E C O P H Y S I C S Practical hints
CLD 780 TR / July 2000 92
7.1 Sampling
For ambient air monitoring the sampling techniques are relatively simple.
To avoid errors, however, a few points should be kept in mind.
All tubing used for transporting the sample should be made of inert mate-
rial, and the tubing inner-surfaces should be smooth. Acceptable materials
are fluorocarbons (PTFE or PFA) and glass. If particle deposits are at any
time detected on the inside walls of the tubing, then it should be replaced
or cleaned immediately. Sampling lines should be kept as short as possi-
ble, since long residence times may introduce systematic errors in the
measurement data, a result of the reaction of NO with O
3
to create NO
2
.
When the sample gas enters the dark intake tubing NO
2
photolysis stops
immediately and ozone is no longer formed due to the absence of light.
Inside the tubing, however, NO continues to be oxidized by the remaining
ozone, successively reducing the concentration of both substances:
NO + O
3
→
NO
2
+ O
2
The NO concentration is thus systematically under-measured by the in-
strument. The NO
x
concentration is, however, not affected, because the
disappearing NO is converted into NO
2
. Since NO
2
is the difference NO
x
-
NO, the NO
2
measurement values are too high.
How significant the error is depends on various factors, including intensity
of (sun) light, actual ozone concentration in the atmosphere and of course
the residence time of the gas sample from the point of extraction to the
reaction chamber.
If several different analyzers are operated simultaneously, a common
sampling line is advisable. Since the sample flow is thus increased, the
residence time in the sampling line is reduced. When selecting the tap-off
point for the individual sample lines to the NO
x
and ozone analyzers re-
member to keep the lines as short as possible.
When supplying calibration gas to the analyzer, it is essential that this gas
is at atmospheric pressure at the cal. gas inlet. This is best accomplished
with a bleed (overflow) line (see Fig. 6.1).
To prevent ambient air back-flow from mixing with calibration gas, the
bleed line should be more than 1 m long and the bleed flow rate at least
50 % of the calibration gas flow required by the analyzer. To be on the
safe side it is always advisable to install a flow meter in the bleed line to
monitor the flow.
To avoid calibration gas in the vicinity of the sampling point affecting sub-
sequent measurements, it is advisable to pass the bleed flow through an
activated charcoal filter. The created pressure drop due to the filter must,
however, be insignificant (<1 mbar).
To Next Page
Loading...