Sensitivity of ozone to bromine in the lower stratosphere

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 similar to 25 years, leading to better agreement between measured and modeled ozone trends. This additional Bry (assumed constant over time) causes more ozone depletion because associated BrO provides a reaction partner for ClO, which increases due to anthropogenic sources. Enhanced Bry causes photochemical loss of ozone below similar to 14 km to change from being controlled by HOx catalytic cycles (primarily HO2+ O-3) to a situation where loss by the BrO+HO2 cycle is also important.