Morphological transformation of soot: investigation of microphysical processes during the condensation of sulfuric acid and limonene ozonolysis product vapors - Soot particle mobility-mass relationship measurement

The morphological transformation of soot particles via condensation of low-volatility materials constitutes a dominant atmospheric process with serious implications for the optical and hygroscopic properties, and atmospheric lifetime of the soot. We consider the morphological transformation of soot aggregates under the influence of condensation of vapors of sulfuric acid, and/or limonene ozonolysis products. This influence was systematically investigated using a Differential Mobility Analyzer-Aerosol Particle Mass Analyzer (DMA-APM) and the Tandem DMA techniques integrated with a laminar flow-tube system. We hypothesize that the morphology transformation of soot results (in general) from a two-step process, i.e., (i) filling of void space within the aggregate; (ii) growth of the particle diameter. Initially, the transformation was dominated by the filling process followed by growth, which led to the accumulation of sufficient material that exerted surface forces, which eventually facilitating further filling. The filling of void space was constrained by the initial morphology of the fresh soot as well as the nature and the amount of condensed material. This process continued in several sequential steps until all void space within the soot aggregate was filled. And then “growth” of a spherical particle continued as long as vapors condensed on it. We developed a framework for quantifying the microphysical transformation of soot upon the condensation of various materials. This framework used experimental data and the hypothesis of “ideal sphere growth” and void filling to quantify the distribution of condensed materials in the complementary filling and growth processes. Using this framework, we quantified the percentage of material consumed by these processes at each step of the transformation. For the largest coating experiments, 6%, 10%, 24% and 58% of condensed material went to filling process, while 94%, 90%, 76% and 42% of condensed material went to growth process for 75 nm, 100 nm, 150 nm and 200 nm soot particles, respectively. We also used the framework to estimate the fraction of internal voids and open voids. This information was then used to estimate the volume equivalent diameter of the soot aggregate containing internal voids and to calculate the dynamic shape factor, accounting for internal voids. The dynamic shape factor estimated based on the traditional assumption (of no internal voids) differed significantly from the value obtained in this study. Internal voids are accounted for in the experimentally derived dynamic shape factor determined in the present study. In fact, the dynamic shape factor adjusted for internal voids was close to 1 for the fresh soot particles considered in this study, indicating the particles were largely spherical. The effective density was strongly correlated with the morphological transformation responses to the condensed material on the soot particle and the resultant effective density was determined by the (i) nature of the condensed material; (ii) morphology and size of the fresh soot. In this work we quantitatively tracked in-situ microphysical changes in soot morphology, providing details of both fresh and coated soot particles at each step of the transformation. This framework can be applied to model development with significant implications for quantifying the morphological transformation (from the viewpoint of hygroscopic and optical properties) of soot in the atmosphere. Purpose: Study of morphological transformation of soot particles Data is collected by instrument Differential Mobility Analyzer-Aerosol Particle Mass Analyzer (DMA-APM), program based on LabVIEW is used to control the instrument and log the data.低挥发性物质冷凝作用引发的炭黑颗粒形貌演化，是一类关键的大气过程，对炭黑的光学特性、吸湿性以及大气停留时长均具有显著影响。本研究聚焦于硫酸蒸汽、柠檬烯臭氧分解产物蒸汽，或二者的混合蒸汽冷凝作用下，炭黑团聚体（soot aggregates）的形貌演化过程。本研究采用集成层流管系统的差分迁移分析仪-气溶胶颗粒质量分析仪（Differential Mobility Analyzer-Aerosol Particle Mass Analyzer, DMA-APM）与串联差分迁移分析（Tandem DMA）技术，对该影响过程开展了系统性探究。 我们提出假设：炭黑颗粒的形貌演化（总体而言）可通过两步过程实现，即：(i) 填充团聚体内部的空隙空间；(ii) 颗粒直径的增长。演化初期，填充过程主导形貌变化，随后转为直径增长；该过程会累积足够多的物质以产生表面作用力，最终进一步促进空隙填充。空隙填充过程受新鲜炭黑的初始形貌、冷凝物质的性质与添加量共同约束。该过程以多步序列方式推进，直至炭黑团聚体内部的所有空隙均被填满。此后，当蒸汽持续在颗粒表面冷凝时，球形颗粒的“生长”过程便会持续进行。 我们构建了一套用于量化多种物质冷凝下炭黑微观物理演化过程的分析框架。该框架结合实验数据与“理想球体生长”及空隙填充假设，对相辅相成的填充与生长过程中冷凝物质的分布情况进行量化。依托该框架，我们量化了形貌演化各步骤中两类过程所消耗的物质占比。针对最大粒径的包覆实验，75 nm、100 nm、150 nm及200 nm炭黑颗粒中，分别有6%、10%、24%与58%的冷凝物质用于空隙填充，剩余94%、90%、76%与42%的冷凝物质用于颗粒直径增长。 我们还利用该框架估算了内部空隙与开放空隙的占比。基于该估算结果，我们进一步计算了含内部空隙的炭黑团聚体的体积等效直径，并结合内部空隙因素推导了动态形状因子。基于传统无内部空隙假设估算得到的动态形状因子，与本研究所得结果存在显著差异。本研究推导的实验动态形状因子已纳入内部空隙的影响。事实上，针对本研究中的新鲜炭黑颗粒，经内部空隙校正后的动态形状因子接近1，表明该类颗粒整体近似球形。 有效密度与炭黑颗粒对冷凝物质的形貌演化响应具有强相关性，最终的有效密度由以下因素决定：(i) 冷凝物质的性质；(ii) 新鲜炭黑的形貌与粒径。本研究实现了炭黑形貌原位微观物理变化的定量追踪，为演化各阶段的新鲜炭黑与包覆炭黑颗粒提供了详细表征数据。该分析框架可应用于模型开发，对量化大气中炭黑的形貌演化（从吸湿性与光学特性角度）具有重要意义。 研究目的：炭黑颗粒的形貌演化研究 实验数据通过差分迁移分析仪-气溶胶颗粒质量分析仪（Differential Mobility Analyzer-Aerosol Particle Mass Analyzer, DMA-APM）采集，基于LabVIEW开发的程序用于控制仪器并记录实验数据。