The chemical composition of remote area aerosols

Vol. 12, No. 4. pp. 367-384, 1981. Printed in Great Britain,

0021-8502/81/040367-17 $02.00]0 ~ 1981PergamonPressLtd.

J. Aerosol Sci.,

THE CHEMICAL C O M P O S I T I O N OF REMOTE AREA AEROSOLS* WILLIAM C. CUNNINGHAM'~ a n d WILLIAM H. ZOLLER Department of Chemistry, University of Maryland, College Park, MD 20742, U.S.A.

Abstract--Studies of background atmospheric aerosols have been conducted at a variety of locations during the past ten years. Most sites that have been studied are dominated by regional aerosols, and have relatively high levels of crustal dust or sea salt. The cleanest place on the earth's surface is the Antarctic, as demonstrated by measurements conducted over the last few years. Atmospheric panicles collected there have been analyzed by neutron activation analysis, and the results show that the major mass of aerosol is sulfate during the summer season. Minor quantities of crustal dust, sea salt and meteoritic debris were also observed along with a rather small component made up of volatile elements, which are believed to be due, in part, to volcanism. During the winter season, the levels of sulfate, crustal dust and meteoritic debris were observed to decrease, and the level of sea salt increased almost twenty fold due to severe storm activity off the coast of Antarctica and enhanced transport from these boundary areas into the center of the continent. At the same time, transport from the upper troposphere into the lower, near surface areas was severely hindered. The chemistry of the aerosol appears to be very similar each summer season and, between 1971 and 1978, no drastic changes in the composition of the aerosols collected at the South Pole can be noted.

INTRODUCTION Since atmospheric aerosols interact with solar radiation and affect micrometeorological processes, they may affect not only the Earth's energy balance, but the climate. The concentrations and composition of aerosols near the surface of the earth are often controlled by local particulate sources. With increasing altitude and distance from the sources, however, the quantities of aerosol and the composition should approach a background limit. From a global point of view, it is the composition of this so called "background aerosol" that is important if one wishes to estimate the impact of anthropogenic sources on the atmosphere. We know that anthropogenic sources dominate the chemistry of atmospheric aerosols in urban regions and we would like to know how wide spread these aerosols are becoming and how they contribute to the global aerosol circulation. Current estimates of the magnitude of anthropogenic emissions are so uncertain that their impact on "global background" aerosols could be from a few percent up to 50% of the masses of some species (SMIC, 1971; SCEP, 1970). Unlike the anthropogenically released gases, such as CO2 or chlorofluoromethane aerosols have relatively brief residence times in the atmosphere and the effects of long range transport will be to limit the amount of large aerosols in remote areas. The residence times of aerosols in the lower atmosphere are measured in days, whereas that of stratospheric aerosols is estimated to be from i to 2 yr depending upon altitude (Junge, 1963). Owing to variations in residence times, "background" levels should be determined as far as possible from major sources and at an altitude above the boundary layer. There are several possible sites that could be used to study "background" aerosols: continental areas, oceanic areas (islands), mountain tops, glacier-covered areas and high altitudes with aircraft. Each of these areas has drawbacks. For example, continental areas are usually dominated by local sources such as wind blown dust, particles from vegetation, or anthropogenic sources, especially in the northern hemisphere. Islands far from continental areas should be relatively free of continental dust contamination unless they are in the line of flow for desert dust transport as occurs from the Sahara Desert (Prospero, 1968). Unfortunately, the marine atmosphere, at least at low elevation, contains a large quantity of sea * Presented at the GAEF Meeting, Schmallenberg, 22-24 October 1980. ¢ Currently at the U.S. Food and Drug Administration, Washington, D.C. 367

368

W . C . CUNN1NGHAMand W. H. ZOLLER

salt which may mask the presence of other aerosols. High areas, such as the tops of mountains above the timber line seem desirable, but, in many cases, they are dominated by local dust blown up the mountain from the surrounding continental land mass or, in the case of islands, by sea salt. Glacier-covered areas offer one of the best possible types of sampling site, but there are only two locations, Greenland and Antarctica. O f these, Greenland is in the northern hemisphere and downwind of industrialized continents; thus, anthropogenic emissions reach the ice cap at certain times of the year. This has been well demonstrated by Murozumi et al. (1969), who observed increases for lead in Greenland ice cores corresponding to modern times. No increase for lead was observed by the same authors in Antarctic ice cores, indicating that anthropogenic aerosols have not reached the Antarctic continent in the abundance they have in Greenland. Aircraft have been used for some studies of the background aerosol. They allow the collection o f samples from many locations with the same equipment in a short time. The main drawback is the limited amount of time available per flight for sampling in very clean air and the lack of continuous sampling. The rising cost of fuel and other aircraft expenses have made this method far from economical. Remote area sampling for "background" aerosols has been done by Rahn (1971, 1973) at several sites in Canada and in northern Norway which are at relatively low altitudes and are usually heavily affected by wind-blown dust and, near the coast, by sea salt. Table 1 gives a summary o f the data for A1, Na and S at several remote sites. Measurements have been conducted at the Jungfraujoch in Switzerland (Dams and DeJong, 1976), Chacaltaya in Boliva (Adams et ai., 1977), and at M a u n a Loa Observatory in Hawaii (Zoiler, 1980). The two continental sites have much higher concentrations of AI than Mauna Loa because of their surroundings, which give significantly higher levels o f crustal material than the Hawaiian site. In addition, the results of Hawaii are selected to eliminate conditions with high levels of sea salt and represent only clean, down slope winds (Zoller, 1980). The data tbr the two glacier-covered areas, Greenland and the South Pole, show the tremendous difference in both crustal dust and sea salt that are found in Greenland as compared to central Antarctica. However, during the Antarctic winter sea salt is transported in rather large quantities to the interior of the Antarctic continent. The lowest levels of crustal dust and sea salt, as indicated by the concentration of AI and Na, are found in the Antarctic and carefully selected samples from islands such as Mauna Loa, Hawaii. Because of these Table 1. Atmospheric concentrations of AI, Na and S at remote locations

