【作者】 黄侃；

【导师】 庄国顺；

【作者基本信息】 复旦大学 ， 环境科学， 2010， 博士

【摘要】 大气气溶胶通过影响大气辐射、大气化学及云和降水过程,改变地水气系统内部的辐射能量收支和水循环,对全球气候和环境变化产生巨大影响。气溶胶长距离输送被认为是全球生物地球化学循环的重要途径之一。亚洲沙尘的长距离传输过程中所携带的大量矿物气溶胶细颗粒物,提供了积聚(经由吸附、表面络合、自由基光化学反应等复相反应)污染物的极好场所,沙尘传输途经人口密集的城市群地区,促进污染气体的气固转化过程并叠加大量污染气溶胶,从而对我国东部地区和海域的生态环境产生重大影响。我国城市群地区近年来频繁发生的严重灰霾就是大气气溶胶传输、混合以至组分转化而导致的大气质量大范围恶化的典型案例。随着城市化进程的加速和机动车的激增,近年来我国已产生的大范围的灰霾区域,已从珠江三角洲、长江三角洲,京津地区和四川盆地,向中国中西部的城市广泛蔓延。长江三角洲地区由于经济的强劲发展,其空气污染程度更是急剧恶化,对生态环境带来了重大挑战。本研究取得了以下几个方面的重要成果：1.提出以SO42-/S、Ca2+/Ca、Na+/Na以及Ca/Al作为区分亚洲三大沙尘来源的示踪物,也即来源于塔克拉玛干沙漠的西部高钙沙尘、来源于黄土高原的西北部高钙沙尘以及来源于蒙古戈壁的东北低钙沙尘。沙尘的传输路径,污染物前体物浓度和气象条件是影响沙尘和污染物混合程度的主要因素。西部沙尘受污染的程度相对最小,其主要来源是塔克拉玛干沙漠的古海洋源,但是仍然在沙尘源区发现污染气溶胶和沙尘存在一定程度的混合。西北部的沙尘则和传输途径中经过的煤矿等释放出的污染物发生了非常强烈的混合反应。2006年的北京沙尘暴显示二次气溶胶离子,SO42-、NO3-和NH4+在较为污染的传输路径中的浓度相较非沙尘时期增加了2至3倍,典型污染元素如Zn、As、Pb、S和Cd的富集系数甚至会高于非沙尘时期,并且发现了生物质燃烧气溶胶和沙尘的混合。东北部的沙尘则在到达下游地区上海后酸度有了明显的增加,其混合机制主要是沙尘、人为污染物以及海盐之间的相互作用。本研究中所阐明的局地、中程以及长程传输过程中沙尘气溶胶和污染物的相互混合机制揭示了亚洲沙尘对区域乃至全球气候变化的可能影响。2.揭示了沙尘时期气溶胶的光学和化学特性以及两者之间的相互关系,从而认识沙尘和人为气溶胶对区域气候的影响。研究发现不同情况下气溶胶在657、870和1020 nm波段范围内的单次反照率SSA主要是由黑碳在颗粒物中的质量百分比所决定。而在沙尘时期439nm处观测到的强吸光性,主要是由于沙尘气溶胶中相当比例的铁氧化物所造成的。在沙尘时期发现气溶胶的光学特性与化学组成存在着线性关系。细模态AOD主要来源于PM2.5的可溶性离子,SO42-、NO3-、NH4和K+的贡献,而总AOD则主要来自于矿物气溶胶的贡献。黑碳作为气溶胶中的强吸光物质,对粗、细颗粒物的消光均有一定的贡献。3.通过对中国的超大城市上海的颗粒物和气态物质研究,阐明了其空气质量存在的急迫问题,揭示了空气质量恶化的成因及灰霾的形成机制。早高峰和晚高峰是一天中的两个主要污染阶段,主要与机动车排放源和气象条件有关。冬季气溶胶的单反照率低达0.79,其吸光性物质黑碳的比例较高。上海气溶胶形成灰霾的主要成分有可溶性离子、有机气溶胶及矿物气溶胶。其中可溶性离子中SO42-、NH4+、NO3-和K+为主要物种,来源可能为光化学反应,云中液相反应以及生物质燃烧等。有机气溶胶占细颗粒物的28.8%,其中二次有机气溶胶占总有机物的比例约为30%。通过元素富集系数分析发现Se和S的富集最为严重,反映了上海煤燃烧带来的严重污染。上海气溶胶中主要的消光物质包括有机气溶胶、硫酸盐、硝酸盐和无机碳,消光比例分别为47%、22%、14%和12%。有机气溶胶和元素碳的主要来源均是机动车排放,因此控制机动车数量的不断增长已成为改善上海乃至中国其他大城市当前大气质量的当务之急。4.通过卫星观测、雷达垂直分布观测以及基于近地面的气溶胶采样分析了2009年长三角地区发生的大规模生物质燃烧事件。通过卫星反演的信号,如火点、氧化碳柱浓度以及气溶胶光学厚度的分布表明这次的生物质燃烧是区域性的。生物质燃烧气溶胶的主要指示组分K+、EC和OC以及OC/EC比值在此时期均有明显的增加。通过估算,有机气溶胶约占PM10质量的50%,而来自生物质燃烧所释放的CO则可贡献其总量的25-35%。由于二次无机气溶胶前体物(SO2、NOx)在氧化过程中造成的损耗,O3的排放量在生物质燃烧时期并没有明显增加。气溶胶的垂直分布表明在这一时期混合层顶处于很低的高度,近地面的气溶胶消光系数(532nm)高达740±280 Mm-1,而整个边界层内的消光系数平均值则为320±250 Mm-1。通过估算PM10的质量消光效率为3.8±0.5m2/g。此次长三角的区域污染不仅来自本地的生物质排放,也可能会受到临近省市如山东、安徽等地的生物质燃烧气溶胶传输的影响。提出了有效控制农业收成时的秸秆燃烧的建议,对提高长三角的空气质量产生了非常积极的影响,将是即将到来的2010上海世博会为空气质量达标所采取的重要措施。5.通过对上海2009年4月至6月的观测,揭示了在短时期内出现了三次能见度较低的污染事件的各自不同形成机制。通过特征气溶胶化学组分、气团后向轨迹以及化学示踪物确定了造成城市灰霾的三种基本类型,即二次无机污染、沙尘以及生物质燃烧。对污染气体的分析发现二次无机污染时期SO2、NOx、和CO均有显著增加,而生物燃烧时期增加最为明显的是CO,沙尘时期的气体浓度则最低。CO/NOx的比值(-14)可以来判断是否有生物质燃烧源的影响,因为其与其他不同时期(9-10)存在显著的区别。线性相关分析表明在二次无机污染时期SO2、NOx、CO与PM2.5存在显著相关性,表明煤燃烧、机动车排放和电厂点源排放是该时期的主要来源。而在生物质燃烧时期只发现CO与PM25存在显著相关性表明有机气溶胶主导了颗粒物的生成。激光雷达对气溶胶的垂直分布观测显示不同污染时期的混合层高度、光学参数垂直分布、以及退偏振比率均有明显区别。卫星反演表明长程传输和区域传输对上海的空气质量均有重要影响,据此评价了城市群气溶胶的区域传输乃至长程传输对局地以至下游地区空气质量的影响。6.通过对2005年上海一整年的降水化学的研究发现上海的酸雨相当严重,年平均pH达到4.49,远远低于定义酸雨的临界值5.60,并且一年中的酸雨降雨频率高达71%。过去的8年来,雨水的pH值呈显著下降的趋势,与1997年相比,上海雨水中的酸度增加了约15倍。由于迅速城市化和机动车所带来的大量SO2和NOx的排放是造成雨水中主要的致酸物质SO42-和NO3-浓度升高的重要原因。SO42-和NO3-主要是以CaSO4和Ca(NO3)2的形式存在,表明了Ca2+在中和过程中的作用比NH4+来得更为重要。源解析表明SO42-、NO3-、NH4+和大部分Ca2+主要是来自人为污染源,K+、Mg2+和部分Ca2+主要来自矿物源,而Cl-和Na+则基本都来自海洋源。上海的降水化学不但受到本地污染源的影响而且也受中程和长程传输的影响。更多还原

【Abstract】 1. Three Asian dust sources, i.e., the western high-Ca-dust in the Taklimakan Desert, the northwestern high-Ca-dust and the northeastern low-Ca-dust in Mongolia Gobi, were identified based on the air mass trajectories plus the elemental tracer analysis (e.g., SO42-/S, Ca2+/Ca, Na+/Na and Ca/Al). The transport pathways of dust, concentrations of pollutant precursors and meteorological conditions were the main factors affecting the mixing extent of pollutants with dust. The western dust source was least polluted in comparison to the other two dust sources, and it mainly derived from the Taklimakan Desert, a