【作者】 马明；

【导师】 王定勇；

【作者基本信息】 西南大学 ， 农业环境保护， 2013， 博士

【摘要】 汞是一种强毒性的重金属元素,在环境中容易通过食物链富集而对人类健康造成极大危害。汞在大气中主要以Hg0形态存在,其含量可占大气总汞90%以上。因此,对于大气汞的来源研究是掌握汞的全球生物地球化学循环的关键。陆生生态系统是汞生物地球化学循环的主要场所,而森林生态系统是其中最大的生态系统,且常被认为是汞的活性库。森林土壤是森林生态系统贮存汞的主要场所,每年可以通过扩散作用向大气排放大量的汞,同时,又经大气干湿沉降过程而将汞输入到土壤中。因此,汞在森林生态系统中的环境行为是汞全球循环的重要组成部分。西南地区是我国森林覆盖率第二大区,其中亚热带森林面积占到四分之三,是西南地区最具代表性的森林生态系统。针对不同生态系统可能发生的汞污染问题,前人已做了大量研究工作,但有关亚热带森林生态系统汞的输入、输出的研究还很匮乏。因此,本文选取具有典型中亚热带森林生态系统的缙云山国家自然保护区和四面山风景名胜区为研究区域,以中亚热带分布最为广泛的针阔混交林、落叶阔叶林等为研究对象,采取野外实地监测的方式,研究典型中亚热带森林生态系统汞的特征,探究森林土壤-大气界面汞交换机制及影响因素,其成果对于揭示不同气候条件下森林与大气间汞交换的规律,估算全球自然汞排放量,大气汞污染防治,研究汞的生物地球化学循环,汞的环境风险预测等有非常重要的意义。结果表明,整个研究期间,缙云山大气气态总汞浓度夏季最高,其次为冬季和春季,秋季最低。缙云山一年四季的气态总汞含量均相对稳定,波动较小。夏季的最大小时平均浓度为4.99ng·m-3,表明缙云山夏季受到更多的人为活动影响。四面山大气气态总汞浓度冬季最高,其次为夏季和春季,秋季最低。冬季的最大小时平均浓度达到3.68ng·m-3,高于春、夏、秋三季。缙云山的气态总汞浓度仅低于贵阳、北京、广州和长三角地区的监测结果,高于上海、韩国首尔、美国芝加哥以及长白山、雷公山、贡嘎山及日本冲绳、美国Great Mountain Forest和美国纽约州阿迪朗达克山脉背景点的监测结果。四面山的气态总汞浓度低于贵阳、北京、广州、长三角地区和贡嘎山的监测结果,高于上海、韩国首尔、美国芝加哥,以及长白山、雷公山、日本冲绳、美国Great Mountain Forest和美国纽约州阿迪朗达克山脉背景点的监测结果。缙云山自然保护区和四面山风景名胜区区,大气汞浓度与气温显著正相关,与光照也是显著正相关；两个采样点大气汞浓度与相对湿度成显著正相关。缙云山和四面山72小时后向轨迹聚类分析确定重庆、贵州、四川、中国中东部地区为可能的汞来源区。分析得到：缙云山和四面山2012年均为四种类型的气团,其中,均为cluster3所占比例最大(缙云山52%和四面山54%)。cluster3与缙云山和四面山大气气态总汞浓度最高值相关(缙云山4.99ng·m-3四面山3.68ng·m-3)。Cluster3气团主要是重庆主城地区这一汞污染最严重区域之一。林地土壤与大气间的汞交换通量表现为双向性,且总体表现以土壤汞释放为主。通过观察四种不同林地不同季节汞交换通量的变化趋势特征发现：(1)所有林分的汞交换通量均在早上6：00开始增加,最大值均出现在正午时分,最小值均出现在午夜时分；(2)所有林分的汞交换通量均为白天高于夜间；(3)四种林分春夏秋冬四季在一天中均表现为土壤向大气释汞,仅冬季的个别时间发生了汞沉降而出现负值：其中缙云山楠竹林在冬季午夜时分表现出最大的汞沉降量,为-4.34ng·(m2·h)-1;(4)据四季汞通量统计结果可知,四种不同林地均在夏季土/气界面释汞通量值较高,缙云山针阔混交林、缙云山灌木林、缙云山楠竹林和四面山落叶阔叶林的平均通量分别达到49.06±12.52ng-(m2-h)-1、67.52±15.49ng·(m2-h)-1、58.34±16.03ng·(m2-h)-1和46.73±12.64ng·(m2·h)-1。(5)缙云山灌木林夏季汞交换通量在一天中变化幅度最大,趋势最明显,最大值达67.52ng·(m2·h)-1,最小值为7.10ng·(m2·h)-1。(6)四种林地均在冬季土/气界面汞交换通量值达到最低,平均通量仅为-3.36ng-(m2·h)-1。四种林地冬季汞通量在一天中的变化幅度较小,趋势较弱,其中汞交换通量的最大值和最小值均出现在缙云山楠竹林,最大值为28.7ng·(m2·h)-1,最小值为-4.34ng-(m2-h)-1.缙云山针阔混交林、灌木林、楠竹林和四面山落叶阔叶林四种林分汞交换通量与环境因子的相关性趋势相似,均表现为与气温、土温和光照强度呈现出正相关关系,与相对湿度呈现出负相关关系。缙云山针阔混交林和四面山落叶阔叶林林外降雨总汞的VMW浓度分别为23.40ng·L-1和24.89ng·L-1,林内降雨总汞的平均体积加权浓度VMW浓度分别为32.13ng·L-1和35.64ng·L-1;缙云山针阔混交林和四面山落叶阔叶林林外降雨甲基汞的平均体积加权浓度VMW分别为0.32ng·L-1、0.29ng·L-1,林内降雨甲基汞浓度分别为0.29ng·L-1、0.43ng·L-1o研究期间总汞变化趋势与降雨量的趋势相反,当降雨量多时,总汞浓度较低,当降雨量少的时候,总汞浓度相对较高。降水中甲基汞的浓度随降雨量变化情况,除了个别雨水样中,甲基汞浓度变化波动不大；两个监测点林内VMW浓度均高于林外降雨,林内降雨中的汞浓度范围为5.4～125.52ng·L-1;缙云山监测点林外降雨汞浓度的体积加权平均值为23.40ng·L-1,林内降雨汞浓度的体积加权平均值为32.12ng·L-1:四面山林外降雨汞浓度的体积加权平均值为24.89ng·L-1,林内降雨汞浓度的体积加权平均值为35.64ng·L-1.缙云山自然保护区林外降雨12个月内总汞平均湿沉降量为23.40μg·m-2;甲基汞平均湿沉降量为0.32μg·m-2.四面山风景名胜区林外降雨12个月内总汞平均湿沉降量为24.89μg·m-2;甲基汞平均湿沉降量为0.32μg·m-2。监测期间,12月的总汞和甲基汞沉降量大于美国大部分地区(除了美国北卡罗莱纳州)的年湿沉降量,但是小于中国北京、长春的总汞年湿沉降量。监测期间,缙云山针阔混交林和四面山落叶阔叶林两种林分林内降雨中的累积总汞通量值为48.19和53.46μg·m-2·yr-1,两种林分林外降雨中的累积总汞通量值为38.03和38.44μg·m-2·yr-1,比林外降雨中的累积汞通量值分别高了21%和28%。研究期间,落叶阔叶林和针叶林树叶枯落物中的总汞浓度和质量均呈现增加的趋势。落叶阔叶林中汞的平均浓度从初始值46.3ng·g-1增加到57.88ng·g-1针阔混交林枯落物中汞的平均浓度从初始值42.40ng·g-1增加到50.88ng·g-1年之后,树叶枯落物中的总汞平均含量增加到其在落叶阔叶林枯落物中初始质量的125%,针叶林枯落物初始质量的120%。在分解过程中甲基汞的变化幅度较小,但较初始甲基汞含量来讲都呈现出了略微增加的趋势。根据指数衰减模型,枯落物质量的半衰期估算为：落叶阔叶林3.1年和针叶林3.7年,这与前人研究的结果是相当的。总体来说,汞质量增加,而枯落物质量会下降。研究期间,在树木生长季节汞不断累积,而枯落物质量却以一个相对一致的速率下降。分段回归模型分析表明汞的累积具有季节差异,在休眠季会出现不显著的下降,在成长季节出现清晰地增加,表明汞季节性累积是一种连续的现象,落叶阔叶林和针叶林层的成长季节均会出现。更多还原

