HO₂ radicals play an important role in tropospheric chemistry. Here we propose a new direct method for measuring HO₂ radicals in the atmosphere using bromide anion chemical ionization mass spectrometry. Ambient measurements in Atlanta are presented. Instrument performance parameters: sensitivity, lower detection limit, and time resolution are discussed. We demonstrate that the technique provides excellent selectivity and is suitable for in situ ground-based HO₂ measurements.