研究生知识库

生物质燃烧是生态系统自然组成的一部分，近年来由于人类活动的影响，全球火灾的速度及规模大幅度加快。生物质燃烧对大气环境危害极大，燃烧过程会向大气输送大量的有毒害气体及颗粒物，例如有机气溶胶、黑碳、褐碳和数千种挥发性有机物（VOCs；Volatile Organic Compounds）。这些混合烟气会与自由基（如NOx、OH-等）发生复杂的光化学反应，生成挥发性较低、可溶性更强的化合物，促进形成新的颗粒物和臭氧，导致雾霾形成，对人类健康造成极大威胁，全球每年大约有700万人由于空气污染而过早死亡。生物质燃烧会向大气中释放大量甲酸（HCOOH，FA）和乙酸（CH3COOH，AA）。作为臭氧和有机气溶胶重要的前体物，甲酸、乙酸在对流层化学中具有重要作用。虽然前人就甲酸、乙酸来源有较为深入了解，但是各个来源的相对输入量化不准确，对影响浓度的过程缺乏了解，导致在自由对流层中观察到的甲酸、乙酸浓度与模型预测浓度之间存在较大差距。文献中已有报道生物质燃烧释放甲酸、乙酸释放因子（EF，Emission Factor），但是描述对象大多为森林火灾，对于农业废弃物焚烧甲酸、乙酸的释放因子报道极少。故本论文主要开展了以下三方面的研究工作：（1）通过实验室搭建小型燃烧装置，改变助燃气浓度，达到模拟不同燃烧条件下农业废弃物燃烧释放的甲酸、乙酸的释放因子为排放源清单提供原始数据；（2）利用针式微萃取方法结合GC-IRMS技术检测出生物质燃烧释放甲酸和乙酸稳定碳同位素组成特点；（3）通过PTR-ToF-MS检测了在美国西部森林大火爆发期间大气中甲酸、乙酸及其前体物在大气中的分布特征。本论文通过对生物质燃烧释放的甲酸、乙酸释放特征及碳同位素组成进行分析研究得出以下结论：（1）搭建实验室小型燃烧平台。在实验室内完成5种常见农业废弃物（小麦秸秆，玉米秸秆，油菜秸秆，水稻秸秆和棉花秸秆）的燃烧实验。建立碱液吸收体系，碱液吸收效率高达88%以上，能有效吸收生物质燃烧释放的气相中甲酸和乙酸。5种生物质燃烧释放的甲酸和乙酸的释放因子分别是：玉米秸秆燃烧释放甲酸、乙酸的平均释放因子为分别为0.36 ± 0.19 g/kg、3.1 ± 1.6 g/kg；水稻秸秆燃烧释放甲酸、乙酸的平均释放因子为分别为0.24 ± 0.18 g/kg、1.042 ± 0.76 g/kg；油菜秸秆燃烧燃烧释放甲酸、乙酸的平均释放因子为分别为0.24 ± 0.14 g/kg、1.76 ± 1.22 g/kg；棉花秸秆燃烧释放甲酸、乙酸的平均释放因子为分别为0.17 ± 0.08 g/kg、1.54 ± 0.81 g/kg；小麦秸秆燃烧释放甲酸、乙酸的平均释放因子为分别为0.36 ± 0.23 g/kg、2.2 ± 1.26 g/kg。5种生物质燃烧释放乙酸和甲酸的的初始比值，即Rt=0=[AA/FA]t=0 ，其中R玉米=8.06；R油菜=9.09；R棉花=9.37；R水稻=4.31；R小麦=6.55。由于生物质燃烧会释放大量的甲酸前体物（如异戊二烯，MVK等），这些前体物在大气中发生光化学反应大量生成甲酸。此外，在大气环境中甲酸比乙酸的寿命长（甲酸寿命为3.2天，乙酸为2.3天）。因此生物质燃烧释放的烟羽，在老化过程中AA/FA的比值会越来越小。秸秆燃烧释放的甲酸、乙酸的初始释放因子及AA/FA比值，不仅为排放源清单提供生物质燃烧源的基础数据，还为田间秸秆燃烧释放烟气老化程度的描述提供依据。（2）检测了实验室内生物质燃烧初始释放甲酸、乙酸的稳定碳同位素比值。即水稻秸秆燃烧释放甲酸和乙酸稳定碳同位素比值分别为-14.49 ± 5.08‰、-22.59 ± 2.55‰；小麦秸秆燃烧释放甲酸和乙酸稳定碳同位素比值分别为-15.59 ± 3.16‰、-23.53 ± 2.49‰；油菜秸秆燃烧释放甲酸和乙酸稳定碳同位素比值分别为-20.26 ± 2.92‰、-31.06 ± 0.96‰；棉花秸秆燃烧释放甲酸和乙酸稳定碳同位素比值分别为-22.33 ± 5.57‰、-31.39 ± 0.81‰；玉米秸秆燃烧释放甲酸和乙酸稳定碳同位素比值分别为-20.18 ± 2.93‰、-30.77 ± 1.75‰；高粱秸秆燃烧释放甲酸和乙酸稳定碳同位素比值分别为-17.07 ± 4.31‰、-22.08 ± 2.85‰。5种生物质在不同氧气条件下燃烧，释放的甲酸存在明显的同位素分馏，例如水稻秸秆燃烧释放的甲酸δ13C值随着氧气流速的增加，甲酸δ13C值越来越负。一方面说明甲酸对于燃烧条件敏感，氧气浓度变化对其同位素分馏产出较大的影响；另一方面说明生物质燃烧过程中，除了燃烧直接释放，还发生了其他化学反应，由于动力学同位素效应（KIE），其13C在反应过程中会被消耗，因而改变其同位素比值。相较之下，乙酸在各个氧气流速下燃烧释放的同位素比值相对稳定，侧面反映了生物质燃烧初级释放是乙酸主要的来源。（3）为了扩大校准的VOC数量，我们在实验室内建立了一套渗透系统，自制渗透管可校准各个类型的VOC。这对于无法存储于气罐中的化合物尤为重要，例如甲酸、乙酸、半挥发性有机化合物（SVOC）以及活性较强的VOCs。除此之外，市售的混合标准气样，浓度难免会有误差。而且随着时间推移，混合标准气瓶中的VOCs浓度会发生变化，也需要一套系统能联合验证VOC浓度。渗透系统的原理是，通过加热渗透管中的VOC从管壁逸出，在流速为20 sccm（sccm，standard cubic centimeter per minute）的载气推送下到达铂催化管，催化成CO2，CO2检测仪测算出CO2浓度，由已知CO2浓度计算VOC浓度。将基于渗透的校准得出的VOC浓度与已知浓度的气瓶标样进行比较，两者的一致性均在5%以内，证明该系统的有效性。同时通过渗透系统有效的检测出甲酸和乙酸在PTR-ToF的灵敏度分别是6.17 ± 0.30 ncps/ppb和7.78 ± 0.29 ncps/ppb，为后续PTR-ToF大气环境中现场实测甲酸和乙酸浓度奠定基础。（4）实测了米苏拉森林大火爆发期间，污染环境中甲酸和乙酸的时空分布特点。发现在此期间甲酸的浓度为0.5-7 μg/m3，乙酸的浓度为2.8-8 μg/m3。相较于之前文献报道的大气中甲酸、乙酸的浓度，本研究监测到的甲酸乙酸浓度更高，说明生物质燃烧可能是甲酸和乙酸的一个重要的来源。（5）通过臭氧与PM2.5的关系以及乙酸与甲酸的比值估算烟羽老化程度。研究发现由于燃烧会消耗臭氧，生物质燃烧释放的臭氧前体物在新鲜的烟羽中未能及时二次生成臭氧，因而生物质燃烧释放的新鲜烟团中臭氧浓度会降低。而生物质燃烧会向大气持续输送颗粒物，因而PM2.5会持续上升；乙酸与甲酸的比值（AA/FA）随着烟羽的老化而变小。因此通过上述两种方法能描述烟羽老化程度。使用参数化方法估算了PTR-ToF监测期间，烟羽的光化学年龄，推算出PTR-ToF捕获的两个烟团P1和P2，其中P1较P2老化程度高，可能来源于远处加拿大哥伦比亚大火释放的烟羽，通过长途运输抵达米苏拉的观测点；P2烟羽来自周围山火释放。（6）通过2016年FIREX实验室生物质燃烧实验提供的美国各种不同生物质燃烧排放因子，评估了生物质燃烧环境中，甲酸、乙酸各个源的贡献比例。发现相较于乙酸大量来自于生物质燃烧直接释放，生物质燃烧对甲酸的直接排放量较低。通过参数化方法及最小二乘法拟合出火灾污染环境下，甲酸的主要来源周围植物生长释放，其次是生物质燃烧初始排放和生物质燃烧二次生成；乙酸最主要来源是生物质燃烧的初始排放，其次是生物源和生物质燃烧二次生成。

