T. Stroyer
Hansen
*,
Mikkel
Kruse
,
Hanne
Nissen†
,
Marianne
Glasius‡
and
Christian
Lohse
University of Southern Denmark, Department of Chemistry, Campusvej
55, DK-5230, Odense M, Denmark. E-mail: tsh@chem.sdu.dk
First published on 4th January 2001
Measurements of nitrogen dioxide using the Palmes diffusion tubes in Uummannaq, Aasiaat, and Nuuk, all located along the west-coast of Greenland, have demonstrated that the levels of pollution at the most heavily impacted sites are comparable to levels in much larger towns in Denmark. The highest concentrations were, in general, observed near sites influenced by car traffic (peak concentrations of up to 16 ppbv), medium concentrations were observed in the residential areas (2–6 ppbv), and very low levels were found at the background locations in the town outskirts (1–2 ppbv). Observations of nitrogen dioxide concentrations less than 0.1 ppbv at a remote site, Akia, 25 km from Nuuk, indicate that, compared to local sources, long-range transport of nitrogen dioxide is not important in western Greenland.
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Fig. 1 Locations for measurement of nitrogen dioxide in West Greenland 1998–2000. |
This work had three main purposes. The first was to find out if the Palmes diffusion tubes could be used for nitrogen dioxide measurements at low temperatures, such as those found in Greenland. The second was to establish the background level in this part of the Arctic. The last was to determine the nitrogen dioxide load on the inhabitants of the towns in Greenland. None of these has been attempted before.
The performance of the Palmes tubes was tested at different temperatures by mounting the tubes in three manifolds and exposing them to known concentrations of nitrogen dioxide. One of these manifolds was kept inside a freezer, where the temperature was controlled. The other two manifolds were placed outside the freezer at room temperature. To regulate the concentration, a known mixture of nitrogen dioxide in pure nitrogen was directed through a mass flow controller. Purified air was directed through another mass flow controller and subsequently humidified by purging it through a reaction vessel containing ice or water, which was placed in the freezer at a controlled temperature. The air streams were then thoroughly mixed at room temperature before entering the first manifold placed outside the freezer. The mixture was then directed through the cold manifold placed in the freezer. Finally, the air stream was purged through the last manifold placed at room temperature. The temperature of the air stream was monitored by thermocouples in the three manifolds. All experiments were carried out over a period of approximately two to three days with a flow-rate of 800 mL min−1 (wind speed, ca. 0.2 cm s−1) and with a concentration of 180 ppbv nitrogen dioxide.
18 stations have been established in Nuuk and are designated N01–N18. The measuring sites are grouped according to their station type: traffic (4 stations), light industry (2), residential (9) and background (3).
10 stations have been established in Aasiaat and are designated A01–A10. The measuring sites are grouped according to their station type: traffic (2 stations), residential areas (6), and background (2).
Owing to Uummannaq's northern position the area is not navigable between
December/January and May/June. The high mountains are the reason why
the sun disappears two weeks earlier and returns two weeks later than one
would expect judging from the latitude at which Uummannaq is located. This
results in midnight sun from the middle of May until the end of July and a
dark period of about 60 days during winter (Nov. 7th–Feb. 4th).5 The annual average temperature is around −5°C.
The coldest months are February and March (−15
°C–−20
°C)
but April also tends to be rather cold.4 It
is comparatively dry in Uummannaq, especially during winter when the inlet
and surrounding waters freeze and an almost continental climate is established.
Below 10 mm precipitation on a monthly basis is common in winter, and
annual precipitation rates of around 150–200 mm and below are
standard. Compared to other locations in Greenland the wind conditions in
Uummannaq are quite calm.6
10 stations have been established in Uummannaq and are designated U01–U10. The measuring sites are grouped according to their station type: traffic (5 stations), residential areas (3), and background (2).
Fig. 2 shows the results obtained.
The x-axis gives the temperature in K in the freezer and the y-axis
is the ratio between the measured concentrations in the freezer and the average
of the measurements before and after the freezer. The data points are clearly
grouped into two sets; one set with temperatures above −8°C
where all ratios are 1.0 and the other set with temperatures below −8
°C
belonging to what appears as a straight line. Applying a linear regression
for the best fit of the latter, the correlation between T and Y
is significant (Y = 0.0338 × T − 7.95
; R2 = 0.98). This is in agreement
with very recently published results of Kirby et al.7
who reported that the TEA absorbtive substrate does not freeze on the steel
meshes at temperatures above −10
°C.
