The concentrations of a suite of halogenated volatile organic compounds (HVOCs) were measured near Barrow, Alaska, from January to April 2005. The HVOCs are produced from the reaction of bromine and chlorine atoms with ethene and propene. During periods of decreasing ozone concentration, increases in the HVOC concentrations allowed for the calculation of the ratio of bromine to chlorine radical concentrations, based on available kinetic data. We use these concentration data to interrogate the chemistry that results in tropospheric ozone depletion in the Arctic, the possible sources of ozone depleting halogen molecules, and the spatial scale in which ozone depletion occurs. We report calculated halogen atom concentration ratios (
/) during partial ozone depletion events. The concentration ratio was observed to range from 80 ¿ 30 to 990 ¿ 300 when ozone concentrations were above 15 ppb. These data make it clear that chlorine and bromine atom chemistry is active in the Arctic troposphere beginning at twilight, even absent large-scale ozone depletion, and that the sources of the chlorine atoms are poorly understood.