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Five years of air chemistry observations in the Canadian Arctic
Amtospheric Environnvnr Printed in Great Britain.
Vol. 19, No. 12. pp. 1995-2010.
1985 0
FIVE YEARS OF AIR CHEMISTRY OBSERVATIONS CANADIAN ARCTIC
0004498l/ES s3.00 + 0.00 1985 Pergamon Press Ltd.
IN THE
L. A. BARRIEand R. M. HOFF Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada (First received 10 December 1984 and infinuIform 29 April 1985) Abstract-Arctic air chemistry observations made in Canada between 1979 and 1984 are discussed. The weekly average concentration of 25 aerosol constituents has been measured routinely at three locations. Anthropogenic pollution typified by SO:- and V has a persistent seasonal cycle. SO:- concentrations are similar at all three locations, although they tend to be. somewhat higher at Alert than at Mould Bay and Igloolik. The seasonal variation of an aerosol constituent depends on its source. There are four distinctive seasonal variations for: (i) anthropogenic constituents Cr. Cu, Mn, Ni, Pb, Sr, V, Zn, H+ , NHf , SO: -, NO;, (ii) halogens (exceptingCl) Br, I, F, (iii) sea salt elements Na, Mg, Cl and (iv) soil constituents Al, Ba, Ca, Fe and Ti. In the Arctic winter, the mean concentrations ofanthropogenic aerosol constituents, except SO:-, are 2-4 times lower than annual mean concentrations in southern Sweden near a major source. region. SOiconcentrations are only 30% lower mainly because of production from SO*. Light scattering (b,,) and SOi- observations indicate that the SOi- fraction of the fine particle mass fluctuates between 3 and 65 9/, du&g the polluted winter’months. Daily mean b tat Mould &y that exceeds 50 x 10e6 m-’ is associated with air originating from the northwest. The solu8e major ion composition of aerosols during winter varies markedly with particle size. H+, NH: and SOi- dominate submicrometre particles while sea-salt ions Mg*+ , Na+ and Cl- predominate in supermicrometre particles. Winter SO2 concentrations at Mould Bay and Igloolik ranged from 0.2 to 1.5ppb(v). The fraction of airborne S as SO1 ranged from 20 to 90 y0and peaked in late December-early January. Theconcentration of total NO; (0.02%0.09Oppb(v)) is much lower than that of SOi- (0.31.2ppb (v)). Key word index: Arctic haze, acidic compounds, visibility, aerosol scattering, heavy metals, aerosol trace elements, Arctic pollution, sulphate, vanadium.
INTRODUCTION
The Arctic atmosphere was once thought to be a pristine environment unaffected by air pollution except in very localized areas around settlements. Based on coordinated ground-level air chemistry observations in the last 10 years (e.g. Arctic Air Chemistry, 1981), on ice core chemistry records (Koerner and Fisher, 1982; Barrie et al., 1985) and on a recent international aircraft study (AGASP, 1984), it is now known that this is not the case. During the winter, visibility-reducing acidic particles originating mainly from fossil fuel combustion and smelting at midlatitudes in Eurasia are widespread at concentrations comparable to those found in rural areas in the south. Glacial ice cores reveal that pollution of the Arctic has occurred since 1912 and has increased significantly since 1956. Beginning in 1979, our studies (Barrie et al., 1981; Hoff et al., 1983; Leaitch et al., 1984; Barrie and Hoff, 1984; Hoff and Trivett, 1984) have been focused on the Canadian Arctic which makes up approximately one-quarter of the northern polar region. The objective has been to investigate the occurrence, nature, origin and effects of man-made air pollutants. The strategy has been to maintain a routine aerosol sampling network for long-term trend information, while mounting intensive shorter-term studies to pro-
vide information on the characteristics, processes, origin and effects of air pollutants. The circumpolar occurrence of Arctic haze has led to a high degree of international cooperation. This paper summarizes our research results for the period 1979-1984, discusses their implications and addresses future research needs.
ROUTINE AEROSOL OBSERVATIONS The Canadian Arctic Aerosol Sampling Network (CAASN), forming part of an international network (Rahn, 1981), consists of three stations; Mould Bay, Igloolik and Alert (Fig. 1). Operations began in April 1979, November 1979 and July 1980, respectively. Aerosol chemistry and aerosol light-scattering measurements are made. Results from the first year of operation (April 1979-May 1980) have been reported by Barrie et al. (1981). They are now augmented by observations up to May 1983. Aerosol chemistry
A. Measurements. At each monitoring site, weekly aerosol samples are collected on 20 x 25 cm Whatman 41 filters using a hi-volume sampler identical to that used at Barrow, Alaska (Rahn and McCaffrey, 1979). Volume flow rate is measured with an accuracy of + 5 % using an orifice calibrated with a ROOTS gas
1995
1996
L. A. BARRIEand R. M. HOFF
Fig. 1. A map of the Arctic showing Canadian, American and Norwegian aerosol sampling sites. A-Alert, I-Igloolik, M-Mould Bay, B-Barrow, S-Spitsbergen.
meter. Air volume is normalized to 1 atmosphere at 20°C. Care has been taken to avoid the influence of local sources. At Mould Bay, a 1Zperson meteorological outpost, the sampler is situated on a plateau in the predominant upwind direction 1.5 km from the camp. At Igloolik, it is located on a hill 1.5 km from the town of 700 people. At Alert, it sits on a 200 m high plateau 10 km from the camp. The camp is situated in a valley that channels air away from the sampler on the plateau. To account for filter matrix and sample handling contaminants, a field blank is collected every 4 weeks and analysed with the exposed filters. Filters containing suspended particles from an air volume of 5 16000m3 are cut into eight strips. Individual strips are analysed for (i) major ions by ion chromatography (IC) and pH electrode after extraction in distilled water, (ii) trace elements soluble in concentrated nitric acid by inductively-coupled plasma emission spectroscopy (ICP) and (iii) total trace elements by neutron activation analysis (NAA). The samples are sufficiently homogeneous across the filter that individual strips are representative of the whole deposit within 10 %. The analytical method and detection limit for each aerosol constituent is listed in Table 1. Detection limits are determined by the background in the filter field blanks
rather than the sensitivity of the analytical technique. This background varies from time to time and from station to station, hence the range in detection limit. The uncertainty in an air concentration varies with concentration. Close to the detection limit the 95% confidence limits are f 30 %, while at concentrations greater than three times the detection limit they are & 20%. For five elements (Cl, Na, Mn, V and Al)comparison between two different techniques is possible. The results, using linear regression analysis (Table 2), show that the agreement between IC and NAA for Cl and Na is excellent. This means that these elements are totally soluble in water and illustrates the validity of the statement made earlier that there is little variation across a filter deposit. The agreement between ICP and NAA for Mn and V is also good. R2 is 0.95 and 0.89, respectively, and the slope is not significantly different than one at the 99% confidence level. This is strong evidence that these elements are in compounds that are completely soluble in concentrated nitric acid and that their determination by either NAA or ICP is reliable. For Al, however, NAA and ICP analyses are well correlated (R2 = 0.84) but the slope is significantly greater than one (2.43 _+0.05). This is probably due to
1997
Five years of air chemistry observations in the Canadian Arctic the presence of nitric-acid-insoluble aluminosilicates or aluminum oxides. Results reported later are those obtained by NAA, that is, total aerosol Al. B. Results. The temporal variation of excess-SO:and excess-V at the three sites is shown in Figs 2 and 3, respectively. Excess-SO; - was obtained by correcting total SOi- for a sea salt component using aerosol Na (which is mostly of marine origin), while total V was corrected for a soil component using Al to yield excessV (Mason, 1966). There is a strong seasonal variation in both these substances of anthropogenic origin. This was first reported and discussed for the Canadian Table I. The anaIytica1 method and weekly-average air concentration detection limit for each aerosol constituent measured
Constituent F-
a-, Cl
1.4-3.7
IC IC, NAA NAA ;: XC, NAA
NH;
IC
12-20 OS-l.9 1.0-1.5 l&3.0 4-4
l-10 2-25 0.05-0.09
K+
I
LA
Mn
NitA, ICP NAA, ICP NAA, ICP ICP ICP ICP ICP ICP ICP ICP ICP ICP ICP ICP ICP
V AI cu
Cr Ba : Zn Pb Mg Sr Fe Ti P
Mould Bay-Alert [SO:-],a
Detection limit (ngm-?
Analysis technique
Br NO; so: Na”, Na
Arctic by Barrie et al. (198 l), for Alaska by Rahn and McCaffrey (1980) and for the Norwegian Arctic by Ottar (1981). The results in Figs 2 and 3 show that the seasonal variation is remarkably persistent from year to year throughout the Canadian Arctic. Differences in the magnitude of the winter peak from year to year are evident and are seen simultaneously at all three sites. For instance, concentrations were lower in the winter of 1980/81 than in 1979/80, 1981/82 or 1982/83. A comparison of weekly-average excess-SO:- concentrations at Mould Bay and Igloolik with those at Alert for the period July 1981-May I983 is shown in Fig. 4. The linear regression lines were as follows:
0.05-G.is 0.003-0.01 0. E.3 O.OH.2 0.040.07 0.01-0.10 5-l 0.91.7 0.2-0.5 58 0.01&0.03 2-5 0.2JJ5 l.Gl.4
IC, ion chromatography; NAA, neutron activation analysis; ICP, inductively-coupled plasma emission spectroscopy. Note: Br- for IC was poor because of large analytical detection limit.
= (87 f: 202)ngm-’ +@78 f .13@0;-IALERT R2 = 0.64, N = 82
Igloolik-Alert [SO:-]i
= (85 f 272)ngm-’ + (0.82 f .16)[SO:-],,,a,
%
xi
Na +(IC)
Na(NAA)
O-1500
CI - (IC) Mn (NAA) V (NAA)
CI (NAA) Mn (ICP)+ v(IcP)* Al(ICP)*
O-3000 G-5 O-5 O-500
Al(NAA)
(2)
RZ = 0.76; N = 38.
Intercepts were not significantly different from zero. Slopes less than 1 (though only si~~~tly so at less than 90% confidence) may indicate that concentrations at Alert are on average about 20-z higher than those at the other sites. This is possible since Alert is closer to Eurasian sources than the other sites. The good correlation between Alert and the other sites has another implication; namely, that for purposes of monitoring long term trends in Arctic haze, Alert is representative of the Canadian Arctic. A comparison of sulphate observations in the Canadian Arctic with those observed in the Norwegian Arctic at Spitsbergen and Bear Island (Joranger and Ottar, 1984) was made, The seasonal variations are similar in both regions. They are in-phase, highest in winter and lowest in summer. However, there are differences. In summer, excess-SO:- levels are much higher in the Norwegian Arctic than in the Canadian Arctic. In winter, they are on average only 15-25 yOlower in the Canadian Arctic.
