We examine the variability of ClO in the Arctic upper troposphere and lowermost stratosphere (UTLS) during the winter of 1999--2000. Data are binned relative to NOy, a species that is a proxy for photochemical age and a photochemical source of NOx. Enhancements in the /y> ratio relative to values expected from gas-phase chemistry alone were observed throughout the region and were largest in the coldest sampled regions, where T < 208 K. At low NOy values, where particles containing NOy and water were often detected, twilight ClO abundances in the afternoon were nearly a factor of 3 larger than those in the morning. At higher NOy values, where much lower particle surface areas were measured, ClO abundances in morning twilight were somewhat larger than those in the afternoon. These observations are consistent with a daytime mechanism of rapid heterogeneous activation of inorganic chlorine in particle-rich, low-NOy regions, with slower deactivation in relatively particle-poor, higher-NOy regions of the lowermost stratosphere. While the data clearly show widespread chlorine activation, knowledge of the precise value of the /y> ratio is limited because of the lack of available data on inorganic chlorine species, notably HCl, believed to be the dominant reservoir of inorganic chlorine at these altitudes.