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Halogens in aerosols in Central Alaska
Atmospheric EnvironmentVol. 27A, No. 17/18, pp. 2969 2977, 1993.
0004 6981/93 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain.
H A L O G E N S IN AEROSOLS IN CENTRAL ALASKA WILLIAM T. STURGES School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
and GLENN E. SHAW Geophysical Institute, University of Alaska, Fairbanks, AK 99775-0800, U.S.A. (First received 10 April 1993 and in final form 30 June 1993)
Abstract--High volume filter samples were collected at Poker Flats in central Alaska, between 1984 and 1987, and analysed for a comprehensive suite of elements. In this report we focus on the results for the halogen elements Br, CI and I, and their correlations with other selected elements (A1,As, Na, Se, and V). Seasonal cycles were observed for the halogens, including a pronounced spring peak in Br and a weak fall peak, pronounced spring and fall peaks in I, and increased winter/spring C1. A significant correlation between Br and Se was shown to be partly due to common transport pathways, and possibly some common sources. Iodine showed enrichments of three orders of magnitude over sea water composition. Correlations to marine elements suggested a marine biogenic source. Chlorine evidently originated from sea salt aerosols, but showed evidence of substantial volatilization, correlated to the degree of pollution of the air mass. Key word index: Aerosols, bromine, chlorine, iodine, selenium, enrichment factors, remote atmospheres,
Arctic, Alaska.
INTRODUCTION There has been interest in halogens in arctic aerosols since Berg et al. (1984) and Barrie and Hoff (1985) showed that bromine, chlorine and iodine undergo regular annual cycles in the Alaskan and Canadian Arctic, respectively. Bromine has been the most studied of these three halogens. There is a pronounced spring pulse of particulate bromine in the Arctic with peak concentrations as much as hundreds of times higher than summer values (Sturges and Barrie, 1988). Interest in bromine in the arctic troposphere increased when Barrie et al. (1988) demonstrated a strong anticorrelation between surface ozone and particulate bromine. High levels of organobromine gases have also been reported in late winter and early spring (Barrie et al., 1988; Cicerone et al., 1988; Berg et al., 1984). The origin of the spring bromine pulse has remained a mystery. Sturges and Barrie (1988) demonstrated that the Br concentrations could not be accounted for by long range transport of Br-containing vehicular pollution, nor Br in sea salt aerosols and crustal material. Coal burning could not be ruled out for lack of a reliable tracer system. Sturges et al. (1992) have shown that polar sea ice microalgae, which are found ubiquitously under annual sea ice during the spring, are prodigious synthesizers of bromoform. It seems likely, therefore, that the spring pulse of both organic
gaseous and particulate bromine has a marine biogenic source. Particulate iodine shows enormous enrichment factors over sea water composition in aerosols (Whitehead, 1984). This has been explained in the marine background atmosphere as arising from fractionation at the sea surface microlayer, or from particle formation from organic iodine gases emitted from the sea surface. Sturges and Barrie (1988) showed that there is an Arctic spring iodine pulse, with similar features to the bromine pulse, and some scant evidence for a secondary fall peak. Barrie and Barrie (1990), using a longer record from Alert, North West Territories, have, however, clearly shown the existence of a fall iodine peak. The causes are unknown, but are thought to be linked to marine biogenic sources. Chlorine was shown by Sturges and Barrie (1988) to peak during the period of winter storminess in the northern oceans. Correlations to sodium confirmed that chlorine was associated almost exclusively with marine aerosols. The ratios to sodium were, however, depleted relative to bulk sea water composition, indicating that volatilization of chlorine had taken place. In all of the above arctic studies, halogens were measured at near-coastal locations. We report here the first extensive measurements of halogen chemistry at an interior subarctic site, in central Alaska. This allows us to further examine some of the above relationships and hypotheses.
2969
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W.T. STURGESand G. E. SHAW
METHODOLOGY Air filter samples were collected at Poker Flats (PF), a site operated by the University of Alaska, about 20 milesnorth of Fairbanks, Alaska. PF is separated from the Fairbanks area by high ground, and is not greatly impacted by emissions from the Fairbanks area, especiallyduring the winter months when strong surface inversions inhibit advection of polluted air out oftbe Chena valleyarea. Air arriving from the north is essentially unperturbed Arctic Basin air. Although PF is about 150 km south of the Arctic Circle, it is frequently within the arctic air mass during the winter and spring as the Arctic Front moves south. At other times there is strong transport from the north Pacific, thus PF is an ideal location from which to examine differences between Pacific marine and Arctic Basin air masses. PF is approximately 600 km from the nearest coastline. Sampling and analytical methodologies are described in detail in Shaw (1991). In brief: high volume air samples (5-10 x 103 m3) were collected by drawing air through 20 x 25 cm Whatman 41 cellulose fiber filters. The collection efficiency for submicron particles was about 70%. Elemental concentrations were determined by instrumental neutron activation analysis at Los Alamos National Laboratory. The neutron flux was 10acm-2s -1. Each filter underwent two irradiations and three gamma counting sequences to derive shortand long-lived isotope abundances.
RESULTS Monthly mean, medians, and confidence limits are plotted as box and whisker plots in Fig. 1. Non-crustal
,o0t
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200
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1
vanadium (ncV) was computed according to the method of Rahn and Lowenthal (1985), i.e. ncV = total V - c r u s t a l V, where crustal V is estimated from measured A1 concentrations and an assumed ratio of V to A1 in crustal material of 1.623 × 10 -3. Correlations between the elements are shown in Tables 1-5 for different data stratification schemes. To reduce the effects of correlations arising from common seasonalities of element pairs, the data were divided into two "seasons": firstly, the period encompassing the elevated bromine levels during the spring (1 February-31 May) (SPR) and, secondly, the remainder of the summer, fall and winter seasons (SFW). Another stratification attempted to group samples that were collected during flow predominantly from the Arctic, and samples collected during flow predominantly from the Pacific. Arctic air masses were identified by mean temperatures during sampling periods that were more than 10°C lower than the long term PF mean for that time of year. Pacific air masses were similarly identified by mean temperatures more than 10 °C above the long-term mean. Two correlation coefficients are given for each element pair in Tables 1-5. The first is the nonparametric Spearman rank correlation coefficient r,. This coefficient is useful where the data are nonnormally distributed, and reduces the effects of outliers. The second is the product moment correlation coefficient G" The last column on the right summarizes the most significant correlations with the element shown in the far left column. Spearman rank correlations with values of 0.5 or greater at a confidence limit > 9 9 % are shown in descending order in the last column. The elements listed in parentheses had correlations less than 0.5 at the same confidence limit. A correlation between ncV and AI has no meaning since AI is used to compute ncV. Instead, the correlation between total V and A1 is shown in the tables. Marine enrichment factors (EF) of an element X were calculated as: EF(X) =
O,S
o
~o
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JFMAMJJASOND
" t . . . . .
Ai
::"'] iI
]
FMAMJJASOND
Fig. l. Box plots of monthly elemental means (dotted lines), medians (center bar), 25 and 75% confidence intervals (extremities of the box), 10 and 90% confidence intervals (exterior bars), and 5 and 95% confidence intervals (dots) for all samples collectedat PF between 1984and 1987.
[X]alr / [Na]alr
[X],ea / [na].ea
i.e. the factor by which the measured ratio of the element to Na in the aerosol exceeded the ratio of the element to Na in bulk sea water (Duce et al., 1983).
