Sensitivity of ozone to bromine in the lower stratosphere

Abstract

Measurements of BrO suggest that inorganic bromine (Br y ) at and above the tropopause is 4 to 8 ppt greater than assumed in models used in past ozone trend assessment studies. This additional bromine is likely carried to the stratosphere by short‐lived biogenic compounds and their decomposition products, including tropospheric BrO. Including this additional bromine in an ozone trend simulation increases the computed ozone depletion over the past ∼25 years, leading to better agreement between measured and modeled ozone trends. This additional Br y (assumed constant over time) causes more ozone depletion because associated BrO provides a reaction partner for ClO, which increases due to anthropogenic sources. Enhanced Br y causes photochemical loss of ozone below ∼14 km to change from being controlled by HO x catalytic cycles (primarily HO 2 +O 3 ) to a situation where loss by the BrO+HO 2 cycle is also important.