Three years of trace gas observations over the EuroSiberian domain derived from aircraft sampling — a concerted action

A three-year trace gas climatology of CO₂ and its stable isotopic ratios, as well as CH₄, N₂O and SF₆, derived from regular vertical aircraft sampling over the Eurasian continent is presented. The four sampling sites range from about 1°E to 89°E in the latitude belt from 48°N to 62°N. The most prominent features of the CO₂ observations are an increase of the seasonal cycle amplitudes of CO₂ and δ¹³C–CO₂ in the free troposphere (at 3000 m a.s.l.) by more than 60% from Western Europe to Western and Central Siberia. δ¹⁸O–CO₂ shows an even larger increase of the seasonal cycle amplitude by a factor of two from Western Europe towards the Ural mountains, which decreases again towards the most eastern site, Zotino. These data reflect a strong influence of carbon exchange fluxes with the continental biosphere. In particular, during autumn and winter δ¹⁸O–CO₂ shows a decrease by more than 0.5‰ from Orléans (Western Europe) to Syktyvkar (Ural mountains) and Zotino (West Siberia), mainly caused by soil respiration fluxes depleted in δ¹⁸O with respect to atmospheric CO₂. CH₄ mixing ratios in the free troposphere at 3000 m over Western Siberia are higher by about 20–30 ppb if compared to Western Europe. Wetland emissions seem to be particularly visible in July–September, with largest signals at Zotino in 1998. Annual mean CH₄ mixing ratios decrease slightly from 1998 to 1999 at all Russian sites. In contrast to CO₂ and CH₄, which show significant vertical gradients between 2000 and 3000 m a.s.l., N₂O mixing ratios are vertically very homogeneous and show no significant logitudinal gradient between the Ural mountains and Western Siberia, indicating insignificant emissions of this trace gas from boreal forest ecosystems in Western Siberia. The growth rate of N₂O (1.2–1.3 ppb yr⁻¹) and the seasonal amplitude (0.5–1.1 ppb) are similar at both aircraft sites, Syktyvkar and Zotino. For SF₆ an annual increase of 5% is observed, together with a small seasonal cycle which is in phase with the N₂O cycle, indicating that the seasonality of both trace gases are most probably caused by atmospheric transport processes with a possible contribution from stratosphere–troposphere exchange.