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Fig. 2 Uptake efficiency of Palmes tubes as a function of temperature. |
From Fig. 2 it is evident that a
strong temperature dependence exists below −8°C. At −28
°C
the uptake rate is only 33% of the rate above −8
°C.
Anomalous structure changes of the TEA when it cools may explain the relatively
large decrease in the uptake rate below −8
°C. Increasing
the viscosity may destroy the active sites at the sorbent steel meshes thus
reducing the available positions for reaction. In Kirby et al.7 and references therein it is indicated that the water
content of the air may be a limiting factor for the speed of reaction. The
vapour pressure over ice drops from 3 Torr at −5
°C
to 0.3 Torr at −28
°C causing a decreasing amount
of water available for the necessary reaction between TEA and nitrogen dioxide.1 Also, the temperature dependence of the rate constant
for the reaction between TEA and nitrogen dioxide is likely to be important.
The experiments clearly show that under similar conditions it is necessary to correct the measured concentrations with respect to temperature. This has been done in the present study by multiplying the measured values by 〈Y−1〉. For this purpose 〈Y−1〉 was calculated for each measuring period (about 4 weeks) by averaging Y−1 for each day in the period using the mean temperature for the day supplied by ASIAQ. In the following discussions, it has been assumed that this correction is sufficient and all numbers refer to data where the decreasing uptake rate as a function of temperature of the Palmes diffusion tubes has been considered in this way.
As an example raw and temperature corrected data are given for one of the most affected sites, U05, in midtown Uummannaq in Fig. 3. It is evident that temperature correction has a marked effect on the magnitude of the results in the cold months.
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Fig. 3 Effect of sub-zero temperature correction of nitrogen dioxide measurements from Uummannaq. |
Heal et al.16,17 showed that the overestimation of nitrogen dioxide measured with diffusion tubes compared to chemiluminescence analysers was due to chemical reactions between NO and O3 inside the tube rather than to shortening of the diffusion path due to wind-induced turbulence at the entrance of the tubes. Through model calculations they accounted for more than 20% overestimation which is also the size of the measured overestimation.
In spite of the excellent results obtained previously, it was decided to make a small scale intercomparison again. Therefore, results from Palmes tubes were validated against results from an NOx monitor in a semi-urban area (Lille Valby near the National Environmental Research Institute (NERI) in Roskilde, Denmark). NOx-monitor data from “The Danish Air Quality Monitoring Programme”18 have kindly been submitted by NERI.19 The regression line was calculated using orthogonal regression and is given by (including 1s):
![]() | (1) |
Very recently published results of Heal et al.20 demonstrate that the chemical overestimation of diffusive sampler results compared with chemiluminescence results is partly compensated for by a net reduction in sampling efficiency in 4 week exposures compared with 1 week exposures. This is in agreement with our conclusion that the 1 month exposures in this work using the Palmes diffusion tubes correlates well with the chemiluminescence monitor results.
Owing to technical difficulties it has not been feasible to make chemiluminescence measurements of nitrogen dioxide in Greenland. Instead we have to rely on comparisons from a rural site in Denmark, where a higher concentration of both NO and O3 should give a greater effect of chemical overestimation. In spite of this we find a good correlation.
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Fig. 4 Nitrogen dioxide concentrations at representative sites in Uummannaq 1998/1999/2000. |
According to a local official,23 most motor
vehicles (85%) in Uummannaq are powered by diesel engines.
It is, therefore, likely that significant amounts of nitrogen dioxide are
emitted directly. The reaction of nitrogen oxide with ozone to form nitrogen
dioxide may not play such an important role as elsewhere, and ozone may not
constitute the limiting factor, as seen in Denmark.18
Future measurements of ozone may confirm this explanation. The drastic increase
in the concentration of nitrogen dioxide in the cold winter months (−12°C–−20
°C)
could be explained by the fact that many car and truck drivers let their engines
run idle during the whole day to avoid problems in starting the cold engines
at the very low temperatures.23
According to the local official, 95% of all family houses are heated by stoves burning kerosine or diesel oil.23 These sources of nitrogen dioxide could also be important during winter.
