Chemistry Reference
In-Depth Information
Early techniques used in regular monitoring were bulky and often relied on air
sampling with subsequent laboratory analysis. Volz and Kley [
22
] describe early
methods to measure O
3
, initially employed in 1876. Here a defined volume of air
was bubbled through a solution of AsO
3
3
which reacted with O
3
to form
AsO
4
3
. The amount of AsO
3
3
was then titrated with I
2
to determine the O
3
concentration. Other early air quality monitoring techniques focused on the
measurement of airborne particles. The use of fibrous filters for sampling airborne
particles was invented around 1920 (see [
23
]) and became standard for ambient
air quality monitoring after 1940. The method of manually weighing a filter prior
to and after sampling a defined volume of air is still in use, and it is currently
the basis of reference methods for the determination of PM
10
and PM
2.5
in the
EU and US.
Measurements using sampling techniques and subsequent laboratory analysis
are labour intensive, costly and do not meet the need for timely dissemination of air
pollution information to the public.
Developments in the 1970s and 1980s allowed the first measurements of gases
to be made using automated measurement techniques. Good examples include the
techniques developed for the measurement of oxides of nitrogen, O
3
,CO
2
, particle
mass and soot (black carbon).
The oxides of nitrogen (NO and NO
2
) can be determined with a method based
on chemiluminescence, compound-specific light emission after excitation. The
principle of this method is based on the following chemical reaction:
NO
þ
O
3
!
NO
2
þ
O
2
(1)
NO
2
!
NO
2
þ
h
(2)
n
The addition of O
3
and measurement of the light intensity at the specific
wavelength (
2
) in a reaction chamber can be used for continuous NO measurements
[
24
] and for NO
2
if this is catalytically reduced to NO as part of the process.
Light absorption at specific wavelengths is another principle often employed for
the online measurement of gaseous pollutants. For example, the absorption of O
3
in
the ultraviolet at 254 nm can be used, though this experiences cross sensitivity to
SO
2
and PAHs, and CO
2
in the infrared at 4.26
m (2,350 cm
1
), which has little
cross sensitivity to other gases. The most common C-H absorption bands are in the
range of 3.33-3.57
m
m (2,800-3,000 cm
1
).
While these methods can readily be employed in “real time”, light absorption of
soot particles was first developed for laboratory measurements of particles collected
on filters and then adapted for online deployment [
25
]. Sampling on filter tapes
allows quasi-continuous measurement by recording the decrease of transmitted
light and moving the tape when the exposed area becomes too dark.
Parallel developments in health research, especially empirical studies, have gone
hand in hand with new air pollution measurement technologies and have led to a
substantial body of evidence on the adverse effects of air pollution. Put simply, new
m