Biomass burning is part of the natural composition of ecosystems. In recent years, the speed and scale of global fires have increased dramatically due to human activities. Biomass combustion is extremely harmful to the atmosphere. The combustion process transports a large amount of toxic gases and particulate matter to the atmosphere, such as aerosols, black carbon, brown carbon and thousands of VOCs ( Volatile Organic Compounds). These mixed flue gases undergo complex photochemical reactions with free radicals (such as NOx, OH-, etc.) to produce less volatile, more soluble compounds, promote the formation of new particulate matter and ozone, leading to haze formation, and human health. This poses a great threat. About 7 million people worldwide die prematurely each year due to air pollution. Biomass combustion releases large amounts of formic acid (HCOOH, FA) and acetic acid (CH3COOH, AA) into the atmosphere. As an important precursor of ozone and organic aerosols, formic acid and acetic acid play an important role in tropospheric chemistry. Although the predecessors have a deeper understanding of the sources of formic acid and acetic acid, the relative input quantization of each source is not accurate, and there is a lack of understanding of the process affecting the concentration. Resulting in the large gap between the real concentration of formic acid, acetic acid in the free troposphere and the predicted concentration of the model observed. It has been reported in the literature that the emission factor of formic acid and acid from biomass burning, but most of the described objects are forest fires. And there are few reports EFs from agricultural waste combustion. Therefore, this paper mainly carried out the following three aspects of research work: (1) The laboratory built a small combustion device. By changing the concentration of the combustion gas, the release factors of formic acid and acetic acid released by burning the agricultural waste under different combustion conditions can be simulated. Provide raw data for the emission inventory;(2) using the needle micro-extraction method combined with GC-IRMS technology to detect the stable carbon isotope composition of formic acid and acetic acid released by burning; (3) detecting the forest fire in the western United States by PTR-ToF-MS The distribution characteristics of formic acid, acetic acid and its precursors in the atmosphere.In this paper, the following conclusions are obtained by analyzing the release characteristics of formic acid, acetic acid and carbon isotope composition released by biomass combustion:(1) Build a small combustion platform in the laboratory. The burning experiments of five common agricultural wastes (wheat straw, corn stover, rape straw, rice straw and cotton straw) were completed in the laboratory. The lye absorption system is established, and the absorption efficiency of the lye is as high as 88% or more, and the formic acid and acetic acid in the gas phase released by the combustion of the biomass can be effectively absorbed. The release factors of formic acid and acetic acid released from the combustion of five biomasses were: the average release factors of formic acid and acetic acid released from corn stover burning were 0.36 ± 0.19 g/kg and 3.1 ± 1.6 g/kg, respectively; The average release factors of acetic acid were 0.24 ± 0.18 g/kg and 1.042 ± 0.76 g/kg, respectively. The average release factors of formic acid and acetic acid from burning and burning of rape straw were 0.24 ± 0.14 g/kg and 1.76 ± 1.22 g/kg, respectively. The average release factors of formic acid and acetic acid released from cotton straw burning were 0.17 ± 0.08 g/kg and 1.54 ± 0.81 g/kg, respectively. The average release factors of formic acid and acetic acid released from wheat straw burning were 0.36 ± 0.23 g/kg, respectively. 