【Abstract】 Mercury (Hg) is a hazardous heavy metal pollutant, which is easily to be bioaccumulated through food chain, leading considerous hazard to human health. Mercury emitted to the atmosphere from natural and anthropogenic sources exists mainly as gaseous elemental mercury (GEM, Hg0), accounting for over90%of the available ambient total gaseous mercury (TGM). The atmospheric residence time of GEM is as long as one year, so Hg0is involved in long range transport through atmospheric circulation, traveling far from the emission sources, being regarded as a global pollutant.Therefore, researches on the sources of gaseous mercury are key to understanding the biogeochemical cycle of mercury. Terrestrial ecological system is the main place for biogeochemical cycle of mercury, whereas forest ecosystem is the largest one and always be seen as an active sink of mercury. Forest soil is the main place for mercury accumulation, emissing a large amount of mercury to the atmosphere every year through dissusion effect. Meanwhile, mercury input to the forest soil includes dry and wet deposition. Therefore, mercury inputs, outputs, cycles and sources in the forest ecosystem are the important parts of the biogeochemical cycle of mercury.Southwest has the second largest forest coverage rate in China, with subtropical forest area accounting for three fourth, being the most representative forest ecosystem in China. Considerable work had been done on possible mercury pollution in different ecosystems previously; however, studies on mercury inputs, outputs of subtropical forest ecosystem are reported far less. In this research, national nature reserves of Mts. Jinyun and Simian were selected as study areas, with coniferous and deciduous forests selected as forest types. The characteristics of mercury of typical subtropical forest ecosystem was studied by field monitoring to investigate soil/air mecury exchange mechanism and influcing factors, results of which has significant meanings in elucidating forest soil/air mecury exchange laws under different atomospheres, evaluating global emission of natural mercury, protecting mercury pollution, researching on biogeochemical cycling of mercury, as well as forcasting environmental risks of mercury.Results of this research indicated that the highest TGM concentration of Mt. Jinyun was observed in summer, followed by winter and spring, and the lowest was in fall. TGM concentrations of Mt. Jinyun in the whole year were relatively stable, with little fluctuation. The highest average concentration of Mt. Jinyun in summer was4.99ng·m-3, which indicated that it was affected more by human activites in summer. As for Mt. Simian, the highest TGM concentration was observed in winter, followed by summer and spring, and the lowest was also in fall.The largest average concentration of Mt. Simian in winter was3.68ng·m-3. TGM concentrations of Mt. Jinyun were only lower than those in Guiyang, Beijing, Guangzhou, and Yangtze Delta, and higher than those in Shanghai, Seoul (Korean), Chicago(the United States), Mts. Changbai, Leigong, Gongga, Okinawan (Japan), Great Mountain Forest(USA), Adirondacks (New York, USA). TGM concentrations of Mt. Simian were lower than those in Guiyang, Beijing, Guanghzou, Yangtze Delta and Mt. Gongga, higher than Shanghai, Seoul (Korean), Chicago (the United States), Mts. Changbai, Leigong, Gongga, Okinawan (Japan), Great Mountain Forest(the United States), Adirondacks (New York, USA). In addition, TGM concentrations of Mts. Jinyun and Simian were both positively correlated with meteorological parameters, with obviously seasonal difference. TGM concentrations of Mts. Jinyun and Simian had remarkably positive correlation with atomospheric temperature, solar radiation, and relative humidity, but no significant correlations were found in air pressure.Cluster