摘要
城市土壤Hg异常/污染是中国普遍存在的重大生态环境问题。文章对北京市近1000km^2范围内的地表土壤、壤中气、大气干湿沉降，大气颗粒物、大气中的Hg含量水平和空间分布模式进行了系统研究，查明北京地表土壤Hg平均含量为0．41mg／kg，大气干湿沉降物中的Hg平均含量为0．194mg／kg，壤中气Hg的平均含量为559．65ng／m^3，大气颗粒物PM10和PM2.5中的Hg含量分别为0．59和0．67ng／m^3，大气中的Hg平均含量为3．13ng／m^3。北京市自2000年起实现了由燃煤转变为燃气的减排措施，导致干湿沉降物中的Hg沉降通量显著减少，2006年大气干湿沉降物中Hg的沉降通量1．837mg·m^-2·a^-1,北京市城区（近1000km2）Hg全年沉降为1837kg，空气中总Hg浓度由1998年的8．3～24．7ng／m^3下降到2006年的3．13ng／m^3，大气颗粒物中Hg含量由2003年的1．18ng／m^3下降到2006年的0．59ng／m^3（PM10）和0．67ng／m^3（PM2.5），表明北京市煤改气减排措施的实施显著改善了大气环境质量。通过对土壤中Hg的存在形式研究，发现土壤中有硫化物（辰砂）及各种Hg盐（HgCl2）的含Hg矿物，Hg也可以各种吸附方式或壤中气方式存在。研究证实北京壤中气Hg与大气Hg存在显著的相关性（n=131,R=0．267，p〈O．01），表明壤中气Hg是大气Hg的重要来源之一。利用2005年地表土壤总Hg与Hg释放速率的线性方程估算，土壤Hg平均释放速率为102．42ng·m^-2·h^-1，2005年土壤释放进大气的Hg通量为936．70kg。在查明土壤中存在大量辰砂矿物的同时，还分布有大量具有高温熔融特征的金属微球粒和玻璃质微球粒，证明燃煤和冶金烟尘是地表土壤Hg的主要来源。土壤中Hg、S、pH和辰砂颗粒浓度在空间上的高度耦合性表明，碱性条件下，土壤中高含量的S和Hg是辰砂形成的重要原因。按国家土壤环境质量标准，北京市I级土壤Hg环境质量的面积为176km^2，II级为808km^2，III级为24km^2，超III为36km^2。III级、超III级主要分布在二环路以内的中心城区。城南（长安街为界）大气Hg环境质量明显优于城北，在北四、北五环之间的部分地区，大气颗粒Hg的环境质量为III级或超III级。在地表土壤Hg含量较高的中心城区，居民每天因呼吸摄入的Hg高达364ng，对人体健康构成潜在风险。根据我国“十一五”规划中每年实现10％节能减排的目标，对北京市未来50年土壤Hg含量的时空演变趋势预测，预测2050年北京因干湿沉降带来的Hg输入量为16．03k，地表土壤释放Hg的输出量为37．36kg，明显大于Hg的输入通量，土壤Hg的环境质量将得到根本改善。预测到2040年III级土壤Hg环境质量的区域将完全消失，到2060年以I级土壤为主。
Mercury enrichments or anomalies in soils in urban and town areas are a general phenomenon in China, and investigation on source, transport cycle and fate of Hg in soil is essential for evaluating environmental risks. Soil Hg may be derived from natural and/or anthropogenic sources and may be deposited by wet or dry processes. Emitted Hg may be also derived from the re-emission of Hg that was previously deposited to the soil from the atmosphere. It is important to understand the relative magnitude of emission, deposition, and reemission of Hg associated with ecosystem of cities with soil Hg anomalies because for these cities being the center districts of human activity, the aggravated Hg pollution would result in serious dangers to environment and human health. The major focus of this paper is to discuss soil-air Hg exchange process, occurrence forms of mercury for high concentration Hg soils in Beijing, China. This information is important for developing eco- geochemical mass balances, assessing the effectiveness of regulatory controls of Hg emission from coal combustion. Data collected from field and laboratory are presented. An extension measurement of mercury in soil covered 1044 km^2 with a density of 1 sample/km^2 were carried out within the Beijing areal 19 samples of dry and wet deposition, and 20 samples of airborne particles （PM10 and PM2.5） with a relatively uniform distribution using a grid design based on 7 km×7 km were collected. 131 atmospheric mercury samples and soil gas mercury samples in mercury enrichments area and 66 samples in mercury background area were sampled. Resuits indicate that the average content of Hg is 0. 410±0. 403 mg/kg in soil, 0. 194±0. 058 mg/kg in dry and wet precipitation, 3.13±2.35 ng/m^3 in atmosphere, 559.65±1177.36 ng/m^3 in soil gas, 0.59±0. 26 ng/m^3 and 0. 67±0. 49 ng/m^3 in PM10 and in PM2.5, respectively. Hg emission from coal combustion has been significantly reduced to 1836 kg in 2006 compared with the emission of 8830 kg in 1999, because the Beijing government in 2000 issued a regulation on changing from coal combustion to natural gas combustion. Consequently, average Hg concentration in atmosphere has been obviously declined from 8. 3-24. 7 ng/m^3 in 1998 to 3. 13 ng/m^3 in 2006, and Hg content in airborne particles has been diminished from 1. 18 ng/m^3 in 2003 to 0. 59 ng/m^3 in PM10 and 0. 67 ng/m^3 in PM2.5 in 2006, respectively. It means that the regulatory measures adopted by the geijing government since 2000 to change the combustion mode from coal combustion to natural gas combustion has been successful in reducing Hg emission. Quantities of cinnabar minerals, metallic and glassy spherical particles characterized by high temperature melting were found in high concentration Hg in soil, which indicated that large ash and mercury emitted from coal combustion and metal smelting industry were de posited in soils in Beijing. Coupling spatial distribution patterns of S, Hg, pH with that of cinnabar minerals showed that the contents of S, Hg and pH in soil are key controlling factors for forming cinnabar minerals. The formation of soil gas Hg is not as well understood as the formation of cinnabar minerals in soil; whether the Hg released from soil was an original component of the soil （geogenic） or the re-emission of Hg previously precipitated from the atmosphere is not known. But a good linear relationship for soil gas Hg versus atmos pheric Hg was confirmed （n=131 ,R=0. 267, p〈0.01）, which indicated that the sources of soil gas Hg may be the more significant contributors of mercury to the atmosphere than previously realized. The area average emission rate of mercury calculated for soil in the Beijing area is 102.42 ng/（m^2·h）, and the total Hg flux emitted to atmosphere from soil is 936.70 kg in 2005, which were obtained based on the linear relationship be tween the total Hg in soil and mercury flux of emission; while 1387 kg Hg was precipitated by the way of dry and wet precipitating processes. A prediction on mercury content in soil, mercury flux of emission from soil and precipitated flux of dry and wet precipitation in Beijing in 2050 has been made, and a scenario of soil environmental quality of mercury in Beijing in 2050 was outlined, which are based on the major objective of reducing 10% of total quantities of pollutant （including Hg） of the previous year, as indicated in China＇s llth 5-year plan.
出处
《地学前缘》
EI
CAS
CSCD
北大核心
2008年第5期126-145,共20页
Earth Science Frontiers
基金
国家重点基础研究发展计划"973"项目(G1999045707)
国土资源大调查项目(200420130002
200414200011)
