Investigation of the atmospheric boundary layer depth variability and its impact on the ²²²Rn concentration at a rural site in France

Continuous monitoring of the atmospheric boundary layer (ABL) depth (z_i) is important for investigations of trace gases with near-surface sources. The aim of this study is to examine the temporal variability of z_i on both diurnal and seasonal time scales over a full year (2011) and relate these changes to the atmospheric ²²²Rn concentrations (C_Rn) measured near the top of a 200m tower at a rural site (Trainou) in France. Continuous z_i estimates were made using a combination of lidar and hourly four-height carbon dioxide (CO₂) profile measurements. Over the diurnal cycle, the 180m C_Rn reached a maximum in the late morning as the growing ABL passed through the inlet height (180 m) transporting upward high C_Rn air from the nocturnal boundary layer. During late afternoon, a minimum in the C_Rn occurred mainly due to ABL-mixing. We argue that ABL dilution occurs in two stages: first, during the rapid morning growth into the residual layer, and second, during afternoon with the free atmosphere when z_i has reached its quasi-stationary height (around 750m in winter or 1700m in summer). An anticorrelation (R² of -0.49) was found while performing a linear regression analysis between the daily z_i growth rates and the corresponding changes in the C_Rn illustrating the ABL-dilution effect. We also analyzed the numerical proportions of the time within a season when z_i remained lower than the inlet height and found a clear seasonal variability for the nighttime measurements with higher number of cases with shallow z_i (<200 m) in winter (67.3%) than in summer (33.9%) and spring (54.5%). Thus, this pilot study helps delineate the impact of z_i on C_Rn at the site mainly for different regimes of ABL, in particular, during the times when the z_i is above the measurement height. It is suggested that when the z_i is well below the inlet height, measurements are most possibly indicative of the residual layer ²²²Rn, an important issue that should be considered in the mass budget approach.