analysis results of TGM of Mts. Jinyun and Simian with72h back-trajectories by Hysplit identified Chongqing, Guizhou, Sichuan and Mideast areas of China as likely Hg sources. And there were both four types of72h trajectories arriving at Mts. Jinyun and Simian from March2012to Febrary2013, with cluster3making up for52%(Mt. Jinyun) and54%(Mt. Simian) in the four types of air masses. Cluster3correlated with the highest value of TGM concentrations, with4.99ng·m-3and3.68ng·m-3for Mts. Jinyun and Simian respectively. Cluster3air mass was from urban areas of Chongqing which had the most serious mercury pollution.The mercury exchange flux between forest soil and atmosphere was bidirectional, with soil mercury release as the main type. The variation of mercury exchange flux observed in four different forest floors in different seasons found that:(1) the mercury exchange flux of all the forest floors started to increase at6:00am in the morning, with the maximum value appearing at noon and the minimum values appearing at midnight;(2) the mercury exchange flux of the four forest floors were all higher by day than those by night;(3) the soil mercury released into the atmosphere was observed almost everyday of the four seasons in the four kinds of forest floors, with only individual winter time occouring mercury deposition (negative values);the bamboo forest of Mt. Jinyun in the winter midnight showed maximum mercury deposition,-4.34ng-(m2·h)-1.(4) According to the statistical results of mercury flux in the soil/air interface of the four seasons, the mercury exchange flux of the four forests were all highest in the summer, with averages of49.06±12.52,67.52±15.49,58.34±16.03and46.73±12.64ng-(m2·h)-1for coniferous and broad-leaved mixed forest, shrub forest, and bamboo forest of Mt. Jinyun, as well as coniferous and broad-leaved mixed forest of Mt. Simian.(5) the fluctuations of mercury exchange flux of shrub forest of Mt. Jinyun reached highest in the summer, having the most obvious trend. The maximum and minimum values were67.52ng·(m2·h)-1and7.10ng·(m2·h)-1individually.(6) soil/gas mercury exchange flux reached the lowest value in the winter for the four forest floors, with average flux of only-3.36ng·(m2·h)-1. The fluctuations of mercury exchange flux of the four forest floors were all smallest in one single day, having the weakest trend. Maximum and minimum values of mercury exchange flux both occurred in the shrub forest of Mt. Jinyun,67.52ng (m2·h)-1and7.10ng-(m2-h)-1individually. The conifer, shrubs, bamboo forest of Mt. Jinyun and the conifer of Mt. Simian had similar relationship with environmental factors, with positive relationship with air temperature, soil temperature and solar radiation, and negative relationship with relative humidity.The VMW concentrations of total mercury in the precipitation of coniferous and broad-leaved mixed forests of Mt.Jinyun and deciduous forests of Mt. Simian were23.40ng·L-1and24.89ng·L-1, and average VMW concentrations of total mercury in the throughfall were32.13ng·L-1and35.64ng·L-1. For the concentrations of MeHg, the VMW concentrations of MeHg in the precipitation and throughfall of coniferous and broad-leaved mixed forests of Mt.Jinyun and deciduous forests of Mt. Simian were0.32ng·L-1and0.29ng·L-1(precipitation) and0.29ng·L-1and0.43ng·L-1(throughfall). During the monitoring period, the total mercury concentration in precipitation was significantly higher than those in the rural and urban of Europe, the United