Seasonality of Air−Water Fluxes of Hexachlorocyclohexanes in Lake Ontario

We investigated the seasonality of the air−water gas transfer of hexachlorocyclohexanes (HCH) in Lake Ontario. These organochlorine pesticides were measured in air and water samples collected on weekly cruises on Lake Ontario from May to October 1993. Air concentrations ranged from 51 to 200 pg m-3 for α-HCH and from 8 to 133 pg m-3 for γ-HCH. Surface water concentrations of α-HCH showed a small seasonal variation:  mean values decreased from 922 ± 73 pg L-1 in May to 679 ± 47 pg L-1 in August and gradually increased to 897 ± 76 pg L-1 by October. Levels of γ-HCH did not show seasonal differences and averaged 357 ± 25 in surface waters. Air−water fugacity gradients of α- and γ-HCH were used to predict the direction of air−water gas exchange of these isomers. Reversals in exchange direction were observed for both compounds on time scales of days, weeks, and months. On average, air−water gas transfer of HCH was depositional in May and early June, reversed to volatilizational by August, and returned to depos...