paleo-marine source. Anyway, it was still found that the dust could have already mixed with pollution aerosol even near dust source regions. The northwestern dust had a considerable mixing and chemical reaction with sulfur precursors from the coal mines on the pathway of the long-range transport. The amount of SO42-, NO3- and NH4+ increased 2-3 times in the relatively polluted transport pathway of 2006 Beijing dust storm compared to the non-dust periods. Typical pollution elements, such as Zn, As, Pb, S and Cd were also more enriched. Additionally, the mixing of biomass burning aerosol with dust was verified. The northeastern dust reached Shanghai with high acidity, and it became the mixed aerosol with the interaction among dust, local pollutants, and sea-salts. The mixing mechanisms of the dust with the pollution aerosol on the local, medium-range, and long-range scale revealed from this study would improve the understanding of the impacts of Asian dust on regional/global climate change.2. The chemical and optical characteristics of dust aerosol and the relation between each other were evaluated to understand the effect of dust and anthropogenic aerosol on the regional climate. The single scattering albedo of dust aerosol in the wavelength of 657,870 and 1020nm was mainly determined by the percentage of black carbon in particles, while the strong light absorbing at 439nm was probably due to the significant proportion of iron oxides in the dust aerosol other than black carbon. Linear relationship between aerosol optical properties and aerosol chemical compositions was found. The results indicated that soluble ions, i.e. SO42-, NO3-, NH4+ and K+, were the main contributors to the light extinction of fine particles while mineral aerosol contributed more to that of coarse particles. Black carbon, as a strong light absorbing species, was found contributing to the light extinction of both fine and 3. The air quality degradation and formation mechanism of haze in big cities of China was elucidated by comprehensive research of atmospheric aerosol and trace gases in Shanghai. The morning and evening rush hours were the two main pollution episodes in one day, which was due to the vehicle emission and meteorological conditions. The single scattering albedo of winter aerosol was as low as 0.79, reflecting the high fraction of the strong light absorbing substance, black carbon in the particles. Major contributors to the aerosol in Shanghai were soluble inorganic ions, organic aerosol and mineral aerosol. SO42-, NH4+, NO3- and K+ were main species of soluble inorganic ions, which derived from photochemical reactions, in-cloud aqueous formation and biomass burning. Organic aerosol contributed 28.8% to PM2.5, of which secondary organic aerosol contributed about 30% to total organic aerosol. The enrichment degree of Se and S was the most significant in all elements, which was attributed to the widely used coal in China. Organic aerosol, sulfate, nitrate and element carbon were the major contributors to light extinction, which accounted for an average of 47%, 22%,14% and 12%, respectively. The major sources of organic and elemental carbon were vehicle emission, thus control measures on the growing vehicle numbers have been urgent need on the improvement of air quality of Shanghai and other big cities in China.4. A synergy of satellite observation, lidar measurement and ground-based aerosol sampling was conducted to monitor the biomass burning in the Yangtze River Delta (YRD), China. Regional biomass burning was indicated by the satellite signal, i.e., fire spots, column carbon monoxide, and