大气汞的干湿沉降及其同位素示踪研究

Mercury (Hg) is considered a global pollutant. Atmospheric deposition is a primary source of Hg to terrestrial and aquatic ecosystems. Deposited inorganic Hg could convert to a more toxic form of methylmercury under the action of microorganisms, which bio-accumulates and threatens ecosystems and human health. China is considered to be the largest anthropogenic Hg source region over the world, but currently, the long-term observation data of atmospheric Hg deposition fluxes are scarce. The scientific problems of characteristics of atmospheric Hg wet and dry deposition, as well as source identification are need to be further studied. This project intends to carry out the dry and wet deposition of Hg in different functional areas (contaminated, urban and background locations). The purposes of this study are to quantified the relative contributions of dry and wet deposition to the total deposition budget and to reveal the key factors influencing atmospheric Hg deposition process. In addition, the method of determining Hg isotope was established to describe Hg isotope fractionation of wet and dry deposition in different functional areas and to identify sources of deposited mercury combined with backward air mass trajectory. The results conduce to further understanding of atmospheric Hg deposition process, enrich the global Hg biogeochemistry cycle theory and to provide scientific support for the policy-making on the global Hg-reduction strategies.

汞(Hg)是一种全球性的污染物。沉降是大气汞进入水陆系统的主要途径,沉降后的无机汞在水陆系统微生物的作用下转化为毒性更高的甲基汞，并对生态环境和人体健康产生严重危害。我国作为人为源汞排放大国，目前还缺乏大气汞沉降通量的长期观测数据的积累，同时大气汞干湿沉降特征及其来源识别等科学问题也有待进一步的研究。本项目以不同环境功能区(污染区、城区和背景区)大气汞沉降为研究对象，阐明干湿沉降对总沉降量的贡献及其变化特征，辨析大气汞沉降过程的关键影响因素。在此基础上，利用同位素示踪方法，构建不同环境功能区干湿沉降汞同位素的成份谱图，结合后向气团轨迹模型，识别大气不同沉降方式的汞来源。研究结果将有助于进一步认识大气汞的沉降规律，丰富大气汞的生物地球化学循环理论，并为制定大气汞减排政策提供科学依据。

沉降过程是大气汞进入陆地和海洋系统，从而产生更高毒性的重要途径。本研究采集了厦门城区、郊区和偏远地区降水样品和大气汞干沉降样品，分析了降水总汞(THg)和干沉降颗粒汞(PBM)和活性汞(RGM)的浓度，估算了大气汞干湿沉降量对总沉降量的贡献，同时也分析了降水和大气PBM中的汞同位素组成。研究发现降水中THg浓度和大气干沉降中PBM和RGM浓度均表现为城区和郊区高于偏远地区。干沉降PBM夏季较高，而降水THg和干沉降RGM均表现为城郊区夏季较高，偏远地区冬季较高。厦门大气汞总沉降量为11.9 μg/m2/yr，其中干、湿沉降量分别占59.7%和40.3%。城区、郊区和偏远地区大气汞干/湿沉降量比值分别是1.82、1.56和1.04，表明本地人为汞源较强的区域，大气干沉降是汞从排放源进入地表的更主要的途径。厦门降水与大气PBM的δ202Hg相当(均为负值)，说明了大气PBM对降水THg的重要贡献。降水δ202Hg接近燃煤烟气的δ202Hg，表明燃煤排放是降水汞的潜在来源。厦门偏远地区大气PBM的Δ199Hg高于城区，原因是偏远地区受到长距离输送的影响，导致光化学还原反应时间增加。大气PBM的Δ199Hg为负值，而降水THg的Δ199Hg为正值，说明湿沉降过程中发生了液相光致还原反应。结合后向气团轨迹结果，发现长距离陆地传输气团影响下的大气PBMΔ199Hg明显较高，短距离陆地传输气团影响下的大气PBMδ202Hg明显较低，而海洋气团影响下大气PBM的Δ199Hg和Δ201Hg相对较高，δ202Hg偏正。研究结果有助于系统、科学地认识大气汞干湿沉降规律，也表明汞同位素有助于定性评估本地源和外来源对大气汞的影响以及探索大气汞的迁移转化过程。