Temperature dependence of Henryʹs law constants of metolachlor and diazinon

A dynamic system based on the water/air equilibrium at the interface within the length of a microporous tube has been used to determine experimentally the Henryʹs law constants (HLC) of two pesticides: metolachlor and diazinon. The measurements were conducted over the temperature range 283–301 K. At 293 K, HLCs values are (42.6 ± 2.8) × 103 (in units of M atm−1) for metolachlor and (3.0 ± 0.3) × 103 for diazinon. The obtained data were used to derive the following Arrhenius expressions: HLC=(3.0 ± 0.4) × 10−11 exp((10 200±1000)/T) for metolachlor and (7.2 ± 0.5) × 10−15 exp((11 900±700)/T) for diazinon. At a cumulus cloud temperature of 283 K, the fractions of metolachlor and diazinon in the atmospheric aqueous phase are about 57% and 11% respectively. In order to evaluate the impact of a cloud on the atmospheric chemistry of both studied pesticides, we compare also their atmospheric lifetimes under clear sky (τgas), and cloudy conditions (τmultiphase). The calculated multiphase lifetimes (in units of hours) are significantly lower than those in gas phase at a cumulus temperature of 283 K (in parentheses): metolachlor, 0.4 (2.9); diazinon, 1.9 (5.0).