Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Environ Sci Technol. 2020 Mar 3;54(5):2595-2605. doi: 10.1021/acs.est.9b05514. Epub 2020 Feb 10.
The diffusivity of semivolatile organic compounds (SVOCs) in the bulk particle phase of a viscous atmospheric secondary organic aerosol (SOA) can have a profound impact on aerosol growth and size distribution dynamics. Here, we investigate the bulk diffusivity of SVOCs formed from photo-oxidation of isoprene as they partition to a bimodal aerosol consisting of an Aitken (potassium sulfate) and accumulation mode (aged α-pinene SOA) particles as a function of relative humidity (RH). The model analysis of the observed size distribution evolution shows that liquid-like diffusion coefficient values of > 10 cm s fail to explain the growth of the Aitken mode. Instead, much lower values of between 2.5 × 10 cm s at 32% RH and 8 × 10 cm s at 82% RH were needed to successfully reproduce the growth of both modes. The diffusivity within the aged α-pinene SOA remains appreciably slow even at 80% RH, resulting in hindered partitioning of SVOCs to large viscous particles and allowing smaller and relatively less viscous particles to effectively absorb the available SVOCs and grow much faster than would be possible otherwise. These results have important implications for modeling SOA formation and growth in the ambient atmosphere.
半挥发性有机化合物 (SVOCs) 在粘性大气二次有机气溶胶 (SOA) 的体相中的扩散系数会对气溶胶的增长和粒径分布动力学产生深远的影响。在这里，我们研究了由异戊二烯光氧化形成的 SVOCs 在双模态气溶胶中分配时的体相扩散系数，该双模态气溶胶由艾肯（硫酸钾）和积聚模态（老化 α-蒎烯 SOA）粒子组成，作为相对湿度 (RH) 的函数。对观察到的粒径分布演化的模型分析表明，大于 10 cm s 的类似液体的扩散系数值无法解释艾肯模式的增长。相反，需要在 32% RH 下为 2.5 × 10 cm s 和在 82% RH 下为 8 × 10 cm s 之间的低得多的值才能成功重现两种模式的增长。即使在 80% RH 下，老化的 α-蒎烯 SOA 内的扩散系数仍然相当缓慢，导致 SVOCs 向大粘性颗粒的分配受阻，从而使较小且相对较不粘性的颗粒能够有效地吸收可用的 SVOCs，并比其他情况下更快地增长。这些结果对模拟环境大气中 SOA 的形成和增长具有重要意义。
