2) We observed GEM depletion before sunrise. Even for bromine-rich volcanic plumes, most bromine is initially in the form of HBr, which does not oxidize mercury. To generate sufficient radical bromine species to completely deplete atmospheric GEM within a few hours, a "bromine explosion" would most likely need to occur . Such a process requires UV radiation. However, the first complete GEM depletion was observed entirely before sunrise (solar zenith angle between 96° and 93°, see Figure ), so that practically no radiation was available (TUV model; UVA < 1 • 10 -3 W m -2 , UVB < 7 • 10 -2 W m -2 ), and photolysis rates for Br-relevant photochemical reactions were very low (TUV model; k < 1.3 • 10 -5 s -1 for Br2 → Br + Br; k < 5.0 • 10 -5 s -1 for BrO → Br + O).