Photo-generated hydroxyl radicals contribute to the formation of halogen radicals leading to ozone depletion on and within polar stratospheric clouds surface.

Polar stratospheric clouds (PSCs), of which the surface is a dynamic liquid water layer and might consist of aqueous HNO ₃ and H ₂ O ₂ , is a well-known key meteorological condition contributing to the ozone hole in the polar stratosphere. PSCs has been considered to provide abundant surface for the heterogeneous reactions causing the formation of the Cl ₂ and HOCl, which are further photolyzed into Cl and ClO radicals leading to the ozone destruction. Here we demonstrated that the sunlight drives the massive and stable production of OH radicals in aqueous HNO ₃ and its main photo-induced byproduct HNO ₂ . We also found that the photo-generated OH radicals in aqueous HNO ₃ , HNO ₂ and H ₂ O ₂ have the remarkable capability to react with the dissolved HCl, Cl ^- and Br ^- to form halogen radicals. In addition, we observed that the H ₂ O ₂ can react with dissolved HCl and Br ^- in darkness to form and release Cl ₂ and Br ₂ gases, which could further be photolyzed into reactive halogen radicals whenever sunlight is available. All these findings suggest that, except for the well-known heterogeneous reactions, photochemical reactions involving the aqueous HNO ₃ and H ₂ O ₂ on and within PSCs surface might constitute another important halogen activation pathway for ozone destruction. This study may shed deeper insights into the mechanism of halogen radicals resulting in ozone depletion in polar stratosphere.
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