2.2 ± 1.26 g/kg.The initial ratio of acetic acid and formic acid released by the combustion of five biomasses, ie, Rt = 0 = [AA / FA] t = 0, where R corn = 8.06; R rape = 9.09; R cotton = 9.37; R rice = 4.31; Wheat = 6.55. Since biomass combustion releases large amounts of formic acid precursors (such as isoprene, MVK, etc.), these precursors undergo a photochemical reaction in the atmosphere to produce large amounts of formic acid. In addition, formic acid has a longer life than acetic acid in the atmosphere (3.2 days for formic acid and 2.3 days for acetic acid). Therefore, the plume released by biomass burning will have a smaller AA/FA ratio during aging. The ratio of the initial release factor of formic acid and acetic acid released by straw burning and the ratio of acetic acid to formic acid not only provides the basic data of biomass burning source for the source list, but also provides a basis for describing the degree of aging of flue gas released from field straw burning.(2) The stable carbon isotope ratio of formic acid and acetic acid in the initial release of biomass combustion in the laboratory was examined. The stable carbon isotope ratios of formic acid and acetic acid released from rice straw burning were -14.49 ± 5.08 ‰ and -22.59 ± 2.55 ‰ respectively; the stable carbon isotope ratios of formic acid and acetic acid released from wheat straw burning were -15.59 ± 3.16 ‰ and -23.53 ± 2.49 respectively. The stable carbon isotope ratios of formic acid and acetic acid released from rapeseed straw burning were -20.26 ± 2.92 ‰ and -31.06 ± 0.96 ‰ respectively; the carbon isotope ratios of formic acid and acetic acid released by cotton straw burning were -22.33 ± 5.57 ‰ and -31.39 ± 0.81, respectively. The stable carbon isotope ratios of formic acid and acetic acid released from corn stover burning were -20.18 ± 2.93 ‰ and -30.77 ± 1.75 ‰, respectively; the stable carbon isotope ratios of formic acid and acetic acid released from sorghum straw burning were -17.07 ± 4.31 ‰ and -22.08 ± 2.85 respectively. The five biomasses were burned under different oxygen conditions, and the released formic acid had obvious isotope fractionation. For example, the δ13C value of formic acid released by rice straw burning increased with the increase of oxygen flow rate, and the δ13C value of formic acid became more and more negative. On the one hand, it indicates that formic acid is sensitive to combustion conditions, and the change of oxygen concentration has a great influence on its isotope fractionation output. On the other hand, in the process of biomass combustion, in addition to the direct release of combustion, other chemical reactions occur due to the kinetic isotope effect. (KIE), its13C is consumed during the reaction, thus changing its isotope ratio. In contrast, the isotope ratio of acetic acid released at various oxygen flow rates is relatively stable, reflecting the primary release of biomass combustion as the primary source of acetic acid.