Bromine
Figure 1 shows that a large increase in particulate bromine occurred from February to May, peaking in April. Concentrations were lowest in summer, and rose somewhat during the fall and early winter. This is quite similar to observations at coastal arctic locations (Berg et al., 1984; Sturges and Barrie, 1988). However, whereas peak values at PF were around 10ngm -3, concentrations at, for instance, Barrow, Alaska, reached as much as 100 ngm -3 during the spring (Berg et al., 1984). The relatively small degree of
Halogens in aerosols in Central Alaska
2971
Table 1. Inter-element correlation coefficients and mean concentrations (ng m-s) for Poker Flats samples collected during the "summer-fall-winter" (SFW) season: 1 June-31 January, 1984-1987. In the correlation matrix the top number is the Spearman rank correlation coefficient, the center number is the product moment correlation coefficient, and the bottom number is the number of sample pairs. Correlation coefficients in bold type are significant at the 99% confidence level. The correlation shown in the space for noV vs AI is actually the correlation for V vs AI (see text). To the fight of the matrix is a summary of the elements that correlated with the element shown in the left column with a significance of 99% and a Spearman rank correlation of 0.5 or greater (99% significance and rs<0.5 in parentheses). The lower two rows show the concentration means and standard deviations AI
As
Br
0,879
0.642
0.561
0.969
0.469
0.418
103 102 0,002 0.664
ncV Se
0,116
Na
95 0.226
CI Br As
94
I
Na
Se
103 0.685
0.151 0.034 103 0.266
0.286 0.165 97 0.381
0.441 0.174 102 0.426
0.521 0.440 95
0.670
0.286
0.404
0.393
95
95
0.423 0.315
0.398 0.518
0.633 0.908
102 101 -0.132 0.258 -0.027 0.233 97 96 -0.179 0.054 -0.051 0.083 103 102 -0.056 0.553 -0.014 0.444 103 102 0.396
102 0.480 0.560 97 0.321 0.461 103
102 0.533 0.854 97
0.195
I
0.608
CI
90
ncV
Correlation summary As, Br, Se (Na, I) Br, As, ncV (Na, I)
95 C1, I (ncV, Se, As, Br)
0.521 0.815
97 C1, Na (Br, Se, ncV) Na, I (Br) Se, ncV, As (I, Na, CI) Se, ncV, Br (Na, CI)
0.684
102 A1 Mean a
As ll7 114
0.133 0.083
0.886 0.461
51.9 38.8
0,446 0.380
66.4 54.9
0.035 0.016
0.258 0.186
Table 2. Inter-element correlation coefficients and mean concentrations (ngm -3) for Poker Flats samples collected during the "spring" (SPR) season: 1 February-31 May, 1984-1987. See legend of Table 1 for explanation of figures
ncV Se
A1
As
Br
0.605 0.746
0.823 0.844
0.653 0.435
97 -0.063 - 0.097
Na
97 -0.002 0.218
97 I
Cl Br As
97 0.648 0.696
99 0.407 0.273
97
97 0.685 0.613
99 0.392 0.418
97
-0.246 - 0.150
0.574 0.479
0.675 0.575
97 0.014 - 0.029 97 -0.057 -0.006 97 0.240 0.200 97
99 -0,117 - 0,049 97 0,595 0.411 99
99 0.059 0.246 97
CI -0.140 -0.103
97 0.297 0.191
97 0.605 0.677
97 0.142
I 0.655 0.461
97 0.692 0.577
99
Na
Se
0.393 0.214
0.683 0.740
97
ncV
As, Se I, Br (Na)
97 I, Br, ncV, I, Na, As (CI)
0.655 0A25
97 I, Se, CI, (As, ncV, Br)
0.661 0.602
97 Se, Br, Na, ncV, As
0.181
97 Na (Se) Se, I, ncV, As (Na) ncV, Se, 1 (Na)
AI Mean O"
Correlation summary
None 139 88
0.269 0.277
4.06 2.99
89.6 82.9
0.827 0.469
116 82
0.067 0.041
0.458 0.302
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W.T. STURGESand G. E. SHAW
Table 3. Inter-element correlation coefficients and mean concentrations (ng m-3) for Poker Flats samples collected during episodes of arctic air mass influence (mean air temperature more than 10 °C below the seasonal norm) for all dates 1984-1987. See legend of Table 1 for explanation of figures
ncV Se Na I CI Br As
AI
As
Br
CI
I
Na
Se
0.742 0.606 18 0.560 0.349 18 0.517 0.551 17 0.379 0.170 18 0.083 0.015 17 0.434 0.476 18 0.595 0.185 18
0.986 0.948 18 0.837 0.798 18 0.642 0.409 17 0.701 0.750 18 0.272 0.320 17 0.651 0.317 18
0.645 0.341 18 0.664 0.566 18 0.767 0.761 17 0.816 0.676 18 0.502 0.494 17
0.235 0.156 17 0.309 0.354 17 0.453 0.509 17 0.485 0.534 17
0.695 0.680 18 0.732 0.752 18 0.770 0.679 1.7
0.654 0.433 17 0.750 0.664 17
0.851 0.835 18
ncV
As, Se I, Br, Na ncV, As, Na, I, Br I, Br. Se, ncV, As Br, Na, Se, As, ncV None I, Na, Se, As, ncV noV, Se, I, Br, AI
AI Mean a
Correlation summary
As 109 109
0.512 0.495
5.39 4.03
84.1 44.1
0.968 0.451
151 84
0.105 0.054
0.595 0.468
Table 4. Inter-element correlation coefficients and mean concentrations (ng m - 3)for Poker Flats samples collected during episodes of Pacific air mass influence (mean air temperature more than 10 °C above the seasonal norm) for all dates 1984-1987. See legend of Table 1 for explanation of figures
noV Se Na I CI Br As
A!
As
Br
CI
I
Na
Se
0.848 0.910 42 -0.096 0.027 39 0.124 --0.125 41 -0.264 -0.249 37 -0.026 -0.142 42 -0.162 - 0.063 42 0,364 0.514 42
0.501 0.223 42 0.435 0.512 39 -0.043 --0.189 41 0.125 0.008 37 -0.275 -0.333 42 0.449 0.383 42
0.501 0.361 42 0.594 0.535 35 0.181 0.226 41 0.564 0.529 37 -0.142 -0.022 42
-0.322 --0.180 42 0.176 0.245 39 0.706 0.891 41 0.202 0.591 37
0.072 0.011 37 0.596 0.581 35 0.445 0.812 37
0.023 -0.073 41 0.280 0.361 39
0.137 0.211 39
ncV
As, Br I, Br (As) CI (I) Se, Br (Na) Na Se, I, ncV (As) ncV (Br)
AI Mean a
Correlation summary
None 135 138
0.121 0.070
1.06 0.80
76.1 55.2
dilution of b r o m i n e c o n c e n t r a t i o n s evident during t r a n s p o r t from the Arctic O c e a n to P F indicates t h a t the high b r o m i n e levels previously observed at coastal arctic locations were not due to a localized effect, b u t
0.460 0.321
82.4 80.9
0.033 0.016
0.290 0.203
are p r o b a b l y representative of the Arctic Basin as a whole. A n u m b e r of studies have s h o w n t h a t high particulate Br levels in the Arctic are only to be found in the
Halogens in aerosols in Central Alaska Table 5. Correlations between the ratio CI/Na and the other elements in this study. See legend to Table 1 for explanation of figures
2973
"pollution" elements Se, ncV and As, and more weakly with the "marine" elements I, Na and C1 (Table 1). The correlation with Se was by far the strongest. Se was, in turn, most strongly associated with Br, As, and ncV, and more weakly with Na and I. The implication is Element SPR SFW that Br during this season may be at least partly A1 -0.044 -0.501 derived from pollution; predominantly coal burning if -0.125 -0.382 the Se in this case is mostly of anthropogenic origin. 97 102 The absolute Br concentrations, however, were small As - 0.585 - 0.423 -0.319 -0.421 (Fig. 1 and Table 1). 97 101 A significantly different picture emerged from Br -0.340 -0.102 correlations during the SPR season (Table 2). The --0.165 -0.045 mean Br concentration was four to five times higher 97 102 than in the SFW season. Br was still most strongly I -- 0.492 0.043 -0.418 0.090 associated with Se, but was closely followed by I. 97 97 Correlations with ncV and As were also high. Se was Se -0.286 -0.184 most strongly correlated with I and Br, followed by -0.174 -0.181 ncV, Na and As. Since I has almost exclusively natural 97 95 origins, probably from marine biogenic sources, as ncV -0.457 -0.514 - 0.354 - 0.428 discussed later, the suggestion is that Br, I and Se all 97 102 have biogenic sources in the Arctic Basin. What of the correlations between Br, As and ncV? That As and ncV are pollution elements of common origin was amply demonstrated by the high intercorrelation (rs surface inversion layer (e.g. Oltmans et al., 1989), =0.823) in Table 2. This could be interpreted as a which has a typical height of 100-500 m. Given that significant additional source of bromine from polluPF is a minimum of 600 km from the nearest arctic tion. It might, however, be equally ascribed to comcoast, and that air from the Arctic Basin must ascend mon transport pathways of arctic air pollution and over the Brooks Range before arriving at PF, it is biogenic emissions from the Arctic Ocean to PF. The perhaps surprising that such a distinct pulse is seen in correlations, therefore, may be biased by an alternacentral Alaska. Either there are terrestrial sources of tion between polluted air containing biogenic emisparticulate bromine in Alaska, in situ formation from sions peculiar to the Arctic, and clean Pacific air gaseous precursors en route to