Site Chacaitaya* (Bolivia) Jungfrauhoch t (Switzerland) Northwest Territories+ (Canada) Norway§ (north coast) Mauna Loa Observatory¶ (Hawaii)

Greenland ii South Pole summer winter

AI

Na

S

(ng/m3) (STP)

150

27

62

51

22

-

66

18

43

440

5.0 32 0.83 0.30

* Adams et al. (1977). t Dams and DeJong (1976). ~: Rahn (1971). § Rahn (1973). ¶ Zoller (1980), clean down slope winds only. [I Neidam (1981).

3.3

74

130 5.1 40

76 29

Chemical composition of remote area aerosols

369

results, the S o u t h Pole S t a t i o n was selected as a clean air m o n i t o r i n g site for s t u d y i n g the chemical c o m p o s i t i o n o f " b a c k g r o u n d " aerosols.

SOUTH

POLE STATION

The Amundsen-Scott Research Station at the geographic South Pole in Antarctica was chosen for this research because it is isolated from any natural sources o f atmospheric aerosols and has the necessary logistic support facilities. This station has been operated by the National Science Foundation for research programs since 1957 and is located approximately 1300 km from the coast of Antarctica (Fig. 1). It is situated on an ice sheet which is nearly 3 km thick, and the nearest land not covered by snow and ice is approximately 650 km away in the vicinity of the transantarctic mountain range, which extends between M c M u r d o and Palmer Stations. As air masses move in an anticyclonic pattern around the Antarctic interior, the size and average altitude of the continent make it extremely difficult for storm systems to move directly across the continent to bring aerosols into the interior as shown in Fig. 1. Because of the existing meteorological patterns, the continent is effectively isolated from many aerosol sources. The nearest other station, Vostok, is almost 1300 km from the Pole Station on the plateau region, so the only possible anthropogenic contaminants are from the station itself and aircraft used to supply it. A photograph o f the new South Pole Station as seen in 1975 is shown in Fig. 2. More recently, the station has become covered with snow so that most of it cannot be seen. The consistency of the surface winds at the South Pole enables one to have control over local aerosols from the station itself. Winds are consistently out of the north-easterly

Io o

e

ANTARTICA • SOUTH

4 Fig. 1. Map of Antarctica showing the tracks of storms that occur in the region.

370

w . c . CUNNING'HAMand W. H. ZOLLER

direction at speeds of approximately 5 m/sec. (By convention, at the South Pole, north is taken as the direction of the 0 ° 0' meridan.) Uncontaminated air can be sampled nearly 95 o of the time if sampling equipment is positioned upwind of the station and controlled by a wind-directional controller. Two sites have been used for sample collection at the new station. Initial samples were collected during November and December, 1970, at the old station using pumps and filter holders out in the open (Zoller et al., 1974). Better samples were collected during the austral summer season of 1974-75 at a remote site located 5 km up-wind of the new station and the results have been discussed by Maenhaut et al. (1979a, 1979b). Other austral summer samples were collected at this same remote site during the 1975-76 and 1977-78 seasons prior to shut-down of this site. Because travel is quite hazardous during the overwinter months, the winter sampling had to be performed close to the South Pole Station itself. For the two overwinter periods discussed in this paper, 1975 and 1976, an under-snow site was set up to the northeast of the station. This temporary site was used for only these two seasons, as a permanent Clean Air Facility (CAF) was constructed for all future work at the South Pole. S A M P L E C O L L E C T I O N AND ANALYSIS At both the remote 5-kin site and the buried overwinter site, sampling was conducted by pulling air into the building through a 30-cm dia polyvinyl chloride (PVC) sampling stack extending approximately 3 m above the snow surface. For the remote sampling site, a 30-cm PVC sampling manifold was used to distribute the air flow to several filter holders. A pump was placed at each end of the manifold to continuously purge air through the system. Several types of samples were collected from the manifold for various studies of Antarctic aerosols. All filter holders were made from high density polyethylene so that no metals would be in contact with the filters or in the air flow. Filter samples were collected using 11-cm dia Nuclepore (0.45-/am pore size) filters and Whatman 41 filters, at flow rates of 10 and 30 m3/hr, respectively. Additional high volume samples were collected using two circular 25-cm Whatman 541 filters which had flow rates of approximately 150 m3/hr. During the overwinter sampling period, only the 25-cm Whatman filter holder was used to minimize the work load on the overwinter personnel. Size-fractionated samples were collected with a modified Sierra Instruments high-volume cascade impactor, Model # 230. The impactor was mounted in a polyethylene container that could be used as an in-line sampler on the sampling manifold. Because chemical analyses were to be carried out on the samples, the substrate chosen was polycarbonate films which could be mounted on each impaction stage. The 50 °~o cutoff diameters under the usual sampling condition (flow rate of 68 m3/hr) were 7.5, 3.7, 1.9, 0.95 and 0.46/am for the 5 stages of the impactor. Scanning electron microscopic (SEM) examination of portions of the impaction surface revealed essentially no particles larger than a few/am except those associated with ice crystals, which act to scavenge fine aerosols as they fall through the air. Impactors made by Scientific Advances (Battelle type) were used for specific single particle studies, e.g. SEM analysis of single particles and the laser micro-Raman analysis of sulfate species discussed elsewhere (Cunningham et al., 1981 and Cunningham, 1979). Each filter and high volume impactor sample was collected for a 10-day period whenever the wind came from the sampling quadrant ( ~ 140° centered on the NE) with a velocity sufficient to turn the anamometer. Blanks were collected approximately once every month and were handled exactly the same as the samples, except that the sampling pumps were turned on for only one minute. For the 25-cm filters, two models of regenerative blower pumps were used: one built by Becker (SV/380) and the other by Cyclonair (DR-6). For the smaller, l l-cm filters and impactor samples, carbon-vane vacuum pumps (Gast Manufacturing Co.) were used exclusively. The filter and impactor samples were returned to the University of Maryland where they were cut in half and packaged for instrumental neutron activation analysis (INAA) in a Class 100 clean room. Prior to neutron irradiation, all samples were counted for natural radioactivity, principally 7Be, and fission products from nuclear weapons tests. For the INAA

