Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8
A performance evaluation of the open tube
diffusion sampler (Palmes sampler) for
monitoring nitrogen dioxide
F. De Santis^, A. Fino^, S. Tiwari^, C. Vazzana^ & I. Allegrini
^CNR - Istituto Inquinamento Atmosferico, Area della Ricerca di
Roma, Italy
* * Rain and Cloud Physics Research Centre, New Delhi, India
Abstract
The Palmes sampler is largely used to carry out indicative measurements for NO]
as considered in EU legislation (Daughter Directive 1999/30/EC). In order to
comply with the quality objectives set out in this directive it is important that the
limits of this technique are fully appreciated. The sampler was tested in a
chamber study and in field trials. Experiments were carried out to study the
effect of wind turbulence, the stability of the NO2-TEA adduct and the selfconsistency of the method. The results of these experiments demonstrated
underestimation in comparison to chemiluminescence probably due to
photodegradation of the NOi-TEA adduct and/or incomplete extraction of nitrite.
1 Introduction
One of the aims of the European Framework Directive [1] on ambient air quality
assessment and management is to assess ambient air quality in member States of
the European Union on the basis of common methods and criteria. Under this
Directive, the Member States are required to assess the spatial concentration
distribution of air pollutants throughout their territory by using screening
techniques and large scale surveys. One of the first priority pollutant that has
been taken into consideration in the Directive is nitrogen dioxide. In principle,
passive sampling represents a perfect tool to characterise those areas where the
limit values are expected to be exceeded and/or where other assessment methods
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C.A. Brebbia, H. Power & J.W.S Longhurst (Editors)
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© 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8
are needed to comply to EC legislation. In contrast to active samplers in which
air is brought into contact with a detector or collector device by means of a
pump, diffusive samplers rely on diffusion to bring the pollutant into contact
with the collector. Compared with the pump-dependent active sampling
procedure, main advantages of the method are cost effectiveness, simplicity and
the potential for large-scale measurements carried out at the same time. The use
of open tube passive samplers (Palmes samplers) [2] for monitoring ambient air
is not new. Originally developed in the field of occupational hygiene, they were
first introduced for monitoring ambient NO2 by Atkins [3] and since then they
have been widely used for studies in urban and rural areas (Campbell et al. [4]
and references therein). A perusal of the literature shows that there are a number
of conflicting results from a number of previous studies which have compared
measurement of NO] carried out by Palmes tubes side-by-side with real-time
continuous techniques such as chemiluminescence. For instance, overestimation
of NO] has been reported by Campbell et al. [4] and by Gair & Penkett [5]. They
have found overestimation of up to 40% that they attributed to a shortening of
the diffusion path due to air velocity across the face of the tube. However, in
contrast with these results, Campbell [6] and Atkins & Lee [7] reported accuracy
within 7% and 3% respectively in comparison to active monitoring. In a study of
Hale & Cape [8] a numerical model of coupled chemistry and diffusion within
the tube was used to investigate the possibility that chemical reactions within the
tube can alter the concentration of NO] leading to the overestimation noticed in
the other studies mentioned above.
In a recent study, Heal et al. [9] have shown that the extent of overestimation
depends on the relative concentration of NO2, NO and Og. Overestimation is
likely to be the greatest in urban locations (i.e. high NO levels) whereas in rural
areas where NO < NO], the overestimation is within the measurement
uncertainty. Recently, a field intercomparison exercise was organised as part of
the quality assurance and control procedures of the UK Department of
Environment Transport and Regions NO] network [10]. Thirty eight laboratories
took part in the exercise. A majority of laboratories (79%) performed to the data
quality objectives set out in the Daughter Directive for NO] [11]. However, there
was a substantial range in the average bias of -39% to + 58% relative to the
chemiluminescence technique. In addition, reanalysis of samples performed by a
single independent laboratory for all participants showed a general increase in
the concentrations and a difficult to explain decrease for some of them. Given
the expected tendency to overestimation in Palmes NO] tubes, the most likely
cause of underestimation was incomplete extraction of absorbed NO] on
triethanolamine. No explanation was given for the systematic analytical positive
bias shown for a minority of laboratories.
If this type of diffusion sampler is to be used with confidence in the air
quality surveys requested by the EU legislation, then it is important that its
performance with regard to the reference method is understood. Results of a
study devised to investigate the problems mentioned before and to evaluate the
suitability of open tube diffusion samplers for the measurement of nitrogen
dioxide are presented. Laboratory scale and field experiments in a urban
background area of Rome (Villa Ada) were carried out to study the effect of
wind turbulence, the role of desorption and photodegradation of triethanolamine
and, finally, the self-consistency of the method.
Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8
K/// 423
2 Experimental
The passive samplers used were based on the original device of Palmes and
Gunnison [12]. They consist of a tube, one end containing a sorbent that fixes the
pollutant at a rate controlled by molecular diffusion. The tubes are made of
acrylic plastic and are 71 mm long and 11 mm across. Two mesh discs coated
with triethanolamine are held at the top in a cap sealing the tube whereas the
lower end is open to allow diffusion. Some experiments were performed by
using tubes having the same length but with a smaller width (inner diameter 8
mm) and with Palmes tubes having a stainless steel mesh placed across the
entrance of the sampler. Laboratory experiments were done in a laminar flow
vessel described elsewhere [13]. The NO] concentrations were measured
continuously with a chemiluminescence monitor (Model 30A Environment,
France).
Field experiments were done by exposing three tubes mounted vertically
under an aluminium flat disk at a height of 1.5 m above the ground. After the
period of deployment the samplers were washed in a suitable amount of a
solution of sodium bicarbonate and sodium carbonate, 2.7 mM Na2CO] and 0.3
mM NaHCOs (the so-called Dionex eluent solution) to extract the collected
analyte. Analysis of the aqueous washings by Ion Chromatography (Model 500,
Dionex, USA) was used to determine the accumulated masses of nitrite.
3 Results and Discussion
3.1 Laboratory studies
Diffusion tubes have been shown to over read relative to chemiluminescence. To
study this effect two different approaches based on a different design of the
standard Palmes sampler have been followed.
a) Palmes tubes manufactured to standard specifications have been compared
with tubes of the same length and material having a smaller width.
b) A mesh wire screen has been placed at the entrance of a standard Palmes
tube as diffusion barrier to attenuate face velocity effects.
To check the degree of attenuation for the face velocity effect under study by
using the approach of different diameters, the two types of tube of different
width have been exposed in a vessel in which turbulence was created by
operating a small fan under the lid of the vessel. The velocity of the fan was
electronically controlled and the resulting wind speeds inside the vessel were
estimated to be in the range of 2 to 4 ms"\ Experiments in triplicate have shown
that at the maximum speed provided by the fan the NO] overestimation was 46 ±
3 % for the conventional Palmes tube, whereas it was 8 ± 0.7 % for the Palmes
tube of reduced width. Other experiments in the laboratory (chamber tests) have
shown that the two tubes behave exactly as described by the Pick's law in
consideration of their geometrical size with uptake rate of 75 ± 5 and 38 ±2
crn^h^ respectively. Figure 1 shows the relationship between the hourly mean
NO2 uptake and the NO: concentration. The test has been conducted on triplicate
Air Pollution
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C.A. Brebbia, H. Power & J.W.S Longhurst (Editors)
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© 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8
samples. The values lie on a straight line with a correlation coefficient of 0.'
and 0.99 for the standard and reduced diameter sampler respectively.
Nitrite collected (ug/h*10-3)
180
160
140
Small
Standard
120
100
80
60
40
20
0
200
400
600
800
1000
1200
NO2 mixing ratio (ppb)
Figure 1 .Hourly mean NC>2 uptake and the NO] concentration (average of three
replicates) for standard and reduced width Palmes tube.
To check the degree of attenuation for the face velocity effect using the
approach of the diffusion barrier, experiments in triplicate with standard Palmes
tubes with and without a stainless steel mesh across the face of the diffusion
sampler were carried out in the laboratory. The samplers were exposed in the
same vessel mentioned before. The results obtained showed that placing the
protective mesh can attenuate the effect of wind turbulence at very low levels. At
maximum fan speed the NO] overestimation was 48 + 3 % for the tube without
mesh whereas was 7 ± 0.8 % for the tube with the stainless steel mesh.
The stability of the adduct NO]-TEA was tested by evaluating the recovery
efficiency of exposed samplers stored under various conditions of potentially
destabilising factors. Two sets of experiments in different periods (Sept - Oct 96
and Nov - Dec 99) have been carried out. Figure 2 gives the fraction of nitrite
remaining in the samples after storage at various times and in different
conditions (i.e. stored in refrigerator or on the shelf; wrapped or not with
aluminium foil as a light shield).
Note from the figure that the exposed samplers stored at room temperature for
two months and half, retained 75% of the initial amount of nitrite in the first
experiment whereas the corresponding fraction was 60% in the second one. The
light exposure and temperature were found to be major factors for the stability of
the NO]-TEA adduct during storage. This result is only partly in agreement with
the findings of Krochmal & Kalina [14] who reported that passive samplers of
K/// 425
Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8
the badge type, made of transparent polythene give results up to 50% lower than
samplers protected against sunlight. The motive of the loss is not clear and, as
noted by Heal et al.9, might involve an elusive biological degradation process
which is difficult to detect and analyse.
1.2
1
o>
•c
0.8
o 0.6
c
1 0.4
0.2
O
Q
A
O
O
Shelf (II)
Refrigerator (II)
Shelf (I)
Refrigerator (I)
Shelf (I, Al foil)
0
20
40
60
80
100
Days of storage
Figure 2.
Fraction of nitrite remaining in the Palmes sampler in different
storage conditions (average of three replicates).
3.2 Field experiments
Palmes diffusion tubes both of the standard type and of reduced diameter were
deployed alongside a chemiluminescence monitor in a park of Rome (Villa Ada).
