Henryʹs law constants measurements of alachlor and dichlorvos between 283 and 298 K

In this work, a dynamic system based on the water/air equilibrium at the interface within the length of a microporous tube was used to determine experimentally the Henryʹs law constants (HLC) of two pesticides: alachlor and dichlorvos. The measurements were conducted over the range 283–298 K. At 298 K, HLC were found to be equal to HLC=(14±2)×103 and HLC=(4.0±0.6)×103 (in units of M atm−1) for alachlor and dichlorvos, respectively. The obtained data were use to derive the following Arrhenius expressions: HLC=(8.0±3.4)×10−10 exp((9200±1600)/T) for alachlor and HLC=(2.8±0.4)×10−13 exp((11 100±1500)/T) for dichlorvos. At a cumulus temperature of 283 K, the fraction of alachlor and dichlorvos in the atmospheric aqueous phase is about 45% and 22%, respectively. Assuming that annual rainfall rate is 1 m/year, the wet deposition lifetimes were then estimated to be of the order of 2.8 days for alachlor and 5.6 days for dichlorvos. These latter are used to compare the relative importance of wet removal towards the lifetime in the gas phase.