Fig. 2. Aerial view of the new Amundsen-Scott Station at the geographic south pole, as it was in January 1975.

O

O

B

O

t~ O

Chemical compositionof remote area aerosols

373

analyses, the samples and the appropriate standards were irradiated in the National Bureau of Standards (NBSJ reactor which has a thermal neutron flux of 6 x 1013 n/cm2.sec. Spectra of the ~,-rays produced by the decay of the induced radioactivity were taken using a Ge(Li) detector coupled to a 4096-channel analyzer system. Two different Tennecomp TP-5000 systems were used, one at NBS for the short-lived isotopes and the other, at the University of Maryland, for the longer-lived species. Each of these computer-based systems is capable of simultaneously collecting data from six detectors and analyzing the spectra. All v-ray spectra were analysed on these same computer systems using standards which were carefully verified against NBS Standard Reference Materials (Ondov et al., 1975 and Germani et al., 1980). Because of the very low levels of atmospheric aerosols in the Antarctic, blank collections were quite significant for many species and limited the number of elements that could be determined in most samples.

RESULTS The results for four austral summer seasons, Deep Freeze (DF) 71, 75, 76 and 78, are discussed and compared with the data from the two overwinter periods, 1975 and 1976. Results of the first experiment (DF 71), presented by Zoller et al. (1974), showed that many trace elements had relative concentrations in the material similar to those for average crustal material, but a group of more volatile elements were identified as being enriched in Antractic aerosols. Concentrations of these elements, including Zn, Cu, Sb, Se, Pb and Br, were orders of magnitude greater than expected if their source were crustal dust. Similar measurements on atmospheric aerosols collected in the North Atlantic by Duce et al. (1975) showed similar enrichments for these elements, but higher absolute concentrations, which led the authors to speculate that there may be a natural source for some of these "anomously enriched elements" and that anthropogenic emissions were not solely responsible for the observed enrichments. Continued measurements at the South Pole by Maenhaut et al. (1979a) on samples collected during the 1974-75 austral summer (DF 75) yielded concentrations similar to those reported for the earlier work (DF 71), but extended the number of elements observed from 22 to 36. Several elements (In, W, Au, As, Ag, Cd and I) in addition to the previously identified elements were found enriched in the aerosols by at least an order of magnitude. These authors also found rather large quantities of sulfate as compared with the masses of other components, amounting to approximately 80-90 ~o of the aerosols mass. Even with the increased number of observed elements and their enrichment patterns, the identification of sources of the enriched and non-enriched elements was not obvious. It was clear that several sources should be investigated further. The source of the sulfate is of major interest as it dominates the aerosol mass, and Maenhaut et al. (1979a) believed that in some way it was linked to the transport of lower stratospheric or upper tropospheric sulfate to the surface at the South Pole because of the close correlation with the radioisotope 7Be, which is produced by cosmic ray bombardment in the upper atmosphere (Maenhaut et al., 1979b). A more definitive answer as to the source ofthe sulfate aerosol had to wait until overwinter data were available. Since then, more data have become available, including results from the two overwinter time periods which showed rather significant seasonal changes in the chemical composition, and hence, the characteristics of the aerosols. Table 2 gives the average summer and winter aerosol concentrations from the four periods for some elements of key interest. As shown, the summer values are rather constant from one summer to another, but differ significantly from the winter values. Sulfur is significantly lower in the winter season, as are the crustal elements A1 and Fe, whereas Na is much higher during the Antarctic winter season. We believe that these changes in the composition of the Antarctic aerosol with the season of the year reflect changes in atmospheric transport from remote source regions rather than local changes. This conclusion is due to the lack of local sources and the vast changes in meteorology during the winter vs summer seasons as discussed by Hogan and Barnard (1981).

374

W. C. CUNNINGHAMand W. H. ZOLLER Table 2. Average aerosol elemental concentrations {ng m 3} ISTP)* Element S Na AI Fe

DF 71 +

DF 75,+

DF 76,~

-7.2-+3.8 0.57_+0.17 0.84 -+ 0.21

49 _+ 10 3.3_+ 1.0 0.82_+0.38 0.62 _+0.23

86 +_21 4.3_+ 1.9 1.40_+0.40 0.92 + 0.38

Winter 76**

Winter Average

21 + 13 32_+ 17 < 0.22 0.12-+0.10

29+ 10 40_+31 < 0.30 0.25_+0.12

Winter 7511 S Na AI Fe

37+ 15 48_+60 0.30 _+0.04 0.38+0.22

DF 78 t

Avg

94 + 2q 5.58_+0.74 0.51 _ + 0 . 1 7 0.92 -+ 0.38

76 -+ 24 5.1 _+ 1.7 0.83+0.41 0.68 -+ 0.25

* The numbers given represent the arithmetic average concentration and standard deviation of the observed concentrations. t Average of 10 samples {Zoller et al., 1974). Average of 5 samples {Maenhaut et al., 1979a). § Average of 20 samples {Cunningham, 1979). ¶ Averageof 15 samples (Cunning,ham, 1979). II Average of 22 samples (Cunningham, 1979). ** Average of 27 samples (Cunningham, 1979).