(3) In order to increase the number of calibrated VOCs, we have established an infiltration system in the laboratory, and the self-made permeation tube can calibrate various types of VOCs. This is especially important for materials that cannot be stored in a gas cylinder mixture during calibration, such as formic acid, acetic acid, semi-volatile organic compounds (SVOC), and more reactive VOCs. In addition, the commercially available mixed standard gas sample will inevitably have errors and the VOC concentration in the mixed standard gas cylinder will change over time. A system combination can also be used to verify the VOC concentration. The osmotic device transports the VOC through a self-made permeation tube to a platinum catalytic tube through a carrier gas having a flow rate of 20 sccm (sccm, standard cubic centimeter per minute), catalyzes the formation of CO2, and then calculates the CO2 concentration by a carbon dioxide detector. The concentration is calculated as the VOC concentration. We validated the PTR-ToF with this osmotic system in conjunction with the standard cylinders of VOCs. The VOC concentration based on the permeation calibration was compared with the known concentration of the cylinder standard, and the consistency of both was within 5%, demonstrating the effectiveness of the system. At the same time, the sensitivity of formic acid and acetic acid in PTR-ToF was 6.17 ± 0.30 ncps/ppb and 7.78 ± 0.29 ncps/ppb, respectively, which was used to determine the concentration of formic acid and acetic acid in the PTR-ToF atmosphere.(4) The temporal and spatial distribution characteristics of formic acid and acetic acid in the polluted environment during the outbreak of the Missoula forest fire were measured. The concentration of formic acid was found to be 0.5-7μg/m3during this period, and the concentration of acetic acid was 2.8-8μg/m3. Compared to the concentration of formic acid in the atmosphere reported in the literature, the concentration of formic acid acetic acid monitored in this study is higher, indicating that biomass burning may be an important source of formic acid and acetic acid.(5) Estimating the degree of aging of the plume by the relationship between ozone and PM2.5 and the ratio of acetic acid to formic acid. The study found that due to the ozone consumed by combustion, the ozone precursor released by biomass combustion failed to generate ozone in a timely manner in fresh plume, so the ozone concentration in the fresh smoke group released by biomass combustion decreased. While biomass combustion continues to transport particulate matter to the atmosphere, PM2.5 will continue to rise; the ratio of acetic acid to formic acid (AA/FA) will decrease as the plume ages. Therefore, the degree of aging of the plume can be described by the above two methods. Using the parametric method to estimate the photochemical age of plume during PTR-ToF monitoring, we estimated the two cigarettes P1 and P2 captured by PTR-ToF, where P1 is more aging than P2 and may be derived from the release of the Canadian Columbia fire in the distance. The plume arrives at the observation point in Missoula by long-distance transportation; the P2 plume is released from the surrounding hill fire. (6) Through the 2016 FIREX laboratory biomass burning experiments provided by the US various biomass burning emission factors, the contribution ratio of formic acid and acetic acid sources in the biomass combustion environment was evaluated. It was found that the direct discharge of formic acid by biomass combustion was lower than the direct release of biomass from biomass combustion. By parameterization method and least squares method, the main source of formic acid is not the direct release of fire source, but the release of surrounding plants. The main source of acetic acid is the initial emission of biomass combustion, followed by biological source. Secondary production with biomass combustion.