School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK.
Phys Chem Chem Phys. 2018 Jun 6;20(22):15560-15573. doi: 10.1039/c8cp01666f.
Mass transfer between the gas and condensed phases in aerosols can be limited by slow bulk diffusion within viscous particles. During the heterogeneous and multiphase reactions of viscous organic aerosol particles, it is necessary to consider the interplay of numerous mass transfer processes and how they are impacted by viscosity, including the partitioning kinetics of semi-volatile organic reactants, water and oxidants. To constrain kinetic models of the heterogeneous chemistry, measurements must provide information on as many observables as possible. Here, the ozonolysis of maleic acid (MA) in ternary aerosol particles containing water and sucrose is used as a model system. By varying the mass ratio of sucrose to MA and by performing reactions over a wide range of relative humidity, direct measurements show that the viscosity of the particle can be varied over 7 orders of magnitude. Measurements of the volatilisation kinetics of MA show that this range in viscosity leads to a suppression in the effective vapour pressure of MA of 3-4 orders of magnitude. The inferred values of the diffusion coefficient of MA in the particle phase closely mirror the expected change in diffusion coefficient from the Stokes-Einstein equation and the change in viscosity. The kinetics of ozonolysis show a similar dependence on particle viscosity that can be further investigated using the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB). Two scenarios, one constraining the diffusion coefficients for MA to those expected based on the Stokes-Einstein equation and the other including the diffusion coefficients as a fit parameter, yield similarly adequate representations of the ozonolysis kinetics, as inferred from the experimental decay in the signature of the vinylic C-H stretching vibration of MA. However, these two scenarios provide very different parameterisations of the compositional dependence of the diffusion coefficients of ozone within the condensed phase, yielding qualitatively different time-dependent internal concentration profiles. We suggest that this highlights the importance of providing additional experimental observables (e.g. particle size, heterogeneity in composition) if measurements and models are to be universally reconciled.
气溶胶中气相和凝聚相之间的质量传递可能会受到粘性颗粒内缓慢的体扩散的限制。在粘性有机气溶胶颗粒的多相和多相反应中，有必要考虑到许多传质过程的相互作用以及它们如何受到粘度的影响，包括半挥发性有机反应物、水和氧化剂的分配动力学。为了约束多相化学动力学模型，测量必须提供尽可能多的可观测值的信息。在这里，马来酸（MA）在含有水和蔗糖的三元气溶胶颗粒中的臭氧化作用被用作模型体系。通过改变蔗糖与 MA 的质量比，并在很宽的相对湿度范围内进行反应，直接测量表明颗粒的粘度可以在 7 个数量级内变化。MA 的挥发动力学测量表明，这种粘度范围导致 MA 的有效蒸气压抑制了 3-4 个数量级。MA 在颗粒相中的扩散系数的推断值与预期的从 Stokes-Einstein 方程得到的扩散系数的变化和粘度的变化非常吻合。臭氧化动力学也表现出对颗粒粘度的类似依赖性，这可以使用气溶胶表面和体化学的动力学多层模型（KM-SUB）进一步研究。两种情况，一种将 MA 的扩散系数约束为基于 Stokes-Einstein 方程的预期值，另一种将扩散系数作为拟合参数包括在内，都可以对臭氧化动力学进行类似的适当表示，这可以从 MA 的乙烯基 C-H 伸缩振动特征的实验衰减中推断出来。然而，这两种情况对臭氧在凝聚相中的扩散系数的组成依赖性提供了非常不同的参数化，产生了定性上不同的随时间变化的内部浓度分布。我们建议，如果要普遍协调测量和模型，那么提供额外的实验可观测值（例如颗粒大小、组成异质性）非常重要。