Table 2. A comparison of aerosof trace eiement concentrations at all stations obtained from two different analytical techniques. A linear regression analysis was performed using the model: X = A + BX; + E; Range of xi, x Owf3)
(1)
A
Linear regression parameters 3 RZ N
5.9f4.1 13.1*t&5 0.05jc.015 0.047 f .011 -17.3Jc3.5
1.03kO.01 1.03f0.01 0.94 50.01 0.95kO.02 2.43 kO.05
0.92 0.93 0.95 0.89 0.84
473 475 47s 466 421
IC, ion chromatography; NAA, + neutron activation analysis; ICP, + plasma emission spectroscopy. * Represent fraction soluble in concentrated nitric acid.
1998
I
MOULD
BAY
IGLOOLIK
INMJSNJMMJSNJMMJSNJMVJSNJM~JSN
979
1980
1981
1982
1983
Fig, 2. A comparison of the ternpod variation of weekly-average excess-SC)- (non-sea salt) concentrations in the atmosphere at three locations in the Canadian Arctic (Fig. 1). Aerosol constituents shown in Table 1 have been grouped according to their seasonal cycles. In making the division, emphasis was placed on the origin of particulate matter during the months of haze. Using plots of the ratio of weekly average aerosol concentration (C) to the annual arithmetic average concen-
tration (e) vs time for 1980/81 and 1981/82, four groups were obtained. 1. Anthropogenic constituents: Cr, Cu, Mn, Ni, Pb, Sr, V, Zn, H+, NH:, SO;-, NO;. The marked seasonal cycle of these substances is simply represented in Fig. 5 by an envelope encompassing the mean curve through
Five years of air chemistry observations in the Canadian Arctic
I
ALERT
MOULD
1999
1 i/i
BAY
1, i,
IGLOOLIK
Fig. 3. A comparison of the temporal variation of weekly-average excess-V (non-soil) concentrations in the atmosphere at three locations in the Canadian Arctic (Fig. I).
the data from each site. It has a large amplitude (winter/summer ratio of lcc40) and a peak in late January and February. The exception was SOi- and Ii+ whose maximum was broader, extending into March and April. The difference is likely due to production of H,SO, from its gaseous precursor SO,
during March and April (Barrie and Hoff, 1984). 2. Halogens (except Cl): Br, I, F. The concentrations of these elements also undergo a marked seasonal cycle (Fig. 6) whose amplitude is as large as that of the anthropogenic constituents. However, it is distinguishable from the latter because it peaks later in the spring
L. A. BARRIEand R. M. HOFF 4r
/
EXCESS
I
E
SULPHATE
(eg/m3)
ALERT
/
1
I
3l
I I
0
1 EXCESS SULPHATE
i 3
2 bglm3)
*
ALERT
Fig. 4. A comparison of weekly-average concentrations of excess-SO:- in aerosols at Mould Bay and Igloolik with those observed simultaneously at Alert.
during March and April. The cycle of bromine is similar to that observed by Berg et al. (1983). They hypothesize that the peak between 15 February and 15
May is associated with the uptake of natural and/or anthropogenic gaseous Br by the aerosol or by direct injection from the sea-surface microlayer. They found Brconcentrations were 7-18 times higher than thoseof particulate Br. During the rest of the year, particulate Br originated from sea salt particles. At Mould Bay and Alert, a seasonal variation in iodine was observed that was similar to that of bromine with two notable differences. The spring maximum was less pronounced for iodine and it had a secondary maximum in the fall. No significant seasonal variation was seen at Igioolilc nor was there one evident at Barrow (Berg et al., 1983). At present we have no explanation for this difference, although it may be associated with Barrow and Igloolik being closer to open oceans than the other high Arctic stations. The spring maximum in iodine suggests a photochemical production mechanism similar to that of bromine. Our routine observations and special studies (Hoff et al., 1983) show that the concentration of bromide ion between November and May is highly enriched with respect to sea water. Simple production of sea-salt is
not sufficient to explain the observations. It should be emphasized that the artifact production of bromide ion on filter paper from passage of reactive gaseous Br species at low temperature has not been ruled out to date. This is true for our cellulose fibre filter medium and for the Nucleopore filters used by Berg et al. (1983). 3. Sea salt constituents: Na, Cl, K, Mg. The sea salt cycle is typified by that of Cl- in Fig. 6. It has a marked variation with the broad peak earlier than that of anthropogenic constituents in September-early January. It is similar to the cycle at Barrow reported by Berg et al. (1983) which had a broad ~ximum in the period September-December. There was a marked difference in the cycle between 1980/81 and 1981/82, presumably due to different transport episodes of sea salt into the high polar region. 4. Soil constituents: Al, Ba, Ca, Fe, Ti and possibly P. One characteristic of soil constituents is the absence of a strong consistent seasonal variation (Fig. 7). Phosphorus tends to peak in winter, coincident with the anthropogenic constituents which inditites that during that season, it may be partially anthropogenic in origin. Note that the seasonal variation of soil constituents in 1981/82differs from that of 1980/81 by having a peak in P, Ba, Fe and Ca in February coincident with that of the anthropogenic elements. This indicates that these elements may have strong anthropogenic components during that winter. The difference between yearly cycles of these constituents may be related to the cause of lower SOi- and V concentrations in 1980/81 than in 1981/82 (Figs 2 and 3). Values of annual arithmetic mean concentrations (c) for 1980/81 and 1981/82 used in producing Figs >7 are shown in Table 3. With a few notable exceptions, 55in a particular year is similar at all three sites. The con~ntration of Pb, I and sea salt elements is somewhat higher at Igloolik than at the other sites. Particulate Br levels at Alert are lower than at the other two sites. This is also true for winter average concentrations (December-April). For the two winters 1980/81 and 1981182, Br concentrations were 13, 21 and 26ngm-’ at Alert, Igloolik and Mould Bay, respectively. One explanation for this difference is that because of lower incident solar radiation, gas-toparticle conversion of Br in late winter occurring by photochemical reactions (Berg et al., 1983-1984) proceeds slowest at Alert, the site furthest north. In order to place the magnitude of air ~llution in the Canadian Arctic during winter in a northern hemispheric perspective, a comparison of the observed concentration of anthropogenic aerosol constituents with those in the Norwegian Arctic and in southern Sweden on the periphery of the European source region was made (Table 4). Mean concentrations of Cu, Mn, Ni, Pb and S during the period December-April are slightly lower in the North American Arctic than in the Norwegian sector. A few differences are notable. Cr and V concentrations are markedly lower at Canadian sites. Wintertime atmos-
2001
Five years of air chemistry observations in the Canadian Arctic I
I
I 5
ANTHROPOOENIC
SO/S1
CONSTITUENTS
Cr Cu Mn NI Pb Sr V Zn Ii+ NH;
SO;
NO;
4
3 c -c 2
1
0 5 81182
4
3 C Y 2
1
0 J
A
s
0
N
0
J
F
M
A
M
J
MONTH
Fig. 5. A schematic representation of the average _seasnal variation of the ratio of weekly-average eon~ntration (C) to annual-average co~ntration(C) for anthropogenic aerosol constituents for two winters 1980/81 and 1981/82 at all Canadian sites (Fig. 1).
pheric concentrations of most of the elements in the North American Arctic are in general lower by a factor of 2-4 than in southern Sweden, which is near a major source region. The exception is aerosol sulphur (S) which is only about 30% lower than in Europe. The contrast between S in the Arctic and Europe is not as great as for the other anthropogenic aerosol constituents because S is the only one with a gaseous precursor, namely SOz. Oxidation of SOz to particulate-S essentiatly replenishes aerosol-S removed by precipitation and dry deposition (Barrie and Hoff, 1984). Other anthropogenic constituents are not replenished and therefore are found at lower concentrations than in the source regions. Aerosol light scattering High volume sampler measurements of trace components of the haze in the Arctic provide a picture of the temporal variation of the haze problem which is sufficient to understand many of the characteristics of haze transport. However, they do not provide insight to the episodicity of Arctic pollution on time scales shorter than I week. Raatz (1984) examined this
episodicity using data obtained with a one day sampling period and, by the use of Fourier analysis, observed a 7-10 day episodicity as well as a longer term, 30-70 day cycle. In order to improve temporal resolution of the haze episodes, a continuous sampling technique is required and there are few continuous techniques which have the analytical precision to detect haze components and are su&ciently reliable to operate at a remote site. This has been the main reason that aerosol optical counters have been wideiy used in the Arctic since i977. A. Measurements. In the CAASN network, MRI Model 1550 integrating nephelometers were installed in April 1979 at Mould Bay in November 1979 at Igloolik. At Mould Bay, the nephelometer intake was on the side of the upper air meteorological building. At Igloolik, the intake was about 4 m above the surface on the side of the &tern Arctic Research Laboratory (EARL). Both nephelometer airstreams were heated. The Mould Bay site was certainly more remote than the Igloolik site since the EARL is located on the southwest edge of the village of Igloolik. It was noted early in the CAASN sampling effort that Iocai con-
L. A. BARRIE and R. M. HOFF
2002
5 HALOGENS
SO/81
Br Cl-
F I
4
3 C i 2
1
0 5 El/82
4
3 c F 2
1
a MONTH
Fig. 6. A schematic representation of the average seasonal variation of the ratio of weekly-average concentration (C)toannual-averageconcentration (C)for halogens for two winters, 1980/81and 1981/S2 at all Canadian sites (Fig. 1). tamination of the nephelometer data at Igloolik was evident during the summer months. This was predominantly due to dust raised by vehicles. During the winter, however, the level of external activity was greatly reduced and local contamination from heating furnace emissions did not have a large effect on the nephelometer results or the large particle size distributions (Leaitch et al., 1984). Nephelometer calibrations were made once a week by the station operators. The regularity of thesechecks was less than ideal since zero drifts have been noted in n~helometers (Ruby and Waggoner, 1981). The frequency of calibrations was limited by the available time of the station observers. Full calibrations of the nephelometers were made with Freon 12 once per year. Lowest detection levels were 5 x 10V6m-’ with the same r.m.s. error in reported b,, values. B. Results. Figure 8 shows the 4-year cumulative results for the nephelometer scattering coefficient, b,,, in units of 10-6m-’ for Mould Bay and Igtoolik. Unlike the sulphate levels in the haze which show a 4@ fold variation over the annual cycle, the variability in the scattering coefficient between winter and summer
is about 8: 1. This is bemuse the b,, data is not corrected for local influences during the summer months. The summer values are about a factor of I-10 higher than at Barrow, where wind-sector filtering is used to remove contaminated data. Such filtering would discard much of the summer data at Igloolik since the EARL is on the edge of the townsite proper. ft is therefore appropriate to regard the summer observations as due to local sources. The usefulness of the nephelometer results can be based on the criteria of whether they are comparable to other chemical and physical m~urements of the haze and whether they add to the temporal resolution of the haze monitoring. With regard to the first criterion, in our earlier paper (Barrie et al., 1981), a correlation between weekly mean b,, and excess-SO:- concentration (in pgrn-‘) was given which showed that b,,=(11f3.2)[S0,Z-]+(6f4.0)x10-6m-1 (3) with R2 = 0.71 and N = 65. Using all data from the two stations obtained since 1979, Fig. 9 shows the
Five years of air chemistry observations in the Canadian Arctic
5
SOIL
SO/81
2003
CONSTITUENTS
Al Ea Ca Fe P Ti
t
MONTH
Fig. 7. A schematic repre~n~tion of the average seasonal variation of the ratio of weekly-average con~ntration (C) to annual average co~ntmtion (c) for soil derived components for two winters, 1980/81 and 1981/82, at all Canadian sites (Fig. 1).