PF, or only limited without the same biogenic components. In Table 3 an attempt has been made to isolate mixing occurs during transport into the interior of instances of flow out of the Arctic based on air Alaska. What is the origin of the particulate bromine? A temperature (see above), to overcome such potential cursory inspection of Fig. 1 shows that the elements I, bias. This selection procedure returned only 18 qualiSe, ncV and As, and to a lesser extent Na and CI, also fying samples. The pattern of correlations was quite had spring maxima. Aluminum, on the other hand, different again. Br showed high correlations with I and was greater in the summer. Iodine is thought to Na, followed by Se, As and ncV. Se, in this instance, originate almost exclusively from natural sources, correlated more strongly with the pollution elements primarily the ocean (Sturges and Barrie, 1988). Sel- ncV and As, followed by Na, I, and Br. This appears to enium has long been considered to have largely confirm that the majority of Br in arctic air masses anthropogenic sources, and has been used as an originates from marine sources, whereas Se evidently effective tracer of emissions from coal burning (Keeler had significant anthropogenic sources. The near-unity and Samson, 1989; Rahn and Lowenthal, 1985). More correlation between As and ncV again illustrated the recently, however, it has been estimated that natural common anthropogenic source of these two elements. The correlation coefficients for Pacific air masses (principally biomethylation) and anthropogenic sources are of approximately equal magnitude (Atkinson (Table 4) are in sharp contrast to those of Table 3. All et al., 1990). Arsenic and ncV originate from industrial of the correlations were weaker. Br correlated to Se, I activities, and have been used extensively as tracers.of and ncV, but Se correlated mostly with marine elelong range transport of pollution (Keeler and Samson, ments (I and Br). The mean Br concentration was 1989; Rahn and Lowenthal, 1985). Particulate C1 and about five times lower than in arctic air masses. Se and Na are believed to originate almost exclusively from I concentrations were also lower. The conclusion here sea salt aerosol, and AI from crustal dust and soil is that Br, Se and I in Pacific air masses were all of marine origin. Pollution levels were also lower, and As (Sturges and Barrie, 1988). Information about the possible sources of partic- and ncV were only weakly correlated. The relationship between Br and Se can be sumulate Br in the Arctic can be gleaned by examining correlations with such elemental tracers. In the SFW marized by a plot of the monthly means of ratios of Br season, Br was most strongly correlated with the to Se (Fig. 2a). Br increased relative to Se during the
2974
W.T. STURGESand G. E. SHAW
150 r~
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•
•
,
,
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,
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(a)
I O0 50
(b)
1.5
~
1.0
g~ 0.5
0.0
L , l , , , , , l J l ,
15
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PF of >161 n g C l m -3. The mean C1 concentration measured at P F was 120ngm -3 and, as discussed below, this C1 was evidently derived solely from marine aerosol, not coal combustion. It is therefore concluded that little or none of the Br measured at P F originates from coal burning• Figure 2a also suggests that there may have been a second peak in excess Br in late fall/early winter. This is also observed in a plot of Br enrichment factor (Fig. 2c). This has not previously been reported from coastal arctic sites• As discussed below, this secondary maximum coincides with a peak in particulate I, suggesting a common biological origin• The aboveunity values of the Br enrichment factors in Fig. 2c confirms that the excess Br did not originate from sea salt aerosol, given that Br enrichment during aerosol formation from sea water has not been observed (Duce and Hoffman, 1976), indeed Br volatilization is more usual (Sturges and Barrie, 1988)• Iodine
J
F
M
A
M
J
J
A
S
O
N
D
Fig. 2. Box plots of monthly means, medians, and confidence intervals for the ratios (a) Br/Se, (b) Se/As, and (c) EF (Br) for all samples collected at PF between 1984 and 1987. See legend to Fig. 1 for explanation of box markers•
SPR season, whereas the ratio Se/As (Fig. 2b) remained relatively constant throughout the year. It can be concluded that there was a source of excess, nonpollution Br during the spring months, and further that Se was mostly of pollutant origin, although the possibility of a natural Se component has been indicated by the correlations above. Germani and Zoller (1988) made measurements of halogens and Se in a coal-fired power plant stack• Applying their ratio of particulate Se to particulate Br in the stack emission (13 + 4/~g m - 3 Se, 9 _ 5 #g m - 3 Br) to the mean P F Se concentration of 0.21 ng m-3, and assuming that the Se at PF originated solely from coal burning, gave a predicted coal-burning-Br concentration of 0.14 ngm -3, well below the measured mean Br concentration of 12 ng m-3. If, however, the Se/Br ratio for the sum of particulate and gaseous phases in the stack emissions was used (33 ___5 #g m - 3 Se, 1310 + 90 pg m - 3 Br), the predicted coal-burningBr concentration (14 ng m-3) was much closer to that measured. In this latter case it must be assumed that both the Se and Br present in vapor form in the stack emissions are converted to particulate matter. This argument appears plausible until the same procedure is applied to predict C1 concentrations at P F due to coal burning. Germani and ZoUer gave a lower limit to total C1 in the stack emissions of 15,000/~g m-3. This yields a predicted coal-burning-C1 concentration at
In Fig. I, iodine is seen to have had two annual peaks. The larger peak occurred at the same time as the spring Br peak. The second I peak coincided with the fall rise in Br concentrations and, more notably, the fall peak in Br/Se and Br enrichment factor (Figs 2a and 2c). A pronounced spring I peak has also been reported by Sturges and Barrie (1988) and Barrie and Barrie (1990). The latter authors also noted a fall peak at Alert• This fall peak occurred between August and October, with the secondary minimum in November. At P F the peak was centered on November. This may be related to the difference in latitude: Alert is at 83 °N, P F at 65°N, and Barrow (Alaskan Arctic coast) at 71 °N. At the higher latitudes the summer is very short. It is entirely plausible that biogenic activity leading to I emission might be correspondingly abbreviated in the most northerly regions of the Arctic Ocean• It is noted that Berg et al. (1984) found no seasonal cycle whatsoever in I at Barrow. In view of the fact that four arctic locations have now been shown to have annual cycles in I, this calls to question the Berg et al. (1984) data, unless some localized source could be demonstrated that is capable of overwhelming the background signal. Unlike Br, absolute I concentrations at P F were little less than those reported for the coastal arctic locations• Sturges and Barrie (1988) reported peak values of about 1-2 ng m-3, depending on the site, while Barrie and Barrie (1990) reported values mostly below 1 ngm -3 at Alert• At PF, peak values were around 2 ng m - 3. This may indicate that I production does not relate solely to surface sources in the Arctic Basin, but has a more widespread origin, and is more uniformly mixed throughout the troposphere. Figure 3 shows the annual cycle of iodine enrichment factor. In all cases the degree of enrichment was enormous• Enrichment factors of around 102-10 3 have been reported from numerous locations in the
Halogens in aerosols in Central Alaska 4000
,
~
,
,
,
1984-
,
,
,
T
,
Chlorine
,
1987
3500 3000 2500 ~" 2000 1500 I000 50O 0
i J
i
J
FMAM
i
J
i
i
J
J
i
i
i
2975
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A S O N D
Fig. 3. Box plot of monthly means, medians, and confidence limits for EF(I) for all samples collected at PF between 1984 and 1987. See legend to Fig. 1 for explanation of box markers.
