Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States.
Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523, United States.
Environ Sci Technol. 2021 Dec 7;55(23):15637-15645. doi: 10.1021/acs.est.1c02984. Epub 2021 Nov 23.
Secondary organic aerosol formation via condensation of organic vapors onto existing aerosol transforms the chemical composition and size distribution of ambient aerosol, with implications for air quality and Earth's radiative balance. Gas-to-particle conversion is generally thought to occur on a continuum between equilibrium-driven partitioning of semivolatile molecules to the pre-existing mass size distribution and kinetic-driven condensation of low volatility molecules to the pre-existing surface area size distribution. However, we offer experimental evidence in contrast to this framework. When catechol is sequentially oxidized by O and NO in the presence of (NH)SO seed particles with a single size mode, we observe a bimodal organic aerosol mass size distribution with two size modes of distinct chemical composition with nitrocatechol from NO oxidation preferentially condensing onto the large end of the pre-existing size distribution (∼750 nm). A size-resolved chemistry and microphysics model reproduces the evolution of the two distinct organic aerosol size modes─heterogeneous nucleation to an independent, nitrocatechol-rich aerosol phase.
二次有机气溶胶通过有机蒸气在现有气溶胶上的凝结而形成，改变了环境气溶胶的化学成分和粒径分布，对空气质量和地球辐射平衡都有影响。一般认为，气相到颗粒相的转化发生在半挥发性分子在预存在的质量粒径分布上的平衡驱动分配和低挥发性分子在预存在的表面积粒径分布上的动力学驱动凝结之间的连续体上。然而，我们提供的实验证据与这一框架相反。当儿茶酚在（NH）SO 种子颗粒存在下依次被 O 和 NO 氧化时，我们观察到一个双模态有机气溶胶质量粒径分布，具有两个不同化学组成的模态，来自 NO 氧化的硝基儿茶酚优先凝结到预存在粒径分布的较大端（约 750nm）。一个尺寸分辨的化学和微物理模型再现了两个不同的有机气溶胶尺寸模态的演化——异质成核到一个独立的、富含硝基儿茶酚的气溶胶相。