The samplers were exposed in triplicate for periods of 1 week at a time over
the period of two weeks. To test self-consistency, a separate triplicate set of
Palmes samplers was operated side-by-side for two weeks rather than for weekly
periods. Figure 3 compares the mean NO] obtained over the same period from
the sum of two parallel exposures. From the figure it can be noted that the
response of the two types of Palmes tube in the field is clearly different and
unexpected on the basis of the laboratory work. Exposure values for the reduced
diameter were always lower than the corresponding values determined by using
the standard configuration. The experiment repeated in another period of the year
again for two weeks gave substantially the same underestimation (experiments
not reported here). On the basis of the laboratory experiments reported before,
the most likely explanation for the underestimation effect exhibited by the
reduced diameter tube is overheating and photo degradation of the TEA-NO]
adduct during sun exposure that occur in the field at a much larger extent in
comparison to laboratory.
It is also important to note that 2-week exposure is much lower than the
corresponding result derived from two subsequent weeks and that this effect is
Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors)
© 2000
WIT Press,
www.witpress.com, ISBN 1-85312-822-8
426 Air
Pollution
VIII
more important for the small tube than for the conventional one. This
observation again points towards the stability of the TEA-NC^ adduct as a source
of the reduced amount of nitrite on the sorbent.
30
n 2*1-week
25
• 2-week
I 20
O xc
z 15
<D
O)
g 10
Standard
Reduced Width
Fig. 3. Comparison between NO2 concentrations derived from 2-week exposure
for standard and reduced width tubes and the 2-week average derived
from 2 consecutive 1-week exposures.
Another effect that could contribute to negative bias in the determination of
NO2 by diffusive samplers is extraction efficiency. The mesh grids carrying the
triethanolamine in the diffusion tube end cap could not release nitrite easily and
make it available in solution for the analysis. This effect could be more
important for the reduced width samplers. Although we have not investigated
specifically this point, reanalysis of some field samples of the two types of
sampler showed very slight changes of nitrite concentration at 4 hours after
extraction. This suggests that incompleteness of extraction was small, if indeed
occurred.
Figure 4 shows the results obtained in two field exercises held in the periods
May - Jul 96 and Dec - Feb 99/00 in which conventional Palmes tube were
exposed side-by-side with a chemiluminescence analyser for consecutive
fortnightly periods over three months. The exercise simulates the use of passive
samplers as tools for the so-called "indicative measurements" mentioned in the
Daughter directive [11]. Although the number of data is small, there is a clear
trend for Palmes tubes to underestimate. Under the Daughter Directive the
measurement accuracy (defined as bias + two times the standard deviation) has
been set out at ± 25% of the reference concentration 11. The average bias for our
data was -19 %. However, accuracy values ranging from 14 to 75 % have been
measured with an average value of 38%. This largely exceeds the limits set in the
i uiiuiiun v 111
Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors)
© 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8
directive. It is difficult to give an interpretation of these data beyond erratic
and/or incomplete extraction of nitrite.
35
.0
CL
30
# May-Jul 96
0)
^
n 25
3
(/> 20
0)
#
"m 15
a.
>»
n 10
CM
O
z 5
# A~
^*
^
4
0
10
20
30
40
NO2 by CL (ppb)
Figure 4. NO2 measured by passive samplers versus NO2 measured by
chemiluminescence.
4 Conclusions
From the present study observations, we conclude that the use of Palmes tubes
for NO2 is likely to be only partially satisfactory. At least in our case
underestimation seems a problem as important as overestimation found by
others. The results reported here underscore some of the difficulties associated
with the use of this device. A careful extraction procedure, the use of a stainless
steel mesh at the entrance of the tube and the use of non transparent plastic to
reduce photodegradation can help to reduce errors. Further investigations is
certainly required to ascertain the utility and limits of Palmes tube for air quality
assessment of NO] in the framework of the Daughter Directive. In addition, these
results show that even a simple technique should be thoroughly investigated
before its widely use for routine pollution monitoring. Since many passive
devices are now available for different pollutants, extensive laboratory and field
tests should be carried out before attempting any monitoring campaign.
Acknowledgements
Resources for this study were provided by CNR within the framework of the
"Programma Finalizzato Beni Culturali" and by the Italian Ministry of
Environment under Contract "Quality Assurance in Environmental
Air Pollution VIII, C.A. Brebbia, H. Power & J.W.S Longhurst (Editors)
428 /if/© 2000 WIT Press, www.witpress.com, ISBN 1-85312-822-8
Measurements". One of the authors (S.T.) thanks the International Centre for
Theoretical Physics, Trieste, Italy for the award of a fellowship and the Rain and
Cloud Physics Research Centre for providing study leave. The authors thank P.
Muthusubramanian for earlier experimental investigation on this subject and
Tiziana Sargolini for the preparation of the manuscript. The assistance of the
City of Rome Department of Air Quality in providing chemiluminescence data is
also gratefully acknowledged.
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