regression
of b,, on total sulphate. The current values
are: b scat= (10.9 f l.l)[SO:-]+(6.8
* 1.4)x 10-6m-’ (4)
with R2 = 0.54 and N = 322. As was noted in Barrie et al. (1981), the slope of this regression (i.e. the specific scattering coefheient) is much higher than could be expected from Mie theory for the known size distribution of the Arctic sulphate aerosol. This suggests that sulphate is responsible for about one third of the aerosol scattering. Using a different technique, Leaitch et al. (1984) reached the similar conclusion that the ratio of soluble to total particle mass varied from 15 to 75 y0 and was generally 1S-35 %. This indicates that too much emphasis is placed on the correspondence of b, to sulphate mass and perhaps that b, should be considered an indicator of total fine particle ( c 2~ diameter) mass. Fine particle mass can be inferred from the nephelometer results using the spec%c scattering coefficient of 3.1 m2 g- ’ suggested by Waggoner and
Weiss (1980). This value is consistent with the Arctic winter grand average at Spitzbergen of 3.2m2 g-‘, measured and seen by Heintzenberg (1982). The ratio of excess-SO:- to this mass ranged from 3 % to 65 % over the spring period with a mean and standard deviation of 22 j, 10%. This supports the conclusion drawn in our earlier paper (Barrie et al., 1981) and that of Leaitch et al. (19&Q. The fraction of fine particle mass consisting of SO:- tends to increase throu~out the winter peaking in March and April. This is probably due to the enhanced production of SOifrom SO2 in early spring as suggested by Barrie and Hoff (1984). The second criterion for appraising the usefulness of the nephelometer is whether it enhances our ability to resolve sub-weekly pollution episodes. To examine this aspect of the program, objective analysis of the starting points for three dimensional S-day back-trajectories of air parcels was done for end-point pressure levels of 1000,925 and 85Omb for the winters 1980-1983. Days which had average b, readings in excess of 50 x 10-6m-’ were flagged and the trajectory starting
L. A. BARRIEand R. M. HOFF
2004
Table 3. Annual arithmetic mean concentrations (Zffaerosol constituents (a year inchtdes July-June) Alert 80/8 1
Substance Anthropogenic *Cr *cu Mn Ni *Pb ‘Sr V *Zn H+ NH: so: NO;
0.16
Halogens Br I F
Soil Al lBa *ca *Fe *Ti *P
Igloolik 81182 80/81 0.15
Mould Bay 81/82 80/81
0.78 1.60 0.32 1.72 0.30 0.36 2.80 6.3 82 890 55
0.30 1.33 1.49 0.38 1.70 0.57 0.62 3.48 7.4 102 1052 14
0.062 0.69 0.45 0.14 3.61 0.35 0.21 3.80 4.0 77 667 62
1.13 0.71 0.27 3.84 0.67 0.47 3.45 5.9 95 966 71
0.31 1.61 0.79 0.40 2.00 0.35 0.28 2.82 6.5 72 738 59
0.32 1.02 0.87 0.45 3.22 0.41 0.56 3.79 9.2 125 1036 67
3.8 0.43 2.0
8.9 0.49 7.0
5.7 0.96 7.2
14 1.03 5.8
7.0 0.45 5.4
16 0.46 6.7
121 114 11 92
Na CI K *Mg
81182
0.50
201 86 0.49 2.79
158 182 16 84
229 368 17 99
313 505 31 129
179 251 15 69
243 354 11 63
146 0.37 139 86 1.31 1.61
13 0.16 83 16 0.68 1.37
0.21 94 21
45 0.42 35 36 0.35 3.11
0.47 29 29 0.24 1.12
c).93
*The fraction soluble in concentrated nitric acid. Table 4. A comparison of the arithmetic mean atmospheric concentration of aethropogenie aerosol constituents (ngm-3) during the winter months (December-April) of 1980/81 and 1981/82 in the Canadian Arctic with measurements elsewhere in the Arctic and in southern Sweden near major sources
Substances Cr cu Mn Ni Pb Sr V Zn S
Alert 0.37 1.88 1.63 0.62 3.49 0.61 0.96 6.30 599
Igloolik 0.27 1.53 0.95 0.42 6.38 0.88 0.76 6.56 520
Mould Bay 0.41 2.01 1.22 0.59 5.42 0.61 0.87 7.15 54+
Spitsbergen* Winter* Marcht (3-weeks) 1983 2.6 1.9 1.5 0.7 4.9 3.2 690
S Sweden3 annual mean
1.2 2.5 2.7 1.3 12.2
2.8 2.4 6.7 1.5 21.0
28 13.7 -
2.5 24.0 800
*Heintzenberg er af. (1981), 2-5 day samples in late winter. ?Pacyna et al. (19851,1 month study. XLannefors et al. (1983).
points identified. Twenty-six 925mb starting points were identified using this criterion. They are plotted in Fig. 10. There is a clear preponderance of trajectories from the northwest of Mould Bay indicating the Asian continent as a source. This is not just an artifact caused by the genera1 system flow since the distribution of 925 mb starting points for all days, flagged and unflag-
ged (Fig. 1l), is more isotropic with about 20 % of the trajectories starting south of Mould Bay in the North American Arctic. None of the high light-scattering days had S-day trajectories starting south of Mould Bay. This adds to the overwhelming evidence of a Eurasian source for the haze. While a similar conclusion has been drawn from weekly-averaged filter data,
Five years of air chemistry observations in the Canadian Arctic
200s
50
40
30
20
IGLOOLIK
1 30
20
10
L
975 3
f”i
1980
1983
1982
1981
Y eoiFig. 8. The temporal variation of the aerosol light ~tte~ngc~~cient Mouid Bay and Igiooiik. El0
I
Total
I
I
Sulphats
(b,)at
Cug/m^33
Fig. 9. The relationship between weekly-average b,,and totalaerosol SO:-. All data from Mould Bay and Igloolik. been the confounding situation in which trajectories over a week period show wide disparity of source regions. The daily results shown here clearly alleviate much of that concern. The results there has often
also point out that not all trajectories that point towards Eurasia as a source classify air parcels which contain high ~~t-~tte~ng aerosols. The trajectories from the Western Soviet Union and those from
L. A. BARRIEand R. M.
HOFF
Fig. 10. The locations of the end points of three dimensional, S-day back trajectories arriving at the 925 mb level at Mouid Bay for days when the daily b,, is greater than 50 x 10-6m-‘. 90”E
180”
0”
90”
w
Fig. 11. The locations of the end points of three-dimensional, S-day back trajectories arriving at the 925 mb level at Mouid Bay for all days in the months considered for Fig. 10.
2007
Five years of air chemistry observations in the Canadian Arctic Eastern Siberia are less likely to be associated with high light-scattering episodes at Mould Bay.
1NTENSIVE SHORT TERM AEROSOL STUDIES
Vertical ~~~tr~but~o~ and mass size ~istrib~~ion~ In February 1982, a study of Arctic aerosol was conducted at Igloolik. Results have been discussed in detail by Hoff et al. (1983) and Leaitch et al. (1984).Of special interest was the vertical profile of aerosol light scattering obtained on 26 February 1982 (Fig. 12), one of the first such profiles ever measured. If one assumes that there is a good correlation between b,, and fine particle mass (Waggoner and Weiss, 1980), it shows that most of the aerosol fine particle mass in the lower troposphere was confined to the lowest 1.5km. An indication of the representativeness of this ‘snapshot’ of the vertical distribution of b,, was given by aircraft m~surements made during March and April 1983 (Schnell and Raatz, 1984; Radke et al., 1984). Of 13 profiles taken in the North American Arctic on different days, 8 showed most b, occurring in the lowest 1.5 km, while 5 showed b,, in layers between 3 and 5 km that was comparable in magnitude to that in the lower 1.5 km. More vertical profiles of aerosol scattering and mass are needed throughout the polluted period January-April. The mass size distributions of major ions in ground level aerosols at Igloolik during February 1982 are shown in Fig. 13. The measurements were made by Hoff et al. (1983) using a low volume Andersen cascade in impactor. NH: and SO:- are pr~ominantly particles of less than 2 pm diameter. Their mass median diameter is approximately 0.5 pm. On the other hand,
NO;, which originates by a similar process to atmospheric SO:- (i.e. gas to particle conversion), is concentrated in larger particles than SO:-. This is consistent with observations elsewhere in the atmosphere. A plausible explanation is that because aerosol NO; is more volatile than SO:-, some of the NO; migrates from the acidic accumulation mode to the more alkaline coarse particle mode, comprised mainly of sea salt and wind blown dust constituents (Bassett and Seinfeld, 1984). A different NO; mass size distribution was observed in aerosol layers aloft by Radke et al. (1984). They found NO; mass almost equally distributed between two modes, centred at 0.3 and 6pm diameter. More measurements are needed to determine the natural variability in NO; size distributions. One explanation for the difference in our ground level size distribution and that of Radke et al. is that more alkaline coarse particle surface area is available for absorption of NO; at the ground than aloft. Also some of our coarse particle NO; might be soil nitrate. The mass size dist~butions of Mg” and Nat (Fig. 14) are consistent with their most probable origin as sea spray. However, chloride shows a deficit in submicrometre particle size ranges which was attributed to volatilization of Cl- in reaction with HzSOd on particles of this size (Hoff et al., 1983; Mohnen et al., 1977). A similar deficit in chloride relative to NaCl is sometimes seen in routine weekly samples during December-April. This volatilization parallels that of NO; discussed above. What the size distribution measurements at Igloolik show is that H+, NHf and SO:- dominate the soluble ions in particles less than 1 pm diameter, while Mg2+, Cl- and Na+ predominate in particles of > 1 pm diameter. NO; is a minor ionic constituent.
9: 24-102 I0 26 Feb 19B2
CN (#/ccl
Temp (Cl -40
-35
-30
-2.5
-20
. .
EC+
(10 -6m-1)
Fig. 12. Vertical profiles of b,, and temperature at Igloolik on 26 February 1982, observed by Leaitch et ai. (1984). The dotted line is dry bulb temperature and squares indicate wet bulb temperatures. Triangles indicate Aitken nuclei concentrations.
L. A. BARRIEand R. M. HOFF
2008
NO,-
PARTICLE
DIAMETER.
I
D ip m)
Fig. 13. The composite mass-size distribution of aerosol major ions at Igloolik during February 1982 observed by Hoff et al. (1983).The ordinate represents mass fraction per unit logarithmic diameter and is plotted vs impactor effective cutoff diameters. The backup filter was assigned a cutoff of 0.1 pm for plotting purposes. The vertical bars represent r.m.s. deviation of four 3-day samples.
