Daily and annual cycle of CO2 concentration near the surface depending on boundary layer structure at a rural site in Spain

CO₂ in the rural atmosphere is related to respiration–photosynthesis processes, although the evolution of the low atmosphere is also a determinant factor. CO₂ concentrations were measured at surface and meteorological variables obtained from a radio acoustic sounding system sodar at a flat rural site during a 3-year campaign. Yearly and daily cycles of CO₂ were described. Maxima were observed in spring and autumn during the night. Wind speed and thermal structure of the lower atmosphere were analysed. Low level jets were observed during the night, their core proving lower in summer. Surface inversions observed with low winds reached up to 100 m. The turbulence layer which developed during the day extended up to 300–400 m and was capped by a stable layer. Median vertical wind speed reached 1 m s^?1 in super-adiabatic conditions in summer. Determination of decoupled low level jets proved difficult with the device used and corresponding concentrations were slightly higher than medians calculated with all the observations. The bulk Richardson number was calculated in the lower atmosphere and four intervals were considered: drainage, transitional, shear flows and unstable conditions. Median CO₂ concentrations were split according to these intervals. Higher values corresponded to drainage flow, which was associated to more stable conditions being less frequent and lower values to shear flow and unstable conditions, revealing a satisfactory link between the bulk Richardson number as a turbulence indicator in the low atmosphere and CO₂ surface concentrations.