Study On Forest Filter Effect Of Persistant Organic Pollutants

Persistent organic pollutants(POPs) are semi-volatile pollutants recalcitrant to degradation and with high affinity for organic carbon. They are generated by human beings and emitted to natural environment. POPs have potential to long-range transport and are finally globally distributed. With the goal of reducing primary emissions of “classical” POPs, re-emission from old capacitors may become the main sources of POPs. Besides, the transition from primary to secondary source may be triggled by global climate change. Prediction of future POP exposure scenarios is therefore necessary, and will improve our understanding of POP environmental distribution.“Global distillation model” is a classical model in predicting the environmental fate of POPs, and has been widely agreed. However, recent publications have questioned the model and believe that the fate of POPs will also be significantly influenced by atmospheric circulation, decomposition and biovector transport. The role of forests in filtering airborne organic pollutants from the atmosphere and transferring them to soil has been termed the “forest filter effect(FFE)”. While the majority of studies on FFE are model work, and field observations are still limited. Considering the influence of Indian moonson and largely forested areas with abundant precipitation, Southern Tibetan forested areas are well suited for examing the “FFE” of POPs.Hence, the first part of our study was conducted on the Hailuogou background area, eastern slope of Mt. Gongga to compare the contribution of “FFE” and high mountain “cold trapping effect(CTE)” on altitudinal distribution of PCBs. We measured concentrations and compositions of PCBs in different environmental matrices from nine elevations. Results showed that atmospheric transport and soil organic carbon(SOC) are two main influence factors, and SOC was a key determinant explaining 75% of the variation in concentration of PCBs along the altitudinal gradient. Based on principle components analysis and multiple linear regression, the average contribution of the FFE(65%) was greater than that of the CTE(35%). Our results deviate from the global distillation theory involving cold condensation at high altitudes of mountain areas and highlight the importance of FFE.The influence of FFE may greatly differ on different types of POPs. In the second part of our study, we investigated the occurrence and distributions of halogenated flame retardants(HFRs) in air and soils on the eastern slope of Mt. Gongga. We evaluated possible sources of HFRs and dominant foctors influencing their altitudinal distributions based on the compound ratio in the technical products and air mass back trajectory analysis. Results showed that those very low volatile compounds, i.e. BDE209, DBDPE, TBB, TBE and DP were detected in such a remote area. DBDPE was detected at relatively high levels up to 171 pg/m3 and 1450 pg/g dry weight in air and soil, respectively. NBFR is the prevalent group of HFRs mainly due to the highest abundance of DBDPE. A general decreasing trend was observed among the HFR concentrations with increasing altitude, and the atmospheric travel distance of PBDE congeners followed the order: BDE47 > BDE99 > BDE209. DBDPE appeared to be easily degraded in the environmental beacuse its data was too sparse even though it was detected at high level. The O-horizon which is formed from vegetation and contains high amounts of soil organic matter accounted for the majority of HFRs, while the concentrations were the highest in C-horizon in the alpine meadow soils. HFRs were likely to be transported with runoff to the piedmont below the timberline particularly during the snow-free season. The altitudinal distribution of HFRs was mainly affected by atmospheric transport and distance of source emissions rather than by environmental conditions(i.e. temperature and soil organic carbon content).The FFE has close relations with vegetation coverage and soil organic carbon. However, because of global climate change and human disturbance, changes in land use/coverage are developing rapidly, thus greatly changing the vegetation coverage and the stock of organic carbon. The planted forest area of China ranks first in the world, and the transformation from natural forests to planted forests in tropical region is an expanding global phenomenon. The loss of soil organic carbon which is caused by soil erosion may result in re-emission of “old” POPs. Therefore, in the third part of our study, we investigated accumulation of POPs in soils under eucalyptus and different ages of rubber forests(5, 15, and 25 years old) as compared with adjacent natural forests in Lanyang State Farm, which is located in Danzhou City, Hainan Island. We also conduceted correlation analysis between soil properties(pH, soil organic carbon, ratio of organic carbon to total nitrogen) and POP concentrations in order to clarify the influence of planted forests on re-transfer of POPs. Results showed that the natural forests generally accumulated more amounts of POPs, and the accumulation increased with increasing stand age of rubber trees. Based on correlation and air-soil equilibrium analysis, we therefore assumed that the elevated mobility especially for more volatile congeners in the planted forests was caused by greater loss of soil organic carbon and leaching exceeding volatilization compared to natural forests. The study highlights the importance of soil organic carbon in POP distributions again.In summary, the results of our study will improve our understanding of POP distribution and global environmental fate.