One way of showing this is to plot the measured concentrations versus the length of periods of calmness (wind speeds below 0.5 m s−1). Fig. 5 shows the correlation of periods of calmness and the average load of nitrogen dioxide for all 10 sites in Uummannaq. The plot has only been made to provide an illustration of this phenomenon. The correlations between percentage calmness and measured concentration, however, appear good for six stations with R2 values of 0.4–0.6, indicating that meteorology could play an especially important role in determining nitrogen dioxide concentrations in Uummannaq. On the other hand, such correlations might be indicative of the measuring device to source distance, showing, for instance, the direct impact from nearby vehicles. This could, as previously mentioned, also be an indication of the possible unusual driving habits of the inhabitants; i.e., leaving their diesel engines running continuously due to problems with cold starting in the cold climate.
![]() | ||
Fig. 5 Correlation between nitrogen dioxide concentrations and percentage calmness in Uummannaq. |
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Fig. 6 Nitrogen dioxide concentrations at representative sites in Aasiaat 1998/99/2000. |
Correlations between the measured concentration of nitrogen dioxide and
periods of calmness are noticeable with R2 values from
0.35 to 0.70 for six of the sites. However, the elevated concentrations measured
in February coincide with this being a period with 10–20% calmness
and an average temperature of only −18°C. It is likely that
a combination of meteorological conditions, i.e., the stability of
the atmosphere, an increase in energy consumption of the residents for heating
and traffic, and inhibition of removal pathways for nitrogen dioxide, have
resulted in the nitrogen dioxide concentration profile over time. There is
polar darkness throughout December until mid-January. However, it is not possible
to conclude to what extent the lack of solar radiation has influenced the
concentrations of nitrogen dioxide. Again, the habit of the local residents
in leaving their diesel engines running continuously during the day may be
important.
![]() | ||
Fig. 7 Nitrogen dioxide concentrations at representative sites in Nuuk 1998/1999/2000. |
Period | Jun–Jul 98 | Aug–Sep 98 | Oct–Dec 98 | Jan–Feb 99 | Mar–Apr99 | May 99 | Jun–Jul 99 |
---|---|---|---|---|---|---|---|
a n.a. = not available. | |||||||
Length/h | 1301 | 983 | 1298 | 1852 | 1150 | 1105 | 1320 |
Temp. corr. | 1.00 | 1.00 | 1.00 | 1.09 | 1.00 | 1.00 | 1.00 |
Station— | |||||||
L01 | 50 | 100 | −10 | 40 | −100 | 110 | 80 |
L02 | 40 | 110 | −20 | 100 | 60 | 30 | 80 |
L03 | 40 | 100 | n.a.a | n.a. | 10 | −30 | n.a. |
L04 | 30 | 60 | 0 | 20 | −40 | 160 | 60 |
L05 | 0 | 70 | 30 | n.a. | n.a. | n.a. | n.a. |
Average conc. (pptv) | 30 | 90 | 0 | 50 | −20 | 70 | 80 |
s | 20 | 40 | 30 | 40 | 60 | 100 | 10 |
Blank average | 170 | 110 | 150 | 60 | 160 | 110 | 160 |
s | 20 | 20 | 10 | 10 | 40 | 60 | 20 |
Det. limit (pptv) | 50 | 50 | 30 | 40 | 120 | 190 | 50 |
This work has shown that the source of the nitrogen dioxide measured in the three towns in West Greenland is to be found locally. Long range transport of nitrogen dioxide is negligible. Its origin is local fossil fuel combustion for heating and traffic with the highest load in the winter months, where the need is highest. Between 50% and 95% of the houses are heated by oil burners. The rest are heated by district heating, where the energy is produced by either oil burners or incineration. Oil burners are known to produce nitrogen dioxide directly. The majority of the motor vehicles are driven by diesel engines, which are known to produce nitrogen dioxide directly, in contrast to petrol engines. This is emphasized by the habit of letting the car engines running idle during the whole day to avoid problems with cold starting. The relatively high concentrations in the two smaller towns are probably due to longer periods of wind calmness when compared to similar towns in Denmark.