To consider the possible sources of the Antractic aerosols and the enriched elements, a more complete comparison of the data from the summer and winter seasons is necessary. In Table 3, average concentrations of all elements observed for these two seasons are given. The observed concentrations vary over at least seven orders of magnitude, ranging from 10's of nanograms to a few femptograms per m 3. ENRICHMENT FACTORS AND CRUSTAL DUST One method of interpreting minor element concentrations is that of comparing relative concentrations of the elements with the elemental composition of sources. As two of the most important sources are crustal dust and sea salt, their average elemental compositions are used in evaluating the sources of the elements. To identify which elements are effectively accounted for by crustal material, crustal enrichment factors (EF~rus,) were calculated. The EFt,us, factors are ratios of observed elemental concentrations to the calculated crustal contributions for those elements. They were calculated using marker elements to represent crustal material as follows: tx/mh,~ EF'~s, -

( x / m l ~ .... "

where x and ra refer to concentrations of the trace element of interest and marker element, respectively. For the data presented, the E F s were calculated under the assumption that all of the observed AI, Sc and V were of crustal origin and that the average composition of crustal material is similar to that given by Taylor (1964). Other crustal abundance patterns could be used, but Taylor uses a slightly higher basaltic content than most others, which agrees well with the Antarctic data. In any case, the differences between other patterns are rather small compared to the large enrichments we observe for many elements. Figure 3 shows crustal enrichment factors for the summer season for the non-enriched elements. Values less than unity indicate depletion relative to average crustal material; whereas, values greater than unity indicate enrichments, which we consider to be slight enrichments up to values of 3 to 5. Some elements have very high enrichments, as shown in Fig. 4. In the case of CI, there are differences between the results obtained for the Whatman and Nuclepore filters which may have been due to the absorption of gaseous chlorine on the filter media. These differences also are found for Br and I (not shown) and are due to the lower efficiency of the Whatman filters for very small aerosols. The impactor results and data

Chemical composition o f remote area aerosols Table 3. Average atmospheric concentration (pg/m 3 STP)

375

in Antarctica

Element

Winter average*

S u m m e r average?

CI Na S Mg Ca K Br AI Fe

68,000 +_ 57,000 40,000_+31,000 29,000 _+ 10,000 5700 _+ 3700 1900 _+ 1600 1300 _+400:1: 320 _+ 220 < 300+40 250 -+ 120

6600 + 5400 5100_+ 1700 76,000 _+ 24,000 930 _+ 280 550 _+ 70 610 _+ 270 800 + 380 830-+410 680 -+ 250 80 -+ 42 1 1 0 + 10 59 + 47 ,~ 49 _+ 38 35 + 5 31 -4- 12 8.4+ 1.1 13_+7 19-+ 11 14_+6 6.3 _+0.6 < 4 2.0 _+0.2 1.9 4- 1.0 0.45 -+0.16 1.6 _+0.6 < 3.0 0.78 _+0.25 0.60+0.21 0.054 -+ 0.021 0.080 + 0.036 0.18+0.11 0.15 + 0.06 0.072 + 0.064 0.16_+0.09 0.10+0.03 0.090 _+0.040 0.020 _+0.007 0.007 _+0.002

I

180 + 150

Ti Cu Cd Zn Sr As Cr Ba Mn Se Rb Ce W Sb V Ag La Co In Au Sc Cs Ta Th Sm Hf Eu Lu

180_+ 40~ 79 _+ 16~ < 200 77 __.39 < 150 17_+ 9:~ 11 _+3 205- 18 6.7_+4.5 6.9 _+ 2.7 3.0 _+ 1.1,* 4.2 _+ 2.8~, < 5 2.1 -+ 1.5:[: ~ 0.9 ~ 1.0 < 2 0.40 -+ 0.13~/ 0.19 + 0.05,1: 0.092 _+0.004 0.037_+0.015 0.086 + 0.040 :[: 0.062 -+ 0.021 0.050_+0.016 < 0.4 0.042 + 0.014 0.009 _+0.004 < 1

* Only the uncontaminated samples from the winters of 1975 (13) and 1976 (1) were used for this average. Uncertainties are the standard deviation of the data points used. t Uncertainties are the standard deviation of all results. ** Upper limits on some samples were lower than this value. Since upper limits were not included in the calculation of the average, the true value is likely to be lower than this value by an unknown amount.

South Pole Aerosols-EFcrust Summer I0

IO

EF v

B~'Mn',H,

Th ~

Mg Ce

I

_

Sc 0.1

F'e

Ca

Ti O.I

Fig. 3. Crustal enrichment factors for the austral s u m m e r seasons for the non-enriched elements.

376

W . C . CUNNINGHAM and W. H. ZOLLER

105~'

PoteAerosols-EFcrustSummernn~~ ~B _lO,

0DF75SOuth

D I

EF 103~,o'L-

B

o DF 78 . ,~

D~

n fle, H T eBM

,o'-,I- oB~ua B~ ~.z~. I

B~

~U8

U

',~"c.'

