Variability of active chlorine in the lowermost Arctic stratosphere

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 NO_y, a species that is a proxy for photochemical age and a photochemical source of NO_x, Enhancements in the [ClO]/[Cl_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 NO_y values, where particles containing NO_y 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 NO_y 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-NO_y regions, with slower deactivation in relatively particle-poor, higher-NO_y regions of the lowermost stratosphere. While the data clearly show widespread chlorine activation, knowledge of the precise value of the [ClO]/[Cl_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.