horiba APNA-370, Operation Manual, Page: 103 / 113

10 APPENDIX
91
10 APPENDIX
10.1 Measurement Principle
When ozone (O
3
) is added to the sample gas containing nitrogen oxide (NOx), a part of nitro-
gen monoxide (NO) in the sample gas is oxidized to nitrogen dioxide (NO
2
). And some of the
generated NO
2
is in the excited state (NO
2
*), and emits light in the de-excitation state. This
phenomenon of light emission is chemiluminescence.
NO
＋
O
3
→
NO
2
*
＋
O
2
NO
2
*
＋
NO
2
＋
hv
This reaction is extremely fast and involves only NO

affected little by the other coexistent
gases. When the NO concentration is low, the light intensity is in proportion to the NO concen-
tration. The method using this reaction to measure NO concentration is known as the
chemiluminescence method (CLD method).
In APNA-370, the sampled gas is divided in two flows: one is used for NOx (NO + NO
2
) con-
centration measurement by reducing NO
2
to NO with a NOx converter; the other is used for
NO concentration measurement directly.
These sample gases are switched to the NOx, NO, and reference gas lines every 0.75 s with
solenoid valves, and are introduced to the reaction chamber in turn.
On the other hand, the open air is separately sucked through the air filter, dried by a self-
reproducing-type silica gel dryer, and used to form ozone in an ozonizer. Then, the generated
ozone is introduced into the reaction chamber.
In the reaction chamber, the sample and ozone react, and the light emission involved in the
reaction is detected by the photodiode.
This instrument calculates NO, NO
2
, NOx concentrations from the outputs obtained by the
photodiode, which are proportion to the NOx and NO concentrations, and outputs the results
as continuous signals.
10.2 Dryer Unit
This instrument incorporates self-reproducing-type silica gel dryer unit to dry the air used to
form ozone.
The dryer unit has two drying cylinders. When one drying cylinder is in use, the other is being
reactivated. In reactivation, the silica gel is heated at about 160°C for 135 minutes to expel the
moisture, and then cooled for 45 minutes.
Continuous drying is achieved by using and reactivating the two lines interchangeably every
180 minutes.