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 ∼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.