global background marine atmosphere (Whitehead, 1984). Sturges and Barrie (1988) reported enrichment factors from arctic sites of up to 104. They attributed these higher factors to the absence of large numbers of freshly generated marine aerosols containing I/Na ratios comparable to sea water values. At PF, the enrichment factors do not reach the highest values reported by Sturges and Barrie (1988), but the mean values from Sturges and Barrie, ranging from 1100 to 1500, are not dissimilar to those from PF. In Fig. 3 the fall peak is as large, if not larger, than the spring peak when expressed as an enrichment factor. This is probably no more than a reflection of the smaller amount of marine aerosol present during the fall compared to the spring (see the plots of Na and CI in Fig. I). It also indicates that I concentrations were essentially independent of marine aerosol concentration, and argues for the overwhelming importance of gas-to-particle reactions in the production of I aerosols, over enrichment during bubble-bursting marine aerosol formation. In Table 1, I is correlated predominantly with CI and Na in the SFW season. In the SPR season (Table 2) it correlates with Se, Br, Na, ncV and As. Since there are no known large anthropogenic sources of I, this appears to be good evidence for the effect of common transport pathways on the elemental correlations. The absence of a correlation to CI, despite the high correlation with Na, is surprising. As discussed below, this may relate to the apparent volatilization of CI from sea salt aerosols. Similar correlations were observed for arctic air masses (Table 3), although in this instance the highest correlation (rs=0.816) was with Br, followed by Na and Se. In Pacific air, all of the correlations to I were weaker; the highest were with Se, Br and Na, suggesting a marine source. Overall it is clear that I had an exclusively marine origin. Given this conclusion, it is further suggested that there is a marine source of Se, at least in Pacific air.
In the SPR and SFW seasons, and in Pacific air masses, Cl correlated almost exclusively with Na (Tables l, 2 and 4), with lesser correlations to I, Br and Se, as would be expected for a marine aerosol origin. In arctic air masses there were no strong correlations: the highest values were for Br, I and Na, again suggesting a marine aerosol origin. In Fig. l, the elevated levels of CI and Na during the winter were due to the greater degree of storminess, and therefore marine aerosol generation, in the northern mid- and high-latitudes at this time of year (Erickson et al., 1986). In the absence of any chemical fractionation effects, the ratio of C1 to Na would be 1.78 (Sturges and Barrie, 1988). Figure 4 shows that, with very few exceptions, the C1/Na ratios observed at PF were below this ratio, indicating substantial loss of C1 from the aerosol. This effect has been noted by other workers (although significantly not by Berg et al., 1984) and has been variously attributed to displacement reactions with acids (Clegg and Brimblecombe, 1985), or reactions with N205 and CIONO2 (Finlayson-Pitts et al., 1989)• In either case, we would expect to see greater CI loss in more polluted air masses. Sturges and Barrie (1988) did, in fact, find an inverse relationship between C1/Na and sulfate. At the highest sulfate concentrations, CI loss amounted to 70%. Sulfate was not measured in this study, but pollutant elements may be used as tracers for air likely to contain acidic compounds and NOx. Table 5 shows that in the SPR season, the highest anticorrelation to CI/Na was with As, confirming the above hypothesis. Other significant correlations included ncV, another pollution tracer, and also I. In the latter case we might speculate that favorable conditions for the photochemical production of acidic species and reactive nitrogen species, and the formation of particulate I,
2,0
1.5
0.0 J F M A M J
J A S O N D
Fig. 4. Box plot of monthly means, medians, and confidence limits for the ratio CI/Na for all samples collected at PF between 1984 and 1987. See legend to Fig• 1 for explanation of box markers.
2976
W.T. STURGESand G. E. SHAW ] : : ~ : : : ] ; [ 7 7.......::i ~ - _ ~ _
\~ ~ ~2_._~ ..... ' ~_ '~ ~_~ _
0.01
0.1
1
As (rig m -3) Fig. 5. CI/Na vs As for all P F data combined, on logarithmic scales.The solid line is a linear fit to the logtransformed data, and the dashed lines are the 9 9 % confidence limits of the regression.
existed in these air masses or, as noted above, that this was an effect of similar transport pathways. During the rest of the year (SFW), the anticorrelation with As and ncV remained, while that with I disappeared. Curiously there was a significant anticorrelation with AI. We can offer no explanation for this at present. Possibly the Al-containing dust in the summer was rich in Na, but this was not reflected in the correlations between A1 and Na in Table 1. Figure 5 shows a plot of C1/Na vs As on a logarithmic scale for the PF data from all seasons. The solid line is a linear fit through the log-transformed data (r = -0.472), bounded by the 99 % confidence interval (dashed lines). At the highest As concentrations, the linear fit indicates that the C1/Na ratios were around 0.3, which equates to about 83 % volatilization of the sea salt chlorine, in good agreement with the abovementioned findings of Sturges and Barrie (1988).
CONCLUSIONS Based on our analysis of the 1984-1987 multielement aerosol data from Poker Flats, Alaska, we have drawn the following conclusions: (a) The arctic spring Br pulse is not restricted to coastal arctic sites, but is also a feature of the interior arctic and subarctic. By inference, it must be a widespread feature throughout the Arctic Basin. (b) The Br pulse originates from an arctic marine source. It does not occur with Pacific marine air. Neither does it appear to originate from man-made sources. The correlation with Se does not indicate a coal-burning origin, rather it indicates common transport pathways, and perhaps a contribution to Se from marine biogenic sources. The lower Br concentrations observed outside of the spring pulse may, on the other hand, have some anthropogenic component. (c) There was some evidence for a fall peak in Br. This has not been reported before. The coincidence
with a fall peak in I may indicate a common biogenic origin. (d) Iodine showed both a spring and fall peak, confirming an earlier report from Alert, North West Territories. It is believed to have a marine biogenic source. Iodine was enormously enriched, generally by three orders of magnitude, over sea water composition. (e) Chlorine originated from marine (sea salt) aerosol. The C1 was, however, depleted relative to Na from its sea water composition. The degree of depletion increased with more polluted air masses, supporting the hypotheses that reactions with certain pollutant species (acids and oxides of nitrogen) lead to displacement of CI. Acknowledgements--WTS acknowledges the support of the
National Science Foundation (under grant DPP-9015614), the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, and the Climate Monitoring and Diagnostics Laboratory of the National Oceanic and Atmospheric Administration. The authors are indebted to the data and graphical analysis work of Geoffery Dutton. Dutton was supported by a Research Experiencefor Undergraduates grant from the National Science Foundation (EAR-9213236/DPP-9015614). REFERENCES
Atkinson R., Aschmann S. M., Hasegawa D., ThompsonEagle E. T. and Frankenberger W. T. Jr (1990) Kinetics of the atmospherically important reactions of dimethyl selenide. Envir. Sci. Technol. 24, 1326-1332. Barrie L. A. and Heft R. M. (1985) Five years of air chemistry observations in the Canadian Arctic.Atmospheric Environment 19, 1995-2010. Barrie L. A. and Barrie M. J. (1990)Chemical components of lower tropospheric aerosols in the High Arctic: six years of observations. J. atmos. Chem. 11, 211-226. Barrie L. A., Bottenheim J. W., Schn¢ll R. C., Crutzen P. J. and Rasmussen R. A. (1988) Ozone destruction and photochemical reactions at polar sunrise in the lower arctic atmosphere. Nature 334, 138-141. Berg W. W., Heidt L. E., Pollock W., Sperry P. D. and Cicerone R. J. (1984) Brominated organic species in the Arctic atmosphere. Geophys. Res. Lett. 11, 429-432. Cicerone R. J., Heidt L. E. and Pollock W. H. (1988) Measurements of atmospheric methyl bromide and bromoform. J. oeophys. Res. 93(D4), 3745-3749. Clegg S. L. and BrimblecombeP. (1985) Potential degassing of hydrogen chloride from acidifiedsodium chloride droplets. Atmospheric Environment 19, 465-470. Duce R. A. and Hoffman E. J. (1976) Chemical fractionation at the air sea interface. Ann. Rev. Earth planet. ScL 4, 187-228. Duce R. A., Arimoto R., Ray B. J., Unni C. K. and Harder P. J. (1983)Atmospheric trace elementsat Enewetak Atoll: 1. Concentrations, sources and temporal variability. J. geophys. Res. 88(C9), 5321-5342. Erickson D. J., Merrill J. T. and Duce R. A. (1986) Seasonal estimates of global atmospheric sea-salt distributions. J. geophys. Res. 91(D1), 1067-1072. Finlayson-Pitts B. J., Ezell M. J. and Pitts J. N. P. Jr (1989) Formation of chemically active chlorine compounds by reactions of atmospheric NaCI particles with gaseous N20 5 and CIONO 2. Nature 337, 241-244. Germani M. S. and Zoller W. H. (1988)Vapor-phase concentrations of arsenic,selenium,bromine, iodine, and mercury
Halogens in aerosols in Central Alaska in the stack of a coal-fired power plant. Envir. Sci. Technol. 22, 1079-1085. Keeler G. J. and Samson P. J. (1989) Spatial representativeness of trace element ratios. Envir. Sci. Technol. 23, 1358-1364. Oltmans S. J., Schnell R. C., Sheridan P. J., Peterson R. E., Li S.-M., Winchester J. W., Tans P. P., Sturges W. T., Kahl J. D. and Barrie L. A. (1989) Seasonal surface ozone and filterable bromine relationship in the High Arctic. Atmospheric Environment 23, 2431-2441. Rahn K. A. and Lowenthal D. H. (1985) Pollution aerosol in
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the Northeast: northeastern-midwestern contributions. Science 228, 275-284. Shaw G. E. (1991) Aerosol chemical components in Alaskan air magscs. J. oeophys. Res. 96, 22,357-22,368. Sturges W. T. and Barrie L. A. (1988) Chlorine, bromine and iodine in arctic aerosols. Atmospheric Environment 22, 1179-1194.
