Chen Zhe, Liu Pai, Wang Weigang, Cao Xue, Liu Yu-Xin, Zhang Yun-Hong, Ge Maofa
Institute of Chemical Physics, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China.
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
Environ Sci Technol. 2022 Jun 21;56(12):7637-7646. doi: 10.1021/acs.est.2c00112. Epub 2022 May 31.
Severe winter haze events in Beijing and North China Plain are characterized by rapid production of sulfate aerosols with unresolved mechanisms. Oxidation of SO by O in the absence of metal catalysts (uncatalyzed autoxidation) represents the most ubiquitous SO conversion pathway in the atmosphere. However, this reaction has long been regarded as too slow to be atmospherically meaningful. This traditional view was based on the kinetic studies conducted in bulk dilute solutions that mimic cloudwater but deviate from urban aerosols. Here, we directly measure the sulfate formation rate via uncatalyzed SO autoxidation in single (NH)SO microdroplets, by using an aerosol optical tweezer coupled with a cavity-enhanced Raman spectroscopy technique. We find that the aqueous reaction of uncatalyzed SO autoxidation is accelerated by two orders of magnitude at the high ionic strength (∼36 molal) conditions in the supersaturated aerosol water. Furthermore, at acidic conditions (pH 3.5-4.5), uncatalyzed autoxidation predominately occurs on droplet surface, with a reaction rate unconstrained by SO solubility. With these rate enhancements, we estimate that the uncatalyzed SO autoxidation in aerosols can produce sulfate at a rate up to 0.20 μg m hr, under the winter air pollution condition in Beijing.
北京及华北平原地区严重的冬季雾霾事件的特点是硫酸盐气溶胶迅速生成，但其机制尚不清楚。在没有金属催化剂的情况下，SO被O氧化（无催化自氧化）是大气中最普遍的SO转化途径。然而，长期以来，该反应一直被认为过于缓慢，在大气中无足轻重。这种传统观点是基于在模拟云水但与城市气溶胶不同的大量稀溶液中进行的动力学研究得出的。在此，我们通过使用气溶胶光镊结合腔增强拉曼光谱技术，直接测量单个(NH)SO微滴中无催化SO自氧化生成硫酸盐的速率。我们发现，在过饱和气溶胶水的高离子强度（约36摩尔）条件下，无催化SO自氧化的水相反应加速了两个数量级。此外，在酸性条件（pH 3.5 - 4.5）下，无催化自氧化主要发生在液滴表面，反应速率不受SO溶解度的限制。有了这些速率增强，我们估计在北京冬季空气污染条件下，气溶胶中无催化SO自氧化生成硫酸盐的速率可达0.20 μgmhr。
