How well do tall-tower measurements characterize the CO₂ mole fraction distribution in the planetary boundary layer?

Abstract. Planetary boundary layer (PBL) CO₂ mole fraction data are needed by transport models and carbon budget models as both input and reference for validation. The height of in situ CO₂ mole fraction measurements is usually different from that of the model levels where the data are needed; data from short towers, in particular, are difficult to utilize in atmospheric models that do not simulate the surface layer well. Tall-tower CO₂ mole fraction measurements observed at heights ranging from 10 to 115 m above ground level at a rural site in Hungary and regular airborne vertical mole fraction profile measurements (136 vertical profiles) above the tower allowed us to estimate how well a tower of a given height could estimate the CO₂ mole fraction above the tower in the PBL. The statistical evaluation of the height-dependent bias between the real PBL CO₂ mole fraction profile (measured by the aircraft) and the measurement at a given elevation above the ground was performed separately for the summer and winter half years to take into account the different dynamics of the lower troposphere and the different surface CO₂ flux in the different seasons. The paper presents (1) how accurately the vertical distribution of CO₂ in the PBL can be estimated from the measurements on the top of a tower of height H; (2) how tall of a tower would be needed for the satisfaction of different requirements on the accuracy of the estimation of the CO₂ vertical distribution; (3) how accurate of a CO₂ vertical distribution estimation can be expected from the existing towers; and (4) how much improvement can be achieved in the accuracy of the estimation of CO₂ vertical distribution by applying the virtual tall-tower concept.