Atmospheric transport and air-surface exchange of pesticides

Atmospheric transport and exchange of pesticides with soil, vegetation, wat er and atmospheric particles are discussed, with an emphasis on applying ph ysicochemical properties of the compound to describe environmental partitio ning. The octanol-air partition coefficient is promoted as a unifying prope rty for describing volatilization of pesticides from soil and sorption to a erosols. Present-day sources of organochlorine (OC) pesticides to the atmos phere are continued usage in certain countries and volatilization from cont aminated soils where they were used in the past. Models are available to pr edict volatilization from soil; however, their implementation is hampered b y lack of soil residue data on a regional scale. The need to differentiate "new" and "old" sources is increasing, as countries negotiate international controls on persistent organic pollutants (POPs). A new technique, based o n the analysis of individual pesticide enantiomers, is proposed to follow e mission of chiral OC pesticides from soil and water. Air monitoring program s in the Arctic show the ubiquitous presence of OC pesticides, PCBs and oth er POPs, and recently a few "modern" pesticides have been identified in fog and surface seawater. Atmospheric loadings of POPs to oceans and large lak es take place mainly by air-water gas exchange. In the case of OC pesticide s and PCBs, aquatic systems are often near air-water equilibrium or even ov ersaturated. Measurement of water/air fugacity ratios suggests revolatiliza tion of PCBs and several OC pesticides in the Great Lakes and, for alpha-he xachloroacyclohexane (alpha-HCH), in the Arctic Ocean. Outgassing of alpha- HCH in large lakes and arctic waters has been confirmed by enantiomeric tra cer studies. The potential for pesticides to be atmospherically transported depends on their ability to be mobilized into air and the removal processe s that take place enroute: wet and dry deposition of gases and particles an d chemical reactions in the atmosphere. Measurement of reaction rate consta nts for pesticides in the gas and particle phase at a range of environmenta l temperatures is a critical research need. The transport distance of a che mical is related to its overall environmental persistence, determined by th e partitioning among different compartments (water, sediment, soil, air), d egradation rates in each compartment and mode of emission (into water, soil , air). Several pesticides found in the arctic environment have predicted l ifetimes in the gas phase of only a few days in temperate climates, pointin g out the need for monitoring and evaluation of persistence in cold regions .