Sturges W. T., Cota G. F. and Buckley P. T. (1992) Bromoform emission from Arctic ice algae. Nature 358, 660-662. Whitehead D. C. (1984) The distribution and transformations of iodine in the environment. Envir. Int. 10, 321-339.
0004 6981/93 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain.
H A L O G E N S IN AEROSOLS IN CENTRAL ALASKA WILLIAM T. STURGES School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
and GLENN E. SHAW Geophysical Institute, University of Alaska, Fairbanks, AK 99775-0800, U.S.A. (First received 10 April 1993 and in final form 30 June 1993)
Abstract--High volume filter samples were collected at Poker Flats in central Alaska, between 1984 and 1987, and analysed for a comprehensive suite of elements. In this report we focus on the results for the halogen elements Br, CI and I, and their correlations with other selected elements (A1,As, Na, Se, and V). Seasonal cycles were observed for the halogens, including a pronounced spring peak in Br and a weak fall peak, pronounced spring and fall peaks in I, and increased winter/spring C1. A significant correlation between Br and Se was shown to be partly due to common transport pathways, and possibly some common sources. Iodine showed enrichments of three orders of magnitude over sea water composition. Correlations to marine elements suggested a marine biogenic source. Chlorine evidently originated from sea salt aerosols, but showed evidence of substantial volatilization, correlated to the degree of pollution of the air mass. Key word index: Aerosols, bromine, chlorine, iodine, selenium, enrichment factors, remote atmospheres,
Arctic, Alaska.
INTRODUCTION There has been interest in halogens in arctic aerosols since Berg et al. (1984) and Barrie and Hoff (1985) showed that bromine, chlorine and iodine undergo regular annual cycles in the Alaskan and Canadian Arctic, respectively. Bromine has been the most studied of these three halogens. There is a pronounced spring pulse of particulate bromine in the Arctic with peak concentrations as much as hundreds of times higher than summer values (Sturges and Barrie, 1988). Interest in bromine in the arctic troposphere increased when Barrie et al. (1988) demonstrated a strong anticorrelation between surface ozone and particulate bromine. High levels of organobromine gases have also been reported in late winter and early spring (Barrie et al., 1988; Cicerone et al., 1988; Berg et al., 1984). The origin of the spring bromine pulse has remained a mystery. Sturges and Barrie (1988) demonstrated that the Br concentrations could not be accounted for by long range transport of Br-containing vehicular pollution, nor Br in sea salt aerosols and crustal material. Coal burning could not be ruled out for lack of a reliable tracer system. Sturges et al. (1992) have shown that polar sea ice microalgae, which are found ubiquitously under annual sea ice during the spring, are prodigious synthesizers of bromoform. It seems likely, therefore, that the spring pulse of both organic
gaseous and particulate bromine has a marine biogenic source. Particulate iodine shows enormous enrichment factors over sea water composition in aerosols (Whitehead, 1984). This has been explained in the marine background atmosphere as arising from fractionation at the sea surface microlayer, or from particle formation from organic iodine gases emitted from the sea surface. Sturges and Barrie (1988) showed that there is an Arctic spring iodine pulse, with similar features to the bromine pulse, and some scant evidence for a secondary fall peak. Barrie and Barrie (1990), using a longer record from Alert, North West Territories, have, however, clearly shown the existence of a fall iodine peak. The causes are unknown, but are thought to be linked to marine biogenic sources. Chlorine was shown by Sturges and Barrie (1988) to peak during the period of winter storminess in the northern oceans. Correlations to sodium confirmed that chlorine was associated almost exclusively with marine aerosols. The ratios to sodium were, however, depleted relative to bulk sea water composition, indicating that volatilization of chlorine had taken place. In all of the above arctic studies, halogens were measured at near-coastal locations. We report here the first extensive measurements of halogen chemistry at an interior subarctic site, in central Alaska. This allows us to further examine some of the above relationships and hypotheses.
2969
2970
W.T. STURGESand G. E. SHAW
METHODOLOGY Air filter samples were collected at Poker Flats (PF), a site operated by the University of Alaska, about 20 milesnorth of Fairbanks, Alaska. PF is separated from the Fairbanks area by high ground, and is not greatly impacted by emissions from the Fairbanks area, especiallyduring the winter months when strong surface inversions inhibit advection of polluted air out oftbe Chena valleyarea. Air arriving from the north is essentially unperturbed Arctic Basin air. Although PF is about 150 km south of the Arctic Circle, it is frequently within the arctic air mass during the winter and spring as the Arctic Front moves south. At other times there is strong transport from the north Pacific, thus PF is an ideal location from which to examine differences between Pacific marine and Arctic Basin air masses. PF is approximately 600 km from the nearest coastline. Sampling and analytical methodologies are described in detail in Shaw (1991). In brief: high volume air samples (5-10 x 103 m3) were collected by drawing air through 20 x 25 cm Whatman 41 cellulose fiber filters. The collection efficiency for submicron particles was about 70%. Elemental concentrations were determined by instrumental neutron activation analysis at Los Alamos National Laboratory. The neutron flux was 10acm-2s -1. Each filter underwent two irradiations and three gamma counting sequences to derive shortand long-lived isotope abundances.
RESULTS Monthly mean, medians, and confidence limits are plotted as box and whisker plots in Fig. 1. Non-crustal
,o0t
s°°"i'"ci'
200
:"
• ~ ""
1
vanadium (ncV) was computed according to the method of Rahn and Lowenthal (1985), i.e. ncV = total V - c r u s t a l V, where crustal V is estimated from measured A1 concentrations and an assumed ratio of V to A1 in crustal material of 1.623 × 10 -3. Correlations between the elements are shown in Tables 1-5 for different data stratification schemes. To reduce the effects of correlations arising from common seasonalities of element pairs, the data were divided into two "seasons": firstly, the period encompassing the elevated bromine levels during the spring (1 February-31 May) (SPR) and, secondly, the remainder of the summer, fall and winter seasons (SFW). Another stratification attempted to group samples that were collected during flow predominantly from the Arctic, and samples collected during flow predominantly from the Pacific. Arctic air masses were identified by mean temperatures during sampling periods that were more than 10°C lower than the long term PF mean for that time of year. Pacific air masses were similarly identified by mean temperatures more than 10 °C above the long-term mean. Two correlation coefficients are given for each element pair in Tables 1-5. The first is the nonparametric Spearman rank correlation coefficient r,. This coefficient is useful where the data are nonnormally distributed, and reduces the effects of outliers. The second is the product moment correlation coefficient G" The last column on the right summarizes the most significant correlations with the element shown in the far left column. Spearman rank correlations with values of 0.5 or greater at a confidence limit > 9 9 % are shown in descending order in the last column. The elements listed in parentheses had correlations less than 0.5 at the same confidence limit. A correlation between ncV and AI has no meaning since AI is used to compute ncV. Instead, the correlation between total V and A1 is shown in the tables. Marine enrichment factors (EF) of an element X were calculated as: EF(X) =
O,S
o
~o
500 000
JFMAMJJASOND
" t . . . . .