Measurements performed of Glasius et al.1 in Odense, Denmark, show that the level of nitrogen dioxide measured at sites relatively close to the city centre are comparable to the concentrations of nitrogen dioxide measured at some of the most heavily influenced sites in Nuuk and Uummannaq at winter time. Considering that the number of inhabitants in Odense is about 15 times greater than in Nuuk or about 200 times greater than in Uummannaq, the level appears high.
It is rather difficult to predict if the current level of air pollution has any significant effect on human health, and it is very unfortunate that there is a persistent lack of knowledge of the health effects of exposure to low concentrations of mixtures of contaminants. It seems fair to say that the direct health effects of inhaled outdoor air in Greenland may be hypothetical, especially if one considers that the measured levels of nitrogen dioxide are more than 25–50 times lower than the lowest observed effect level in asthmatics as reported by Lundquist.26
On 15 May 1990 the Greenland Homerule decided that the air quality in Greenland should be better than in Europe, and that the long-time air quality should be better than 25 µg NO2 m−3 (13 ppbv) observed as the median (50th percentile) of 1 h averages over one year.
For one of the most heavily loaded stations in Uummannaq, U05, the median (50th percentile) for all measurements (September 1998–July 2000) is 10 µg NO2 m−3 (5.4 ppbv).
The short-time air quality may be expressed by the 98th percentile. Annual average and six monthly mean concentrations, measured using diffusion samplers, are often used to extrapolate the 98th percentile concentrations by multiplying by a factor of 2.4.13 The average concentration for the measurements (September 1998–July 2000) at U05 in Uummannaq is 6.2 ppbv, resulting in an estimated 1 h mean 98th percentile of 15 ppbv. The Greenland Homerule decided on 15 May 1990 to follow the European Council Directive 85/203/EEC of 7 March 1985 on air quality standards for nitrogen dioxide. The recommended guide value is 135 µg NO2 m−3 (72 ppbv).
Hence the guide values are exceeded for neither short-time nor long-time load of nitrogen dioxide for any of the stations in Uummannaq. Conclusions are similar for Nuuk and Aasiaat.
When assessing the possible impact of air pollution on human health, the exposure to indoor air pollution needs to be considered, since this is where the majority of people spend most of their time. A number of air pollutants are present at concentrations significantly higher indoors than outdoors. For example, the average contribution from gas stoves has been estimated to yield an additional background level of 45 µg m−3 (24 ppbv) nitrogen dioxide to the indoor air compared to the level in homes with electric stoves.27 Therefore, it is possible that the level of indoor air pollution in the majority of Greenlandic homes using gas appliances is higher than the outdoor concentrations measured in this study. The high consumption of tobacco in Greenland is likely to be another important contributor to indoor air pollution. It would be very interesting to combine measurements of indoor and outdoor air pollution in Greenland.
Regarding the measurements at Akia, it is difficult to conclude anything, except for the fact that the level of nitrogen dioxide is very low, i.e., less than 100 pptv. This is not surprising considering that the location was chosen as to not be influenced by local emissions from Nuuk or other towns, and that the lifetime of nitrogen dioxide is relatively short. Measurements from the Canadian Arctic generally present low concentrations of NOx, and as a result support the findings at Akia.28 Obviously, a consequence of the measurements at Akia is that long-range transport of nitrogen dioxide cannot be important at inhabited locations in Greenland. If an evaluation of the relative importance of long-range transport in proportion to local sources in the more desolate areas of Greenland is desired, measurement of other more long-lived species is required.
At remote sites long-range transport of air pollutants might very well constitute an important part of the “background level”. For example, measurements performed at Station Nord at the northern coast of Greenland clearly show that during wintertime, the concentration of air pollutants increase by several orders of magnitude due to long-range transport.24 It could therefore be very interesting to measure, e.g., PAN or SO2, at Akia.
Footnotes |
† Present address: NOVO Nordisk, Ndr. Fasanvej 215, DK-2200 Copenhagen N, Denmark. |
‡ Present address: National Environmental Research Institute, Department of Environmental Chemistry, Frederiksborgvej 399, DK-4000 Roskilde, Denmark. |
This journal is © The Royal Society of Chemistry 2001 |