Variation of CO₂ mole fraction in the lower free troposphere, in the boundary layer and at the surface

Abstract. Eight years of occasional flask air sampling and 3 years of frequent in situ measurements of carbon dioxide (CO₂) vertical profiles on board of a small aircraft, over a tall tower greenhouse gases monitoring site in Hungary are used for the analysis of the variations of vertical profile of CO₂ mole fraction. Using the airborne vertical profiles and the measurements along the 115 m tall tower it is shown that the measurements at the top of the tower estimate the mean boundary layer CO₂ mole fraction during the mid-afternoon fairly well, with an underestimation of 0.27–0.85 μmol mol⁻¹ in summer, and an overestimation of 0.66–1.83 μmol mol⁻¹ in winter. The seasonal cycle of CO₂ mole fraction is damped with elevation. While the amplitude of the seasonal cycle is 28.5 μmol mol⁻¹ at 10 m above the ground, it is only 10.7 μmol mol⁻¹ in the layer of 2500–3000 m corresponding to the lower free atmosphere above the well-mixed boundary layer. The maximum mole fraction in the layer of 2500–3000 m can be observed around 25 March on average, two weeks ahead of that of the marine boundary layer reference (GLOBALVIEW). By contrast, close to the ground, the maximum CO₂ mole fraction is observed late December, early January. The specific seasonal behavior is attributed to the climatology of vertical mixing of the atmosphere in the Carpathian Basin.