Ai
::"'] iI
]
FMAMJJASOND
Fig. l. Box plots of monthly elemental means (dotted lines), medians (center bar), 25 and 75% confidence intervals (extremities of the box), 10 and 90% confidence intervals (exterior bars), and 5 and 95% confidence intervals (dots) for all samples collectedat PF between 1984and 1987.
[X]alr / [Na]alr
[X],ea / [na].ea
i.e. the factor by which the measured ratio of the element to Na in the aerosol exceeded the ratio of the element to Na in bulk sea water (Duce et al., 1983).
Bromine
Figure 1 shows that a large increase in particulate bromine occurred from February to May, peaking in April. Concentrations were lowest in summer, and rose somewhat during the fall and early winter. This is quite similar to observations at coastal arctic locations (Berg et al., 1984; Sturges and Barrie, 1988). However, whereas peak values at PF were around 10ngm -3, concentrations at, for instance, Barrow, Alaska, reached as much as 100 ngm -3 during the spring (Berg et al., 1984). The relatively small degree of
Halogens in aerosols in Central Alaska
2971
Table 1. Inter-element correlation coefficients and mean concentrations (ng m-s) for Poker Flats samples collected during the "summer-fall-winter" (SFW) season: 1 June-31 January, 1984-1987. In the correlation matrix the top number is the Spearman rank correlation coefficient, the center number is the product moment correlation coefficient, and the bottom number is the number of sample pairs. Correlation coefficients in bold type are significant at the 99% confidence level. The correlation shown in the space for noV vs AI is actually the correlation for V vs AI (see text). To the fight of the matrix is a summary of the elements that correlated with the element shown in the left column with a significance of 99% and a Spearman rank correlation of 0.5 or greater (99% significance and rs<0.5 in parentheses). The lower two rows show the concentration means and standard deviations AI
As
Br
0,879
0.642
0.561
0.969
0.469
0.418
103 102 0,002 0.664
ncV Se
0,116
Na
95 0.226
CI Br As
94
I
Na
Se
103 0.685
0.151 0.034 103 0.266
0.286 0.165 97 0.381
0.441 0.174 102 0.426
0.521 0.440 95
0.670
0.286
0.404
0.393
95
95
0.423 0.315
0.398 0.518
0.633 0.908
102 101 -0.132 0.258 -0.027 0.233 97 96 -0.179 0.054 -0.051 0.083 103 102 -0.056 0.553 -0.014 0.444 103 102 0.396
102 0.480 0.560 97 0.321 0.461 103
102 0.533 0.854 97
0.195
I
0.608
CI
90
ncV
Correlation summary As, Br, Se (Na, I) Br, As, ncV (Na, I)
95 C1, I (ncV, Se, As, Br)
0.521 0.815
97 C1, Na (Br, Se, ncV) Na, I (Br) Se, ncV, As (I, Na, CI) Se, ncV, Br (Na, CI)
0.684
102 A1 Mean a
As ll7 114
0.133 0.083
0.886 0.461
51.9 38.8
0,446 0.380
66.4 54.9
0.035 0.016
0.258 0.186
Table 2. Inter-element correlation coefficients and mean concentrations (ngm -3) for Poker Flats samples collected during the "spring" (SPR) season: 1 February-31 May, 1984-1987. See legend of Table 1 for explanation of figures
ncV Se
A1
As
Br
0.605 0.746
0.823 0.844
0.653 0.435
97 -0.063 - 0.097
Na
97 -0.002 0.218
97 I
Cl Br As
97 0.648 0.696
99 0.407 0.273
97
97 0.685 0.613
99 0.392 0.418
97
-0.246 - 0.150
0.574 0.479
0.675 0.575
97 0.014 - 0.029 97 -0.057 -0.006 97 0.240 0.200 97
99 -0,117 - 0,049 97 0,595 0.411 99
99 0.059 0.246 97
CI -0.140 -0.103
97 0.297 0.191
97 0.605 0.677
97 0.142
I 0.655 0.461
97 0.692 0.577
99
Na
Se
0.393 0.214
0.683 0.740
97
ncV
As, Se I, Br (Na)
97 I, Br, ncV, I, Na, As (CI)
0.655 0A25
97 I, Se, CI, (As, ncV, Br)
0.661 0.602
97 Se, Br, Na, ncV, As
0.181
97 Na (Se) Se, I, ncV, As (Na) ncV, Se, 1 (Na)
AI Mean O"
Correlation summary
None 139 88
0.269 0.277
4.06 2.99
89.6 82.9
0.827 0.469
116 82
0.067 0.041
0.458 0.302
2972
W.T. STURGESand G. E. SHAW
Table 3. Inter-element correlation coefficients and mean concentrations (ng m-3) for Poker Flats samples collected during episodes of arctic air mass influence (mean air temperature more than 10 °C below the seasonal norm) for all dates 1984-1987. See legend of Table 1 for explanation of figures
ncV Se Na I CI Br As
AI
As
Br
CI
I
Na
Se
0.742 0.606 18 0.560 0.349 18 0.517 0.551 17 0.379 0.170 18 0.083 0.015 17 0.434 0.476 18 0.595 0.185 18
0.986 0.948 18 0.837 0.798 18 0.642 0.409 17 0.701 0.750 18 0.272 0.320 17 0.651 0.317 18
0.645 0.341 18 0.664 0.566 18 0.767 0.761 17 0.816 0.676 18 0.502 0.494 17
0.235 0.156 17 0.309 0.354 17 0.453 0.509 17 0.485 0.534 17
0.695 0.680 18 0.732 0.752 18 0.770 0.679 1.7
0.654 0.433 17 0.750 0.664 17
0.851 0.835 18
ncV
As, Se I, Br, Na ncV, As, Na, I, Br I, Br. Se, ncV, As Br, Na, Se, As, ncV None I, Na, Se, As, ncV noV, Se, I, Br, AI
AI Mean a
Correlation summary
As 109 109
0.512 0.495
5.39 4.03
84.1 44.1
0.968 0.451
151 84
0.105 0.054
0.595 0.468
Table 4. Inter-element correlation coefficients and mean concentrations (ng m - 3)for Poker Flats samples collected during episodes of Pacific air mass influence (mean air temperature more than 10 °C above the seasonal norm) for all dates 1984-1987. See legend of Table 1 for explanation of figures
noV Se Na I CI Br As
A!
