森林地区持久性有机污染物的沉降和释放

Deposition and release of persistent organic pollutants in forests

持久性有机污染物(POPs)是一类具有毒性、难降解性和生物富集性的污染物。由于具有半挥发性,POPs可以随大气进行长距离传输并富集到极地等寒冷的偏远地区。大气沉降和挥发是POPs全球传输的关键过程。森林是POPs的重要储库,其POPs的强吸收能力加速了POPs的沉降,使得森林成为POPs大气沉降研究的重要介质。本文综述了当前森林POPs沉降研究的主要成果,重点阐述了大气POPs向植被(树冠)沉降和植被POPs向林下土壤迁移沉降这两大关键过程的特征和机制,并简要总结了林下POPs的后沉降过程(循环及再释放)和机制。最后,提出森林POPs沉降研究应着眼于观测技术开发、微观机制解析、关键规律总结、释放特征观测和多污染物耦合等方向,进而拓展大气污染物干湿沉降研究范围、丰富大气污染物干湿沉降研究的理论和方法,力争为气候变化背景下污染防治提供科学依据和理论支撑。

Background,aim,and scope Persistent organic pollutants(POPs)are a class of pollutants with high toxicity,low degradation,and bioaccumulation.Due to their semi-volatilities and persistence,POPs can undergo long-range atmospheric transport to cold/remote areas(e.g.,the Polar regions).Atmospheric deposition and volatilization are the key processes during the global transport of POPs.Forests have high uptake capacity of atmospheric POPs,thus is the important reservoir of POPs.As the key medium for atmospheric deposition of POPs,the forests can accelerate the deposition of POPs from air to ground.The aim of this review is to report the recent advances in the deposition and release of POPs in forests.Materials and methods Previous studies on the deposition of POPs in forests were collected and reviewed,focusing on the patterns and mechanisms of deposition both from air to foliage and from foliage to the ground under canopy.The post-deposition processes of POPs were also summarized briefly.Result(1)Leaves absorb gas-phase POPs via waxes and stomata,while barks absorb both gas-phase and particle-phase POPs via lenticels.Besides,mosses and lichens under canopy can uptake the POPs in wet deposition.(2)POPs in leaves,barks and understory vegetation can be further deposited to the ground surface via litterfall and throughfall.While,the soil under canopy can also absorb gaseous POPs directly.(3)The losses of POPs occur in post-deposition processes.Low molecular-weight(or high water-solubility)POPs can release from soil through infiltration and leaching.Moreover,in the context of climate change,forest fires could be a new and important pathway to release POPs stored in forests.Discussion(1)Foliage mainly uptakes POPs from atmospheric dry deposition.The influencing factors include physicochemical properties,and atmospheric concentrations of POPs,as well as traits of leaf/bark/moss/lichen,and environmental/climatic conditions.However,there are some shortcomings in previous studies.For example,the mechanism of deposition of POPs from organ-/molecular-scales to plants is not clear.In addition,there is a lack of high-precision online observations of cycling process of POPs in forests.The summary of the distribution pattern of POPs in different environmental media of forests is missing.(2)Compared to the strong deposition and storage of POPs in forests,the losses of POPs from forests in three ways were relatively small,including the release/re-evaporation from vegetation,degradation,and leaching from soil.Considering the climate change,it is worth pointing out that the release of POPs from forest fires is likely to become a“secondary source”.Conclusions Although the processes of atmospheric POPs deposition to plants were studied thoroughly,the mechanisms of the deposition on organ-/molecular-scales are uncertain.Recommendations and perspectives Future researches on POPs deposition in forests should focus on the development of observation technologies,and deposition mechanisms on fine scales.The observations of POPs release from forests,and the co-occurrences of pollutants will expand the research field and theories of dry/wet deposition of atmospheric pollutants.