Knowledge Management System Of Institute of Earth Environment, CAS
|导师||安芷生 ; 曹军骥 ; 顾兆林|
|关键词||黄土 黑碳 焦炭 烟炱 火灾历史和干旱化|
|其他摘要|| 黑碳是生物质和化石燃料不完全燃烧生成的富含碳物质的连续统一体，包括微焦化的植物体、木炭、焦炭、烟炱和石墨态碳。黑碳的形成与类型受气候条件、燃料种类、燃烧强度、燃烧温度、燃烧持续时间等因素的影响，因此环境中黑碳的化学组分与形态结构也存在很大的差异。环境中的黑碳主要可以分成两大类：焦炭与烟炱。焦炭保留了来源植物体的细胞结构特征，一般粒径为5-100 µm或更大；而烟炱是高温下气态物质浓缩凝聚而成，主要是亚微米或纳米级颗粒。这些不完全燃烧的产物大多数在原地土壤中保存，少部分黑碳经风力搬运、沉降、雨水冲刷和地表径流等作用，最终在河流、湖泊、海洋、冰雪等环境中沉积下来。土壤和沉积物中的焦炭因粒径较大，通常被用来反映局地的火事件，而烟炱组分则可以反映大气黑碳气溶胶的变化历史。|
土壤表层0-20 cm和20-100 cm有机碳库分别为0.741 Pg和2.888 Pg，分别占1 m深度土壤有机碳库的20.42%和79.58%，这说明黄土高原深层土壤有机碳的储量要明显高于表层土。土壤表层0-20 cm黑碳储量为0.073 Pg，焦炭和烟炱储量则分别为0.053和0.02 Pg，分别占1 m土壤焦炭和烟炱储量的18.4%和11.9%。由此可见，深层土壤中黑碳、焦炭和烟炱储量要明显大于表层土。土壤黑碳与有机碳含量之间存在明显的相关性，说明黑碳对土壤有机碳的稳定起到十分重要的作用。所有柱状剖面黑碳与焦炭都有极其显著的相关性，说明了土壤中的焦炭对黑碳含量的贡献较大。土壤烟炱含量与各理化指标之间并不存在明显的相关性，可能反映了烟炱来源的多变性和复杂性。黄土高原地区土壤黑碳的来源主要是生物质燃料和化石燃料燃烧的排放，除了受到局地野火、机动车和人为排放的黑碳影响外，远源输送的黑碳气溶胶也可能是黄土黑碳的一个重要来源。
第四纪以来，洛川黄土古土壤剖面的黑碳、焦炭和烟炱含量虽然波动频繁，但整体上都呈现阶段性增加的变化趋势。900 kyr BP以来，焦炭含量有明显的冰期间冰期旋回变化规律，即：间冰期阶段发育古土壤时，黑碳和焦炭含量较高；而冰期黄土堆积时，黑碳和焦炭含量较低，表明黄土高原较高频率/较强火事件主要发生于间冰期阶段，这可能与燃料的大量积累有关。焦炭含量存在准100 kyr（偏心率）、41 kyr（黄赤交角）和23 kyr（岁差）变化周期，说明黄土高原中部地区自然火灾的发生与地球轨道要素引起的太阳辐射量变化密切相关。此外，植被演替、人类活动和全球冰量变化驱动的东亚冬夏季风变迁可能也对火事件产生重要的影响。黑碳的超显微特征表明黄土高原的黑碳主要来源于草本植物燃烧，主要反映了局地的火灾演化历史。
洛川黄土剖面烟炱含量变化记录了1200 kyr BP、900 kyr BP、500 kyr BP前后亚洲内陆干旱化加剧的几次重要气候事件。整体上，烟炱含量呈现冰期高，间冰期低的变化规律，与黄土质量沉积速率和石英颗粒粒径变化一致，说明烟炱在一定程度上可以作为指示东亚冬季风强弱的指标。1800-2600 kyr BP，1200-1800 kyr BP和600-1200 kyr BP时段，烟炱记录所反映的气候干旱化过程主要存在黄赤交角41 kyr变化周期，而600 kyr BP以后，周期由41 kyr向100 kyr发生转变，可能反映了气候的重大转变。全球冰量变化可能是导致亚洲内陆干旱化发展的重要影响因素。
; Black carbon (BC) is generally defined as the carbonaceous material that forms during the incomplete combustion of fossil fuels and biomass burning, which consists of a continuum of compounds ranging from slightly-charred plant biomass, to charcoal, char, soot and ultimately graphite. The formation and type of BC is controlled by many factors, such as the climatic conditions, fuel type, fire intensity, combustion temperature, duration of the fire, and so on. Thus the chemical composition and morphology of BC is quite different. Char and soot are two most important components of BC in the environment. Char retains some plant chemistry and morphology, mainly composed of micrometer-sized or much larger particles; while soot is composed mainly of submicron particles formed by the condensation of gas phase intermediates. A majority of these incomplete combustion materials were stored in the soils, while a small part of them were underwent aeolian transport, dry and wet deposition, and ubiquitously deposited on all the earth’s surface like rivers, lakes, oceans, ice and snow. Chars in the soils and sediments were always being used to indicate local fire regime, while soot ccould be used to reflect the variation history of BC aerosol.
