Active and passive discontinuous methods are normally used for daily to monthly
sampling. The manual discontinuous methods are relatively cheap and simple, but
have several disadvantages including the need for manpower for sampling and
analysis, the limited time resolution and the time delay until results are available.
For the daily measurements of NO2, two active methods, the potassium iodide and the
Saltzmann methods have. Both methods can be operated on a 24-hour basis, although
for the Saltzmann-method, the sampling time is recommended to be shorter. These
daily sampling are used in many European rural areas, for example in the EMEP
programme. In some countries it is also the main monitoring method at urban
The potassium iodide method is based on the absorption of NO2 on a potassium
iodide impregnated sintered glass filters (Ferm and Sjödin 1993). NO2 is absorbed and
reduced to nitrite by the iodide on the filter. The nitrite formed is extracted with deionised
water and determined spectro-photometrically with the Griess method.
The Saltzmann/modified Saltzmann method is based on the direct Griess reaction
during sampling (ISO, 1981). A pink colour is produced during sampling. The
intensity is measured spectro-photometrically (Mücke et al., 1995).
More details are found, e.g. in the new EMEP manual (EMEP-CCC1995) and in the
GEMS/AIR´s - WMO, UNEP handbooks.
Diffusion denuders combined with an analysis by ion chromatography may also be
used for the determination of NO2. Carbon coated denuders allow for the simultaneous
determination of NO2 and PAN at concentration levels which approach 50 ppt (parts
per trillion) on a 24 hour basis which makes the technique very suitable for the
measurement of NO2 in rural areas (De Santis et al., 1996).
Diffusive, or passive, sampling methods are normally used for longer sampling
periods, on weekly (usually in more polluted areas) or monthly (usually in less
polluted areas) basis. The sampling technique is based on molecular diffusion of the
gas molecules into the sampler, where they are quantitatively collected on an
impregnated filter. No electricity, pump or other equipment (except for a rain shelter)
is needed. The method is used routinely for NO2 (Svanberg et al., (1994) and (1995);
Atkins et al., (1995); Campbell et al., (1994), Brauer and Brook, (1995).
Diffusive sampling is suitable for two or three-dimensional mapping - weekly to
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