The relative abundance sulphates and nitrate
ofgaseous and particuiate phase
A. Measurements. In the winter of 1983/&l, measurements of the concentration of SOa, SO:-, HNO, and NO; were undertaken at Mould Bay. On a weekly basis, air was drawn through three filters in series: a Teflon particulate filter, a nitric-acid-trapping nylon filter and a Whatman 41 filter impregnated with glycerol and potassium carbonate to trap SOz. Filters were extracted in distilled water which was then analyzed by ion chromatography or the modified Thorin method (for SO2 and SO,”-). The voIatil~tion of particulate NO; to HNOa causing an artifact of HNOs on the nylon filter cannot be ruled out. Nitrate results are reported as gaseous and particulate frac-
tions for completeness, however, they should be interpreted as total nitrate. B. SO,/SO:- results. Concentrations for SO2 and SOi - observed at Mould Bay and fgloolik are shown in Fig. 14. SO, concentrations range from 0.2 to 1.5 ppb. They are higher in late January and February than in early December. They are similar in magnitude and temporal variation to those observed at Igloolik (Hoff et al., 1983; Batrie and Hoff, 1984) and at Bear Island and Spitsbergen (Rahn et al., 1981; Joranger and Ottar, 1984). Using an aircraft, Radke et al. (1984) measured SO2 concentrations of 0.2-0.6 ppb in Arctic haze layers during April. The fraction of airborne sulphur comprised of SO* (Fso,) is of interest because it is an indication of
2009
Five years of air chemistry observations in the Canadian Arctic p--7---
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Fig. 14. The average concentration of SO2 and SO:observed at Mould Bay (weeklyaverages)and Igloolik
(l-3 day averages). ‘potential’ SO:- that could be formed if Sol is oxidized. Furthermore, it can be used in conjuction with the ratios of excess-SOi-/excess-V to estimate the mean oxidation rate of SO* between Eurasian sources and the North American Arctic (Barrie and Hoff, 1984). Fso, at Igloolik in 1981/82 and at Mould Bay in 1983/84 ranged from 20% to 90% and tended to be higher in December, January and February than in April. C. HNO,/NO; observations. The concentrations of ‘so-called gaseous and particulate NO; observed at Mould Bay are shown in Fig. 15. Particulate concentrations exceed the gas phase ones early in the winter but they are much lower later in the winter. That is, the fraction of airborne NO; comprised of HNOs increases from _ 30% in December to u 75 % in February. As mentioned above, one cannot be sure that the gaseous HNOl observed on the nylon filter is not from volatilization of particulate matter. An important conclusion based on these measurements is that the concentration of total NO; (0.0250.090 ppb) is much lower than that of SOi- particulate matter (0.3-1.2 ppb). CONCLUSION
Fig. 15. The weekly average atmospheric concentration of nitrate observed on a Teflon filter (P-NO; ) and on a nylon backup filter (G-NO;)at Mould Bay from December 1983 to February 1984. Part of G-NO; may be volatilized aerosol nitrate rather than HNOs.
enon of Arctic haze and acidic air pollution. This insight is summarized in the following conclusions: (1) In the Canadian Arctic, there is a persistent, strong seasonal variation in anthropogenic aerosol concentrations that is in-phase with a similar variation observed in Alaska and the Norwegian Arctic. Year-toyear variability is evident and is consistent at all three locations. In particular, excess-SO:- and excess-V concentrations for the period January-April were on average twice as high in 1982 as in 1981 and 1983. (2) SOi- concentrations at Alert are well correlated with-but about 20% higher than-those at Mould Bay and Igloolik. Seasonal variations are similar throughout the Canadian and Norwegian Arctic. Winter average concentrations in Canada are 15-25 % lower than in the Norwegian Arctic. (3) The 24 aerosol constituents measured routinely can be classified into four groups according to their seasonal variation: (i) Anthropogenic elements: peaking in January and February, Cr, Cu, Mn, Ni, Pb, Sr, V, Zn, NO;, NH: with the exception of H+ and SOithat peak for a longer period, December-April. (ii) Halogens (excepting Cl): Br, I, F, peaking in March-April. (iii) Sea salt elements: Na, Mg, Cl broad maximum between September and April. (iv) Soil elements: Al, Ba, Fe, Ti (possibly P) having no consistent seasonal variation. (4) Mean concentrations of anthropogenic aerosol constituents between December and April are, with the exception of SO:-, 2-4 times lower than annual means in southern Sweden near major European sources. SOi- concentrations are only 30 % lower than in southern
Air chemistry observations made at Alert, Mould Bay and Igloolik in the Canadian high Arctic on a routine and special studies basis between 1979 and 1984 have provided greater insight into the phenom-
Sweden.
(5) The broad winter maximum in concentration of aerosol H+ and SOi- noted in 3 (i) above and the small difference in SOi- concentrations between the Arctic and the Eurasian source region noted in 4 above
L. A. BARRIE and R. M. HOFF
2010
are explained by seasonal variations in SO2 oxidation
rate. Rates are higher in early winter and late spring than they are in mid-winter. This is consistent with the observation that the fraction of airborne sulphur as SO2 peaks in January and February at Mould Bay and Igloolik. Depending on the winter month, 20-90 % of airborne sulphur exists as SOz. Concentrations range from 0.2 to 1.5ppb(v). (6) Aerosol light scattering observations are useful. Not only do they enable one to examine the episodicity of haze aerosols but they also allow one to estimate the average fraction of fine particle mass that is comprised of SO:- to be 22 f 10%. It is concluded that high light-scattering days at Mould Bay are genera!ly associated with transport from the north and northwest. (7) The soluble major ion composition of Arctic haze aerosols varies with particle size. H+, NH:, SOi- predominate in submicrometre particles while Na’, Mg” and Cl from sea salt dominate the su~rmi~rometre particles. (8) The concentration of total NO; (HNO; plus during the Arctic particulate NO;) in December-February (O.O2~.~ppb) is much lower than that of SO:- particulate matter (0.3-1.2ppb). The future direction of Arctic air chemistry research in tinada will likely follow a three-pronged approach. Routine observations of aerosol chemistry, aerosol light scattering and SO2 will be continued at Alert. Operations were terminated at Mould Bay and Igloolik in May 1984 after 4 years of simultaneous measurements at three locations which showed that one Canadian station was adequate for monitoring Arctic air ~llution from distant sources. It is particularly important to understand the vertical structure of haze transport throughout the winter period. The ‘snapshots’ of haze structure seen in aircraft studies to date have been extremely useful in identifying layers of haze but a ‘climatology’ of these profiles is needed. To this end, a monostatic ruby lidar is operating at Alert, during the 1984/85 winter to obtain verticle profiles of haze on a longer term basis. Special studies will be mounted in a continuing effort to understand the processes affecting the pathways of Arctic haze particles through the atmosphere, the origin of pollutants and potential effects. In addition, our expanding knowledge of the long range transport of anthro~genjc pollutants will be integrated into a chemical transport model.
REFERENCES AGASP (1984)(edited by Schnell R.). Geophys. Res. Letters 11, 359472. Arctic Air Chemistry (1981) (edited by Rahn K. A.). Atmospheric Environment 15, 8. JSarrieL. A., Hoff R. M. and Daggupaty S. M. (1981) The
inffuence of mid-~titudina~ ooifution sources on haze in the Canadian Arctic. At~spheric Env~ro~~nt 15, 1407- 1420.
Barrie L. A. and Hoff R. M. (1984) The oxidation rate and residence time of sulphur dioxide in the Arctic atmosphere. Atmospheric Environment 18, 271l-2722. Barrie L. A., Fisher D. and Koerner R. M. (1985) Twentieth century trends in Arctic air pollution revealed by conductivity and acidity observations in snow and ice in the Canadian high Arctic. Atmospheric Environment 19, 2055-2063. BassettM. E. and Seinfeld J. H. (1984) Atmospheric equiiibrium model of suipbate and nitrate aerosols-II. Particle size analysis. Atmospheric ~nviro~nt l%, 1163-l 170. Berg W. W. Sperry P., Rabn K. A. and Gladney E. S. (1983) Atmospheric bromine in the Arctic. J. ~e0phys. Res. 88, 67196736. Berg W. W., Heidt L. E., Policck W., Sperry P. D., Ciirone R. J. and Gladney E. S. (1984) Brominated organic species in the Arctic atmosphere. Geophy. Rex Lett. 5, 429-432. Heintzenberg, J., Hansson H. C. and Lannefors H. (1981) The chemical composition of Arctic haze at Ny-Alesund, Spitsbergen. Tellus 33, 162-171. Heintzenberg J. (1982) Size-segregated measurements of particulate elemental carbon and aerosol light absorption at remote Arctic location. Atmospheric Environment 16, 2461-2469. Hoff R. M., Leaitch W. R., Fellin P. and Barrie L. A. (1983) Mass-size dist~butions of chemical constituents of the winter Arctic aerosol. .I. geophys. Res. 88, 10947-10956. Hoff R. M. and Trivett N. B. A. 119841 Ground-based measurements of Arctic haze madk at Alert, N. W. T., Canada, during the Arctic Gas and Aerosol Sampling Project (AGASP). Geophy. Res. Mt. 11, 389-392. Lannefors H., Hansson H. C. and Granat L. (1983) Background aerosol composition in southern Swedenfourteen micro and macro constituents measured in seven particle size intervals at one site during one year. Atmospheric Environment 17, 87-101. Leaitch W. R., Hoff R. M., Melnichuk S. and Hogan A. (1984) Some physical and chemical properties of the Arctic winter aerosol in northeastern Canada. J. Ctim. appi. Met. 23, 916-928. Koemer R. M. and Fisher D. (1982) Acid snow in the Canadian high Arctic. Nature 295, 137-140. Mason B. (1966) P&&pies of Geochemistry 3rd edn, John Wiley, New York. Ottar B. (1981) The transfer of airborne pollutants to the Arctic region. Atmospheric Environment 15, 143+1446. Pacyna J. M., Ottar B., Tomza U. and Maenhaut W. (1985) Long-range transport of trace elements to Ny-Alesund, Spitsbergen. Atmospheric Environment 19, 857-865. Radke L. F., Lyons J. H., Hegg D. A., Hobbs P. V. and Bailey I. H. (1984) Airborne observations of Arctic aerosols-I. Characteristics of Arctic haze. Geophys. Res. Lett. 5, 393-396. Rahn K. A. and McCaffrey R. J. (1979) Long range transport of pollution aerosol to the Arctic. A problem without borders. Proc. WMO Symp. on the Z.ung Range Transport of FOll~t~ts wrd Its Relation to General Circu~arion uncivil ~tratosp~r~~ropospher~c Exehtqe Processes, Sofia, l-5 October, WMO. No. 538, pp. 25-35. Rahn K. A., Joranger E., Semb A. and Conway T. (1980) High winter concentrations of SO2 in the Norwegian Arctic and transport from Eurasia. Nature 287. 824-826. Rahn %. A. (1981) The Arctic air-sampling network in 1980. Atmospheric Environment 15, 1349-l 352. Ruby M. G. and Waggoner A. P. (1981) Intercomparison of intergrating nephelometer measurements. En&. Sci. Technol. 15, 109. Schnell R. C. and Raatz W. E. (1984) Vertical and horizontal characteristics of Arctic haze during AGASP: Alaskan Arctic. Geophys. Rex Lett. 5, 369-372. Waggoner A. P. and Weiss R. E. (1980) Comparison of fine particle mass con~ntration and light scattering extinction in ambient aerosol. Atmospheric ~~~iro~n~ b&623-626.
