Volcanic Plume Chemistry: Formation of BrO, ClO and OClO

Reactive halogen species BrO, ClO and OClO have recently been observed at significant concentrations in volcanic plumes in the lower troposphere. To understand their formation, a more detailed knowledge of plume composition and its evolution via physical and chemical processing in the atmosphere is required. A volcanic chemical plume model has been developed for this purpose. Results from model runs in 0-D and 2-D are presented and the formation of BrO, ClO and OClO discussed. Experiments show that the plume chemistry depends on both the chemical initialization and how the plume disperses into the background atmosphere. The model runs cover a range of chemical initializations including recent crater rim measurements and high- temperature model outputs. A set of plume-air mixing scenarios are then used to explore the effect of plume dilution on the chemistry. In addition, the plume heterogeneity and its effects are also considered. The results are combined to give an overview of how BrO, ClO and OClO can be formed in volcanic plumes, what controls their chemistry, and the implications for the atmosphere on regional scales.