E C O P H Y S I C S Practical hints
CLD 780 TR / July 2000 93
7.2 Quenching
Quenching occurs when excited NO
2
* molecules created by the reaction
between NO and ozone collide with other molecules in the carrier gas
and are deactivated without emitting a photon. This results in a reduction
of the optical signal measured by the photomultiplier, the magnitude of
which depends strongly on the kind colliding molecules. These compo-
nents must be present in appreciable concentrations (%-range) in order to
give a measurable quenching effect.
Known quenching components are water vapor, N
2
and O
2
. The latter two
are of course the main constituents of air, the usual carrier gas during en-
vironmental measurements. If the instrument is calibrated with air or N
2
as
the carrier gas this quenching effect is automatically compensated for and
causes no further problem.
However, when ambient air is being monitored the carrier gas is air with
varying humidity. Calibrations, however, rare usually made with dry gas.
Since water vapor is an effective quencher, a decline in signal is to be ex-
pected when absolute air humidity is high. The overall effect of the H
2
O
quenching is indicated in the technical specifications of the instrument un-
der „Interferences“ (see 2.1).
This phenomena can be corrected by either applying a compensation fac-
tor to the measurement data (if the humidity is known) or by the controlled
humidification of the carrier gas (diluent gas) during calibration.
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