Gas-phase reaction of dichlorvos, carbaryl, chlordimeform, and 2,4-D butyl ester with OH radicals

Widespread use of pesticides has caused serious environmental concern. In order to evaluate the fate of organic pesticides in the atmosphere, rate constants for gas phase reactions of OH radicals with dichlorvos, carbaryl, chlordimeform, and 2,4-D butyl ester were measured using the relative rate method at ambient temperature and 101 kPa total pressure. On-line FTIR spectroscopy was used to monitor the concentrations of pesticides as a function of time. The reaction rate constants with OH radicals (in units of cm ³ molecule^-1 s^-1) have been determined as (2.0±0.4) × 10^-11 for dichlorvos, (3.3±0.5) × 10^-11 for carbaryl, (3.0±0.7) × 10 ^-10 for chlordimeform, and (1.5 ± 0.2) × 10 ^-11 for 2,4-D butyl ester. These rate constants agree well with those estimated based on the structure-activity relationship. The group rate constant for N=C group (k(N=c)) was estimated as 2.7 × 10^-10 cm³ molecule^-1 s^-1. Dimethyl phosphite has been tentatively identified as a product of the reaction of dichlorvos with OH radicals. Atmospheric lifetimes due to the reactions with OH radicals were also estimated (in units of h): 14±3 for dichlorvos, 8±1 for carbaryl, 1.0±0.3 for chlordimeform, and 19±3 for 2,4-D butyl ester. These short atmospheric lifetimes indicate that the four organic pesticides degrade rapidly in the atmosphere, and they themselves are unlikely to cause persistent pollution. Further studies are needed to identify the potential hazard of their degradation products.