Spatial analysis of CO2 concentration in an unpolluted environment in northern Spain

CO2 transport in the low atmosphere near the surface at night was analysed using spatial procedures applied to back trajectories. Concentration and meteorological data were obtained at a rural site during a seven and a half month campaign. Daily evolution of CO2 skewness showed positive values during the night and negative values during the day. One concentration and one recirculation factor, an indicator of local recirculation, were associated with each back trajectory calculated during the night to study source and meteorological influences on concentrations recorded. Moreover, four procedures were used to analyse the trajectories, and their strengths and weaknesses were also investigated. (1) The nonparametric trajectory analysis applied with two weight functions successfully marked the most contributing region. (2) Hexagonal cells were used to account for radial distribution of trajectories. The potential source contribution function calculated in these cells highlighted the influence of the source against meteorology, this procedure therefore proving the best to mark the source direction. (3) Trajectory sector analysis revealed the most contributing wind sector and emphasised the role of recirculation in the E–S sectors. (4) Cluster analysis grouped neighbouring trajectories and was the most flexible procedure to classify them, providing a contrast of around 12 ppm between medians obtained in the SE cluster and the least contributing group.

► Using four procedures, CO2 transport at a rural site was analysed with back trajectories.
► Nonparametric trajectory analysis successfully described the most contributing region.
► Potential source contribution function in hexagonal cells marked the source direction.
► Trajectory sector analysis marked the recirculation sector.
► Cluster analysis proved useful for classifying trajectories following proximity.