Ne Cr

' S '

~

rO 4

J

-4,o' i

-,o' i

Fig. 4. Crustal enrichment factors for the austral s u m m e r season for enriched elements. The symbols N and W for chlorine refer to Nuclepore and W h a t m a n filters, respectively.

published previously on Antarctic aerosols(Maenhaut et al., 1979a) show that these elements are all borne by very small aerosols. For the halogens, CI, Br and I, Duce et al. (1973) have found that there are large components in the vapor phase which will eventually become associated with the smaller sized aerosols, which have the largest surface-to-mass ratios (Junge, 1963). SEA SALT The high enrichments found for Na and CI and the slightly high enrichments for Mg, K and, in some cases, Ca are due to sea salt. In fact, during the winter months, crustal enrichments for all these elements are much higher when the quantity of sea salt increases. Sodium can be used to calculate the mass of sea salt by first correcting the Na concentration for crustal Na and assuming that the rest of it is due to sea salt. Because information on the fractionation of the elements in marine aerosols relative to sea water is poor, the assumption is made that no fractionation occurs and that the sea salt composition is as reported by Brewer (1975). By multiplying the excess Na by the ratio of an element to Na from Brewer's data, the contribution of each element due to sea salt can be calculated. Likewise, using the fraction of sea salt that is due to Na (31.29 %), the mass of the sea salt aerosol can be calculated. As has been shown by Maenhaut et al. (1979a), the contributions of elements other than Na, Mg, Ca, K and CI from sea salt are negligible. If one considers gas-phase species and the known enrichments of species such as I and Br, the sources of the enriched halogens may also be the oceans surrounding Antarctica. Variations of the crustal dust and sea salt components of the Antarctic aerosol are shown in Fig. 5. The mass of crustal dust decreases each winter season by a factor of approximately 5 relative to the austral summer months as indicated by the data in Tables 2 and 3. Sea salt concentrations are lower during the summer months and have a maximum during the winter season. MI~TEORITES AND VOLCANOES Since neither crustal material or sea salt can account for the enriched elements, other potential sources have been considered. Two of the most important are meteorites and volcanism. As there is much information on chemical compositions of meteorites, we can use the known composition for comparison with that observed in Antarctic aerosols, using data on chondrites as adopted by Mason (1979). A similar comparison can also be made with data on the composition of volcanic clouds. Unfortunately, little good data are available on volcanic emissions. Data are available for Heimaey in Iceland (Mroz and Zoller, 1975), Kilauea in Hawaii (Duce et al., 1976), St. Augustine in Alaska (Lepel et al., 1978), Etna in

Chemical composition of remote area aerosols

377

South Polar Aerosols Component Concentrations

'°I c,u,,\

-I0

E

--

,~

I00

E

Sea

Z) I

I00

~ ;a

I0 I I. . . . . . .

DF 71 I1

]

1975

I

II DF78

1976

Fig. 5. Temporal variation of the crustal dust and sea salt components of the Antarctic aerosol at the South Pole between 1971 and 1978.

Sicily (Buat-Menard and Arnold, 1978) and Mt. St. Helens in Washington (Vossler et al., 1981). Differences of compositions of particles from various volcanoes are so great that no clear trends stand out except that volcanoes do enrich the volatile elements in eruptive clouds and fumarolic gases. We have chosen results of one study (St. Augustine) which are representative of moderate enrichments for comparison with the chemical composition of meteorites and Antarctic aerosols. In Fig. 6, EFt,u,, values for meteorites and volcanic clouds are compared with the average summer Antarctic aerosols. Several elements namely Fe, Mn, Co and Mg, are enriched far more in meteorites than in volcanic or atmospheric samples. The presence of meteoritic

CRUSTAL ENRICHMENTFACT?I

IO'

,0'

PI South Polar Aerosols x Meteorites

,o'

104

,o'

IOs

,o'

I0 ~

EF

90

.

x

x ~...~

x Zn

x

x

~

~

Se

CI

S

'~

x I~

-'110'

Sb

In

W x

x

x

tO

i -

a-on on onXOOxoUon o[ x

0

-

S¢ ~

Fe

V

Co

Sm

Co

x

Mn Ti

x

O0

00nDnBal x x

T~

~

Mg

x

x

~

K

--10

--

I

00I

Fig. 6. Comparison of crustal enrichment factors for south polar aerosols, volcanic emissions and meteorites.

378

W . C . CUNNINGHAMand W. H. ZOLLER

material in South Pole particulates would have the greatest influence on the observed enrichments of these elements. The more volatile elements, for example Se, As. etc., have a much smaller impact on the aerosol composition, as they are enriched less in meteorites than in atmospheric aerosols. If one were to assume all of the Se or As comes from meteorites. then the mass predicted for Co and Fe would be much larger than observed values. In general, the chemical composition pattern of volcanic emissions more closely fits the aerosol enrichments than does the meteorite pattern. Meteorites are enriched in some of the volatile elements, but the volcanic enrichments are usually much larger. There are also rather marked depletions of the rare earths, Th, Ba and K in meteorites as compared to volcanic and Antarctic aerosols. We can test the ability of the crustal dust, marine aerosol and meteoritic material to account for the elements observed in Antarctica by use of chemical element balances (CE Bs). This method has been used extensively to determine sources of elements on urban aerosol (e.g. Kowalczyk et al., 1978; Friedlander, 1973). The approach assumes that the observed concentration pattern is a linear combination of the concentration patterns of particles from the sources assumed to be important. The strengths of the sources are obtained by performing a least-squares fit to the observed concentrations of several carefully chosen "marker" elements. We have performed CEBs on the Antarctic samples using a program developed by Kowalczyk (1979). In the case of the Antarctic aerosol, we have selected the marker elements AI, V and Sc, which are sensitive to the crustal source, Na, which is a measure of marine contributions and Co and Fe for the meteoritic component. Examples of the fits for some representative elements are shown in Figs. 7 and 8. In Fig. 7 most elements are fit fairly well because they are marker or major elements in the samples. The sea is a dominant source for Na, CI and minor source for Mg, K and Ca, whereas meteorites have only a minor impact on Mg, A1 and Ti. The reason for underprediction of K and Mg may be an enrichment of these elements in marine aerosols produced from the biologically rich ocean around Antarctica, but there is little good evidence for this. Our crustal component may also be incorrect, but since we really don't know from which continent the crustal dust is coming, it is difficult arbitrarily to choose a different component. The elements shown in Fig. 8 are all metals and meteorites do have a significant impact, at least on Fe and Co, and can explain the usually observed excesses in these elements. The sea is a negligible source of these elements and we severely unpredict the Cs and Ba concentrations. The volatile elements are severely underpredicted by the CEB based on meteorites, crustal dust and marine aerosols, but volcanism appears to be quite significant. As volcanism is a potentially significant source of Antarctic aerosols, we must ask if Mt. Erebus, the active

