High Levels of Molecular Chlorine found in the Arctic Atmosphere

Tuesday, 16 December 2014: 10:35 AM
Jin Liao1,2, Greg Huey2, Zhen Liu2,3, David Tanner2, Christopher A Cantrell4, John Joseph Orlando5, Frank M Flocke5, Paul B Shepson6, Andrew John Weinheimer5, Samuel R Hall5, Harry Beine7, Yuhang Wang2, Ellery D Ingall2, Chelsea R Thompson4,6, Rebecca S Hornbrook5, Eric C Apel5, Alan Fried4, Lee Mauldin4, James N Smith5, Ralf M Staebler8, J A Neuman1,9 and John B Nowak10, (1)NOAA ESRL, Boulder, CO, United States, (2)Georgia Institute of Technology, Atlanta, GA, United States, (3)Sandia National Laboratories, Albuquerque, NM, United States, (4)Univ of Colorado, Boulder, CO, United States, (5)NCAR, Boulder, CO, United States, (6)Purdue Univ, West Lafayette, IN, United States, (7)University of California, Davis, Davis, CA, United States, (8)Environment Canada Toronto, Toronto, ON, Canada, (9)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (10)Aerodyne Research Inc., Billerica, MA, United States
Chlorine radicals are a strong atmospheric oxidant, particularly in polar regions where levels of hydroxyl radicals can be quite low. In the atmosphere, chlorine radicals expedite the degradation of methane and tropospheric ozone and the oxidation of mercury to more toxic forms. Here, we present direct measurements of molecular chlorine levels in the Arctic marine boundary layer in Barrow, Alaska, collected in the spring of 2009 over a six-week period using chemical ionization mass spectrometry. We detected high levels of molecular chlorine of up to 400 pptv. Concentrations peaked in the early morning and late afternoon and fell to near-zero levels at night. Average daytime molecular chlorine levels were correlated with ozone concentrations, suggesting that sunlight and ozone are required for molecular chlorine formation. Using a time-dependent box model, we estimated that the chlorine radicals produced from the photolysis of molecular chlorine on average oxidized more methane than hydroxyl radicals and enhanced the abundance of short-lived peroxy radicals. Elevated hydroperoxyl radical levels, in turn, promoted the formation of hypobromous acid, which catalyzed mercury oxidation and the breakdown of tropospheric ozone. Therefore, we propose that molecular chlorine exerts a significant effect on the atmospheric chemistry in the Arctic. While the formation mechanisms of molecular chlorine are not yet understood, the main potential sources of chlorine include snowpack, sea salt, and sea ice. There is recent evidence of molecular halogen (Br2 and Cl2) formation in the Arctic snowpack. The coverage and composition of the snow may control halogen chemistry in the Arctic. Changes of sea ice and snow cover in the changing climate may affect air-snow-ice interaction and have a significant impact on the levels of radicals, ozone, mercury and methane in the Arctic troposphere.