The surface of the alveoli of the human body is covered with a layer of liquid, or alveolar surface lining fluid. This layer is like a natural line of defense, as it contains several important antioxidants, such as ascorbic acid (commonly known as vitamin C), glutathione, and uric acid. The human body uses them to resist strong oxidizing pollutants from the environment, such as ozone and other free radicals. However, if the acidity of this layer of liquid increases, for example, the human body inhales high-acid aerosols, it may affect the antioxidant capacity of these antioxidant molecules. According to liquid-phase experimental observations in the literature, the reactivity of ascorbic acid to ozone will indeed decrease significantly in a high acidic condition. However, the experimental observations of the reaction characteristics of ascorbic acid in a heterogeneous phase, such as the gas-liquid interface formed by the thin film liquid, are still divergent and need to be further clarified. Therefore, the research team of our center exploited the characteristics of the size of micro-droplets to be similar to the thickness of the alveolar surface lining fluid and used the technology of aerosol optical tweezers to capture single aqueous ascorbic acid micro-droplets, thereby investigating the reactivity of ascorbic acid in micro-droplets to the ozone in the atmosphere. Our research results confirmed that if the pH of the aerogel is greater than the acid dissociation constant of ascorbic acid (pKa = 4.1), the ascorbic acid in the aerosol has a high reaction rate to the ozone. However, if the pH of the aerosol is less than the pKa of ascorbic acid, the reaction rate of the ascorbic acid in the aerosol to ozone will decrease by as much as two orders of magnitude. These results imply that if the acidity of the lining fluid on the alveolar surface is increased, the antioxidant capacity of ascorbic acid in the lining fluid will be drastically reduced, allowing the unreacted ozone to diffuse to the cells and tissues on the alveoli, causing further damages further. The research results have been published in the international scientific journal Physical Chemistry Chemical Physics (link).