Air-water exchange of semivolatile organic chemicals in the Great Lakes

S J Eisenreich; K C Hornbuckle; D R Achman

Air-water or air-sea exchange describes the processes (addition to, transfer across, and emission from the interface) between the local atmosphere and water. Thus, it describes atmospheric deposition of particles and gases to the interface from rain and snow scavenging, dry deposition of particles, the exchange of gases and particles at the air-water interface, the sorption of gases at the interface, and the emission of particles and gases into the atmosphere. This chapter will concentrate on the exchange of gases at the air-water interface emphasizing the semivolatile organic chemicals (SOCs) such as the chlorinated hydrocarbons (polychlorinated biphenyls [PCBs]; hexachlorocyclohexanes [HCHs]) and the combustion-derived polycyclic aromatic hydrocarbons (PAHs). SOCs are characterized by subcooled liquid vapor pressures (P-L degrees) less than 10(-4) atm (similar to 10 Pa) and aqueous solubilities less than 10(-4) moles/L. These two physical-chemical parameters imply that SOCs are distributed between the gas and particle phase in the urban and rural atmosphere [1,2] and the dissolved and particle phase in the water column [3,4]. Little attention has been paid to the role of air-water exchange of SOCs in lakes, estuaries, and oceans of planet Earth. Whereas the transfer of low molecular weight, volatile, and biologically mediated gases such as O-2, CO2, CH4, H2S, DMS and 1 and 2 carbon halocarbons has received considerable attention for water quality, carbon cycling, global warming, and other reasons [5], the SOCs such as PCBs, DDTs, HCHs, PAHs, and toxaphene have been largely ignored until recently (see [6-13]). These compounds are derived from major anthropogenic emissions in urban/industrial centers and world-wide agriculture, transported globally, and often concentrated in the northern hemispheric atmosphere from 20 degrees N to 45 degrees N [14,15]. Recent studies suggest that air-water exchange of SOCs plays an important role in the mass balancing of inputs in large aquatic systems such as the Great Lakes and the world's oceans [6,8-10,12,13,16-22]. For example, the calculated volatilization flux of PCBs out of the North American Great Lakes [18,21-24] has been estimated to be comparable to sedimentation losses and may, in fact, be substantially greater in low sedimentation ecosystems such as Lake Superior [21]. There must be an appreciable cycling of SOCs between air and water with the magnitude and direction of net transfer changing with season and even with wind direction and speed. There is now convincing evidence that concentrations of SOCs and their residence times in large lakes are influenced, if not controlled, by air-water exchange [21,22,25,26]. Air-water exchange is also recognized as a dominant process for SOC input to the world's oceans [8,10,13,27]. The direction of gas transfer is not obvious unless the air and water concentrations of the SOCs are measured or accurately predicted and Henry's Law equilibrium is established, for it is the departure from equilibrium that drives the processes of gas volatilization and absorption.

Air-water exchange of semivolatile organic chemicals in the Great Lakes

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