is particularly noticeable with smaller quantities of atmospheric gases. The
sensible heat flux is not influenced by fluctuations in air density.
The effect of changes in the density of dry air q a , in the vertical fluxes under
constant pressure, wherein the vertical flux of dry air is assumed to be zero, is given
by the following expression (Webb et al. 1980):
q a w ¼ q a w þ q 0
a w 0 ¼ 0
ð3:177Þ
where q a is the density of dry air so that Eq. (3.177) yields
w ¼
Àq 0
a w
0
q a
ð3:178Þ
This results in a non-zero mean vertical velocity w, whenever there are sensible
or latent heat fluxes, which cause correlations of fluctuations of q a with w. This
velocity will induce an additional convective flux, which needs to be eliminated
with the correction proposed by Webb et al. (1980) denoted by WPL, for the change
in air density. The WPL correction is used in both open- and closed-path systems.
In open-path systems, WPL correction for carbon dioxide flux can be expressed as
(Burba and Anderson 2010)
F ¼ F un þ 1:61
E
q d
À
c
 Ã
1 þ 1:61
q V
q d
!
þ
H
qc p
À
c
 Ã
T a
ð3:179Þ
Fig. 3.12 Laboratory
calibration of open-path
analyzer
3.7 Eddy Covariance Method
85