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Environmental fate & pathways

Phototransformation in air

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Reference
Endpoint:
phototransformation in air
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
accepted calculation method
Justification for type of information:
QSAR prediction
1. SOFTWARE
EPIWIN v4.10/ SRC AOP v1.92

2. MODEL
The estimation methods used by AOPWIN are based upon the structure-activity relationship (SAR) methods developed by Dr. Roger Atkinson and co-workers (Atkinson, 1985, 1986, 1987, 1991; Atkinson and Carter, 1984; Biermann et al, 1985; Kwok et al, 1992, Kwok and Atkinson, 1995; Kwok et al, 1996).   AOPWIN incorporates updated fragment and reaction values as cited in Kwok and Atkinson (1995).  In addition, Syracuse Research Corporation has derived some additional fragment and reaction values from more recent experimental data.

3. SMILES OR OTHER IDENTIFIERS USED AS INPUT FOR THE MODEL
O=C(OC)C(=C)C

4. ACCURANCY & DOMAIN
Estimation Accuracy - Hydroxyl Radical Estimation Accuracy of AOPWIN

An Appendix is available which is a compilation of experimentally determined gas-phase, hydroxyl radical rate constants for more than 660 organic chemicals at room temperatures. Most experimental values were taken from Atkinson (1989, 1994) and Kwok and Atkinson (1995). This Appendix also lists the rate constant estimations made by the Atmospheric Oxidation Program (AOPWIN).

A figure is available which is a graphical representation of the correlation between the experimental OH rate constants and the rate constants estimated by the AOP program.  The correlation includes 667 compounds; most experimental values containing a "less than" sign (<) were excluded.  Since the range of experimental rate constants spans nearly six orders of magnitude, a statistical correlation was computed on a logarithmic basis ... comparing experimental to estimated values:

 AOPWIN  
correlation coefficient (r2)   0.963
standard deviation (sd)        0.218
absolute mean error (me)       0.127

For the 667 AOPWIN estimations, 90% are within a factor of two of the experimental value and 95% are within a factor of three.
Qualifier:
no guideline followed
Principles of method if other than guideline:
Calculation using SRC AOP (v1.92)
GLP compliance:
no
Estimation method (if used):
PHOTOCHEMICAL REACTION WITH OH RADICALS
- Concentration of OH radicals: 500000 molecules/cm3
- Degradation rate constant: 18.34xE-12 cm3/molecules*s
- Computer programme: SRC AOP v1.92
- Other: assuming a 24-hour day
DT50:
20.99 h
Test condition:
for reactions with hydroxyl radicals
DT50:
24.179 h
Test condition:
for reactions with ozone

SMILES : O=C(OC)C(=C)C

CHEM : 2-Propenoic acid, 2-methyl-, methyl ester

MOL FOR: C5 H8 O2

MOL WT : 100.12

------------------- SUMMARY (AOP v1.92): HYDROXYL RADICALS (25 deg C) --------

Hydrogen Abstraction = 0.3536 E-12 cm3/molecule-sec

Reaction with N, S and -OH = 0.0000 E-12 cm3/molecule-sec

Addition to Triple Bonds = 0.0000 E-12 cm3/molecule-sec

Addition to Olefinic Bonds = 17.9900 E-12 cm3/molecule-sec

Addition to Aromatic Rings = 0.0000 E-12 cm3/molecule-sec

Addition to Fused Rings = 0.0000 E-12 cm3/molecule-sec

OVERALL OH Rate Constant = 18.3436 E-12 cm3/molecule-sec

HALF-LIFE = 0.875 Days (24-hr day; 1.5E6 OH/cm3)

HALF-LIFE = 20.991 Hrs

------------------- SUMMARY (AOP v1.91): OZONE REACTION (25 deg C) -----------

OVERALL OZONE Rate Constant = 1.137500 E-17 cm3/molecule-sec

HALF-LIFE = 1.007 Days (at 7E11 mol/cm3)

HALF-LIFE = 24.179 Hrs

Experimental Database Structure Match:

Chem Name : Methyl methacrylate

CAS Number: 000080-62-6

Exper OH rate constant : 26 E-12 cm3/molecule-sec

Exper OH Reference: SAUNDERS,SM ET AL. (1993)

Exper Ozone rate constant: --- cm3/molecule-sec

Exper NO3 rate constant : --- cm3/molecule-sec

Fraction sorbed to airborne particulates (phi):

4.7E-008 (Junge-Pankow, Mackay avg)

3.61E-008 (Koa method)

Note: the sorbed fraction may be resistant to atmospheric oxidation

Conclusions:
Using EPA's standard modelling method, the half-life was determined to be 21 h for the reaction with hydroxyl radicals, 24.2 h for the reaction with ozone and 9.2 h combined.
Executive summary:

Using EPA's standard modelling method, the half-life was determined to be 21 h for the reaction with hydroxyl radicals, 24.2 h for the reaction with ozone and 9.2 h combined.

Description of key information

After evaporation or exposure to air, methyl methacrylate will be rapidly degraded by photochemical processes.

Key value for chemical safety assessment

Half-life in air:
20.99 h
Degradation rate constant with OH radicals:
0 m³ molecule-1 s-1

Additional information

The phototransformation of methyl methacrylate was investigated in several studies. The results of Saunders et al. (1992) and Howard (1991) showed a atmospheric halftime clearly below one day. Another calculation using EpiWin v4.10 AOP v1.92 showed a rate constant for the reaction of methyl methacrylate with hydroxyl radicals in the atmosphere of 18.34E-12 cm³/molecule*s. From this rate constant half-life for the reaction of OH radicals with the test substance is calculated as 20.99 h assuming a 24 h-day.

In conclusion, after evaporation or exposure to air, methyl methacrylate will be rapidly degraded by photochemical processes.