Nucleation step in soot formation can be defined as the transition from gas phase to condensed phase state at flame temperature. This aspect of particle formation in flame is the least understood despite of the great effort of the last decades. Most of the models and of the experimental setups are not able to isolate the single step of nucleation. The formation of the first particles or species in condensed phase is studied in combustion environments in which many other processes – namely PAH growth from other small hydrocarbons and oxidation – are taking place. A tubular flow reactor working in the temperature range of 800–1100 K with a stream containing pure PAH – pyrene in the current work - was used to isolate the pathways leading from gas to condensed phase. To avoid the formation of smaller compounds from the pyrene pyrolysis, temperature was kept lower than 1100 K allowing a residence time long enough to favour mass growth. DMA measurements at the exit of the reactor showed the presence of particles. Particle formation was found to be sensitive to the temperature. In particular, the formation rate was higher at the higher temperature, suggesting the intervention of some chemical-driven pathway for the formation of the first condensed phase species. The gas and the particles exiting the reactor were trapped in ethanol and analyzed by UV-Visible and fluorescence spectroscopy, and size exclusion chromatography (SEC). The formed particles have lost molecular spectral features and have a molecular weight (MW) >= 400: these characteristics allow to consider them the smallest species in condensed phase. Their MW evaluated by SEC and the analysis of their absorption spectra hinted to the predominance of oligomers formation, from dimers to tetramers, being trimers the most favored and stable species in the investigated conditions.

烟灰形成中的成核步骤可以定义为在火焰温度下从气相到冷凝相的转变。尽管最近几十年来付出了巨大的努力，但人们对火焰中颗粒形成的这一方面的了解最少。大多数模型和实验设置都无法隔离成核的单个步骤。在燃烧环境中研究了冷凝相中第一个颗粒或物质的形成，在燃烧环境中，发生了许多其他过程，即其他小烃类化合物的多环芳烃的生长和氧化。在800-1100 K的温度范围内工作的管式流动反应器，其流中含有纯PAH（在当前工作中为pyr），用于隔离从气体到冷凝相的通路。为避免the热解形成较小的化合物，温度保持在低于1100 K，允许停留时间足够长，有利于质量增长。在反应器出口处的DMA测量表明存在颗粒。发现颗粒形成对温度敏感。特别地，在较高温度下，形成速率较高，表明某些化学驱动途径对第一冷凝相物种的形成的干预。离开反应器的气体和颗粒被捕获在乙醇中，并通过紫外可见光谱和荧光光谱以及尺寸排阻色谱法（SEC）进行分析。形成的颗粒失去了分子光谱特征，分子量（MW）> = 400：这些特征使它们成为凝聚相中最小的物质。