States, as well as Japan, being slightly lower than those in Guiyang and Wujiang, being obviously lower than those in other cities in China, such as Beijing and Changchun. MeHg content was about7times of that in Carolina, the United States, and over twice of that in several areas of the United States. The trend of total mercury reversed with that of rainfall during the study period. When rainfall for a long time, the total mercury concentration was low, whereas when rainfall was less, the total mercury concentration was relatively high. The concentration of MeHg in precipitation varied with rainfall, with little fluctuations except individual samples. VMW concentrations in the two monitoring sites were both higher than that in precipitation. Ranges for the the mercury concentration in the throughfall was5.4-125.52ng·L-1. The volume weighted value of precipitation in Mt. Jinyun was23.40ng·L-1, and32.12ng·L-1for throughfall. For Mt. Simian, the volume weighted value of precipitation in Mt. Simian was24.89ng·L-1, and35.64ng-L’1for throughfall. The average wet deposition of total mercury in the precipitation within the12months in Mt. Jinyun was23.40μg·m-2, and0.32μg·m-2for MeHg. For Mt. Simian, the average wet deposition of total mercury in the precipitation within the12months in Mt. Jinyun was24.89μg·m-2, and0.32μg·m-2for MeHg.During the monitoring period,12-month wet deposition of total mercury and MeHg were larger than those in most areas of the USA (except North Carolina), but were lower than those in Beijing and Changchun, China. During the monitoring period, the cumulative total mercury flux in throughfall in the coniferous and broad-leaved mixed forests of Mt.Jinyun and deciduous forests of Mt. Simian were48.19and53.46μg m-2·yr-1repectively. While for precipitation, the accumulative total mercury flux in the coniferous and broad-leaved mixed forests of Mt.Jinyun and deciduous forests of Mt. Simian were38.03and38.44μg m-2·yr-1, which were higher than those in precipitation by21%and28%respectively.During the study period, both the concentration and mass of total mercury in leaf litter of deciduous and coniferous and broad-leaved mixed forests increased. The mean concentration of mercury in deciduous forests increased from46.3ng·g-1to57.88ng·g-1. And for coniferous and broad-leaved mixed forests, it increased from42.40ng·g-1to50.88ng·g-1. One year later, the average concentration of total mercury in the leaf litter of deciduous as wellas coniferous and broad-leaved mixed forests increased to125%and120%respectively. During the process of decomposition, the concentration of MeHg varied slightly, but comparing with the initial values, the concentration of MeHg increased slightly. According to the exponential attenuation model, the half-life of the mass of litter estimated as3.1and3.7years for deciduous as well as coniferous and broad-leaved mixed forests individually,which were similar with previous research results. In general, the mercury mass increased, but the mass of litter declined. During the study period, growing season mercury content accumulated gradually, while the mass of litter dropped by a relatively consistent rate. Piecewise regression model analysis showed that the accumulation of mercury varied with the season, with not significant decrease in non-growing season and clear increase in growing seasons. This indicated that seasonal accumulation of mercury was continuous, occouring in both deciduous and coniferous and broad-leaved mixed forests.更多还原