aerosol optical depth. K+, EC, OC, and OC/EC were significantly enhanced in the biomass burning episode. Organic aerosol contributed-50% to PM10 and biomass derived CO contributed-25-35% of total CO. No significant increase of O3 was observed, which was attributed to depletion due to the oxidation formation of sulfate and nitrate. Vertical profile showed low mixing height and extremely high aerosol extinction coefficient (532nm) of 740±280 Mm-1 near the ground with average of 320±250 Mm-1 within the boundary layer. Mass extinction efficiency of PM10 was 3.8±0.5 m2/g. The air pollution in YRD during the biomass burning episode not only derived from the local agricultural burning, but also affected by the transport of smoke aerosol from far regions, such as Shandong and Anhui provinces. It is suggested that effective controls on biomass burning emission caused by the post-harvest anthropogenic activities could have positive effect on air quality in YRD, and will be an efficient way on the meet of air quality in the forthcoming 2010 Shanghai World Expo.5. The different formation mechanisms of haze in three pollution episodes during April to June,2009 in Shanghai were elaborated. Three types of haze, i.e., secondary inorganic pollution, dust, and biomass burning were confirmed by using the typical aerosol chemical species, air mass backward trajectory and chemical tracers analysis. SO2, NOx and CO were significantly elevated in the secondary inorganic pollution episode, while CO increased obviously in the biomass burning episode. The ratio of CO/NOx (-14) could be used as the tracer to indicate the possible influence of biomass burning, as it is distinctly distinguished from the other periods (CO/NOx: 9-10). Linear regression analysis suggested the significant correlation between SO2, NOx, CO and PM2.5 in the secondary inorganic pollution episode, respectively, indicating the sources from coal burning, vehicle emission and power plants. While it was found that PM2.5 only correlated with CO in the biomass burning episode, indicating the dominance of organic aerosol. Vertical structure of aerosol by LIDAR suggested distinct differences of mixing height, depolarization ratio, vertical distribution of aerosol optical parameters in different episodes. The impact of long-range and regional transport on the air quality of Shanghai was evaluated by satellite observations.6. The acidity level of the precipitation in Shanghai was considerably high with the annual mean pH value of 4.49 and the frequency of acid rain was 71%. In 2005 the acidity of rain in Shanghai increased more than 15 times in the past 8 years compared to 1997. The concentration of SO42- was higher than the most polluted cities abroad, indicating Shanghai has been a severe polluted city over the world. The high coal/fuel consumption from urbanization and the rapid increasing amount of motor vehicles from motorization resulted in the high emission of SO2 and NOx, the precussors of the high concentration of acidic ions, SO42- and NO3-, which were the main reason of the severe acid rain in Shanghai. SO42- and NO3- existed mainly in the form of CaSO4 and Ca(NO3)2, indicating the neutralization capability of Ca2+ dominated over NH4+. The principal component analysis indicated that SO42-, NO3-, NH4+ and partial Ca2+ derived from anthropogenic sources, K+, Mg2+ and Ca2+ mostly originated from mineral sources, and almost all the Cl- and Na+ derived from the sea. The chemistry of precipitation in Shanghai was under the influence of local pollution sources and the long-and moderate-range transport through back trajectory analysis.更多还原