Atmospheric transport of persistent organic pollutants to aquatic ecosystems

Abstract: The load of persistent organic pollutants (POPs) is considered high in the Baltic ecosystem. The Baltic Sea spans over 12 latitudes and the regional differences in climate affect the behavior of POPs. Therefore spatial and temporal variability of the concentrations of POPs in air and precipitation within this area has been investgated at 16 (mostly rural) stations around the Baltic Sea between 1990-1993. In addition, the deposition of gaseous and particulate associated POPs to the Baltic Sea is estimated from empirical data. This atmospheric input of POPs is compared with the input from rivers. Additionally, data from Ross Island, Antarctica and Lake Kariba, Zimbabwe, Africa is presented, and all results are discussed and explained using the ”global fractionation hypothesis” as a framework. In the Baltic Sea, concentrations of individual POPs in air were found to be influenced by their physical-chemical properties, ambient air temperature and location. A latitudinal gradient, with higher levels in the south was found for PCBs and the gradient was more pronounced for the low volatility congeners. As a result, the high volatility congeners in air increased in relative importance with latitude. Generally, PCB concentrations increased with temperature, but slopes of the partial pressure in air versus reciprocal temperature were different between congeners and between stations. In general, the low volatility congeners were more temperature dependent than the high volatility PCB congeners. Steep slopes at a sampling location indicate that the concentration in air is largely determined by diffusive exchange with soils. Lack of a temperature dependence may be due to the influence of long-range transported air masses at remote sites and due to the episodic, or random nature of PCB sources at urban sites. The concentrations of individual congeners in precipitation were found to be influenced by atmospheric concentrations of PCBs, ambient temperature, precipitation volume and the compounds physical-chemical properties. Concentrations of PCBs in precipitation decreased with temperature, and the slopes of this relationship were the same for all stations but differed between congeners. High volatility congeners showed steeper slopes for the temperature relationship than did low volatility PCB congeners. Annual wash out ratios were calculated and found to decrease with congener volatility. Wash out ratios generally decreased with temperature. At snow scavenging events the wash out ratio of PCBs increased with a factor of 2. Latitudinal trends for PCB concentrations in precipitation and for deposition were not statistically significant, but tended to be higher in the south for low volatility congeners. For high volatility congeners the latitudinal trend was reversed. Deposition of PCB congeners varied seasonally, with a factor between 2.0-3.2, and was generally highest during fall. The calculated yearly deposition of PCBs to the Baltic Sea was 387 kg and 5-18 kg for individual congeners, with PCB-138 having the highest flux.