Measurements of nitrogen dioxide in Greenland using Palmes diffusion tubes

Abstract

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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Introduction

Greenland is part of the North American continent and is situated in the North Atlantic Ocean. It is the largest island in the world covering 2 175 600 km². Greenland extends about 2 670 km from north to south and more than 1 050 km from east to west at its widest point. A large portion (82%) of the country is permanently covered with ice. In this study, staff from ASIAQ (Greenland Survey, www.asiaq.gl) selected the main locations for the measurements of nitrogen dioxide. Four main locations were chosen: Nuuk, Uummannaq, Aasiaat, and Akia. All sites are situated on the west coast of Greenland (Fig. 1). The sites were chosen to represent different climatic areas. The local climate in Nuuk is dominated by the town activities, the semi-regional climate of Uummannaq is dominated by the inlet Uummannaq Fjord, and the regional climate of Aasiaat is dominated by the larger Disco Bay. Akia was chosen as a remote site far from local sources of nitrogen dioxide. With the exception of Uummannaq being the northernmost location and well within the arctic region, the sites are all in the subarctic region.

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.

Experimental

Preparation and analysis of diffusion tubes

The procedures used in this work for preparation and analysis of the tubes have been described previously by Glasius et al..¹ The diffusion tube consisted of a hollow acrylic tube and two closely fitting caps. Three small steel meshes were placed between the tube and the cap at one end of the diffusion tube. The tubes were 70.98 ± 0.09 mm long with a diameter of 10.98 ± 0.03 mm (all materials were supplied from Gradko Int., UK). To prepare the diffusion tubes, steel meshes were coated by dipping them in a 33% solution of triethanolamine (TEA) in acetone and then drying them for at most 20 min on absorbent paper. The assembled diffusion tubes were always stored in a freezer and storage time could be at least one year before exposure and at least 5 months after exposure. The exposure times were unless otherwise stated approximately 1 month. Every measurement was made with 5 tubes mounted in a protecting, upside-down stainless-steel bowl (diameter 20 cm). The open ends of the tubes were level with the rim of the bowl to minimize turbulence from the passing air. Four tubes were used for collecting the nitrogen dioxide. The last tube was not exposed to the air and was used to establish the blank value for the whole procedure (of preparing, exposing, and analysing the tubes). Every measurement was the average of the results from 4 exposed tubes corrected for the blank value. After exposure of the diffusion tubes, the amount of nitrogen dioxide collected was determined colorimetrically as nitrite with Griess-Saltzman reagent. The absorbance was measured at 540 nm. All chemicals used were analytical grade supplied by Sigma-Aldrich.

Temperature dependency

One of the primary objectives of this study was to investigate the performance of the Palmes diffusive samplers¹ at sub-zero temperatures, i.e., under arctic conditions. The performance of Palmes tubes at normal ambient temperatures is known to be slightly temperature dependent, which is accounted for in the calculation of the diffusion coefficient.¹ However, it is to be expected that at the sub-zero temperatures in Greenland there may be additional effects. From a kinetic point of view, the rate constant for the reaction of nitrogen dioxide with the absorptive substrate TEA in the Palmes tube is likely to depend on temperature. Also, the behaviour of the solvent matrix at sub-zero temperatures may influence the uptake rate. It has been difficult to find any reports on the performance of diffusive samplers at low temperatures. However, it has been reported that low temperatures in the range 251–283 K result in an underestimation of the concentration of nitrogen dioxide, where it was ascribed to the possible anomalous behaviour of TEA below its freezing point.²

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.

Specification of the monitoring network

Nuuk (64° 10′ N, 51° 45′ W). Nuuk, situated on the south-west coast, is the southernmost location of the four sites. The municipality has a total area of 105 000 km², of which 19 000 km² is free of ice and 6 500 km² is ocean. With a population of 13 427 the community is by far the biggest in the country.³ The majority of the population live in the town of Nuuk.⁴ The climate in Nuuk is heavily influenced by the rather warm West Greenland current and is thus relatively humid. The average annual temperature in 1996 and 1997 was just below zero. Nuuk is located south of the Arctic Circle (66.5° N). The winds experienced in Nuuk are relatively high when compared to the prevailing wind forces in Greenland.

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).

Aasiaat (68° 43′ N, 52° 53′ W). The total area of the municipality of Aasiaat is 4 000 km², 400 km² of which is land and the remainder ocean. Situated on an island at the edge of the southern Disko Bay area, Aasiaat is the fourth largest town in Greenland with a population of 3 147. Aasiaat is slightly industrialized. The climate in Aasiaat is low arctic but relatively dry. The annual average temperature is about −4 °C.⁴ Aasiaat is located about 250 km north of the Arctic Circle and therefore from May to July there is midnight sun (about 55 days). During winter, the period of darkness lasts for about 27 days.

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).

