night hours. NO3 radical concentration is about 400 times of that of OH radical.

Table 3. Global average concentration (molecule/cm3)

   Day light (Hewitt and Harrison)  Night hours (Sabljic and Gursten)
 OH radical  6.5 x 105  
 NO3 radical    2.4 x 108

The calculation of phototransformation half-lives (both OH and NO3 radical reactions) is performed by using equations (3) and the results are listed in Table 4. OH radical reactions are dominant photodegradation process with shorter half-lives than NO3 radical reactiosn (13 hrs vs 29 hrs). Comparing to photodegradation in water, the half lives of RH-573 in troposphere (12.7) is much shorter than those in water (11 days, See Section 5.1.3).

Table 4. Photodegradation half-life of RH-573

 Hours  RH-573
 t1/2(OH)  12.7
 t1/2(NO3)  29.9

Metabolites

SAR calculations are conducted for all potential metabolites using Equations 1 and 3 (Table 5). Since the metabolites do not have the unstable isothiazoline ring, they will be more stable than the parent compounds. However, since all potential metabolites have either S and/or N type bonding, they are still very reactive towards tropospheric photodegradation with half-lives ranging 19.4 - 24.4 hours.

Table 5. Potential RH-573 metabolites

 Potential RH-573 Metabolites  kOH (10 -13 cm3/molecule/s  t1/2 (hour)
 

CH3NHC(O)CH=CHSO3H

 152.72  19.4
 CH3NHC(O)CHHC(O)SO2H  142.03  20.9
 CH3NHC(O)CHHCOOH  122.03  24.3
 CH3NHC(O)COOH  121.44  24.4
 CH3NHC(O)CH3  121.86  24.3
 CH3NHCOOH  121.13  24.4

Validity criteria fulfilled:
yes
Conclusions:
RH-573 has a short half-life of about 13 hours and is expected to be quickly photodegraded in the troposphere. Potential metabolites are also very reactive towards photodegradation with half-lives ranging from 19 to 25 hours.
Executive summary:

Phototransformation rate constants and half-lives of RH-573 was calculated using a SAR (Structure-Activity Relationships) method. The calculation shows that it has a short half-life of about 13 hours and is expected to be quickly photodegraded in the troposphere. Potential metabolites are also very reactive towards photodegradation with half-lives ranging from 19 to 25 hours. Due to very small annual production volume of isothiazolone products, their effect on global warming is negligible.

Description of key information

The photochemical oxidative degradation half-life of MIT in air was estimated using the Atmospheric Oxidation Program AOPWIN. The reaction with hydroxyl radicals dominates the decay of MIT during daylight while at night the much slower decay due to ozone may dominate. The half-life and rate constant for the photochemical oxidative degradation of MIT in air via the hydroxyl

reaction was estimated to be 4.78 hours and 26.84 x 10E–12 cm3 molecule-1 s-1, respectively, based on 1.5 x 10E6 OH radicals per cm3 which is considered to be a representative value for daylight hours (12 hours per day). The MIT half-life of 4.78 hours is equivalent to 0.399 daylight periods (12 h day). During night-time, OH radical concentrations will be much lower and hence the decay rate will decrease correspondingly. Similarly, the half life and rate constant for the photochemical oxidative degradation of MIT in air via the ozone reaction was estimated to be 157 hours and 0.175 x 10E17cm3 molecule-1 s-1, respectively based on 7 x 10E11 mol ozone/cm3 (=55 μg ozone/m3) which is considered to be a representative value for ozone over a 24 hours period. The MIT half-life of 157 hours is equivalent to 6.549 days (24 h day). The half-life of MIT for the reaction with OH-radicals taking into account the concentration of OH-radicals in atmosphere of 5 x 10E5 molecules cm-3 as recommended in the TGD (2203) is equivalent to 14.35 hours or 1.196 days (12-h day).

Key value for chemical safety assessment

Half-life in air:
14.35 h
Degradation rate constant with OH radicals:
26 840 000 000 000 000 000 cm³ molecule-1 s-1

Additional information