Publication Date:
2015-01-09
Description:
Continuous monitoring of the atmospheric boundary layer (ABL) depth ( z i ) is important for investigations of trace gases with near-surface sources. This study investigates the temporal variability of z i on both diurnal and seasonal time scales over a full year (2011) and relates these changes to the atmospheric 222 Rn concentrations ( C Rn ) measured near the top of a 200 m tower at a rural site (Trainou) in France. Continuous z i estimates were made using a combination of lidar and hourly 4-height carbon dioxide (CO 2 ) profile measurements. Over the diurnal cycle, the 180 m 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: firstly, during the rapid morning growth into the residual layer, and secondly, during afternoon with the free atmosphere when z i has reached its quasi-stationary height (around 750 m in winter, or 1700 m in summer). An anti-correlation (R 2 of −0.49) was found while performing a regression analysis between the daily z i growth rates and the corresponding changes in the C Rn illustrating ABL-dilution effect. We also investigated 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 222 Rn; an important issue that should be considered in the mass budget approach.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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