Conversely, the manual absorbing solution method based
on direct Griess reaction during sampling (Saltzman, 1954)
has also been allotted an ISO standard (ISO, 1985). The method
is sensitive and more selective than the chemiluminescence
method, but the colour to be measured spectrophotometrically
is developed during sampling, and the measurements
have to be performed immediately after sampling due to the
instability of the colour. The field inter comparison studies has
shown that the Saltzman method fails to detect concentrations
below 1 μg/m3
. Other absorbing solutions have been
used in which nitrogen dioxide is absorbed and transformed
to nitrite (EMEP/CHEM 3/77, 1977; EMEP/CCC-Report 3/93,
1993). These methods are generally not sensitive enough in
background areas, and also have the problem of instability of
the exposed absorption solution during transport when the
sunlight and temperature are not controlled. There has also
been considerable uncertainty about the absorption efficiency
of the absorbing solutions, and to which extent this varies
with concentration. The use of experimentally determined
absorption efficiencies has shown to be inadequate. Jacobs
and Hochheiser (1958) have used a 0.1 N NaOH solution as an
absorption solution in presence of a surface active agent viz.
butanol for the enhancement in the absorption efficiency
(Jacobs and Hochheiser, 1958; U. S. Protection Agency, 1971).
This is a multi-step and multi-reagent procedure with the
increased possibility of the experimental error. The interference
due to SO2 also restricts the use of this method.