Aircraft and satellite observations indicate the presence of ppt (ppt ≡ pmol/mol) levels of BrO in the free troposphere with important implications for the tropospheric budgets of ozone, OH, and mercury. We can reproduce these observations with the GEOS‐Chem global tropospheric chemistry model by including a broader consideration of multiphase halogen (Br‐Cl) chemistry than has been done in the past. Important reactions for regenerating BrO from its nonradical reservoirs include HOBr + Br−/Cl− in both aerosols and clouds, and oxidation of Br− by ClNO3 and ozone. Most tropospheric BrO in the model is in the free troposphere, consistent with observations and originates mainly from the photolysis and oxidation of ocean‐emitted CHBr3. Stratospheric input is also important in the upper troposphere. Including production of gas phase inorganic bromine from debromination of acidified sea salt aerosol increases free tropospheric Bry by about 30%. We find HOBr to be the dominant gas‐phase reservoir of inorganic bromine. Halogen (Br‐Cl) radical chemistry as implemented here in GEOS‐Chem drives 14% and 11% decreases in the global burdens of tropospheric ozone and OH, respectively, a 16% increase in the atmospheric lifetime of methane, and an atmospheric lifetime of 6 months for elemental mercury. The dominant mechanism for the Br‐Cl driven tropospheric ozone decrease is oxidation of NOx by formation and hydrolysis of BrNO3 and ClNO3. Key Points Recent BrO observations are interpreted using a new GEOS‐Chem coupled Br‐Cl simulation Multiphase oxidation of Br‐ by ozone is critical for maintaining the high observed levels of BrO Br and Cl lower the global burden of O3 by 14% by increasing NOx loss and shortening the O3 lifetime