大气环境科学实验室知识库

      汞作为一种具有持久性和全球性的有毒重金属 正受到全球人们的广泛关注。大气中的汞依据其操作上的定义主要分为气态单质汞 GEM 颗粒态汞 PBM和活性气态汞 RGM 。 PBM和 RGM统称为活性汞 RM 。在活性汞中也有一部分汞是以高毒性的甲基汞 MeHg 形态存在。每种形态的汞都具有独特的物理和化学特征 进而导致它们在大气中具有不同的迁移和转化过程。我国已经明确将汞列为重点管控的污染物之一 但是目前 对我国大气汞污染特征以及其迁移转化的认识还非常有限 有必要通过相关研究全面、科学地揭示大气汞的变化特征、 转化过程和 来源。
      本研究通过在秋、冬、春、夏四个季节对北京城市地区大气中GEM、 PBM和 RGM浓度的观测 分析 了 其 浓度特征 季节变化和日变化规律 ，探究了 活性汞在气态和颗粒态间的分配特征及其影响因素；此外 获得了北京城市地区大气细颗粒物中 MeHg的浓度水平和季节变化规律 并 初步探究了大气细颗粒物中MeHg的潜在来源；最后 分析了北京城市地区大气汞的湿沉降特征以及降雨对大气细颗粒物中 MeHg的冲刷 效率 。通过以上研究 揭示了在复杂城市大气污染的情况下 大气中不同形态汞的污染特征、转化 过程 及潜在来源 以利于完善模型对大气汞的评估以及更准确的认识大气汞对整个生态系统的影响。
      大气中不同形态汞的观测结果表明北京城市地区大气中 GEM、 PBM和RGM的浓度分别为 4.70 ± 3.53 ng m-3、 85.18 ± 95.34 pg m-3和 18.47 ± 22.27 pg m-3。 三种不同形态汞浓度 均明显高于偏远地区 表明北京城市地区受到 一定程度的人为汞 排放 污染 。在观测期间 GEM、 PBM和 RGM均显示出明显的季节变化。秋、夏、春、冬四个季节大气中 GEM的 浓度依次降低； RGM的平均浓度在春季最高 夏季最低； PBM浓度在冬季、秋季和春季没有明显不同 夏季显著低于其它季节。此外 随着 PM2.5浓度的增加 PBM浓度明显增加 但是 RGM浓度逐渐降低。 PBM浓度受颗粒物污染影响严重 污染天 PBM的浓度是清洁天的3.34倍 。北京的污染天通常都伴随着较高的相对湿度 。 高的相对湿度会增加气溶胶表面的含水量 进而促进更多的汞吸附到颗粒物上。
       在不同气象条件和颗粒物组成情况下活性汞 在气态和颗粒态间 具有不同的分配特征 。 温度 是影响活性汞气粒分配的一个重要因素 且在秋季和夏季 温度对活性汞气粒分配过程的影响大于冬季和春季。除了温度 相对湿度也影响着活性汞的气粒分配过程。随着相对湿度的增加 快速增长的气溶胶含水量会促进更多的 RGM溶解到颗粒相。 颗粒物组成成分对活性汞气粒分配系数也具有一定的影响 颗粒物上高浓度的 Cl-和 BC会导致更多的活性汞吸附到颗粒物上。 此外我们得到了四个不同季 节 分配系数 与气象参数 温度和相对湿度 的线性关系这 有助于模型对大气中活性汞的评估。
       通过改进水样中 MeHg浓度的分析方法 得到大气细颗粒物中 MeHg的浓度为 0.21 ± 0.17 pg m-3 MeHg在大气细颗粒物中具有很高的富集程度 。 大气细颗粒物上 MeHg浓度也具有明显的季节变化 表现为 夏季 >秋季 >春季 >冬季。随着 PM2.5浓度的增加 大气细颗粒物中 MeHg的 浓度逐渐升高 但是 质量分数显著降低。 MeHg在大气细颗粒物中的富集程度在清洁天最高。清洁天时 在大气中存在 MeHg的生成过程 随后富集到大气细颗粒物上。随着污染程度的增加 MeHg在 大气细颗粒物中的富集程度逐渐下降。 此外 通过采用 EPA PMF 5.0受体模型对 北京 城市大气细颗粒物中 MeHg的潜在来源进行探究 发现二甲基汞降解和甲基汞的表面挥发 贡献 率 最大 燃烧和海洋源 贡献扬尘对大气细颗粒物上 MeHg的贡献为 此外 非均相反应 具有 相对 稍低的贡献 。
      研究期间北京城市地区降水中总汞和甲基汞的浓度分别为 13.25 ± 9.28 ng L-1和 0.40 ± 0.31 ng L-1。 降水中总汞和甲基汞的浓度春季和夏季均明显高于秋季。湿沉降通量主要受降雨量的影响。北京城市地区大气中总汞和甲基汞最高的湿沉降通量均发生在夏季。 降雨可以有效移除大气 细颗粒物 中的 MeHg 其冲刷效率在 29.4%-77.0%之间 大气细颗粒物中 MeHg的移除是湿沉降中 MeHg的重要来源。

