Atmospheric fate of hymexazol (5-methylisoxazol-3-ol): Simulation chamber studies

The gas-phase degradation of hymexazol (5-methylisoxazol-3-ol), a widely used fungicide, was investigated under atmospheric conditions at the European Photoreactor (EUPHORE) in Valencia, Spain, a large outdoor simulation chamber. The rate coefficient for the reaction of hydroxyl radicals with hymexazol was measured using a conventional relative rate technique. A value of the rate coefficient for the reaction of OH radicals with hymexazol, kOH(hymexazol) = (4.9 ± 0.4) × 10−12 cm3 molecule−1 s−1 was determined at 300 ± 5 K and atmospheric pressure. Rate coefficient data for the reaction of OH radicals with isoxazole, the unsubstituted analogue of hymexazol, was also obtained using the relative rate method, kOH (isoxazole) = (1.1 ± 0.1) × 10−12 cm3 molecule−1 s−1. The rate coefficients for photolysis of hymexazol, J(hymexazol) < 5 × 10−6 s−1, and reaction of ozone with hymexazol, k O 3 (hymexazol) = (3.2 ± 0.6) × 10−19 cm3 molecule−1 s−1, under atmospheric conditions were also determined. The results showed that removal of hymexazol from the atmosphere by photolysis or by reaction with ozone is slow compared to loss by reaction with OH radicals. The available kinetic data suggest that the gas-phase tropospheric degradation of hymexazol will be mainly controlled by reaction with OH and possibly NO3 radicals. The data provide the basis of an estimate for the tropospheric lifetime of approximately 1 day. The atmospheric implications of the use of hymexazol as a fungicide are discussed.