1Chinese Academy of Meteorological Sciences (CAMS), CMA, Beijing 100081, China
2Graduate University of Chinese Academy of Sciences, Beijing 100049, China
3Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
4Environment Canada, 4905 Dufferin Street, Toronto, Ont., M3H 5T4, Canada
5Empa, Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland
6Federal Department of Home Affairs FDHA, Federal Office of Meteorology and Climatology MeteoSwiss, Kraehbuehlstrasse 58, P.O. Box 514, 8044 Zurich, Switzerland
7Qinghai Meteorological Bureau, CMA, Xining 810001, China
Abstract. High frequency measurements of carbon monoxide (CO) recorded over three years at Mount Waliguan (WLG), a global background station in remote western China, were examined using back trajectory analysis. Corrections for the drift in reference gases were also included in the data revision. Between July 2004 and June 2007, a time series of CO exhibited large fluctuations and the 5%, 50% and 95%-percentiles of relevant CO mixing ratios were 102 ppb, 126 ppb and 194 ppb. Approximately 50% of all observed data have been selected as CO background data using a mathematical procedure of robust local regression with the remainder affected by regional-scale pollution. The monthly mean background CO mixing ratios showed a minimum in summer and a maximum in late winter, although all seasons were effected by short-term enhancements that exceeded background levels two or more times. The CO data were compared to the values observed at the high alpine research station Jungfraujoch, Switzerland. Smaller seasonal amplitudes were observed at WLG compared to the Jungfraujoch due to lower winter and spring CO levels, however, episodic enhancements of polluted air were much greater at the site in China. The air parcels arriving at WLG came predominately from the West, except in summer when advection from the East and Southeast prevailed. Transport from the East typically brought polluted air to the site, having passed over populated urban areas upwind. A large number of elevated CO mixing ratios could also be associated with advection from the Northwest of WLG via the central Xinjiang Uygur Autonomous Region (XUAR) and the Ge'ermu urban area where growing industrial activities as well as crops residue burning provide large sources of CO. These background conditions were observed most frequently when air masses originated from remote Tibet west of WLG. The probability that air parcels pass over regions of clean or polluted regions was further identified using potential source contribution function (PSCF) analysis.