As
Br
CI
I
Na
Se
0.848 0.910 42 -0.096 0.027 39 0.124 --0.125 41 -0.264 -0.249 37 -0.026 -0.142 42 -0.162 - 0.063 42 0,364 0.514 42
0.501 0.223 42 0.435 0.512 39 -0.043 --0.189 41 0.125 0.008 37 -0.275 -0.333 42 0.449 0.383 42
0.501 0.361 42 0.594 0.535 35 0.181 0.226 41 0.564 0.529 37 -0.142 -0.022 42
-0.322 --0.180 42 0.176 0.245 39 0.706 0.891 41 0.202 0.591 37
0.072 0.011 37 0.596 0.581 35 0.445 0.812 37
0.023 -0.073 41 0.280 0.361 39
0.137 0.211 39
ncV
As, Br I, Br (As) CI (I) Se, Br (Na) Na Se, I, ncV (As) ncV (Br)
AI Mean a
Correlation summary
None 135 138
0.121 0.070
1.06 0.80
76.1 55.2
dilution of b r o m i n e c o n c e n t r a t i o n s evident during t r a n s p o r t from the Arctic O c e a n to P F indicates t h a t the high b r o m i n e levels previously observed at coastal arctic locations were not due to a localized effect, b u t
0.460 0.321
82.4 80.9
0.033 0.016
0.290 0.203
are p r o b a b l y representative of the Arctic Basin as a whole. A n u m b e r of studies have s h o w n t h a t high particulate Br levels in the Arctic are only to be found in the
Halogens in aerosols in Central Alaska Table 5. Correlations between the ratio CI/Na and the other elements in this study. See legend to Table 1 for explanation of figures
2973
"pollution" elements Se, ncV and As, and more weakly with the "marine" elements I, Na and C1 (Table 1). The correlation with Se was by far the strongest. Se was, in turn, most strongly associated with Br, As, and ncV, and more weakly with Na and I. The implication is Element SPR SFW that Br during this season may be at least partly A1 -0.044 -0.501 derived from pollution; predominantly coal burning if -0.125 -0.382 the Se in this case is mostly of anthropogenic origin. 97 102 The absolute Br concentrations, however, were small As - 0.585 - 0.423 -0.319 -0.421 (Fig. 1 and Table 1). 97 101 A significantly different picture emerged from Br -0.340 -0.102 correlations during the SPR season (Table 2). The --0.165 -0.045 mean Br concentration was four to five times higher 97 102 than in the SFW season. Br was still most strongly I -- 0.492 0.043 -0.418 0.090 associated with Se, but was closely followed by I. 97 97 Correlations with ncV and As were also high. Se was Se -0.286 -0.184 most strongly correlated with I and Br, followed by -0.174 -0.181 ncV, Na and As. Since I has almost exclusively natural 97 95 origins, probably from marine biogenic sources, as ncV -0.457 -0.514 - 0.354 - 0.428 discussed later, the suggestion is that Br, I and Se all 97 102 have biogenic sources in the Arctic Basin. What of the correlations between Br, As and ncV? That As and ncV are pollution elements of common origin was amply demonstrated by the high intercorrelation (rs surface inversion layer (e.g. Oltmans et al., 1989), =0.823) in Table 2. This could be interpreted as a which has a typical height of 100-500 m. Given that significant additional source of bromine from polluPF is a minimum of 600 km from the nearest arctic tion. It might, however, be equally ascribed to comcoast, and that air from the Arctic Basin must ascend mon transport pathways of arctic air pollution and over the Brooks Range before arriving at PF, it is biogenic emissions from the Arctic Ocean to PF. The perhaps surprising that such a distinct pulse is seen in correlations, therefore, may be biased by an alternacentral Alaska. Either there are terrestrial sources of tion between polluted air containing biogenic emisparticulate bromine in Alaska, in situ formation from sions peculiar to the Arctic, and clean Pacific air gaseous precursors en route to PF, or only limited without the same biogenic components. In Table 3 an attempt has been made to isolate mixing occurs during transport into the interior of instances of flow out of the Arctic based on air Alaska. What is the origin of the particulate bromine? A temperature (see above), to overcome such potential cursory inspection of Fig. 1 shows that the elements I, bias. This selection procedure returned only 18 qualiSe, ncV and As, and to a lesser extent Na and CI, also fying samples. The pattern of correlations was quite had spring maxima. Aluminum, on the other hand, different again. Br showed high correlations with I and was greater in the summer. Iodine is thought to Na, followed by Se, As and ncV. Se, in this instance, originate almost exclusively from natural sources, correlated more strongly with the pollution elements primarily the ocean (Sturges and Barrie, 1988). Sel- ncV and As, followed by Na, I, and Br. This appears to enium has long been considered to have largely confirm that the majority of Br in arctic air masses anthropogenic sources, and has been used as an originates from marine sources, whereas Se evidently effective tracer of emissions from coal burning (Keeler had significant anthropogenic sources. The near-unity and Samson, 1989; Rahn and Lowenthal, 1985). More correlation between As and ncV again illustrated the recently, however, it has been estimated that natural common anthropogenic source of these two elements. The correlation coefficients for Pacific air masses (principally biomethylation) and anthropogenic sources are of approximately equal magnitude (Atkinson (Table 4) are in sharp contrast to those of Table 3. All et al., 1990). Arsenic and ncV originate from industrial of the correlations were weaker. Br correlated to Se, I activities, and have been used extensively as tracers.of and ncV, but Se correlated mostly with marine elelong range transport of pollution (Keeler and Samson, ments (I and Br). The mean Br concentration was 1989; Rahn and Lowenthal, 1985). Particulate C1 and about five times lower than in arctic air masses. Se and Na are believed to originate almost exclusively from I concentrations were also lower. The conclusion here sea salt aerosol, and AI from crustal dust and soil is that Br, Se and I in Pacific air masses were all of marine origin. Pollution levels were also lower, and As (Sturges and Barrie, 1988). Information about the possible sources of partic- and ncV were only weakly correlated. The relationship between Br and Se can be sumulate Br in the Arctic can be gleaned by examining correlations with such elemental tracers. In the SFW marized by a plot of the monthly means of ratios of Br season, Br was most strongly correlated with the to Se (Fig. 2a). Br increased relative to Se during the
2974
W.T. STURGESand G. E. SHAW
150 r~
,
,
•
•
,
,
,
,
,
,
,
,
,
,
(a)
I O0 50
(b)
1.5
~
1.0
g~ 0.5
0.0
L , l , , , , , l J l ,
15
(~)-
PF of >161 n g C l m -3. The mean C1 concentration measured at P F was 120ngm -3 and, as discussed below, this C1 was evidently derived solely from marine aerosol, not coal combustion. It is therefore concluded that little or none of the Br measured at P F originates from coal burning• Figure 2a also suggests that there may have been a second peak in excess Br in late fall/early winter. This is also observed in a plot of Br enrichment factor (Fig. 2c). This has not previously been reported from coastal arctic sites• As discussed below, this secondary maximum coincides with a peak in particulate I, suggesting a common biological origin• The aboveunity values of the Br enrichment factors in Fig. 2c confirms that the excess Br did not originate from sea salt aerosol, given that Br enrichment during aerosol formation from sea water has not been observed (Duce and Hoffman, 1976), indeed Br volatilization is more usual (Sturges and Barrie, 1988)• Iodine
J
F
M
A
M
J
J
A
S
O
N
D
Fig. 2. Box plots of monthly means, medians, and confidence intervals for the ratios (a) Br/Se, (b) Se/As, and (c) EF (Br) for all samples collected at PF between 1984 and 1987. See legend to Fig. 1 for explanation of box markers•
SPR season, whereas the ratio Se/As (Fig. 2b) remained relatively constant throughout the year. It can be concluded that there was a source of excess, nonpollution Br during the spring months, and further that Se was mostly of pollutant origin, although the possibility of a natural Se component has been indicated by the correlations above. Germani and Zoller (1988) made measurements of halogens and Se in a coal-fired power plant stack• Applying their ratio of particulate Se to particulate Br in the stack emission (13 + 4/~g m - 3 Se, 9 _ 5 #g m - 3 Br) to the mean P F Se concentration of 0.21 ng m-3, and assuming that the Se at PF originated solely from coal burning, gave a predicted coal-burning-Br concentration of 0.14 ngm -3, well below the measured mean Br concentration of 12 ng m-3. If, however, the Se/Br ratio for the sum of particulate and gaseous phases in the stack emissions was used (33 ___5 #g m - 3 Se, 1310 + 90 pg m - 3 Br), the predicted coal-burningBr concentration (14 ng m-3) was much closer to that measured. In this latter case it must be assumed that both the Se and Br present in vapor form in the stack emissions are converted to particulate matter. This argument appears plausible until the same procedure is applied to predict C1 concentrations at P F due to coal burning. Germani and ZoUer gave a lower limit to total C1 in the stack emissions of 15,000/~g m-3. This yields a predicted coal-burning-C1 concentration at
In Fig. I, iodine is seen to have had two annual peaks. The larger peak occurred at the same time as the spring Br peak. The second I peak coincided with the fall rise in Br concentrations and, more notably, the fall peak in Br/Se and Br enrichment factor (Figs 2a and 2c). A pronounced spring I peak has also been reported by Sturges and Barrie (1988) and Barrie and Barrie (1990). The latter authors also noted a fall peak at Alert• This fall peak occurred between August and October, with the secondary minimum in November. At P F the peak was centered on November. This may be related to the difference in latitude: Alert is at 83 °N, P F at 65°N, and Barrow (Alaskan Arctic coast) at 71 °N. At the higher latitudes the summer is very short. It is entirely plausible that biogenic activity leading to I emission might be correspondingly abbreviated in the most northerly regions of the Arctic Ocean• It is noted that Berg et al. (1984) found no seasonal cycle whatsoever in I at Barrow. In view of the fact that four arctic locations have now been shown to have annual cycles in I, this calls to question the Berg et al. (1984) data, unless some localized source could be demonstrated that is capable of overwhelming the background signal. Unlike Br, absolute I concentrations at P F were little less than those reported for the coastal arctic locations• Sturges and Barrie (1988) reported peak values of about 1-2 ng m-3, depending on the site, while Barrie and Barrie (1990) reported values mostly below 1 ngm -3 at Alert• At PF, peak values were around 2 ng m - 3. This may indicate that I production does not relate solely to surface sources in the Arctic Basin, but has a more widespread origin, and is more uniformly mixed throughout the troposphere. Figure 3 shows the annual cycle of iodine enrichment factor. In all cases the degree of enrichment was enormous• Enrichment factors of around 102-10 3 have been reported from numerous locations in the
Halogens in aerosols in Central Alaska 4000
,
~
,
,
,
1984-
,
,
,
T
,
Chlorine
,
1987
3500 3000 2500 ~" 2000 1500 I000 50O 0
i J
i
J
FMAM
i
J
i
i
J
J
i
i
i
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L .L.
