Appendix i. useful equations, 10 p – Campbell Scientific AP200 CO2/H2O Atmospheric Profile System User Manual
Page 111
Appendix I. Useful Equations
Dewpoint Temperature Calculations
The IRGA reports the dewpoint temperature (public variable
Td_cell), but this
represents the conditions in the sample cell. The AP200 converts this to
dewpoint temperature in the ambient air,
Td_ambient. This conversion is a
three-step process: 1) calculate vapor pressure in the sample cell, 2)
compensate for the pressure difference between the ambient and the sample
cell, and 3) calculate ambient dewpoint temperature from ambient vapor
pressure. The equations are adapted from:
Buck, A.L. 1981, “New equations for computing vapor pressure and
enhancement factor”, Journal of Applied Meteorology,
20, 1527-1532.
Calculate vapor pressure from dewpoint temperature:
+
=
cell
cell
T
T
cell
e
f
e
97
.
240
502
.
17
3
1121
.
6
)
1
.
0
(
Buck, eq. 8
(
)
amb
P
f
10
10
46
.3
0007
.1
6
3
−
×
+
=
Buck, eq. 7c
where:
e
cell
is the vapor pressure in the sample cell
the factor 0.1 converts the vapor pressure from mb to kPa.
f
3
is the enhancement factor
T
cell
is the dewpoint temperature in the sample cell
10P
amb
is the ambient pressure, converted from kPa to mb
Compensate for the reduced pressure in the sample cell:
=
cell
amb
cell
amb
P
P
e
e
where:
e
amb
is the ambient vapor pressure
P
cell
is the pressure in the sample cell
I-1