CIRAS-3 Operation Manual V. 1.09
162
(5.1)
IRGA CO
2
readings are corrected for water vapor, temperature, and atmospheric pressure. The
addition of transpirational water vapor dilutes the air leaving the cuvette (C
out
), and this is compensated
for in the calculation:
𝑨
= (
𝑪
𝒊𝒊
×
𝑾
)
−
[
𝑪
𝒐𝒐𝒐
× (
𝑾
+
𝑬
)]
(5.2)
To calculate net CO2 assimilation we rearrange equation (
5.1
) to:
𝑨
=
−��
(
𝑪
𝒐𝒐𝒐
− 𝑪
𝒊𝒊
) ×
𝑾�
+ (
𝑪
𝒐𝒐𝒐
×
𝑬
)
�
CIRAS-3 calculates and displays the CO
2
difference (C
out
- C
in
). As relates to the calculated values in the
CIRAS-3 display:
𝑪
𝒐𝒐𝒐
= (
𝑪𝑯𝟐
𝒓
+
𝑪𝑯𝟐
𝒅
) =
𝑪𝑯𝟐
𝒂
Intercellular CO
2
Concentration
Calculate CO
2
concentration in the sub-stomatal cavity (C
i
) using the equation derived by von Caemmerer
& Farquhar, 1981
(6.1)
The
sub-stomatal CO2 concentration, C
i
, is given by:
𝑪
𝒊
�𝝁𝒎𝒐𝒎 𝒎𝒐𝒎
−𝟏
�
=
��𝒈
𝒄
−
𝑬
𝟐
�
×
𝑪
𝒐𝒐𝒐
� − 𝑨
�𝒈
𝒄
+
𝑬
𝟐
�
(6.2)
Where g
c
is the total conductance to CO
2
transfer:
𝒈
𝒄
�𝒎𝒎𝒐𝒎 𝒎
−𝟐
𝒔
−𝟏
�
=
�
𝟏
(
𝟏
.
𝟓𝟓𝟓
×
𝒓
𝒔
) + (
𝟏
.
𝟑𝟐
×
𝒓
𝒃
)
�
×
𝟏𝟎
𝟑
(1.585 is the diffusion ratio of CO
2
and water in
air
, and 1.37 is the diffusion ratio of CO
2
and water in the
boundary layer
).
Note:
These calculations are based on the following assumptions:
•
the leaf is exposed on both upper and lower leaf surfaces
•
the upper and lower boundary layer resistances are similar
•
stomata are evenly distributed on both upper and lower leaf surfaces.