Vol. 19, No. 12. pp. 1995-2010.
1985 0
FIVE YEARS OF AIR CHEMISTRY OBSERVATIONS CANADIAN ARCTIC
0004498l/ES s3.00 + 0.00 1985 Pergamon Press Ltd.
IN THE
L. A. BARRIEand R. M. HOFF Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada (First received 10 December 1984 and infinuIform 29 April 1985) Abstract-Arctic air chemistry observations made in Canada between 1979 and 1984 are discussed. The weekly average concentration of 25 aerosol constituents has been measured routinely at three locations. Anthropogenic pollution typified by SO:- and V has a persistent seasonal cycle. SO:- concentrations are similar at all three locations, although they tend to be. somewhat higher at Alert than at Mould Bay and Igloolik. The seasonal variation of an aerosol constituent depends on its source. There are four distinctive seasonal variations for: (i) anthropogenic constituents Cr. Cu, Mn, Ni, Pb, Sr, V, Zn, H+ , NHf , SO: -, NO;, (ii) halogens (exceptingCl) Br, I, F, (iii) sea salt elements Na, Mg, Cl and (iv) soil constituents Al, Ba, Ca, Fe and Ti. In the Arctic winter, the mean concentrations ofanthropogenic aerosol constituents, except SO:-, are 2-4 times lower than annual mean concentrations in southern Sweden near a major source. region. SOiconcentrations are only 30% lower mainly because of production from SO*. Light scattering (b,,) and SOi- observations indicate that the SOi- fraction of the fine particle mass fluctuates between 3 and 65 9/, du&g the polluted winter’months. Daily mean b tat Mould &y that exceeds 50 x 10e6 m-’ is associated with air originating from the northwest. The solu8e major ion composition of aerosols during winter varies markedly with particle size. H+, NH: and SOi- dominate submicrometre particles while sea-salt ions Mg*+ , Na+ and Cl- predominate in supermicrometre particles. Winter SO2 concentrations at Mould Bay and Igloolik ranged from 0.2 to 1.5ppb(v). The fraction of airborne S as SO1 ranged from 20 to 90 y0and peaked in late December-early January. Theconcentration of total NO; (0.02%0.09Oppb(v)) is much lower than that of SOi- (0.31.2ppb (v)). Key word index: Arctic haze, acidic compounds, visibility, aerosol scattering, heavy metals, aerosol trace elements, Arctic pollution, sulphate, vanadium.
INTRODUCTION
The Arctic atmosphere was once thought to be a pristine environment unaffected by air pollution except in very localized areas around settlements. Based on coordinated ground-level air chemistry observations in the last 10 years (e.g. Arctic Air Chemistry, 1981), on ice core chemistry records (Koerner and Fisher, 1982; Barrie et al., 1985) and on a recent international aircraft study (AGASP, 1984), it is now known that this is not the case. During the winter, visibility-reducing acidic particles originating mainly from fossil fuel combustion and smelting at midlatitudes in Eurasia are widespread at concentrations comparable to those found in rural areas in the south. Glacial ice cores reveal that pollution of the Arctic has occurred since 1912 and has increased significantly since 1956. Beginning in 1979, our studies (Barrie et al., 1981; Hoff et al., 1983; Leaitch et al., 1984; Barrie and Hoff, 1984; Hoff and Trivett, 1984) have been focused on the Canadian Arctic which makes up approximately one-quarter of the northern polar region. The objective has been to investigate the occurrence, nature, origin and effects of man-made air pollutants. The strategy has been to maintain a routine aerosol sampling network for long-term trend information, while mounting intensive shorter-term studies to pro-
vide information on the characteristics, processes, origin and effects of air pollutants. The circumpolar occurrence of Arctic haze has led to a high degree of international cooperation. This paper summarizes our research results for the period 1979-1984, discusses their implications and addresses future research needs.
ROUTINE AEROSOL OBSERVATIONS The Canadian Arctic Aerosol Sampling Network (CAASN), forming part of an international network (Rahn, 1981), consists of three stations; Mould Bay, Igloolik and Alert (Fig. 1). Operations began in April 1979, November 1979 and July 1980, respectively. Aerosol chemistry and aerosol light-scattering measurements are made. Results from the first year of operation (April 1979-May 1980) have been reported by Barrie et al. (1981). They are now augmented by observations up to May 1983. Aerosol chemistry
A. Measurements. At each monitoring site, weekly aerosol samples are collected on 20 x 25 cm Whatman 41 filters using a hi-volume sampler identical to that used at Barrow, Alaska (Rahn and McCaffrey, 1979). Volume flow rate is measured with an accuracy of + 5 % using an orifice calibrated with a ROOTS gas
1995
1996
L. A. BARRIEand R. M. HOFF
Fig. 1. A map of the Arctic showing Canadian, American and Norwegian aerosol sampling sites. A-Alert, I-Igloolik, M-Mould Bay, B-Barrow, S-Spitsbergen.
meter. Air volume is normalized to 1 atmosphere at 20°C. Care has been taken to avoid the influence of local sources. At Mould Bay, a 1Zperson meteorological outpost, the sampler is situated on a plateau in the predominant upwind direction 1.5 km from the camp. At Igloolik, it is located on a hill 1.5 km from the town of 700 people. At Alert, it sits on a 200 m high plateau 10 km from the camp. The camp is situated in a valley that channels air away from the sampler on the plateau. To account for filter matrix and sample handling contaminants, a field blank is collected every 4 weeks and analysed with the exposed filters. Filters containing suspended particles from an air volume of 5 16000m3 are cut into eight strips. Individual strips are analysed for (i) major ions by ion chromatography (IC) and pH electrode after extraction in distilled water, (ii) trace elements soluble in concentrated nitric acid by inductively-coupled plasma emission spectroscopy (ICP) and (iii) total trace elements by neutron activation analysis (NAA). The samples are sufficiently homogeneous across the filter that individual strips are representative of the whole deposit within 10 %. The analytical method and detection limit for each aerosol constituent is listed in Table 1. Detection limits are determined by the background in the filter field blanks
rather than the sensitivity of the analytical technique. This background varies from time to time and from station to station, hence the range in detection limit. The uncertainty in an air concentration varies with concentration. Close to the detection limit the 95% confidence limits are f 30 %, while at concentrations greater than three times the detection limit they are & 20%. For five elements (Cl, Na, Mn, V and Al)comparison between two different techniques is possible. The results, using linear regression analysis (Table 2), show that the agreement between IC and NAA for Cl and Na is excellent. This means that these elements are totally soluble in water and illustrates the validity of the statement made earlier that there is little variation across a filter deposit. The agreement between ICP and NAA for Mn and V is also good. R2 is 0.95 and 0.89, respectively, and the slope is not significantly different than one at the 99% confidence level. This is strong evidence that these elements are in compounds that are completely soluble in concentrated nitric acid and that their determination by either NAA or ICP is reliable. For Al, however, NAA and ICP analyses are well correlated (R2 = 0.84) but the slope is significantly greater than one (2.43 _+0.05). This is probably due to
1997
Five years of air chemistry observations in the Canadian Arctic the presence of nitric-acid-insoluble aluminosilicates or aluminum oxides. Results reported later are those obtained by NAA, that is, total aerosol Al. B. Results. The temporal variation of excess-SO:and excess-V at the three sites is shown in Figs 2 and 3, respectively. Excess-SO; - was obtained by correcting total SOi- for a sea salt component using aerosol Na (which is mostly of marine origin), while total V was corrected for a soil component using Al to yield excessV (Mason, 1966). There is a strong seasonal variation in both these substances of anthropogenic origin. This was first reported and discussed for the Canadian Table I. The anaIytica1 method and weekly-average air concentration detection limit for each aerosol constituent measured
Constituent F-
a-, Cl
1.4-3.7
IC IC, NAA NAA ;: XC, NAA
NH;
IC
12-20 OS-l.9 1.0-1.5 l&3.0 4-4
l-10 2-25 0.05-0.09
K+
I
LA
Mn
NitA, ICP NAA, ICP NAA, ICP ICP ICP ICP ICP ICP ICP ICP ICP ICP ICP ICP ICP
V AI cu
Cr Ba : Zn Pb Mg Sr Fe Ti P
Mould Bay-Alert [SO:-],a
Detection limit (ngm-?
Analysis technique
Br NO; so: Na”, Na
Arctic by Barrie et al. (198 l), for Alaska by Rahn and McCaffrey (1980) and for the Norwegian Arctic by Ottar (1981). The results in Figs 2 and 3 show that the seasonal variation is remarkably persistent from year to year throughout the Canadian Arctic. Differences in the magnitude of the winter peak from year to year are evident and are seen simultaneously at all three sites. For instance, concentrations were lower in the winter of 1980/81 than in 1979/80, 1981/82 or 1982/83. A comparison of weekly-average excess-SO:- concentrations at Mould Bay and Igloolik with those at Alert for the period July 1981-May I983 is shown in Fig. 4. The linear regression lines were as follows:
0.05-G.is 0.003-0.01 0. E.3 O.OH.2 0.040.07 0.01-0.10 5-l 0.91.7 0.2-0.5 58 0.01&0.03 2-5 0.2JJ5 l.Gl.4
IC, ion chromatography; NAA, neutron activation analysis; ICP, inductively-coupled plasma emission spectroscopy. Note: Br- for IC was poor because of large analytical detection limit.
= (87 f: 202)ngm-’ +@78 f .13@0;-IALERT R2 = 0.64, N = 82
Igloolik-Alert [SO:-]i
= (85 f 272)ngm-’ + (0.82 f .16)[SO:-],,,a,
%
xi
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Na(NAA)
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RZ = 0.76; N = 38.
Intercepts were not significantly different from zero. Slopes less than 1 (though only si~~~tly so at less than 90% confidence) may indicate that concentrations at Alert are on average about 20-z higher than those at the other sites. This is possible since Alert is closer to Eurasian sources than the other sites. The good correlation between Alert and the other sites has another implication; namely, that for purposes of monitoring long term trends in Arctic haze, Alert is representative of the Canadian Arctic. A comparison of sulphate observations in the Canadian Arctic with those observed in the Norwegian Arctic at Spitsbergen and Bear Island (Joranger and Ottar, 1984) was made, The seasonal variations are similar in both regions. They are in-phase, highest in winter and lowest in summer. However, there are differences. In summer, excess-SO:- levels are much higher in the Norwegian Arctic than in the Canadian Arctic. In winter, they are on average only 15-25 yOlower in the Canadian Arctic.
Table 2. A comparison of aerosof trace eiement concentrations at all stations obtained from two different analytical techniques. A linear regression analysis was performed using the model: X = A + BX; + E; Range of xi, x Owf3)
(1)
A
Linear regression parameters 3 RZ N
5.9f4.1 13.1*t&5 0.05jc.015 0.047 f .011 -17.3Jc3.5
1.03kO.01 1.03f0.01 0.94 50.01 0.95kO.02 2.43 kO.05
0.92 0.93 0.95 0.89 0.84
473 475 47s 466 421
IC, ion chromatography; NAA, + neutron activation analysis; ICP, + plasma emission spectroscopy. * Represent fraction soluble in concentrated nitric acid.