The methodology for BC quantification in the soils and sediments is still a big problem in the world. Until now, there is none standardized and common method for BC determination. Due to imprecise definition of BC and positive and negative artifacts of different methods, method intercomparisons become very difficult. Thermal optical reflectance (TOR) method was successfully applied in the measurement of BC both in soils and sediments, and by using this method we can differentiate between char and soot. The comparison study of TOR method with other two commonly used BC quantification methods showed that the BC content measured with wet chemical oxidation (Cr2O7) seem generally higher than TOR and chemothermal oxidation method (CTO-375). Though it is suited for the measurement of different components of BC concinuum, the pretreatment experiment is hard to control. BC content yielded by CTO-375 method was the lowest, thus it is well suited for the analysis of soot which is produced at high temperatrue. Instead, a standard addition approach suggested the robustness and feasibility of TOR method for the BC determination in soils and sediments.
The spatial distribution of organic carbon (OC) and total nitrogen (TN) contents in the soils from Chinese Loess Plateau are following the order: loess zone > clayey loess zone > sandy loess zone. Generally, the concentrations of different BC components in the clayey loess zone were the highest, the second and third were in the loess zone and in the sandy loess zone, respectively. The average densities of OC, TN, BC, char and soot are in the order of clayey loess zone, loess zone and sandy loess zone, presenting the variation characteristics of gradual reduction from the southeast to the northwest. This may be related to the effects of climatic conditions and human activities, such as emissions from motor veichle, coal-fired power generation, heating and straw burning, etc. The vertical distributions of OC, TN, BC, char and soot contents and densities were generally much higher in the top soil, and decreased with the soil depth. But in some soil profiles, the highest BC values were found in the deep soil layer，which may be affected by many factors like translocation of BC particles, physical and chemical properties of the soil (the protection of clay minerals and micro-aggregate), and geological fire regimes.
The soil organic carbon (SOC) pools in the 0-20 cm the 20-100 cm layer are 0.741 Pg and 2.888 Pg, respectively, accounting for 20.42% and 79.58% of the total 100 cm SOC pool, which indicate that a large amount of C may be stored in the deeper layers of Chinese Loess Plateau. The BC stored in the 0-20 cm is 0.073 Pg. Char and soot pools in the 0 to 20 cm layer are 0.053 Pg and 0.02 Pg, which amount to 18.4% and 11.9% of the total 100 cm char and soot storages, respectively, showing that deeper soil layers store much larger amount of char and soot. The close correlation between SOC and BC suggests the role of BC in the stabilization of SOC. Char and BC contents in all the soil profiles are well correlated, implying the contribution of char to the BC content in the soil. Soot content has poor correlation with other physiochemical properties, which possibly attribute to the complexity and variability of the sources of soot. The main sources of BC in the topsoil from Chinese Loess Plateau are emissions from biomass burning and fossil fuels combustion. Apart from the effects of emissions from local wildfires, motor vehicle and human activities, BC aerosols transported from far away are also an important source.
Though the contents of BC, char and soot in the Luochuan loess-paleosol vary with frequent fluctuations, they increase periodically as a whole since the Quaternary. During the last 900 kyr BP, BC and char contents have evidently glacial/ interglacial cycles, that is, when paleosol developed in the interglacial, BC and char contents are high, and when the loess deposited in the glacial, BC and char contents are low. This suggests that natural fire occurrence was much more intensive and frequent in the period of interglacial, which may be related to the accumulation of biomass fuels. Spectral analysis results proved that the occurrence of natural fire changed with 100 kyr, 41 kyr and 23 kyr cycles, which correlate to the earth orbital elements, implying that the fire regimes in the central Loess Plateau had a close relationship with the variation of solar radiation. Furthermore, vegetation change and human activities also could cause the fire regimes. The shift of East Asia summer and winter monsoon which is driven by the change of global ice volume may be another factor affecting the fire occurences. The mophological characteristic of BC indicated the main source of BC was from the grassland fires and mainly reflected local fires.
The variation trend of soot content in Luochuan section recorded several significant climatic events that aridification of Asian inland were happened at 1200 kyr BP, 900 kyr BP, and 500 kyr BP. Generally, soot contents were higher in glacials but lower in interglacials, which were similar to the variation of loess mass sedimentation rate and grain size of quartz particles. This suggests that the record of soot to some extent can be used as an indicator for change of East Asia winter monsoon. During the periods of 1800-2600 kyr BP, 1200-1800 kyr BP, and 600-1200 kyr BP, spectral analysis showed 41-kyr cycle was dominant for the history of aridification, and then a transition from 41 kyr to 100 kyr cycle was happened during the 600-0 kyr BP, possibly indicating large fluctuations of the climate. Global cooling driven by the change of global ice volume may be an controlling factor which is responsible for the drying of Asian inland.
|学科领域||大气科学 ; 环境科学|
|占长林. 黄土高原黑碳分布、储量估算及其气候环境指示意义[D]. 北京. 中国科学院研究生院,2013.|