OPENÕs System Variables
Band Broadening Correction for Water Vapor
Using the LI-6400
14-23
Band Broadening Correction for Water Vapor
Absorption in the infrared involves vibrational and rotational energy transi-
tions. The 4.26 µm CO
2
absorption band is due to infrared energy absorption
by a particular bond stretching mode that is coupled to rotational energy tran-
sitions that produce a large number of individual absorption lines. Individual
absorption line widths are sensitive to intermolecular collisions and become
broader with increasing pressure. Therefore, total absorption across a band
per mole of absorber increases with pressure.
Full description of an absorption band is complex, but approximate expres-
sions can be used over limited ranges of absorber mole fraction, pressure and
pathlength. It can be shown that the Ònon-overlapping line approximationÓ
applies at ambient pressure and CO
2
mole fraction over the short pathlengths
found in LI-COR infrared gas analyzers (Wolfe and Zissis, 1978). This leads
to a Òscaling lawÓ that allows absorption measured under one set of condi-
tions to be scaled to other conditions (Jamieson, et al., 1963),
(14-33)
where
A
is total band absorption,
P
is total pressure (kPa),
u
is absorber
amount (mol m
-2
) =
; is molar density (mol m
-3
), and
L
is pathlength
(m);
g
is a general unspecified function.
From the ideal gas law, the absorber mole density
can be expressed as
(14-34)
where
p
is absorber partial pressure and
X
is absorber mole fraction (mol ab-
sorber / mol air). Therefore,
(14-35)
Substituting (14-35) into (14-33) and incorporating the constants
L
and
R
into
a new function
h
gives,
(14-36)
A
P
---
g u P
¤
(
)
=
r
L
r
r
r
p
RT
-------
=
XP
RT
-------
=
u
P
---
XL
RT
-------
=
A
P
---
h
X
T
----
( )
=
Summary of Contents for LI-6400
Page 1: ...Using the LI 6400 Portable Photosynthesis System ...
Page 15: ...Part I The Basics ...
Page 16: ......
Page 174: ...Making Measurements Answers to Questions 4 56 Using the LI 6400 4 ...
Page 175: ...Part II Useful Details ...
Page 176: ......
Page 200: ...Standard Tools Power ON Hooks 5 24 Using the LI 6400 5 ...
Page 214: ...Real Time Data Real Time Graphics 6 14 Using the LI 6400 6 ...
Page 234: ...Environmental Control Light Control 7 20 Using the LI 6400 7 ...
Page 244: ...Light Sensor Considerations Gallium Arsenide Phosphide GaAsP Sensor 8 10 Using the LI 6400 8 ...
Page 288: ...Data Logging Making Your Own AutoPrograms 9 44 Using the LI 6400 9 ...
Page 289: ...Part III Working With Files ...
Page 290: ......
Page 312: ...The LPL File System Troubleshooting 10 22 Using the LI 6400 10 ...
Page 340: ...Downloading Files Using a Data Capture Program 11 28 Using the LI 6400 11 ...
Page 375: ...Part IV Configuration Issues ...
Page 376: ......
Page 420: ...Defining User Variables Old Style vs New Style 15 18 Using the LI 6400 15 ...
Page 454: ...Using an Energy Balance Further Reading 17 12 Using the LI 6400 17 ...
Page 455: ...Part V Maintenance Troubleshooting ...
Page 456: ......
Page 572: ...Troubleshooting Useful Information 20 46 Using the LI 6400 20 ...
Page 593: ...Part VI Programming ...
Page 594: ......
Page 622: ...Programming with LPL Compiler Directives 22 28 Using the LI 6400 22 ...
Page 846: ...Index I 16 Using the LI 6400 ...