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Investigation of the atmospheric boundary layer depth variability and its impact on the 222 Rn concentration at a rural site in France
Author(s) -
Pal S.,
Lopez M.,
Schmidt M.,
Ramonet M.,
Gibert F.,
XuerefRemy I.,
Ciais P.
Publication year - 2015
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2014jd022322
Subject(s) - morning , planetary boundary layer , atmospheric sciences , diurnal cycle , boundary layer , environmental science , trace gas , atmosphere (unit) , dilution , climatology , meteorology , geology , geography , physics , astronomy , thermodynamics
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 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 four‐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: 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 750 m in winter or 1700 m in summer). An anticorrelation ( R 2 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 222 Rn, an important issue that should be considered in the mass budget approach.

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