E C O P H Y S I C S Measurement principle
CLD 780 TR / July 2000 14
3.1 Measurement principle
The reference measurement principle specified by the U.S. Environmental
Protection Agency (EPA) for nitrogen dioxide (NO
2
) is the gas phase
chemiluminescent reaction of nitric oxide (NO) with ozone (O
3
). NO is
measured directly, NO
2
indirectly. The NO
2
-to-NO reduction is achieved
by the use of a converter. The reactions between NO and an excess
amount of O
3
are detailed in the following formulae:
NO + O
3
NO
2
+O
2
[1]
NO + O
3
NO
2
*
+O
2
[2]
NO
2
*
NO
2
+h
ν
[3]
NO
2
*
+M NO
2
+M [4]
NO
2
*
denotes the excited nitrogen dioxide molecule, M for deactivating
collision partners such as N
2
, O
2
and H
2
O. The spontaneous deactivation
of NO
2
occurs with emission of light [3]. By far the larger fraction of NO
2
*
loses its excitation energy without light emission by colliding with other
molecules [M) [4]. In order to achieve a high yield of light the reaction of
NO with O
3
must take place under low pressure.
The light intensity generated by the chemiluminescent reaction [3] is pro-
portional to the mixing ratio of NO. A photomultiplier tube [PMT) is used
to convert the light energy emitted from [3] into electrical impulses. A
counter counts the electrical impulses over a chosen integration time inter-
val (ITI) and a microprocessor calculates the signal (S
NO
) in ppbv.
For the correct measurement of low levels of NO (pptv) it is important to
recognize that there are other sources of the signal (S
1
). The instrument’s
measured signal (S
1
) contains, beside S
NO
, also the dark current signal of
the PMT (S
D
). Since the detector registers the chemiluminescent emission of
other reactions than that between NO and O
3
, S
1
also contains S
In
(signal
from interfering reactions). This can be significant, for example in rural ar-
eas at night where S
In
can be higher than S
NO
.
The total signal S
1
therefore is:
S
1
= S
NO
+ S
In
+ S
D
where: S
D
= dark current
S
In
= chemical interferences
S
NO
= the NO concentration.
If S
In
= 0 and S
D
is known for a specific ITI
∆
t, the dark current S
D
gives the
detection limit of the NO concentration. The interference signal S
In
can be
very simply determined by use of a pre-chamber. This is possible because
most of the interference reactions that produce the signal S
In
are signifi-
cantly slower than reactions [1] to [3].
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