Uummannaq (70° 40′ N, 52° 08′ W). The area of the municipality of Uummannaq is 93 000 km² of which 12 500 km² is free of ice. About half of the population of Uummannaq municipality live in the town itself (1 481). The town is situated at the foot of a mountain on a small island in Uummannaq Fjord north of the Nuussuaq peninsula in the Baffin Bay area.

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).

Akia (64° 20′ N, 51° 50′ W). 25 km north of Nuuk, five stations have been established within an area of 100 m² in the hills of Akia. The barren area is free of direct sources of nitrogen dioxide, and as such should not be influenced by Nuuk due to local weather conditions. For that reason the stations at Akia could help determine the significance of long-range transport of air pollution with nitrogen dioxide.

Results

Temperature dependency

One of the primary objectives of this study was to investigate the performance of the Palmes type diffusive samplers at sub-zero temperatures, i.e., under Arctic conditions. The tubes were exposed to known amounts of nitrogen dioxide in the laboratory while varying the temperature.

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.

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.

Fig. 3 Effect of sub-zero temperature correction of nitrogen dioxide measurements from Uummannaq.

Intercalibration

Several studies have examined the performance of diffusion tubes in comparison with active measuring methods, i.e., chemiluminescence monitoring. Using diffusion tubes (of any kind) Krochmal and Kalina,^8,9 Roeyset¹⁰ and Ayers et al.¹¹ all measured concentrations very close to those measured by the monitor used for comparison. Several other studies, however, indicate quite the contrary (Moscheandras et al.,² Campbell et al.,¹² Gair and Penkett,¹³ Hedley et al.¹⁴). Here, diffusion tubes were shown to overestimate the concentrations of nitrogen dioxide by up to 15–40%. It was concluded that this behaviour could be ascribed to the shortening of the diffusion path by wind induced turbulence. In a study by Glasius et al.,¹ more than 25 measurements performed at three different locations demonstrated that measurements of nitrogen dioxide with the Palmes tubes correlated well with monitor results. The study also showed that the passive samplers, on average, overestimated the concentrations of nitrogen dioxide by about 10%, which is quite low compared with the results mentioned above. According to Glasius,¹⁵ the influence of wind-induced turbulence had been overcome in her study by the use of a special mounting device sheltering the diffusion tubes, which is also used in this work.

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 NO_x monitor in a semi-urban area (Lille Valby near the National Environmental Research Institute (NERI) in Roskilde, Denmark). NO_x-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):

Since the data is based on monthly measurements, only seven observations are available. It is obvious, however, that there is good correlation between the monitor and the Palmes results. Comparing the slope of the curve with the measurements of Glasius et al.¹ also shows a very good agreement.

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.

Results from ambient measurements in Greenland

Here we present the most important results of the Palmes diffusion tube measurements of nitrogen dioxide which were performed in Uummannaq, Aasiaat, Nuuk, and at Akia from June 1998 to July 2000.

Uummannaq. Fig. 4 shows concentrations of nitrogen dioxide at three representative sites in Uummannaq as well as the average concentrations of all 10 sites in the town. In Uummannaq the overall average concentration of nitrogen dioxide from September 1998 to July 2000 was 4 ppbv. Relatively low concentrations were measured during spring, summer, and autumn. However, at all sites the concentrations increase abruptly in winter. The highest concentrations were observed in February, when peaks of up to 14 ppbv were measured. With the exception of the winter months, the level of nitrogen dioxide generally stays below 4 ppbv in the residential areas. The highest concentrations were found at the sites by the parking lot of the local supermarket. At the other sites that should be influenced by traffic, the concentrations are only slightly higher or similar to those found in the residential areas (1–4 ppbv in spring, summer, and autumn, and 7–14 ppbv during winter).

Fig. 4 Nitrogen dioxide concentrations at representative sites in Uummannaq 1998/1999/2000.

Sources. In background areas of the Arctic it has been found that concentrations of nitrogen dioxide were higher during winter due to a decrease in the height of the boundary layer reducing the volume for dilution of emissions. Furthermore the removal processes are inhibited due to a decrease in photochemical activity and low precipitation.²¹ It has not been possible to gain any data on annual variation of nitrogen dioxide in towns in the Arctic, and therefore our data cannot be compared with measurements conducted under similar conditions. However, measurements of nitrogen dioxide in Danish cities do, in general, show the lowest concentrations in summer, increasing concentrations during autumn, and elevated concentrations in winter followed by a maximum in spring.^1,22

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.

Influence by calmness on nitrogen dioxide concentrations. One of the most important reasons for the very high concentrations measured during winter could be that it was up to 26% calm (wind speed below 0.5 m s⁻¹) during these months. The lifetime of the locally emitted nitrogen dioxide is, both in winter and in summer, longer than its residence time in the street canyon. Therefore, the measured concentrations of nitrogen dioxide might be determined by meteorological conditions, particularly the stability of the atmosphere.