ELEMENTAL SOURCE PREDICTIONS-DF 76,NUCLEPORE FILTERS No

Mg

(x05)

AI

(xlS)

(xl 5)

CI

(x05}

K

(~2}

Ca

(x3)

,~

(x6000)

Ti

(x20)

Total

0

L

.

.

.

.

.

Crust ~ ok~ I--- 4 I Z w 3~

D

br 0 I-

.

~

--

Sea

Meteoritesi

Fig. 7. Elemental source prediction for Deep Freeze 76 Nuclepore filter samples. The horizontal lines indicate the measured concentrations.

379

Chemical composition of remote area aerosols

ELEMENTAL SOURCE PREDICTIONS-DF 76, NUCLEPORE FILTERS v

(xSO0)

Mn (xso)

Fe

Co Cs (='rso) (x5ooo)

Ba (x5o)

Th

( x4OOOI

Total ~

.o 05

~

o

0 I-r,," I--

Crust

i.o 0.5 0

Sea :7

1.0

L)

05 0

Meteorites

~

Fig. 8. Elemental source prediction for Deep Freeze 76 Nuclepore filters.

volcano on Ross Island (1300 km from the South Pole), is important as a source? Samples collected and analyzed at Mt. Erebus and reported by Germani (1980) show that the pattern of enrichments of Mt. Erebus plume aerosols is unique and quite different than those of the South Polar aerosol. Therefore, the impact of this volcano is minimal and not the dominant volcanic source. The mass for each of the aerosol types that have been identified and the percent of total mass are given in Table 4. Sulfate dominates the aerosol mass, accounting for more than 90 ~o during the summer and 60 % in winter. The crustal aerosol mass is usually a few percent as is the sea salt mass except in the winter, when the crustal dust decreases and the marine aerosol mass significantly increases up to 30-40 % of the mass. The meteorite component always accounts for less than 1% of total mass as do the dominant volatile element Br and the rest of the volatiles. During the winter of 1975 there was an anomously large increase in the mass of Table 4. Mass of aerosol source types in South Polar aerosols (ng/m a and % of total) Sulfate*

Crust*

DF 71 DF 75 DF 76

-180 (92%) 330 (90%)

5.2 8.6 (4.4%) 18.3 (5.O%)

DF 78

350 (96 %)

Sea~

Meteorite§

Bromine¶

VolatilesH

< 10 2.6 0.3%) 12.8 (3.5%)

1.47 0.67 (O.34%) 2.2 (0.60~)

0.63 2.6 (1.3%) 1.6 (0.44~o)

0.10 0.13 (O.07%) 0.27 (0.07 ~)

5.0 (1.4%)

4.5 (1.2%)

0.83 (0.23 ~)

2.2 (0.61%)

0.16 (0.O4%) 2.6 (1.13%)

Summer

Winter

1975

140 (61%)

2.4 (1.0%)

a4 (36%)

1.07 (0.46%) 0.35 (0.15%)

1976

80 (60°/g)

1.2 (0.90~o)

52 (39%)

0.25 (0.19%)

Crust

Non-sulfate mass percentage Sea Meteorite

Bromine

0.19 (0.14~o) 0.23 (0.17%)

Volatiles

Summer

DF 75 DF 76 DF 78

59°0 52'7g 39 0o

18~o 36°g 35 o~,

4.6~o 6.3% 6.5 ~o

2.6 ° 0 2.2°o

93 5o 96°~,

1.2 % 0.46~o

18% 4.6~o 17 ~o

0.89~ 0.77% 1.26 %

Winter

1975 1976

0.39 % 0.35%

2.9 °/o 0.43~

* Based on 20!'o H2SO4 and 80°0 (NH4)2SO4. Calculated from AI, V and Sc concentrations based on crustral abundances from Taylor (1964). Calculated from excess Na assuming that Na accounts for 31 ~o of the mass of sea salt. § Calculated from chondritic meteorite composition based on excess Co and Fe. • Particulate only. II Sum of other volatile elements probably mostly volcanic.

380

w . c . CUNNINGHAMand W. H. ZOLLER

volatile elements (not including Br) which is mostly due to increases in As. Se. Zn and Sb. This increase, as is discussed below, is believed due to volcanism. If one examines only the non-sulfate aerosol mass it is dominated by crustal dust (50 °o) and sea salt (30 °o) during the summer. During the winter sea salt accounts for 94 ° o of the mass. It is interesting that the Br concentration decreases during the winter when sea salt is at its highest concentration, possibly indicating that the Br is not coming directly from the sea. When one compares the temporal variation of the meteorite component with that of VBe, which we know originates in the stratosphere, and with sulfate, we find some interesting comparisons (Fig. 9). The 7Be has a definite decrease during the winter season and the sulfate and meteorite components show similar seasonal variations. Although the decrease for these components does not appear to be as great during the winter months, many of the sulfate values are less than our detection limit. The meteorite component follows the trend of the VBe very closely except for early in the winter of 1975. This higher meteorite component may have been due to an increase in meteorite b o m b a r d m e n t in the southern hemisphere or some other variation as yet unidentified. F r o m these results we believe that there is a coupling of these three components and, therefore, that at least a portion of the sulfate and meteoritic component do come from the stratosphere. SEASONALVAmATIONS-Stratospheric Components

°, °

,'°:

Meteorite -

o° ° °° "° •

. •

0.1 2O0 IO0

o *.

