No abstract

[1] This paper describes background characteristics of atmospheric CO 2 and stable isotopic ratios (d 13 C and d 18 O) as well as their possible drivers at Waliguan Baseline Observatory (WLG) (36°17 0 N, 100°54 0 E, 3816 m above sea level) in the inland plateau of western China. The study is based on observational CO 2 data (NOAA Climate Monitoring and Diagnostics Laboratory discrete and WLG continuous measurements) obtained at WLG for the period from May 1991 to December 2002. Over this period the change in monthly means is $+16 ppm for CO 2 , $À0.2% for d 13 C, and $À0.5% for d 18 O. The overall increase of CO 2 and subsequent decline of d 13 C, with a Dd 13 C/DCO 2 ratio (À0.011 ± 0.105) % ppm À1 at WLG, reflect the persistent worldwide influence of fossil fuel emissions. The negative secular trend of d 18 O at WLG is probably due to vigorous 18 O exchange with soils in the Northern Hemisphere (NH) and conversion from C 3 to C 4 plants via land use change. The CO 2 , d 13 C, and d 18 O mean annual cycles with peak-to-peak annual amplitudes of $10.5 ppm, $0.499 %, and $0.819%, respectively, at WLG show typical middle-to-high NH continental features that correspond to the seasonal cycle of the terrestrial biosphere. The significant CO 2 and d 13 C interannual variability at WLG is very likely caused by worldwide climate anomalies and associated regional fluctuation in biospheric CO 2 uptake in the Asian inland plateau as well as long-range air mass transport. The results of this study help to provide a basic understanding of the individual sources and sinks of carbon in this area and help us to better address the role of the Asian inland terrestrial biosphere in the global carbon cycle.

This paper describes the continuous measurements of atmospheric carbon dioxide at Mt. Waliguan (36°17′N, 100°54′E, 3816 m asl) in western China over the period 1994–2000. The CO2 hourly mixing ratios were segregated by horizontal wind direction/speed and vertical winds, respectively, merged by season over the entire measurement period. The short‐term variability in CO2 was examined mainly from the point of view of local winds observed at this station and isobaric back trajectory cluster‐concentration analysis as for local and long‐range transport influence, to permit the selection of hourly average data that is representative of background conditions. From the selected hourly data, daily, monthly and annual averages that are not influenced by local CO2 sources and sinks be computed by discriminating the local and regional impact on the Waliguan CO2 records. On the basis of these results, background CO2 data were then analyzed to evaluate the averaged diurnal variation, monthly mean time series, CO2 mixing ratio distribution in different seasons as well as averaged seasonal cycle. Annual mean and growth rate of CO2 at Waliguan during the period of 1991 to 2000 were further discussed by supplement with NOAA/CMDL flask air sampling records at this station and other monitoring stations located at similar latitudinal band in the Northern Hemisphere. The results from this study can provide atmospheric CO2 characteristics in Asian inland regions, and be used in other studies to improve the understanding of carbon source and sink distributions.