Heterogeneous oxidation of folpet and dimethomorph by OH radicals: A kinetic and mechanistic study

- The heterogeneous reaction between dimethomorph and OH radicals leads to the formation of a methanone.
- The life time of folpet against OH radicals is of about two months.
- The life time of dimethomorph against OH radicals is of about two years.
- The (4-chlorophenyl)(3,4-dimethoxyphenyl)methanone is highly reactive against OH radicals.

This study investigates the heterogeneous OH oxidation of folpet and dimethomorph, two fungicides identified in the atmosphere of the Champagne-Ardenne region. Kinetic experiments were conducted in the relative mode, using terbuthylazine as a reference compound. The experimental method employed makes use of a simulation chamber coupled to a GC/MS analytical system. Meanwhile, the identification of (4-chlorophenyl)(3,4-dimethoxyphenyl)methanone (CPMPM), a degradation product of the OH oxidation of dimethomorph, is achieved using SPME/GC/MS. Moreover, a degradation mechanism of dimethomorph is proposed, and the heterogeneous OH reactivity of CPMPM is evaluated. The obtained OH reaction rate constants (cm3 molecule−1 s−1) are: kZ-dimethomorph = (2.0 ± 1.2) 10−14, kE-dimethomorph = (1.7 ± 1.2) 10−14, kFolpet = (1.6 ± 0.9) 10−13 and kCPMPM = (1.9 ± 1.0) 10−12. The implicated tropospheric life-times are up to 2 months, which shows that the investigated pesticides are relatively persistent towards oxidation removal processes. CPMPM, the identified product of OH oxidation of dimethomorph, is less persistent with a life time of only 6 days relative to heterogeneous oxidation by OH radicals.

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