1998
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1980
1981
1982
1983
Fig, 2. A comparison of the ternpod variation of weekly-average excess-SC)- (non-sea salt) concentrations in the atmosphere at three locations in the Canadian Arctic (Fig. 1). Aerosol constituents shown in Table 1 have been grouped according to their seasonal cycles. In making the division, emphasis was placed on the origin of particulate matter during the months of haze. Using plots of the ratio of weekly average aerosol concentration (C) to the annual arithmetic average concen-
tration (e) vs time for 1980/81 and 1981/82, four groups were obtained. 1. Anthropogenic constituents: Cr, Cu, Mn, Ni, Pb, Sr, V, Zn, H+, NH:, SO;-, NO;. The marked seasonal cycle of these substances is simply represented in Fig. 5 by an envelope encompassing the mean curve through
Five years of air chemistry observations in the Canadian Arctic
I
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MOULD
1999
1 i/i
BAY
1, i,
IGLOOLIK
Fig. 3. A comparison of the temporal variation of weekly-average excess-V (non-soil) concentrations in the atmosphere at three locations in the Canadian Arctic (Fig. I).
the data from each site. It has a large amplitude (winter/summer ratio of lcc40) and a peak in late January and February. The exception was SOi- and Ii+ whose maximum was broader, extending into March and April. The difference is likely due to production of H,SO, from its gaseous precursor SO,
during March and April (Barrie and Hoff, 1984). 2. Halogens (except Cl): Br, I, F. The concentrations of these elements also undergo a marked seasonal cycle (Fig. 6) whose amplitude is as large as that of the anthropogenic constituents. However, it is distinguishable from the latter because it peaks later in the spring
L. A. BARRIEand R. M. HOFF 4r
/
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E
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Fig. 4. A comparison of weekly-average concentrations of excess-SO:- in aerosols at Mould Bay and Igloolik with those observed simultaneously at Alert.
during March and April. The cycle of bromine is similar to that observed by Berg et al. (1983). They hypothesize that the peak between 15 February and 15
May is associated with the uptake of natural and/or anthropogenic gaseous Br by the aerosol or by direct injection from the sea-surface microlayer. They found Brconcentrations were 7-18 times higher than thoseof particulate Br. During the rest of the year, particulate Br originated from sea salt particles. At Mould Bay and Alert, a seasonal variation in iodine was observed that was similar to that of bromine with two notable differences. The spring maximum was less pronounced for iodine and it had a secondary maximum in the fall. No significant seasonal variation was seen at Igioolilc nor was there one evident at Barrow (Berg et al., 1983). At present we have no explanation for this difference, although it may be associated with Barrow and Igloolik being closer to open oceans than the other high Arctic stations. The spring maximum in iodine suggests a photochemical production mechanism similar to that of bromine. Our routine observations and special studies (Hoff et al., 1983) show that the concentration of bromide ion between November and May is highly enriched with respect to sea water. Simple production of sea-salt is
not sufficient to explain the observations. It should be emphasized that the artifact production of bromide ion on filter paper from passage of reactive gaseous Br species at low temperature has not been ruled out to date. This is true for our cellulose fibre filter medium and for the Nucleopore filters used by Berg et al. (1983). 3. Sea salt constituents: Na, Cl, K, Mg. The sea salt cycle is typified by that of Cl- in Fig. 6. It has a marked variation with the broad peak earlier than that of anthropogenic constituents in September-early January. It is similar to the cycle at Barrow reported by Berg et al. (1983) which had a broad ~ximum in the period September-December. There was a marked difference in the cycle between 1980/81 and 1981/82, presumably due to different transport episodes of sea salt into the high polar region. 4. Soil constituents: Al, Ba, Ca, Fe, Ti and possibly P. One characteristic of soil constituents is the absence of a strong consistent seasonal variation (Fig. 7). Phosphorus tends to peak in winter, coincident with the anthropogenic constituents which inditites that during that season, it may be partially anthropogenic in origin. Note that the seasonal variation of soil constituents in 1981/82differs from that of 1980/81 by having a peak in P, Ba, Fe and Ca in February coincident with that of the anthropogenic elements. This indicates that these elements may have strong anthropogenic components during that winter. The difference between yearly cycles of these constituents may be related to the cause of lower SOi- and V concentrations in 1980/81 than in 1981/82 (Figs 2 and 3). Values of annual arithmetic mean concentrations (c) for 1980/81 and 1981/82 used in producing Figs >7 are shown in Table 3. With a few notable exceptions, 55in a particular year is similar at all three sites. The con~ntration of Pb, I and sea salt elements is somewhat higher at Igloolik than at the other sites. Particulate Br levels at Alert are lower than at the other two sites. This is also true for winter average concentrations (December-April). For the two winters 1980/81 and 1981182, Br concentrations were 13, 21 and 26ngm-’ at Alert, Igloolik and Mould Bay, respectively. One explanation for this difference is that because of lower incident solar radiation, gas-toparticle conversion of Br in late winter occurring by photochemical reactions (Berg et al., 1983-1984) proceeds slowest at Alert, the site furthest north. In order to place the magnitude of air ~llution in the Canadian Arctic during winter in a northern hemispheric perspective, a comparison of the observed concentration of anthropogenic aerosol constituents with those in the Norwegian Arctic and in southern Sweden on the periphery of the European source region was made (Table 4). Mean concentrations of Cu, Mn, Ni, Pb and S during the period December-April are slightly lower in the North American Arctic than in the Norwegian sector. A few differences are notable. Cr and V concentrations are markedly lower at Canadian sites. Wintertime atmos-
2001
Five years of air chemistry observations in the Canadian Arctic I
I
I 5
ANTHROPOOENIC
SO/S1
CONSTITUENTS
Cr Cu Mn NI Pb Sr V Zn Ii+ NH;
SO;
NO;
4
3 c -c 2
1
0 5 81182
4
3 C Y 2
1
0 J
A
s
0
N
0
J
F
M
A
M
J
MONTH
Fig. 5. A schematic representation of the average _seasnal variation of the ratio of weekly-average eon~ntration (C) to annual-average co~ntration(C) for anthropogenic aerosol constituents for two winters 1980/81 and 1981/82 at all Canadian sites (Fig. 1).
pheric concentrations of most of the elements in the North American Arctic are in general lower by a factor of 2-4 than in southern Sweden, which is near a major source region. The exception is aerosol sulphur (S) which is only about 30% lower than in Europe. The contrast between S in the Arctic and Europe is not as great as for the other anthropogenic aerosol constituents because S is the only one with a gaseous precursor, namely SOz. Oxidation of SOz to particulate-S essentiatly replenishes aerosol-S removed by precipitation and dry deposition (Barrie and Hoff, 1984). Other anthropogenic constituents are not replenished and therefore are found at lower concentrations than in the source regions. Aerosol light scattering High volume sampler measurements of trace components of the haze in the Arctic provide a picture of the temporal variation of the haze problem which is sufficient to understand many of the characteristics of haze transport. However, they do not provide insight to the episodicity of Arctic pollution on time scales shorter than I week. Raatz (1984) examined this
episodicity using data obtained with a one day sampling period and, by the use of Fourier analysis, observed a 7-10 day episodicity as well as a longer term, 30-70 day cycle. In order to improve temporal resolution of the haze episodes, a continuous sampling technique is required and there are few continuous techniques which have the analytical precision to detect haze components and are su&ciently reliable to operate at a remote site. This has been the main reason that aerosol optical counters have been wideiy used in the Arctic since i977. A. Measurements. In the CAASN network, MRI Model 1550 integrating nephelometers were installed in April 1979 at Mould Bay in November 1979 at Igloolik. At Mould Bay, the nephelometer intake was on the side of the upper air meteorological building. At Igloolik, the intake was about 4 m above the surface on the side of the &tern Arctic Research Laboratory (EARL). Both nephelometer airstreams were heated. The Mould Bay site was certainly more remote than the Igloolik site since the EARL is located on the southwest edge of the village of Igloolik. It was noted early in the CAASN sampling effort that Iocai con-
L. A. BARRIE and R. M. HOFF
2002
5 HALOGENS
SO/81
Br Cl-
F I
4
3 C i 2
1
0 5 El/82
4
3 c F 2
1
a MONTH
Fig. 6. A schematic representation of the average seasonal variation of the ratio of weekly-average concentration (C)toannual-averageconcentration (C)for halogens for two winters, 1980/81and 1981/S2 at all Canadian sites (Fig. 1). tamination of the nephelometer data at Igloolik was evident during the summer months. This was predominantly due to dust raised by vehicles. During the winter, however, the level of external activity was greatly reduced and local contamination from heating furnace emissions did not have a large effect on the nephelometer results or the large particle size distributions (Leaitch et al., 1984). Nephelometer calibrations were made once a week by the station operators. The regularity of thesechecks was less than ideal since zero drifts have been noted in n~helometers (Ruby and Waggoner, 1981). The frequency of calibrations was limited by the available time of the station observers. Full calibrations of the nephelometers were made with Freon 12 once per year. Lowest detection levels were 5 x 10V6m-’ with the same r.m.s. error in reported b,, values. B. Results. Figure 8 shows the 4-year cumulative results for the nephelometer scattering coefficient, b,,, in units of 10-6m-’ for Mould Bay and Igtoolik. Unlike the sulphate levels in the haze which show a 4@ fold variation over the annual cycle, the variability in the scattering coefficient between winter and summer
is about 8: 1. This is bemuse the b,, data is not corrected for local influences during the summer months. The summer values are about a factor of I-10 higher than at Barrow, where wind-sector filtering is used to remove contaminated data. Such filtering would discard much of the summer data at Igloolik since the EARL is on the edge of the townsite proper. ft is therefore appropriate to regard the summer observations as due to local sources. The usefulness of the nephelometer results can be based on the criteria of whether they are comparable to other chemical and physical m~urements of the haze and whether they add to the temporal resolution of the haze monitoring. With regard to the first criterion, in our earlier paper (Barrie et al., 1981), a correlation between weekly mean b,, and excess-SO:- concentration (in pgrn-‘) was given which showed that b,,=(11f3.2)[S0,Z-]+(6f4.0)x10-6m-1 (3) with R2 = 0.71 and N = 65. Using all data from the two stations obtained since 1979, Fig. 9 shows the
Five years of air chemistry observations in the Canadian Arctic
5
SOIL
SO/81
2003
CONSTITUENTS
Al Ea Ca Fe P Ti
t
MONTH
Fig. 7. A schematic repre~n~tion of the average seasonal variation of the ratio of weekly-average con~ntration (C) to annual average co~ntmtion (c) for soil derived components for two winters, 1980/81 and 1981/82, at all Canadian sites (Fig. 1).
