青藏高原纳木错地区大气气溶胶光学和理化特征研究.doc

青藏高原纳木错地区大气气溶胶光学和理化特征研究
摘要：青藏高原平均海拔约4000 m，是世界上面积最大，平均海拔最高的高原，被喻为“世界屋脊”和“地球第三极”。由于其巨大的面积和突出的高度，青藏高原对邻近地区乃至全球的大气环流系统和天气气候均有显著影响。青藏高原自身大气环境非常洁净，不存在较大的污染源，但青藏高原周边广泛分布着大气污染严重区域，如中国西南、南亚和东南亚等。这些长期存在的大气污染物在一定的条件下，通过大气环流传输进入高原内陆，与高原的冰冻圈联系在一起，有可能对区域气候和环境产生深刻的影响。
    纳木错（N30°30′~30°56′, E90°16′~91°03′）位于青藏高原中南部，是青藏高原第二大湖，远离人类工农业生产区，夏季受印度季风影响，冬季受西风环流的控制。洁净的自然环境和独特的大气环流系统，使得该区域对全球气候和环境变化极为敏感。中国科学院青藏高原研究所2005年在纳木错湖东南岸建立了野外综合观测站，为大气环境现代过程的长期系统监测提供了良好保障。本论文针对纳木错地区大气气溶胶进行了长期连续的观测和分析，初步明确了纳木错地区大气气溶胶的光学特征、元素组成和单颗粒特征，并对纳木错地区大气气溶胶的来源进行了探讨。
    根据气溶胶自动观测网（AErosol RObotic NETwork, AERONET）的反演数据，研究了纳木错地区的大气气溶胶光学特征并分析了其季节变化。纳木错地区与其它地区相比，气溶胶光学厚度小，年均值为0.05，是AERONET观测网络中大气环境最为清洁的站点之一。该地区大气气溶胶光学厚度具有明显的季节变化，春、夏季大，秋季次之，冬季最小。春季大气中的主要成分是沙尘，夏季的主要成分是人为排放的细颗粒，冬季由于被大雪所覆盖，气溶胶光学厚度最低。气溶胶Angstrom指数的年均值为0.42±0.27，说明从全年来看，气溶胶中粗粒子占主导地位。纳木错夏季受印度季风的影响，大气水汽含量很高，而冬季最低。此外纳木错地区气溶胶不具备显著的吸湿性。
    在纳木错观测站连续收集了从2006年8月到2007年7月共52个大气气溶胶样品。纳木错地区大气气溶胶（TSP）的质量浓度变化范围为0.48~36.11 μg/m3，平均为6.74 μg/m3。纳木错地区具有和南极类似的气溶胶浓度水平，是世界上大气环境最为洁净的站点之一。地壳中的常量元素Ca、Fe、Ti、K、Al、Mg、Na是纳木错大气气溶胶中的主要组成元素，约占元素总质量的92％。而其它微量元素如Li、Sc、V、Cr、Mn、Co、Ni、Cu、Zn、Ga和Pb等含量相当低。富集因子计算结果说明，对于地壳元素，季风期和非季风期不存在明显的差别，而Cr、Ni、Cu、Zn和Pb季风期的富集因子显著高于非季风期。主因子分析结果显示纳木错大气气溶胶的元素来源可归为三大类：土壤沙尘，工业排放和交通排放。从Pb同位素比值分析结果看，纳木错具有和中国内地城市（如北京等）迥然不同的特征，并与同属青藏高原的瓦里关站存在较大差异，但由于印度所能得到的参考数值过少，仅根据Pb同位素比值尚不能确定印度大气污染对青藏高原的贡献程度。
    采用场发射扫描电镜和能谱联用技术分析了2400个气溶胶粒子的单颗粒特征。根据显微形貌和元素特征，纳木错大气气溶胶颗粒大致可分为烟尘、Tar ball、硅铝酸盐/石英、硫酸钙、碳酸盐、Fe/Ti氧化物和生物颗粒7大类。其中，硅铝酸盐/石英为最主要的类型，说明纳木错地区大气气溶胶主要受地壳物质的影响。粒径较小的烟尘和Tar ball的相对比例在夏季明显增加，说明纳木错在夏季受到了较为明显的人为活动影响。这些人为源颗粒物既有可能是南亚污染物随印度季风传输而进入青藏高原内陆，也有可能来自当地夏季较多的放牧和旅游活动。纳木错大气气溶胶分析并没有发现其它较为明显的人为源特别是工业排放气溶胶类型，如燃煤飞灰等，说明尽管纳木错在夏季受到一定的人为活动影响，但从整体上看，大气环境较为洁净，可以作为世界偏远地区大气环境背景的代表。

Abstract：   The Tibetan Plateau is one of the most imposing topographic features on the surface of the Earth, often called as “the roof of the world” or “the third pole”. With an immense area (about 2,500,000 km2) and mean elevation of more than 4000 m above sea level, the Tibetan Plateau plays a key role in the central Asia climatology and atmospheric circulation. Meanwhile, the atmosphere over the plateau is probably the least affected by human activities in the Asian continent due to the sparse population and minimal industries. However serious atmospheric pollution are widely distributed at Southwest China, Southeast and South Asia, the adjacent area of Tibetan Plateau. Such atmospheric pollutant may be transported into the inland of plateau with specific atmospheric circulation, leading to profound consequence related to the fragile ecology and environment.