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.

The polar darkness. In Uummannaq there is polar darkness from the beginning of November until the start of February. Considering that several air pollutants have been observed in the Arctic atmosphere in relatively high concentrations at the end of the dark season,²⁴ it is likely that the absence of sunlight has played a role in the manifestation of the detected winter maxima. It is tempting to speculate on the possible influence of the polar sunrise, considering that the return of sunlight may trigger an almost explosive increase in photochemical activity.²⁵ Unfortunately, it is not possible to conclude whether the observed pattern may partly be ascribed to the absence or presence of sunlight. A better time resolution of the measurements (e.g., days) would enable detection of more rapid variations in concentrations and thus would be better suited for this purpose.

Aasiaat. Graphs featuring the concentration of nitrogen dioxide at representative sites as well as the average of all 10 sites in Aasiaat are shown in Fig. 6. The overall average for Aasiaat was 3.6 ppbv (September 1998–July 2000). Generally, the concentrations of nitrogen dioxide were found to be in the range 0.5–12 ppbv, with the highest concentrations at sites influenced by traffic. The lowest concentrations of nitrogen dioxide were, as expected, found at a background station. In the residential areas the concentration of nitrogen dioxide ranges on average between 3 and 4 ppbv and stays in general below 8 ppbv.

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.

Nuuk. As a result of the relatively high average temperatures throughout the winter, ca. −10 °C, correction of the data with respect to temperature has only a small effect. In Nuuk the overall average concentration of nitrogen dioxide from June 1998 to July 2000 was 5.5 ppbv. Fig. 7 shows the concentrations of nitrogen dioxide at four representative sites as well as the average of all 18 sites in Nuuk. The highest concentrations were measured at the locations influenced by traffic, especially near the centre of Nuuk where relatively high levels of nitrogen dioxide were observed (peak concentration of up to 18 ppbv). In general terms, relatively low concentrations were measured during summer at the sites influenced by traffic. At most stations, however, the concentration of nitrogen dioxide increased during autumn up until winter, at which point the level of nitrogen dioxide seems to stabilize. From January onwards the concentration of nitrogen dioxide fluctuates. In the industrial area the concentration profile of nitrogen dioxide over time differs from that observed at other stations in Nuuk, again probably reflecting the local activities in this area. In the residential areas the measured concentrations of nitrogen dioxide stay below 6 ppbv.

Fig. 7 Nitrogen dioxide concentrations at representative sites in Nuuk 1998/1999/2000.

Akia. Located 25 km north of Nuuk, the concentrations of nitrogen dioxide at Akia were expected to be very low. The site can only be accessed by helicopter, which was landed downwind of the measuring stations. Measurements have only been made in seven periods of varying lengths. The exposure times (in hours) are for single samples. The detection limit has been calculated as three times the standard deviation of all the blanks for the five stations in the period. The resulting detection limits in pptv are given in Table 1 together with the averaged values of the nitrogen dioxide concentrations. The measured values are larger than the detection limit for only three periods. In any case, the values are all less than 100 pptv. This indicates that there is no long-range transport of nitrogen dioxide to this part of Greenland.

Table 1 Nitrogen dioxide concentrations and detection limits (pptv) for Akia. Detection limits have been defined as three times the standard deviation of all the blanks for the period. The average concentration exceeds the detection limit for only three periods (written in bold italic)

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

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Discussion

As previously mentioned, the amount of relevant accessible data to this study is infinitesimal and the literature that is available mostly takes its starting point in measurements performed in the remote Arctic, far from any direct anthropogenic influence. Hence the data obtained in this study can not be compared with measurements performed under similar conditions. Nevertheless, several theoretical studies have attempted to arrive at estimates for the Arctic. An example is that carried out by Lundquist,²⁶ according to whom the Arctic peak concentrations of nitrogen dioxide are not likely to exceed 1 µg m⁻³ (0.5 ppbv). Our measurements at Akia confirm this with nitrogen dioxide levels lower than 0.1 ppbv.

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.

Are the measurements performed in this study representative?

The station net covers three of the most populous towns in Greenland. All are located in areas where different climatic conditions prevail and should as such represent different conceivable scenarios in Greenland. It can be argued that the very wet climate at the south-east coast has not been considered. However, here concentrations of nitrogen dioxide are likely to be lower than the levels measured in the present study due to increased removal of NO_x as a consequence of the predominant high precipitation rates. Furthermore, North and North-east Greenland have also been left out, but since the area is only sparsely populated, concentrations are probably relatively low. Therefore, it seems that the three main locations can be regarded as representative of Greenlandic towns.

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 NO_x, 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.

Acknowledgements

We thank DANCEA, Danish Cooperation for Environment in the Arctic, The Danish Ministry of Environment and Energy, for economic support. We thank ASIAQ (Greenland Survey) for good collaboration in maintaining the monitoring stations.

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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.

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