Mecoprop

Phototransformation in air

Link to relevant study record(s)

Reference

Endpoint:

phototransformation in air

Type of information:

other: Calculation by Atkinson Method

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:

according to guideline

Guideline:

other: BBA guidelines for the official testing of plant protection products, Part IV, 1-6

Version / remarks:

July 1990

Deviations:

no

GLP compliance:

no

Estimation method (if used):

PHOTOCHEMICAL REACTION WITH OH RADICALS
Using the incremental values published by Atkinson, the rate constant for reactions of the test material with hydroxyl radicals is calculated based on the structural formula. Assuming a pseudo-first order reaction, the degradation half-life via this reaction channel is calculated taking into account the diurnally and seasonally averaged concentration of hydroxyl radicals in the troposphere.

Calculation of the rate constant:

k = k°prim · F(C6H5) + k°tert · F(-O-) · F(CH3-) · F(-C(O)OR) + K°prim · F(-CH<) + kadd,ar

For the three potential reaction sites at the aromatic ring, the following sums of electrophilic substituent constants are obtained:
Σσ^+1 = σ^+m(-CH3) + σ^+m(-C1) + σ^+p(-OCH3)
= -0.066 + 0.399 – 0.778 = 0.445

Σσ^+2 = σ^+p(-CH3) + σ^+p(-C1) + σ^+m(-OCH3)
= -0.311 + 0.114 + 0.047 = -0.150

Σσ^+3 = Σσ^+2

As the most negative value has to be considered for further calculations, the rate constant is:

k = [ 0.144 · 1.0 + 1.83 · 6.1 · 1.00 · 0 + 0.144 · 1.29 + 10^-11.69 - (1.35 · (-0.445)) + 12] ·10^-12 cm^3 s^-1
= 0.144 + 0 + 0.186 + 8.14] · 10^-12 cm^3 s^-1
= 8.5 · 10^-12 cm^3 s^-1

Calculation of half-life for tropospheric degradation by hydroxyl radicals:
Degradation of the test material by OH radicals follows second order kinetics:

-d[test material]/dt = k · [OH radicals] · [test material]

Assuming a stationary steady state and thus constant) concentration of OH-radicals in the atmosphere, a new rate constant can be defined:

k’ = k · [OH-radicals]

Thus, degradation follows pseudo-first order kinetics:

-d[test material] / dt = k’ · [test material]

The half-life of this process is easily calculated by the equation:

T½ = 1n 2 / k’ = 1n 2 / (k · [OH-radicals])

The diurnally and seasonally averaged tropospheric hydroxyl concentration for the northern hemisphere is 5 · 10^5 cm^-3. Therefore, the half-life for degradation of the test material by OH-radicals is:

T½ = 0.693 / (8.5 · 10^-12 · 5 · 10^5) s
= 1.6 · 10^5 s = 45 h

Reference substance:

no

Remarks:

Calculation method.

DT50:

45 h

Test condition:

Atkinson calculation method

Transformation products:

not measured

Validity criteria fulfilled:

not applicable

Conclusions:

The rate constant for reactions of the test material with OH-radicals was calculated according to Atkinson’s method. The atmospheric degradation half-life of the test material via this reaction channel is: T½ = 45 h.

Executive summary:

The rate constant for reactions of the test material with OH-radicals was calculated according to Atkinson’s method.

Based on the resulting value of:

K = 8.5·10^-12 cm^3 s^-1

The atmospheric degradation half-life of the test material via this reaction channel is:

T½ = 45 h.

Description of key information

Key Study: Sarafin (1991)

The rate constant for reactions of the test material with OH-radicals was calculated according to Atkinson’s method. The atmospheric degradation half-life of the test material via this reaction channel is: T½ = 45 h.

Key value for chemical safety assessment

Half-life in air:

45 h

Additional information

Key Study: Sarafin (1991)

The rate constant for reactions of the test material with OH-radicals was calculated according to Atkinson’s method. The study was awarded a reliability score of 2 in accordance with the criteria set forth by Klimisch et al. (1997).

Based on the resulting value of: k = 8.5·10^-12 cm^3 s^-1. The atmospheric degradation half-life of the test material via this reaction channel is: T½ = 45 h.