Temperature dependence of Henry’s law constants of metolachlor and diazinon Val  erie Feigenbrugel, St  ephane Le Calv e * , Philippe Mirabel Centre de G eochimie de la Surface/CNRS and Universit e Louis Pasteur, 1 rue Blessig, F-67084 Strasbourg Cedex, France Received 4 September 2003; received in revised form 22 April 2004; accepted 10 May 2004 Abstract 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 Henry’s 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) · 10 3 (in units of M atm 1 ) for metolachlor and (3.0 ± 0.3) · 10 3 for diazinon. The obtained data were used to derive the following Arrhenius expressions: HLC ¼ (3.0±0.4) · 10 11 expðð10200  1000Þ=T Þ for metolachlor and (7.2 ± 0.5) · 10 15 expðð11900  700Þ=T Þ 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 (s gas ), and cloudy conditions (s multiphase ). 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). Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Pesticides; Metolachlor; Diazinon; Henry’s law constants 1. Introduction Chloroacetanilides such as metolachlor are used in agriculture since 1952 as pre-emergence herbicides to control annual grass weeds and broadleaf weeds in corn, cotton, peanuts, soybeans and beans. Estimated United States annual use of metolachlor according to Chesters et al. (1989) was approximately 22 · 10 6 kg. Organophosphorus compounds, like diazinon that was formulated in the 1950s (Bruce et al., 1955), are also used as pesticides in agriculture where they have re- placed organochlorine compounds because of the per- sistence and accumulation of the latter in the environment. Organophosphorus compounds do not accumulate in organisms and they degrade easily in the environment. However, organophosphate pesticides are of great concern for the environment primarily because they are more toxic to mammals and birds than the chlorinated hydrocarbon pesticides (Ku et al., 1998). They inhibit acetylcholinesterase activity, not only in insects but can also affect the nervous system of humans (Chambers, 1992; Racke, 1992). Contamination of surface water by pesticides use is mainly due to runoff, usually within a few weeks after application. Once in an aqueous environment, a pesticide is subject to volatilization, hydrolysis, or photolysis. Metolachlor possesses a long hydrolysis half-life (Chiron et al., 1995) while its photolysis seems to be an important abiotic degradation pathway in an aqueous environment. * Corresponding author. Tel.: +33-3-90-24-03-68; fax: +33-3- 90-24-04-02. E-mail address: slecalve@illite.u-strasbg.fr (S. Le Calv e). 0045-6535/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2004.05.013 Chemosphere 57 (2004) 319–327 www.elsevier.com/locate/chemosphere