A trace-gas climatology above Zotino, central Siberia

Using light aircraft and at intervals of approximately 14 days, vertical profiles of temperature, humidity, CO2 concentration and C-13/C-12 and O-18/O-16 ratio, as well as concentrations of CH4, CO, H-2 and N2O, from about 80 to 3000 m above ground level have been determined for the atmosphere above a flux measurement tower located near the village of Zotino in central Siberia (60degrees45'N, 89degrees23'E). As well as being determined from flask measurements (typically at heights of 100, 500, 1000, 1500, 2000, 2500 and 3000 m) continuous CO2 concentration profiles at 1 Hz have also been obtained using an infrared gas analyser. This measurement program is ongoing and has been in existence since July 1998. Data to November 2000 are presented and show a seasonal cycle for CO2 concentration of about 25 mumol mol(-1) within the atmospheric boundary layer (ABL) and about 15 mumol mol(-1) in the free troposphere. Marked seasonal cycles in the isotopic compositions Of CO2 are also observed, with that of oxygen-18 in CO2 being unusual: always being depleted in the ABL with respect to the free troposphere above. This is irrespective of whether the CO2 concentration is higher or lower in the free troposphere. We interpret this as indicating a net negative discrimination being associated with the net terrestrial carbon exchange, irrespective of whether photosynthesis or respiration dominates the net carbon flux in this region. During winter flights, large fluctuations in CO2 concentration with height are often observed both within and above the stable ABL. Usually (but not always) these variations in CO2 concentrations are associated with more or less stoichiometrically constant variations in CO and CH4 concentrations. We interpret this as reflecting the frequent transport of polluted air from Europe with very little vertical mixing having occurred, despite the large horizontal distances traversed. This notion is supported by back-trajectory analyses. Vertical profiles Of CO2 concentration with supplementary flask measurements allow more information on the structure and composition of an air mass to be obtained than is the case for flask measurements or for ground-based measurements only. In particular, our data question the notion that there is usually anything like "well mixed background air" in the mid-to-high northern latitudes during the winter months.