.°

xx

x

'

.~ 200 Io0 2 50

t' fl

~o~

o°o

.'.-.-. oF I oF I ,9~ ? I

78

rBe

..'....-. 10~l,grsloF ?11

re

Fig. 9. Seasonal variation of 7Be, sulfate and the meteorite component, all of which appear to be linked to a stratospheric source. VOLATILE ELEMENTS AND VOLCANISM If one examines the time variations of two typical volatile elements such as As and Se, as shown in Fig. 10, it appears that they do not show strong seasonal trends. Arsenic appears to have about the same concentration each summer season, but during the winter of 1975, a large increase in the As concentration occurred and, in the second winter, it was much lower than any of the summer samples. Selenium shows rather constant values, with the only feature being a small increase early in the winter of 1975 which correlates well with the increase in the As. This increase occurs only a few weeks after the eruption of Ngauruhoe volcano in New Zealand. This same increase is also observed for other volatile elements such as Zn, Sb and In. It would appear that, during the eruption, m~_terial was injected into the lower stratosphere or upper troposphere and transported toward Antarctica. This material slowly dispersed, and as shown by the As concentrations, was removed from the atmosphere during the next year and a half, finally reaching very low levels later in 1976. Since As and the rest of the volatile elements have been shown to be enriched in volcanic plumes, it is possible that the lower levels of these elements are due to residual aerosols from other volcanic eruptions that usually occur every few years. More measurements over the next few years,

381

Chemical composition of remote area aerosols

SOUTH POLE AEROSOL TRACE

ELEMENT CONCENTRATIONS

?

I000As

(xlO) o

'!

IOO0000

"X

Se

,; I •

10-

t

*°',= d \ !

°'~ °

~J

DF 71

'1

i

I

1975

DF 78

1976

Fig. 10. Seasonal variation of As and Se in the south pole aerosol.

when we can expect to have different levels of volcanism, should help decipher the importance of volcanic emissions for maintaining the background levels of the volatile elements.

NEW

CRUSTAL

AND

BACKGROUND

AEROSOL

COMPONENTS

We have subtracted the marine and meteoritic contribution from each of the samples to calculate a new, or Antarctic crustal component from out data. The results of these calculations for the crustal elements are given in Table 5 along with our best estimate of the average background aerosol composition for the austral summer season. In this later component we have not included sea salt, but have left in the volatile elements, meteorite debris, volcanism and crustal dust. These components should be of use in comparing the compositions observed in other background studies in the results of this study. Table 5. New crustal and background aerosol components (pg/m 3)

Element

S Al K Fe Ca Mg Na Ti Mn V Th Sc Co Br I Cu Zn As Co Se Sb Cs In

Background aerosol

86,000 1370 630 720 530 610 245 115 16.7 2.16 0.30 0.23 1.26 1860 820 91 43 20.5 19 7.5 2.64 0.200 0.088

New crustal -1130 630 480 470 465 235 110 13.6 2.20 0.29 0.23 0.14

Taylor 3.6 1130 29 770 570 320 320 78 13.0 1.85 0.13 0.30 0.34

382

W.C. CUNNINGHAMand W. H. ZOLLER SIZE D I S T R I B U T I O N S OF A N T A R C T I C A E R O S O L S

Size distribution data from the high volume cascade impactor samples are of limited value in resolving the sources of the volatile elements and the meteoritic debris, but they do confirm the earlier data on aerosol sizes reported by Maenhaut et al. (1979a). Only samples from the austral summer season have been collected. In general, most of the mass of each element is on particles which are collected on the backup filter, i.e. less than 0.6/~m in diameter, the 50 ° o cutoff of the last stage. Because of relatively high blanks and random contamination of some elements by the sample handling techniques employed, for only a few elements were the data reliable. The elements which had the most appreciable amounts of material in the larger size fraction were Sc and Mn from crustal dust and Na and K from sea salt. For these elements, as much as 30-40 ~o of the mass of a given element was found on particles between 0.6 and 2.5 #m, but most of the mass was still less than 0.6 #m. Other elements usually associated with sea salt or crustal dust showed similar patterns, although the quality of the data is not as good. The elements S, As, Br, In, Se and Zn were all found to be overwhelmingly on the backup filter with only a few percent on the stages. The radionuclide 7Be also was found exlusively, with the S and other volatile elements. For these elements more than 90 g~ is usually found on the backup filter and we don't believe that bounce-offcould be responsible for the remainder, as the majority of the aerosols are sulfate-sulfuric droplets (Cunningham, 1980 and Cunningham et al., 1981). In general, more than 95 ~ of the As, S and 7Be were found on the _< 0.6-/~m size fraction. Some elements, such as Br and Se, that have vapor-phase components, have a slightly larger fraction on the larger sized particles, probably due to gasto-particle conversion processes, so that some of the larger crustal and sea salt aerosols have some of these elements on their surfaces. As 0.4-#m Nuclepore filters have a very high efficiency for aerosols down to 0.1-0.3 #m in diameter, and Whatman filters are not nearly as efficient, it is not surprising that for some of the elements found on the smaller sized particles we do see some differences for the two filter types. This difference has been discussed by Maenhaut et al. (1979a) and was also observed in this study.