    Nam Co （N30°30′~30°56′, E90°16′~91°03′） is the largest lake in Tibet as well as the highest great lake in the world. There is sparse anthropogenic activities in Nam Co region. In summer, this region is under influence of India Monsoon, while in winter, it is dominated by westerlies. Due to the unique atmospheric circulation regime and clean environment, Nam Co is an ideal region to monitor atmospheric environmental change. In the summer of 2005, a comprehensive observation and research station was set up at the southeast shore of Nam Co by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. The establishment of Nam Co Station makes it practicable to long-term monitor the aerosol properties in central Tibet. In this paper, based on the continues monitoring results from August 2006 to July 2007, the optical properties, elemental composition, individual characteristics of aerosols from Nam Co as well as their potential sources were preliminary studied.
Very low aerosol optical values were observed with annual mean Aerosol Optical Depth (AOD) of 0.05 at 500 nm during August 2006 to July 2007, representing one of the most pristine site in the AERONET network. The seasonal variation of the monthly average AOD shows maximum values in the spring (April, May) and remains high values in summer monsoon season (June, July, August, and September). The former corresponds to the prevalence of soil dust particle in spring and the later reflects the enhanced human activities in summer. Angstrom parameters were also low with an average of 0.42±0.27, indicating that coarse particle is the dominant aerosol mode. The WVC has a clear seasonal pattern with maximum values in summer and minimum values during autumn and winter, which reflects the regional climate characteristic. In addition, AOD and Angstrom parameter had little correlation with water vapor content, suggesting hygroscopic growth of aerosols is insignificant.
    In this study, totally 52 TSP sample were collected at Nam co Station from August 2006 to July 2007. The concentrations of TSP range from 0.48 to 36.11 μg m-3, with the average of 6.74 μg/m3, which are generally comparable to other remote area like Antarctic. The seven major crustal elements (Ca、Fe、Ti, K、Al、Mg、Na) account for major proportion of the total elements concentration (92％), while other trace element (Li、Sc、V、Cr、Mn、Co、Ni、Cu、Zn、Ga and Pb) constitute minor proportion. Enrichment factor calculation shows that there are no distinct difference between monsoon and non-monsoon seasons for crustal elements.
    Size, morphology and chemical composition of 2400 individual aerosol particles were determined by scanning electron microscopy and energy-dispersive X-ray microanalysis (SEM-EDX). Based on morphology and elemental composition, the particles were clustered into 7 groups: soot, tar ball, alumosilicates/silica, calcium sulfate, carbonate, Fe/Ti-rich particles, and biological particles. Alumosilicates/silica is the major particle group, reflecting crustal material is the most important contributor of aerosol constituent. The proportions of soot and tar ball in fine mode increase significantly in summer, indicating Nam Co receive more influence of anthropogenic activities. Combined with the result of the backward air mass trajectory, it could be deduced that those anthropogenic aerosols may be transported from South Asia with the monsoon or induced by the increasing local resident and tourist. There is no other typical aerosol particle group which closely related to industrial process like fly ash was determined. Generally, it can be concluded that Nam Co is relative pristine and can be the typical background site worldwide, though it receive some influence of anthropogenic activities in summer.