Red‐Light‐Induced Decomposition of an Organic Peroxy Radical: A New Source of the HO 2 Radical

The gas‐phase decomposition of the α ‐hydroxy methylperoxy radical has been theoretically examined, and the results provide insight into a new source of the hydroperoxy radical (HO 2 ) in the troposphere. Bimolecular peroxy decomposition is promoted by the red‐light or near‐IR radiation excitation. The calculations suggest for the first time, an important chemical role for the H 2 O⋅HO 2 radical complex that exist in significant abundance in the troposphere. In particular, the reaction of organic peroxy radicals with the HO 2 radical and the H 2 O⋅HO 2 radical complex represent an autocatalytic source of atmospheric HO 2 . This reaction is a new example of red‐light‐initiated atmospheric chemistry that may help in understanding the discrepancy between the observed and measured levels of the HO x at sunrise.