Air/sea gas exchange of PCBs in the southern Baltic Sea

Air/sea gas exchange is an important process for the environmental fate of many semivolatile organic compounds (SOCs). The ability to quantify this flux can be very useful, e.g. for understanding the role of seas as a sink or a source for SOCs. In this work the air/sea gas exchange of polychlorinated biphenyls (PCBs) was investigated in the Baltic Sea, a particularly contaminated marine ecosystem. Air and water samples were collected during three cruises in the southern Baltic Sea and analysed for eight PCB congeners. Water/air fugacity ratios were calculated using literature values for the air/water partition coefficients. A sensitivity analysis indicated that it was not possible to identify the direction of net air/sea exchange of gaseous PCBs with 95% certainty due to the uncertainty in the air/water partition coefficients. Even when this uncertainty was reduced by using a recent set of internally consistent physical–chemical properties of PCBs, the direction of the net flux was only ascertainable for PCB 28/31. The uncertainty in the fugacity ratio rendered a quantification of the air/sea exchange flux impossible. Nevertheless, the observation that the PCBs in the air/sea system were relatively close to a partitioning equilibrium was useful in identifying discrepancies and weaknesses in existing estimates of the PCB mass balance for the Baltic Sea. Furthermore, the observation of lower water/air fugacity ratios for the more lipophilic PCB congeners suggests that vertical export with particulate organic material can cause a fugacity deficit of lipophilic substances in surface water of the Baltic Sea.