Analysis of CO2 daily cycle in the low atmosphere at a rural site.

Directional analysis procedures were extended to study the CO(2) daily cycle in the low atmosphere at a rural site. Weighted average, asymmetric circular functions, and conditional probability curves were applied to concentrations recorded at three levels near surface. Wind speed and potential temperature gradient were the meteorological variables used to establish a link to CO(2). The highest weighted concentration was obtained for wind speed below 4 m s(-1) and for potential temperature gradient above 0.05 °C m(-1). Asymmetric circular functions were fitted to selected percentiles, the generalized von Mises function providing the lowest RMSE. Slight differences among levels were observed in the transition between day and night, and higher gradients were observed during the night. Four intervals were proposed when concentrations were binned by intervals of meteorological variables. Wind speed was only relevant to separate intermediate concentrations. Finally, conditional probability curves provided differences between the highest and lowest levels in up to 20% of cases at 20-22 GMT.