regression
of b,, on total sulphate. The current values
are: b scat= (10.9 f l.l)[SO:-]+(6.8
* 1.4)x 10-6m-’ (4)
with R2 = 0.54 and N = 322. As was noted in Barrie et al. (1981), the slope of this regression (i.e. the specific scattering coefheient) is much higher than could be expected from Mie theory for the known size distribution of the Arctic sulphate aerosol. This suggests that sulphate is responsible for about one third of the aerosol scattering. Using a different technique, Leaitch et al. (1984) reached the similar conclusion that the ratio of soluble to total particle mass varied from 15 to 75 y0 and was generally 1S-35 %. This indicates that too much emphasis is placed on the correspondence of b, to sulphate mass and perhaps that b, should be considered an indicator of total fine particle ( c 2~ diameter) mass. Fine particle mass can be inferred from the nephelometer results using the spec%c scattering coefficient of 3.1 m2 g- ’ suggested by Waggoner and
Weiss (1980). This value is consistent with the Arctic winter grand average at Spitzbergen of 3.2m2 g-‘, measured and seen by Heintzenberg (1982). The ratio of excess-SO:- to this mass ranged from 3 % to 65 % over the spring period with a mean and standard deviation of 22 j, 10%. This supports the conclusion drawn in our earlier paper (Barrie et al., 1981) and that of Leaitch et al. (19&Q. The fraction of fine particle mass consisting of SO:- tends to increase throu~out the winter peaking in March and April. This is probably due to the enhanced production of SOifrom SO2 in early spring as suggested by Barrie and Hoff (1984). The second criterion for appraising the usefulness of the nephelometer is whether it enhances our ability to resolve sub-weekly pollution episodes. To examine this aspect of the program, objective analysis of the starting points for three dimensional S-day back-trajectories of air parcels was done for end-point pressure levels of 1000,925 and 85Omb for the winters 1980-1983. Days which had average b, readings in excess of 50 x 10-6m-’ were flagged and the trajectory starting
L. A. BARRIEand R. M. HOFF
2004
Table 3. Annual arithmetic mean concentrations (Zffaerosol constituents (a year inchtdes July-June) Alert 80/8 1
Substance Anthropogenic *Cr *cu Mn Ni *Pb ‘Sr V *Zn H+ NH: so: NO;
0.16
Halogens Br I F
Soil Al lBa *ca *Fe *Ti *P
Igloolik 81182 80/81 0.15
Mould Bay 81/82 80/81
0.78 1.60 0.32 1.72 0.30 0.36 2.80 6.3 82 890 55
0.30 1.33 1.49 0.38 1.70 0.57 0.62 3.48 7.4 102 1052 14
0.062 0.69 0.45 0.14 3.61 0.35 0.21 3.80 4.0 77 667 62
1.13 0.71 0.27 3.84 0.67 0.47 3.45 5.9 95 966 71
0.31 1.61 0.79 0.40 2.00 0.35 0.28 2.82 6.5 72 738 59
0.32 1.02 0.87 0.45 3.22 0.41 0.56 3.79 9.2 125 1036 67
3.8 0.43 2.0
8.9 0.49 7.0
5.7 0.96 7.2
14 1.03 5.8
7.0 0.45 5.4
16 0.46 6.7
121 114 11 92
Na CI K *Mg
81182
0.50
201 86 0.49 2.79
158 182 16 84
229 368 17 99
313 505 31 129
179 251 15 69
243 354 11 63
146 0.37 139 86 1.31 1.61
13 0.16 83 16 0.68 1.37
0.21 94 21
45 0.42 35 36 0.35 3.11
0.47 29 29 0.24 1.12
c).93
*The fraction soluble in concentrated nitric acid. Table 4. A comparison of the arithmetic mean atmospheric concentration of aethropogenie aerosol constituents (ngm-3) during the winter months (December-April) of 1980/81 and 1981/82 in the Canadian Arctic with measurements elsewhere in the Arctic and in southern Sweden near major sources
Substances Cr cu Mn Ni Pb Sr V Zn S
Alert 0.37 1.88 1.63 0.62 3.49 0.61 0.96 6.30 599
Igloolik 0.27 1.53 0.95 0.42 6.38 0.88 0.76 6.56 520
Mould Bay 0.41 2.01 1.22 0.59 5.42 0.61 0.87 7.15 54+
Spitsbergen* Winter* Marcht (3-weeks) 1983 2.6 1.9 1.5 0.7 4.9 3.2 690
S Sweden3 annual mean
1.2 2.5 2.7 1.3 12.2
2.8 2.4 6.7 1.5 21.0
28 13.7 -
2.5 24.0 800
*Heintzenberg er af. (1981), 2-5 day samples in late winter. ?Pacyna et al. (19851,1 month study. XLannefors et al. (1983).
points identified. Twenty-six 925mb starting points were identified using this criterion. They are plotted in Fig. 10. There is a clear preponderance of trajectories from the northwest of Mould Bay indicating the Asian continent as a source. This is not just an artifact caused by the genera1 system flow since the distribution of 925 mb starting points for all days, flagged and unflag-
ged (Fig. 1l), is more isotropic with about 20 % of the trajectories starting south of Mould Bay in the North American Arctic. None of the high light-scattering days had S-day trajectories starting south of Mould Bay. This adds to the overwhelming evidence of a Eurasian source for the haze. While a similar conclusion has been drawn from weekly-averaged filter data,
Five years of air chemistry observations in the Canadian Arctic
200s
50
40
30
20
IGLOOLIK
1 30
20
10
L
975 3
f”i
1980
1983
1982
1981
Y eoiFig. 8. The temporal variation of the aerosol light ~tte~ngc~~cient Mouid Bay and Igiooiik. El0
I
Total
I
I
Sulphats
(b,)at
Cug/m^33
Fig. 9. The relationship between weekly-average b,,and totalaerosol SO:-. All data from Mould Bay and Igloolik. been the confounding situation in which trajectories over a week period show wide disparity of source regions. The daily results shown here clearly alleviate much of that concern. The results there has often
also point out that not all trajectories that point towards Eurasia as a source classify air parcels which contain high ~~t-~tte~ng aerosols. The trajectories from the Western Soviet Union and those from
L. A. BARRIEand R. M.
HOFF
Fig. 10. The locations of the end points of three dimensional, S-day back trajectories arriving at the 925 mb level at Mouid Bay for days when the daily b,, is greater than 50 x 10-6m-‘. 90”E
180”
0”
90”
w
Fig. 11. The locations of the end points of three-dimensional, S-day back trajectories arriving at the 925 mb level at Mouid Bay for all days in the months considered for Fig. 10.
2007
Five years of air chemistry observations in the Canadian Arctic Eastern Siberia are less likely to be associated with high light-scattering episodes at Mould Bay.
1NTENSIVE SHORT TERM AEROSOL STUDIES
Vertical ~~~tr~but~o~ and mass size ~istrib~~ion~ In February 1982, a study of Arctic aerosol was conducted at Igloolik. Results have been discussed in detail by Hoff et al. (1983) and Leaitch et al. (1984).Of special interest was the vertical profile of aerosol light scattering obtained on 26 February 1982 (Fig. 12), one of the first such profiles ever measured. If one assumes that there is a good correlation between b,, and fine particle mass (Waggoner and Weiss, 1980), it shows that most of the aerosol fine particle mass in the lower troposphere was confined to the lowest 1.5km. An indication of the representativeness of this ‘snapshot’ of the vertical distribution of b,, was given by aircraft m~surements made during March and April 1983 (Schnell and Raatz, 1984; Radke et al., 1984). Of 13 profiles taken in the North American Arctic on different days, 8 showed most b, occurring in the lowest 1.5 km, while 5 showed b,, in layers between 3 and 5 km that was comparable in magnitude to that in the lower 1.5 km. More vertical profiles of aerosol scattering and mass are needed throughout the polluted period January-April. The mass size distributions of major ions in ground level aerosols at Igloolik during February 1982 are shown in Fig. 13. The measurements were made by Hoff et al. (1983) using a low volume Andersen cascade in impactor. NH: and SO:- are pr~ominantly particles of less than 2 pm diameter. Their mass median diameter is approximately 0.5 pm. On the other hand,
NO;, which originates by a similar process to atmospheric SO:- (i.e. gas to particle conversion), is concentrated in larger particles than SO:-. This is consistent with observations elsewhere in the atmosphere. A plausible explanation is that because aerosol NO; is more volatile than SO:-, some of the NO; migrates from the acidic accumulation mode to the more alkaline coarse particle mode, comprised mainly of sea salt and wind blown dust constituents (Bassett and Seinfeld, 1984). A different NO; mass size distribution was observed in aerosol layers aloft by Radke et al. (1984). They found NO; mass almost equally distributed between two modes, centred at 0.3 and 6pm diameter. More measurements are needed to determine the natural variability in NO; size distributions. One explanation for the difference in our ground level size distribution and that of Radke et al. is that more alkaline coarse particle surface area is available for absorption of NO; at the ground than aloft. Also some of our coarse particle NO; might be soil nitrate. The mass size dist~butions of Mg” and Nat (Fig. 14) are consistent with their most probable origin as sea spray. However, chloride shows a deficit in submicrometre particle size ranges which was attributed to volatilization of Cl- in reaction with HzSOd on particles of this size (Hoff et al., 1983; Mohnen et al., 1977). A similar deficit in chloride relative to NaCl is sometimes seen in routine weekly samples during December-April. This volatilization parallels that of NO; discussed above. What the size distribution measurements at Igloolik show is that H+, NHf and SO:- dominate the soluble ions in particles less than 1 pm diameter, while Mg2+, Cl- and Na+ predominate in particles of > 1 pm diameter. NO; is a minor ionic constituent.
9: 24-102 I0 26 Feb 19B2
CN (#/ccl
Temp (Cl -40
-35
-30
-2.5
-20
. .
EC+
(10 -6m-1)
Fig. 12. Vertical profiles of b,, and temperature at Igloolik on 26 February 1982, observed by Leaitch et ai. (1984). The dotted line is dry bulb temperature and squares indicate wet bulb temperatures. Triangles indicate Aitken nuclei concentrations.
L. A. BARRIEand R. M. HOFF
2008
NO,-
PARTICLE
DIAMETER.
I
D ip m)
Fig. 13. The composite mass-size distribution of aerosol major ions at Igloolik during February 1982 observed by Hoff et al. (1983).The ordinate represents mass fraction per unit logarithmic diameter and is plotted vs impactor effective cutoff diameters. The backup filter was assigned a cutoff of 0.1 pm for plotting purposes. The vertical bars represent r.m.s. deviation of four 3-day samples.
