Atmospheric Hydroxyl Radical Source: Reaction of Triplet SO₂ and Water

The reaction of electronically excited triplet state sulfur dioxide (³SO₂) with water was investigated both theoretically and experimentally. The quantum chemical calculations find that the reaction leads to the formation of hydroxyl radical (OH) and hydroxysulfinyl radical (HOSO) via a low energy barrier pathway. Experimentally the formation of OH was monitored via its reaction with methane, which itself is relatively unreactive with ³SO₂, making it a suitable probe of OH production from the reaction of ³SO₂ and water. This reaction has implications for the formation of OH in environments that are assumed to be depleted in OH, such as volcanic plumes. This reaction also provides a mechanism for the formation of OH in planetary atmospheres with little or no oxygen (O₂) or ozone (O₃) present.