TRANSPORT INTO THE ANTARCTIC INTERIOR From the results on the temporal variation in aerosol chemistry, a number of important conclusions can be drawn with regard to the transport into the Antarctic continent from the surrounding area. First, there are strong seasonal trends in all of the detected components such as crustal dust, sea salt, meteoritic debris and sulfate, but little or no change in the concentrations of most of the volatile elements. Based on the observed chemistry and meteorology of the region, a model of transport into the Antarctic can be postulated. During the Antarctic austral summer season the surface inversion over the polar ice cap is relatively weak and aerosols from the upper troposphere ( > 5 km) can easily be mixed down to the surface. Since the south polar tropopause is relatively weak, there is some transport from the lower stratosphere into the upper troposphere that probably occurs near the margins of the continent, as well as over the interior. As vertical mixing is relatively slow, most of this material is mixed thoroughly throughout the upper troposphere, yielding a uniform aerosol burden and composition over the Antarctic continent. This uniform air mass generally descends over the polar plateau and because of the cooling on the surface, flows off the high plateau, i.e. the katabatic wind. This wind flows from the higher locations in Antarctica, the polar plateau, down to the south pole and onto the edges of the continent. During the summer when this pattern is in effect, the interior is continuously flushed by a relatively clean air mass from the slowly decending upper tropospheric air mass. The composition of the summer aerosol reflects the influence of high altitude input (meteoritic debris and fission products), long range transport from continents (crustal dust) and sea salt as well as volcanism. Most of the long range transport probably takes place in the mid to

Chemical composition o f remote area aerosols

383

upper levels of the troposphere ( > 4-5 km) and the material is probably from all areas surrounding Antarctica. Mixing occurs as the anticyclonic flow mixes air masses around the continent, until a rather uniform higher altitude air mass is produced. During the Antarctic winter, the area near the surface is isolated from the upper layers of the troposphere and stratosphere as the quantities of 7Be, fission products, sulfate, meteoritic debris and crustal material are decreased drastically. The katabatic winds usually increase in iritensity, and off the coast of Antarctica, increase in storm activity generates vast quantities of marine aerosol. Most of the sea salt generated by storm activity off the coast is probably removed by precipitation processes around the coast of Antarctica. A few of the sea salt aerosols do reach a sufficiently high altitude that they can escape the wet removal processes and eventually move with the stronger upper altitude flow over the Antarctic, where some mixes down toward the surface and is found in the vicinity of the South Pole. In a few cases, as observed by drastic increases in temperature, aerosol mass and sea salt, air masses move rapidly over Antarctic from the coast driven by severe storms.

SULFATE AEROSOLS IN ANTARCTICA Sulfate, the dominant aerosol in Antarctica, probably comes from many sources. By using the chemistry of the total aerosol, and the radioactive isotopes found with the aerosols, one can make estimates of the contributions from different sources. The fraction from the ocean (primary aerosols) can easily be determined as we know the mass of sea salt. In general, it amounts to only 0.4-1.8 % during the austral summer and from 4 to 11% during the winter season. If one uses the SO4FBe ratio of from stratospheric samples collected at 12 km in the northern hemisphere (summer) along with the ~Be measurements at the south pole, it is possible to estimate the fraction of sulfate that could be of stratospheric origin. With this method, the winter and summer results are identical, predicting between 4 and 6 % of the sulfate as being of stratospheric origin. This is probably a very low estimate, but we don't have any estimate of the transport time involved. If one assumes it is about 1-2 months, then the percentages should double (because of the 53-day half life of 7Be), so as to account for approximately 10 % of the sulfate. The rest of the sulfate is probably formed by gas-toparticle conversion throughout the troposphere from sulfur gases produced by many sources as indicated by Friend (1973). In fact, it appears as if most of the sulfate is from the upper troposphere, with little or no gas-to-particle conversion occurring in the Antarctic during the summer time and minimal direct stratospheric injection. It is interesting to note that the SO4=/TBe ratio is slightly higher in the winter, but not very different than the ratio observed in clean downslope air sampled at Mauna Loa Observatory in Hawaii (Zoller 1980). This similarity strongly suggests a rather uniform upper troposphere with small (~ 10-20 %) changes due to local sources.

CONCLUSION The results of this study of the chemical composition of background aerosols in central Antarctica has led to several important conclusions. During the past 10 yr a great deal of information has been gathered with respect to the chemistry of aerosols in the region and the record of atmospheric aerosols contained in the snow column. The following points are the basic conclusion of this work: The Antarctic aerosol during the austral summer season is mostly sulfate. During the winter the aerosol found at the South Pole has much more sea salt and less crustal dust, sulfate and meteoritic debris than in summer. Transport to the interior of Antarctica is usually through the mid or upper troposphere. There are indications that the concentrations of the volatile elements are strongly influenced by volcanic activity.

384

W.C. CUNNINGHAMand W. H. ZOLLER

T h e r e is n o e v i d e n c e for a c h a n g e in the g e n e r a l c o m p o s i t i o n o f the A n t a r c t i c a e r o s o l d u e 1o anthropogenic emissions.

Acknowledgements--We wish to thank GMCC-NOAA personnel for collecting the overwinter samples at the South Pole. Stephen Kott and George Engleman collected the sampies during 1975 and Valentine Szwarc and James Jorden did the work in 1976. We also wish to thank the staffof the NBS reactor for their help with the neutron irradiations. This work was supported by the National Science Foundation, Division of Polar Programs under grants = DPP-767908964 and DPP-76-23423-A01.

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