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Environmental fate & pathways

Phototransformation in air

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phototransformation in air
Type of information:
Calculation of the Chemical Lifetime of Pencycuron in the Troposphere
Adequacy of study:
supporting study
Study period:
2 (reliable with restrictions)
no guideline followed
Principles of method if other than guideline:
A theoretical calculation of the photo-oxidation of Pencycuron in the atmosphere, using the method of Atkinson (1989), updated by Kwok & Atkinson (1995).
GLP compliance:
GLP is not relavant for studies using QSAR models to calculate endpoint
Specific details on test material used for the study:
There are different reaction mechanisms that may result in degradation of organic trace substances in the air being gaseous or bound to particles. According to the present knowledge mainly reactions with photochemical produced hydroxyl radicals, with nitrate radicals and ozone as well as direct photolysis are possibilities. The abiotic degradability and/or the reversal of that, the persistence of a substance X, can be predicted if the reaction rate ki and the concentrations c(yi) of the potential reaction partners are known.

On account of the molecular structure of Pencycuron, it can be taken for granted with great certainty, based on studying the relevant literature that mainly reactions with photochemical produced hydroxyl radicals determine its degradation rate K(total) and chemical lifetime (t) in the air. A value of 5 x 105 OH radi­cals per cm3 is generally regarded nowadays as the mean OH radical concentration in the troposphere (global 24-hrs-mean). A value of 1.5 x 106 OH radicals per cm3 is regarded as the global 12-hrs-day-time concentration (excluding the night). Consequently the following assumptions can be made for compounds that are emitted into the air during the day and that are degradable within a period of a few hours.

Ktotal = kOH [cm3 / molecules sec]

Chemical lifetime (t) = 1/(kOH 1.5 x 106) [sec]

Half life (t1/2) = 0.69 X (t) [sec]

The measurement of kOH is experimentally very laborious and can hardly be carried out in the gas phase for larger molecules with lower vapour pressures. Therefore, proce­ dures have been developed to calculate this important tropospheric parameter. They are based on the molecular structure of a compound.

Starting out from a comprehensive set of experimental data Roger Atkinson developed such a calculation procedure by means of quantitative structure reactivity relations (QSAR). The transformation of the calculation procedure into a personal computer program (Atmospheric Oxidation Program, AOP) was carried out by W. Meylan & P. Howard.

The calculation according to Atkinson divides the total reactivity in four single contribu­tions that depend on different chemical reaction mechanisms.

kadd: addition of HO* to olefin bonds

kar: addition of HO* to aromatic rings

kabs: abstraction of hydrogen

kNSP: reaction with N, S or P atoms

Key result
6.4 h
Test condition:
QSAR model. Concentration of OH radicals in air: 5.0E+05 molecules/cm3.

Using the method of Atkinson (1989), updated by Kwok & Atkinson (1995), gave a DT50 of 2.1 hours corresponding to a chemical lifetime of Pencycuron in air = 3.0 hours. This value was derived from a rate constant of 60.5E-12 cm3/(moleculesxsec), based on reaction with hydroxyl radicals, assuming a concentration in the atmosphere of 1.5x106 OH radicals/cm3

Following guidance under REACH the degradation in air is based on the average concentration of OH radicals in the atmosphere of 5.0E+05 molecules/cm3. This would result in a chemical lifetime of Pencycuron in the air of 6.4 hrs.

Validity criteria fulfilled:
not applicable
QSAR calculation
The chemical stability of Pencycuron in air is not determined by an attack at one single site, but at different parts of the molecule. This should result in the formation of various primary radicals leading to secondary oxidation products, which can be eliminated from the air by wet and/or dry deposition.

On account of the short chemical lifetime of Pencycuron in the air it is to be expected that the active ingredient can not be transported in gaseous phase over large distances and can not accumulate in the air.
Executive summary:

The photochemical stability of pencycuron in air was calculated using QSAR following the method of Atkinson. A value for the DT50 in air of 6.4 hours was obtained using a concentration of OH radicals in air of 5E+05 molecules/cm3.

Description of key information

Test TypeResultAssessmentReference
Degradation in air to assess LRT using QSARRate constant of 60.5E-12 cm3/molecule/s obtained, giving a DT50 of 6.4 hour (used [OH] of 5.0E+05 radicals/cm3Supporting studyHellpointner (2000) 

Key value for chemical safety assessment

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

Additional information