Water is the most familiar material known to humankind; therefore, the common expectation is that scientists understand water’s chemistry and physics quite well. As a corollary, whenever a new feature/property of water is discovered, it has a wide-ranging impact on our understanding of numerous environmental processes. Recently, it has been claimed that water spontaneously transforms to hydrogen peroxide (H₂O₂) at the air–water interface of microdroplets. How covalent bonds in water could break under such mild conditions necessitates a paradigm shift in our understanding of water science and physical chemistry, in general. Notably, these reports have received much attention; also, their implications on cloud chemistry, seasonality of diseases, the origin of life, and green chemistry have been suggested. In response, we conducted a comprehensive experimental investigation of H₂O₂ formation in (i) water microdroplets sprayed over a range of liquid flow rates, the (shearing) airflow rates, and the air composition (ii) water microdroplets condensed on hydrophobic substrates formed via hot water or humidifier under controlled air composition. Specifically, we assessed the contributions of the evaporative concentration and shock waves in sprays and the effects of trace O₃(g) on the H₂O₂ formation. Glovebox experiments revealed that the H₂O₂ formation in water microdroplets was most sensitive to the air-borne ozone (O₃) concentration. In the absence of O₃(g), we could not detect H₂O₂(aq) in sprays or condensates (detection limit 250 nM). In contrast, microdroplets exposed to atmospherically relevant O₃(g) concentration (10–100 ppb) formed 2–30 M H₂O₂(aq), increasing with the gas–liquid surface area, mixing, and contact duration. Thus, the water surface area merely facilitates the O₃(g) mass transfer, which is followed by the chemical transformation of O₃(aq) into H₂O₂(aq). These findings explain the water-to-peroxide conundrum and should also help us understand its implications in natural and applied contexts.

Himanshu Mishra

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