The relative abundance sulphates and nitrate
ofgaseous and particuiate phase
A. Measurements. In the winter of 1983/&l, measurements of the concentration of SOa, SO:-, HNO, and NO; were undertaken at Mould Bay. On a weekly basis, air was drawn through three filters in series: a Teflon particulate filter, a nitric-acid-trapping nylon filter and a Whatman 41 filter impregnated with glycerol and potassium carbonate to trap SOz. Filters were extracted in distilled water which was then analyzed by ion chromatography or the modified Thorin method (for SO2 and SO,”-). The voIatil~tion of particulate NO; to HNOa causing an artifact of HNOs on the nylon filter cannot be ruled out. Nitrate results are reported as gaseous and particulate frac-
tions for completeness, however, they should be interpreted as total nitrate. B. SO,/SO:- results. Concentrations for SO2 and SOi - observed at Mould Bay and fgloolik are shown in Fig. 14. SO, concentrations range from 0.2 to 1.5 ppb. They are higher in late January and February than in early December. They are similar in magnitude and temporal variation to those observed at Igloolik (Hoff et al., 1983; Batrie and Hoff, 1984) and at Bear Island and Spitsbergen (Rahn et al., 1981; Joranger and Ottar, 1984). Using an aircraft, Radke et al. (1984) measured SO2 concentrations of 0.2-0.6 ppb in Arctic haze layers during April. The fraction of airborne sulphur comprised of SO* (Fso,) is of interest because it is an indication of
2009
Five years of air chemistry observations in the Canadian Arctic p--7---
Ei
~
k
’
50
I
I
I
MOULD
BAY
I
F :
I
I MOULD BAY 1983/84
1903184 40-
f5 5
30-
E zw 0 *o$ E < ‘+_+-__+
-
+/+ +,+/+\+,
lo-
/‘.,*,
&:f’--+
D
J
”
MONTH
I
I
IGLOOLIK
1981182
6? c
p so,
a
i’
\
I
-1 i
I
I
I
F
J
‘\
‘\
,*i
I
F
M
A
MONTH
21’
D
/
i
A
oi
:
p - Noi
M
A
MONTH
Fig. 14. The average concentration of SO2 and SO:observed at Mould Bay (weeklyaverages)and Igloolik
(l-3 day averages). ‘potential’ SO:- that could be formed if Sol is oxidized. Furthermore, it can be used in conjuction with the ratios of excess-SOi-/excess-V to estimate the mean oxidation rate of SO* between Eurasian sources and the North American Arctic (Barrie and Hoff, 1984). Fso, at Igloolik in 1981/82 and at Mould Bay in 1983/84 ranged from 20% to 90% and tended to be higher in December, January and February than in April. C. HNO,/NO; observations. The concentrations of ‘so-called gaseous and particulate NO; observed at Mould Bay are shown in Fig. 15. Particulate concentrations exceed the gas phase ones early in the winter but they are much lower later in the winter. That is, the fraction of airborne NO; comprised of HNOs increases from _ 30% in December to u 75 % in February. As mentioned above, one cannot be sure that the gaseous HNOl observed on the nylon filter is not from volatilization of particulate matter. An important conclusion based on these measurements is that the concentration of total NO; (0.0250.090 ppb) is much lower than that of SOi- particulate matter (0.3-1.2 ppb). CONCLUSION
Fig. 15. The weekly average atmospheric concentration of nitrate observed on a Teflon filter (P-NO; ) and on a nylon backup filter (G-NO;)at Mould Bay from December 1983 to February 1984. Part of G-NO; may be volatilized aerosol nitrate rather than HNOs.
enon of Arctic haze and acidic air pollution. This insight is summarized in the following conclusions: (1) In the Canadian Arctic, there is a persistent, strong seasonal variation in anthropogenic aerosol concentrations that is in-phase with a similar variation observed in Alaska and the Norwegian Arctic. Year-toyear variability is evident and is consistent at all three locations. In particular, excess-SO:- and excess-V concentrations for the period January-April were on average twice as high in 1982 as in 1981 and 1983. (2) SOi- concentrations at Alert are well correlated with-but about 20% higher than-those at Mould Bay and Igloolik. Seasonal variations are similar throughout the Canadian and Norwegian Arctic. Winter average concentrations in Canada are 15-25 % lower than in the Norwegian Arctic. (3) The 24 aerosol constituents measured routinely can be classified into four groups according to their seasonal variation: (i) Anthropogenic elements: peaking in January and February, Cr, Cu, Mn, Ni, Pb, Sr, V, Zn, NO;, NH: with the exception of H+ and SOithat peak for a longer period, December-April. (ii) Halogens (excepting Cl): Br, I, F, peaking in March-April. (iii) Sea salt elements: Na, Mg, Cl broad maximum between September and April. (iv) Soil elements: Al, Ba, Fe, Ti (possibly P) having no consistent seasonal variation. (4) Mean concentrations of anthropogenic aerosol constituents between December and April are, with the exception of SO:-, 2-4 times lower than annual means in southern Sweden near major European sources. SOi- concentrations are only 30 % lower than in southern
Air chemistry observations made at Alert, Mould Bay and Igloolik in the Canadian high Arctic on a routine and special studies basis between 1979 and 1984 have provided greater insight into the phenom-
Sweden.
(5) The broad winter maximum in concentration of aerosol H+ and SOi- noted in 3 (i) above and the small difference in SOi- concentrations between the Arctic and the Eurasian source region noted in 4 above
L. A. BARRIE and R. M. HOFF
2010
are explained by seasonal variations in SO2 oxidation
rate. Rates are higher in early winter and late spring than they are in mid-winter. This is consistent with the observation that the fraction of airborne sulphur as SO2 peaks in January and February at Mould Bay and Igloolik. Depending on the winter month, 20-90 % of airborne sulphur exists as SOz. Concentrations range from 0.2 to 1.5ppb(v). (6) Aerosol light scattering observations are useful. Not only do they enable one to examine the episodicity of haze aerosols but they also allow one to estimate the average fraction of fine particle mass that is comprised of SO:- to be 22 f 10%. It is concluded that high light-scattering days at Mould Bay are genera!ly associated with transport from the north and northwest. (7) The soluble major ion composition of Arctic haze aerosols varies with particle size. H+, NH:, SOi- predominate in submicrometre particles while Na’, Mg” and Cl from sea salt dominate the su~rmi~rometre particles. (8) The concentration of total NO; (HNO; plus during the Arctic particulate NO;) in December-February (O.O2~.~ppb) is much lower than that of SO:- particulate matter (0.3-1.2ppb). The future direction of Arctic air chemistry research in tinada will likely follow a three-pronged approach. Routine observations of aerosol chemistry, aerosol light scattering and SO2 will be continued at Alert. Operations were terminated at Mould Bay and Igloolik in May 1984 after 4 years of simultaneous measurements at three locations which showed that one Canadian station was adequate for monitoring Arctic air ~llution from distant sources. It is particularly important to understand the vertical structure of haze transport throughout the winter period. The ‘snapshots’ of haze structure seen in aircraft studies to date have been extremely useful in identifying layers of haze but a ‘climatology’ of these profiles is needed. To this end, a monostatic ruby lidar is operating at Alert, during the 1984/85 winter to obtain verticle profiles of haze on a longer term basis. Special studies will be mounted in a continuing effort to understand the processes affecting the pathways of Arctic haze particles through the atmosphere, the origin of pollutants and potential effects. In addition, our expanding knowledge of the long range transport of anthro~genjc pollutants will be integrated into a chemical transport model.
REFERENCES AGASP (1984)(edited by Schnell R.). Geophys. Res. Letters 11, 359472. Arctic Air Chemistry (1981) (edited by Rahn K. A.). Atmospheric Environment 15, 8. JSarrieL. A., Hoff R. M. and Daggupaty S. M. (1981) The
inffuence of mid-~titudina~ ooifution sources on haze in the Canadian Arctic. At~spheric Env~ro~~nt 15, 1407- 1420.
Barrie L. A. and Hoff R. M. (1984) The oxidation rate and residence time of sulphur dioxide in the Arctic atmosphere. Atmospheric Environment 18, 271l-2722. Barrie L. A., Fisher D. and Koerner R. M. (1985) Twentieth century trends in Arctic air pollution revealed by conductivity and acidity observations in snow and ice in the Canadian high Arctic. Atmospheric Environment 19, 2055-2063. BassettM. E. and Seinfeld J. H. (1984) Atmospheric equiiibrium model of suipbate and nitrate aerosols-II. Particle size analysis. Atmospheric ~nviro~nt l%, 1163-l 170. Berg W. W. Sperry P., Rabn K. A. and Gladney E. S. (1983) Atmospheric bromine in the Arctic. J. ~e0phys. Res. 88, 67196736. Berg W. W., Heidt L. E., Policck W., Sperry P. D., Ciirone R. J. and Gladney E. S. (1984) Brominated organic species in the Arctic atmosphere. Geophy. Rex Lett. 5, 429-432. Heintzenberg, J., Hansson H. C. and Lannefors H. (1981) The chemical composition of Arctic haze at Ny-Alesund, Spitsbergen. Tellus 33, 162-171. Heintzenberg J. (1982) Size-segregated measurements of particulate elemental carbon and aerosol light absorption at remote Arctic location. Atmospheric Environment 16, 2461-2469. Hoff R. M., Leaitch W. R., Fellin P. and Barrie L. A. (1983) Mass-size dist~butions of chemical constituents of the winter Arctic aerosol. .I. geophys. Res. 88, 10947-10956. Hoff R. M. and Trivett N. B. A. 119841 Ground-based measurements of Arctic haze madk at Alert, N. W. T., Canada, during the Arctic Gas and Aerosol Sampling Project (AGASP). Geophy. Res. Mt. 11, 389-392. Lannefors H., Hansson H. C. and Granat L. (1983) Background aerosol composition in southern Swedenfourteen micro and macro constituents measured in seven particle size intervals at one site during one year. Atmospheric Environment 17, 87-101. Leaitch W. R., Hoff R. M., Melnichuk S. and Hogan A. (1984) Some physical and chemical properties of the Arctic winter aerosol in northeastern Canada. J. Ctim. appi. Met. 23, 916-928. Koemer R. M. and Fisher D. (1982) Acid snow in the Canadian high Arctic. Nature 295, 137-140. Mason B. (1966) P&&pies of Geochemistry 3rd edn, John Wiley, New York. Ottar B. (1981) The transfer of airborne pollutants to the Arctic region. Atmospheric Environment 15, 143+1446. Pacyna J. M., Ottar B., Tomza U. and Maenhaut W. (1985) Long-range transport of trace elements to Ny-Alesund, Spitsbergen. Atmospheric Environment 19, 857-865. Radke L. F., Lyons J. H., Hegg D. A., Hobbs P. V. and Bailey I. H. (1984) Airborne observations of Arctic aerosols-I. Characteristics of Arctic haze. Geophys. Res. Lett. 5, 393-396. Rahn K. A. and McCaffrey R. J. (1979) Long range transport of pollution aerosol to the Arctic. A problem without borders. Proc. WMO Symp. on the Z.ung Range Transport of FOll~t~ts wrd Its Relation to General Circu~arion uncivil ~tratosp~r~~ropospher~c Exehtqe Processes, Sofia, l-5 October, WMO. No. 538, pp. 25-35. Rahn K. A., Joranger E., Semb A. and Conway T. (1980) High winter concentrations of SO2 in the Norwegian Arctic and transport from Eurasia. Nature 287. 824-826. Rahn %. A. (1981) The Arctic air-sampling network in 1980. Atmospheric Environment 15, 1349-l 352. Ruby M. G. and Waggoner A. P. (1981) Intercomparison of intergrating nephelometer measurements. En&. Sci. Technol. 15, 109. Schnell R. C. and Raatz W. E. (1984) Vertical and horizontal characteristics of Arctic haze during AGASP: Alaskan Arctic. Geophys. Rex Lett. 5, 369-372. Waggoner A. P. and Weiss R. E. (1980) Comparison of fine particle mass con~ntration and light scattering extinction in ambient aerosol. Atmospheric ~~~iro~n~ b&623-626.