       As a persistent and global pervasive toxic metal mercury has drawn worldwide attention. In the atmosphere, mercury is operationally defined as three operational forms: gaseous elemental mercury (GEM), particulate bound mercury (PBM), and reactive gaseous mercury (RGM). The sum of PBM and RGM is reactive mercury (RM). Some of the RM is in the form of highly toxic methylmercury (MeHg). Each form of mercury has unique physical and chemical characteristics, which lead to different migration and transformation processes in the atmosphere. Mercury has been clearly listed as one of the key pollutants under control in China. However, at present, the understanding of atmospheric mercury pollution characteristics and its migration and transformation in China ia still limited. It is necessary to comprehensively and scientifically reveal the variation characteristics, sources and transformation mechanism of atmospheric mercury through relevant research.
       In this study, based on the observation of GEM, PBM and RGM in atmosphere at an urban site in Beijing in autumn, winter, spring and summer, we explored the characteristics of concentrations, seasonal and diurnal variation of atmospheric mercury in different forms. The distribution characteristics and its influencing factors of RM in the gaseous and particulate phases were also discussed. Furthermore, we obtained the concentrations and the seasonal variation of MeHg in atmospheric fine particle in Beijing urban, and the potential sources of MeHg in atmospheric fine particles were preliminarily clarified. Finally, we studied the wet deposition characteristics of atmospheric mercury in Beijing urban and the scavenging efficiency of MeHg by precipitation. The results show the pollution characteristics, transformation mechanism and potential sources of different forms of atmospheric mercury in urban area that with serious complex air pollution, which is beneficial to the  valuation of atmospheric mercury by models and the understanding of the impact of atmospheric mercury on the whole ecosystem.The observation results of different forms of atmospheric mercury show that the concentrations of GEM, RGM and PBM were 4.70 ± 3.53 ng m−3, 18.47 ± 22.27 pg m−3 and 85.18 ± 95.34 pg m−3, respectively, which were all higher than those in remote areas. This indicating that the Beijing urban was polluted by anthropogenic mercury emissions. GEM, RGM and PBM all showed distinct seasonality over the course of observation. The concentrations of GEM decreased in the order of autumn, summer, spring and winter. The highest concentration of RGM was in spring and the lowest value was in summer. There were no significant differences of the PBM levels among winter, autumn and spring, whereas the concentration of PBM in summer was significantly low in comparison with other seasons. Furthermore, increasing concentration of PM2.5 led to an increase of PBM concentration, whereas the concentration of RGM gradually decreased. The PBM concentration was most seriously affected by the pollution of atmospheric particulate matter. The average PBM concentration during polluted periods was 3.34 times that during clean periods. In Beijing, most of the polluted period were accompanied by relatively high relative humidity. The high relative humidity during polluted periods increased the aerosol liquid water content, which promotes the adsorption of mercury onto particles.
      In the case of different meteorological conditions and particle composition, RM has different distribution characteristics between gaseous and particulate phases. Temperature has been shown to be a key parameter for determining the gas-particle partitioning of RM, and the influence of temperature on the partitioning process in autumn and summer was greater than that in winter and spring. In addition to temperature, relative humidity also affected the gas-particle partitioning of RM. Particle growth with the increasing of relative humidity presented an aqueous phase for adsorption of RGM. Particle composition was also a factor that influence the gas-particle partitioning of RM. The Cl− and BC more easily adsorb the RGM and bind to atmospheric particulate matter. Furthermore, the linear relationships between gas-particle partitioning coefficient and meteorological factors (air temperature and relative humidity) were obtained over the four seasons, which would help the evaluation of RM in the atmosphere by models.

      Through improving the analytical method of MeHg in water, we obtained the concentration of MeHg in atmospheric fine particles, which was 0.21 ± 0.17 pg m−3, and MeHg was highly enriched in atmospheric fine particles. The seasonal mean concentrations of MeHg decreased in order: summer > autumn> spring > winter. In addition, with the increase of PM2.5 concentrations, MeHg volume concentrations elevated, whereas, the mass compositions of MeHg in PM2.5 were significantly decreased. MeHg had a high enrichment rate on atmospheric fine particles during clean days, when there was formation of MeHg and then enriched on atmospheric fine particles. With the increase of pollution, the enrichment of MeHg on atmospheric fine particles gradually decreased. Moreover, EPA PMF 5.0 receptor model was applied to apportion the sources of MeHg in atmospheric fine particles. The results show that DMeHg decomposition and MeHg surface volatilization had the largest contributions (47.8%), combustion emission contributed 18.7%, dust contributed 16.4% to MeHg, and heterogeneous reaction had relatively low contributions (13.2%).
      During the period of study, the concentration of total mercury and MeHg in precipitation in Beijing urban were 13.25 ± 9.28 ng L-1 and 0.40 ± 0.31 ng L-1. The concentration of total mercury and MeHg in precipitation were higher in spring and summer than in autumn. The wet deposition flux of mercury is mainly affected by precipitation amounts. In Beijing urban, the highest wet deposition fluxes of total mercury and MeHg occurred in summer. Moreover, the precipitation could scavenge 29.4%-77.0% MeHg that bound to atmospheric fine particles. Scavenging of MeHg in atmospheric fine particles is a significant source of MeHg in wet deposition.