We report a new method to investigate water transport kinetics in aerosol particles by using rapid scan FTIR spectroscopy combined with a custom-built pulse relative humidity (RH) control system. From real time in situ measurements of RH and composition using high time resolution infrared spectroscopy (0.12 s for one spectrum), and through achieving a high rate of RH change (as fast as 60% per second), we are able to investigate the competition between the gas and condensed phase diffusive transport limits of water for particles with mean diameter ∼3 μm and varying phase and viscosity. The characteristic time (τ) for equilibration in particle composition following a step change in RH is measured to quantify dissolution timescales for crystalline particles and to probe the kinetics of water evaporation and condensation in amorphous particles. We show that the dissolution kinetics are prompt for crystalline inorganic salt particles following an increase in RH from below to above the deliquescence RH, occurring on a timescale comparable to the timescale of the RH change (<1 s). For aqueous sucrose particles, we show that the timescales for both the drying and condensation processes can be delayed by many orders of magnitude, depending on the viscosity of the particles in the range 10¹ to 10⁹ Pa s considered here. For amorphous particles, these kinetics are shown to be consistent with previous measurements of mass transfer rates in larger single particles. More specifically, the consistency suggests that fully understanding and modelling the complex microphysical processes and heterogeneities that form in viscous particles may not be necessary for estimating timescales for particle equilibration. A comparison of the kinetics for crystalline and amorphous particles illustrates the interplay of the rates of gas and condensed phase diffusion in determining the mass transport rates of water in aerosols.

中文翻译：

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快速扫描真空FTIR法测定粘性和玻璃态气溶胶颗粒中的扩散系数

我们报告了一种通过使用快速扫描FTIR光谱结合定制脉冲相对湿度（RH）控制系统来研究气溶胶颗粒中水输送动力学的新方法。从实时原位使用高时间分辨率红外光谱（0.12 S代表一个谱）RH的测量和组成，并通过实现RH高变化率（每秒速度为60％），我们可以探讨竞争平均直径约3μm且相和粘度变化的粒子在水的气相和冷凝相扩散输运极限之间的关系。特征时间（τ），以测量RH的阶跃变化后颗粒组成的平衡，以量化结晶颗粒的溶解时间尺度，并探究无定形颗粒中水蒸发和冷凝的动力学。我们表明，溶出动力学对于结晶无机盐颗粒是迅速的，其相对湿度从下到上潮解RH的增加是随时间的，该时间尺度与RH变化的时间尺度可比（<1 s）。对于蔗糖水溶液颗粒，我们表明干燥和冷凝过程的时间尺度可以延迟许多数量级，具体取决于颗粒的粘度范围为10 ¹至10 ⁹Pa在这里考虑过。对于无定形颗粒，这些动力学与先前在较大的单个颗粒中的传质速率测量结果一致。更具体地讲，一致性表明，对于估计颗粒平衡的时间尺度，可能不需要完全理解和建模在粘性颗粒中形成的复杂的微物理过程和异质性。结晶颗粒和无定形颗粒的动力学比较表明，气体和冷凝相扩散速率之间的相互作用决定了气溶胶中水的质量传输速率。