The pH of a solution is one of its most fundamental chemical properties, impacting reaction pathways and kinetics across every area of chemistry. The atmosphere is no different, with the pH of the condensed phase driving key chemical reactions that ultimately impact global climate in numerous ways. The condensed phase in the atmosphere is comprised of suspended liquid or solid particles, known as the atmospheric aerosol, which are differentiated from cloud droplets by their much smaller size (primarily <10 μm). The pH of the atmospheric aerosol can enhance certain chemical reactions leading to the formation of additional condensed phase mass from lower volatility species (secondary aerosol), alter the optical and water uptake properties of particles, and solubilize metals that can act as key nutrients in nutrient-limited ecosystems or cause oxidative stress after inhalation. However, despite the importance of aerosol acidity for climate and health, our fundamental understanding of pH has been limited due to aerosol size (by number >99% of particles are <1 μm) and complexity. Within a single atmospheric particle, there can be hundreds to thousands of distinct chemical species, varying water content, high ionic strengths, and different phases (liquid, semisolid, and solid). Making aerosol analysis even more challenging, atmospheric particles are constantly evolving through heterogeneous reactions with gases and multiphase chemistry within the condensed phase. Based on these challenges, traditional pH measurements are not feasible, and, for years, indirect and proxy methods were the most common way to estimate aerosol pH, with mixed results. However, aerosol pH needs to be incorporated into climate models to accurately determine which chemical reactions are dominant in the atmosphere. Consequently, experimental measurements that probe pH in atmospherically relevant particles are sorely needed to advance our understanding of aerosol acidity.

中文翻译：

---

气溶胶酸度：大气化学的新型测量方法和意义。

溶液的pH值是其最基本的化学性质之一，会影响整个化学领域的反应途径和动力学。大气没有什么不同，冷凝相的pH值驱动着关键的化学反应，这些化学反应最终以多种方式影响全球气候。大气中的凝结相由悬浮的液体或固体颗粒（称为大气气溶胶）组成，这些颗粒通过小得多的尺寸（主要是<10μm）与云滴相区别。大气气溶胶的pH值可以增强某些化学反应，从而导致较低挥发性物质（次级气溶胶）形成额外的冷凝相质量，从而改变颗粒的光学和吸水特性，并溶解金属，这些金属可以在营养有限的生态系统中充当关键营养物质，或者在吸入后引起氧化应激。然而，尽管气溶胶酸度对于气候和健康至关重要，但由于气溶胶尺寸（数量大于99％的颗粒小于1微米），我们对pH值的基本理解受到了限制。在单个大气颗粒中，可能有成百上千种不同的化学物质，不同的水含量，高离子强度和不同的相（液体，半固体和固体）。使气溶胶分析更具挑战性的是，大气颗粒通过与气体的异相反应和冷凝相内的多相化学物质不断演化。基于这些挑战，传统的pH测量方法不可行，而且多年来，间接法和替代法是估算气溶胶pH值的最常用方法，结果不一。但是，需要将气溶胶pH值纳入气候模型中，以准确确定哪些化学反应在大气中占主导地位。因此，迫切需要进行实验测量以探测与大气有关的颗粒中的pH值，以增进我们对气溶胶酸度的了解。