ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-5195-2011 Evaluation of in situ measurements of atmospheric carbon monoxide at Mount Waliguan, China Zhang F. 1 2 Zhou L. X. 1 Novelli P. C. 3 Worthy D. E. J. 4 Zellweger C. 5 Klausen J. 6 Ernst M. 4 Steinbacher M. 5 Cai Y. X. 7 Xu L. 1 Fang S. X. 1 Yao B. 1 Chinese Academy of Meteorological Sciences (CAMS), CMA, Beijing 100081, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA Environment Canada, 4905 Dufferin Street, Toronto, Ont., M3H 5T4, Canada Empa, Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland Federal Department of Home Affairs FDHA, Federal Office of Meteorology and Climatology MeteoSwiss, Kraehbuehlstrasse 58, P.O. Box 514, 8044 Zuerich, Switzerland Qinghai Meteorological Bureau, CMA, Xining 810001, China 01 06 2011 11 11 5195 5206 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/5195/2011/acp-11-5195-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/5195/2011/acp-11-5195-2011.pdf

Quasicontinuous measurements of carbon monoxide (CO) recorded over three years at Mount Waliguan (WLG), a global baseline station in remote western China, were examined using back trajectory analysis. The data include a revision to correct the working reference scale to the WMO2000 scale and corrections for drift in the reference gases. Between July 2004 and June 2007, 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 were 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 affected by short-term enhancements that exceeded background levels. The CO data were compared to values observed at the high alpine research station at 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 greater at WLG. 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 or southeast 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 sources of CO. Air masses passing over northwestern Gansu were associated with relatively high CO values suggesting an anthropogenic influence, which was likely due to anthropogenic emissions from northwestern China (based on back-trajectory and potential source contribution analysis and on the INTEX-B: intercontinental Chemical Transport Experiment-Phase B). Background conditions were observed most frequently in air parcels 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.

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