An Experimental Data Base and Modeling Effort to Describe the Atmospheric Gas-Aerosol Mass Transfer of Semi-Volatile Organics

本研究的目的是提高我们对控制半挥发性有机化合物气相到气溶胶相分布的因素的认识。该程序的结果将为建模者提供工具，以预测气体和颗粒浓度随时间变化时半挥发性物质的蒸发和冷凝。目前预测有机分配的大多数努力都是基于平衡理论。尽管这些努力大大提高了我们对理想条件下的分配过程的理解，但有强烈的迹象表明，对于大多数感兴趣的化合物，在相对较短的时间尺度（几分钟到几小时）内，气体和颗粒相之间的大气平衡可能无法建立。为了获得实验数据来开发和测试气相和颗粒相之间的半挥发性传质模型，燃烧排放将在一个190cm3的大型室外特氟龙膜环境室中进行老化。多核芳烃（PAHs）将作为示例化合物，但该方法将适用于其他有机半挥发性有毒物质。将监测颗粒和气相中多环芳烃的净损失率。多环芳烃从颗粒中传质的速率将使用一个新的颗粒内部扩散模型来估计，该模型考虑了颗粒周围液体有机层的传质。将使用三种不同类型的粒子。它们将由木材、柴油和煤炭燃烧产生，以代表颗粒类型，这些颗粒类型具有不同数量的“液相”，围绕着内部碳核心。预计形成颗粒外层的有机物质的数量和类型将强烈影响内部有机颗粒的扩散，从表面解吸的焓，以及最终颗粒半挥发性吸收和损失的速率。通过改变腔室中半挥发性蒸汽和/或颗粒的浓度，从而扰乱气相和颗粒相有机之间的平衡，这些研究人员将能够研究半挥发性吸附（冷凝）和颗粒之间的演化速率。为了从根本上改变平衡条件，一些实验将通过将燃烧排放物通过大型气提塔/剥蚀器来进行，以在颗粒进入腔室之前去除气相半挥发性成分。将半挥发性氘化多环芳烃加入气相以观察粒子的传质速率。

The purpose of this research is to improve our understanding of the factors that control the gas to aerosol phase distribution of semi-volatile organic compounds. Results from this program will provide modelers with tools to predict vaporization and condensation of semi-volatiles as gas and particle concentrations change with time. Most of the current efforts to predict organic partitioning have been based on equilibrium theory. Although these efforts have substantially advanced our understanding of the partitioning process under ideal conditions, there are strong indications that atmospheric equilibrium between the gas and particle phases may not be established for most compounds of interest over relatively short time scales (minutes to hours). To obtain experimental data to develop and test models of semi-volatile mass transfer between the gas and particle phases, combustion emissions will be aged in a large 190cm3 outdoor Teflon film environmental chamber. Polynuclear aromatic hydrocarbons (PAHs) will be used as example compounds, but the methodology will be applicable to other organic semi-volatile toxics. Net PAH loss rates from particles and the gas phase will be monitored. Rates of PAH mass transfer to the from the particles will be estimated using a new inner particle diffusion model which considers mass transfer in a liquid organic layer surrounding the particle. Three different types of particles will be used. They will be generated from wood, diesel and coal combustion to represent particle types which have different amounts of a "liquid phase" surrounding an inner carbon core. It is expected that the amount and type of organic material that forms this outer layer of the particle will strongly influence inner organic particle diffusion, enthalpies of desorption from the surface, and ultimately rates of particle semi- volatile uptake and loss. By changing the concentration of semi-volatile vapors and/or particles in the chamber, an d thereby perturbing the equilibrium between gas and particle phase organic, these researchers will be able to investigate rates of semi-volatile sorption (condensation) and evolution to and from particles. To radically alter equilibrium conditions, some experiments will be conducted by passing combustion emissions through a large gas stripper/denuder to remove the gas phase semi-volatile component before particles enter the chamber. Semi-volatile deuterated PAH will be added to the gas phase to observe rates of mass transfer to the particles.