A S O N D
Fig. 3. Box plot of monthly means, medians, and confidence limits for EF(I) for all samples collected at PF between 1984 and 1987. See legend to Fig. 1 for explanation of box markers.
global background marine atmosphere (Whitehead, 1984). Sturges and Barrie (1988) reported enrichment factors from arctic sites of up to 104. They attributed these higher factors to the absence of large numbers of freshly generated marine aerosols containing I/Na ratios comparable to sea water values. At PF, the enrichment factors do not reach the highest values reported by Sturges and Barrie (1988), but the mean values from Sturges and Barrie, ranging from 1100 to 1500, are not dissimilar to those from PF. In Fig. 3 the fall peak is as large, if not larger, than the spring peak when expressed as an enrichment factor. This is probably no more than a reflection of the smaller amount of marine aerosol present during the fall compared to the spring (see the plots of Na and CI in Fig. I). It also indicates that I concentrations were essentially independent of marine aerosol concentration, and argues for the overwhelming importance of gas-to-particle reactions in the production of I aerosols, over enrichment during bubble-bursting marine aerosol formation. In Table 1, I is correlated predominantly with CI and Na in the SFW season. In the SPR season (Table 2) it correlates with Se, Br, Na, ncV and As. Since there are no known large anthropogenic sources of I, this appears to be good evidence for the effect of common transport pathways on the elemental correlations. The absence of a correlation to CI, despite the high correlation with Na, is surprising. As discussed below, this may relate to the apparent volatilization of CI from sea salt aerosols. Similar correlations were observed for arctic air masses (Table 3), although in this instance the highest correlation (rs=0.816) was with Br, followed by Na and Se. In Pacific air, all of the correlations to I were weaker; the highest were with Se, Br and Na, suggesting a marine source. Overall it is clear that I had an exclusively marine origin. Given this conclusion, it is further suggested that there is a marine source of Se, at least in Pacific air.
In the SPR and SFW seasons, and in Pacific air masses, Cl correlated almost exclusively with Na (Tables l, 2 and 4), with lesser correlations to I, Br and Se, as would be expected for a marine aerosol origin. In arctic air masses there were no strong correlations: the highest values were for Br, I and Na, again suggesting a marine aerosol origin. In Fig. l, the elevated levels of CI and Na during the winter were due to the greater degree of storminess, and therefore marine aerosol generation, in the northern mid- and high-latitudes at this time of year (Erickson et al., 1986). In the absence of any chemical fractionation effects, the ratio of C1 to Na would be 1.78 (Sturges and Barrie, 1988). Figure 4 shows that, with very few exceptions, the C1/Na ratios observed at PF were below this ratio, indicating substantial loss of C1 from the aerosol. This effect has been noted by other workers (although significantly not by Berg et al., 1984) and has been variously attributed to displacement reactions with acids (Clegg and Brimblecombe, 1985), or reactions with N205 and CIONO2 (Finlayson-Pitts et al., 1989)• In either case, we would expect to see greater CI loss in more polluted air masses. Sturges and Barrie (1988) did, in fact, find an inverse relationship between C1/Na and sulfate. At the highest sulfate concentrations, CI loss amounted to 70%. Sulfate was not measured in this study, but pollutant elements may be used as tracers for air likely to contain acidic compounds and NOx. Table 5 shows that in the SPR season, the highest anticorrelation to CI/Na was with As, confirming the above hypothesis. Other significant correlations included ncV, another pollution tracer, and also I. In the latter case we might speculate that favorable conditions for the photochemical production of acidic species and reactive nitrogen species, and the formation of particulate I,
2,0
1.5
0.0 J F M A M J
J A S O N D
Fig. 4. Box plot of monthly means, medians, and confidence limits for the ratio CI/Na for all samples collected at PF between 1984 and 1987. See legend to Fig• 1 for explanation of box markers.
2976
W.T. STURGESand G. E. SHAW ] : : ~ : : : ] ; [ 7 7.......::i ~ - _ ~ _
\~ ~ ~2_._~ ..... ' ~_ '~ ~_~ _
0.01
0.1
1
As (rig m -3) Fig. 5. CI/Na vs As for all P F data combined, on logarithmic scales.The solid line is a linear fit to the logtransformed data, and the dashed lines are the 9 9 % confidence limits of the regression.
existed in these air masses or, as noted above, that this was an effect of similar transport pathways. During the rest of the year (SFW), the anticorrelation with As and ncV remained, while that with I disappeared. Curiously there was a significant anticorrelation with AI. We can offer no explanation for this at present. Possibly the Al-containing dust in the summer was rich in Na, but this was not reflected in the correlations between A1 and Na in Table 1. Figure 5 shows a plot of C1/Na vs As on a logarithmic scale for the PF data from all seasons. The solid line is a linear fit through the log-transformed data (r = -0.472), bounded by the 99 % confidence interval (dashed lines). At the highest As concentrations, the linear fit indicates that the C1/Na ratios were around 0.3, which equates to about 83 % volatilization of the sea salt chlorine, in good agreement with the abovementioned findings of Sturges and Barrie (1988).
CONCLUSIONS Based on our analysis of the 1984-1987 multielement aerosol data from Poker Flats, Alaska, we have drawn the following conclusions: (a) The arctic spring Br pulse is not restricted to coastal arctic sites, but is also a feature of the interior arctic and subarctic. By inference, it must be a widespread feature throughout the Arctic Basin. (b) The Br pulse originates from an arctic marine source. It does not occur with Pacific marine air. Neither does it appear to originate from man-made sources. The correlation with Se does not indicate a coal-burning origin, rather it indicates common transport pathways, and perhaps a contribution to Se from marine biogenic sources. The lower Br concentrations observed outside of the spring pulse may, on the other hand, have some anthropogenic component. (c) There was some evidence for a fall peak in Br. This has not been reported before. The coincidence
with a fall peak in I may indicate a common biogenic origin. (d) Iodine showed both a spring and fall peak, confirming an earlier report from Alert, North West Territories. It is believed to have a marine biogenic source. Iodine was enormously enriched, generally by three orders of magnitude, over sea water composition. (e) Chlorine originated from marine (sea salt) aerosol. The C1 was, however, depleted relative to Na from its sea water composition. The degree of depletion increased with more polluted air masses, supporting the hypotheses that reactions with certain pollutant species (acids and oxides of nitrogen) lead to displacement of CI. Acknowledgements--WTS acknowledges the support of the
National Science Foundation (under grant DPP-9015614), the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, and the Climate Monitoring and Diagnostics Laboratory of the National Oceanic and Atmospheric Administration. The authors are indebted to the data and graphical analysis work of Geoffery Dutton. Dutton was supported by a Research Experiencefor Undergraduates grant from the National Science Foundation (EAR-9213236/DPP-9015614). REFERENCES
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