Summertime aerosol measurements in the ross sea region of Antarctica

Atmospheric Environment Vol. 25A, No. 3/4, pp. 569-580, 1991. Printed in Great Britain.

SUMMERTIME

0004-6981/91 $3.00+0.00 Pergamon Press pie

AEROSOL MEASUREMENTS IN THE ROSS SEA REGION OF ANTARCTICA

M. J. HARVEY, G. W. FISHER, I. S. LECHNERand P. ISAAC New Zealand Meteorological Service, P.O. Box 722, Wellington, New Zealand N. E. FLOWER Physics and Engineering Laboratory, D.S.I.R., P.O. Box 31-313, Lower Hurt, New Zealand

and A. L. DICK Chemistry Division, D.S.I.R., Private Bag, Petone, New Zealand (First received 1 Auoust 1989 and in final form 21 June 1990)

Almtraet--The physical and chemical characteristics of atmospheric aerosol were determined at a site remote from anthropogenic influences, on the edge of the Antarctic continent. The number concentration (0.12-3.12/an diameter) ranged between 9 and 90cm -3 and the corresponding mass between 0.1 and 3.7/~g m- 3. The concentration of sulphate in two filter samples was 0.29 and 0.48/~g m- 3. Size distributions at the site were remarkably invariant. The two major factors affecting the size distribution and concentration were occurrence of precipitation and atmospheric stability, respectively. Modes in the volume distribution occurred at about 0.2 and 2.0/an diameter. The smallest particles <0.1 lan diameter were composed entirely of sulphur species whereas particles above about 0.5/~m diameter consisted mainly of sea-salt minerals. Similarities in size distribution and composition were observed between aerosols <0.5/zm diameter collected in this study and those sampled in the free troposphere of the Southwest Pacific. Key word index: Remote aerosol, aerosol size distribution, aerosol chemistry, Antarctica.

1. I N T R O D U C T I O N

The background aerosol interacts directly with solar and terrestrial radiation and may also affect radiation budgets by influencing the albedo of clouds (Charlson et aL, 1987). The strength of these interactions is dependent on the size and chemical composition of the aerosol. In order to understand what effect the background aerosol has on global radiation budgets, it is necessary to make 'baseline' measurements at sites remote from anthropogenic pollution. The Antarctic is a unique natural laboratory for the study of aerosols generated by natural processes: transport of particles in polluted air from southern mid-latitudes is inhibited by a vigorous circumpolar circulation and storm belt around 62-64 ° which occurs just north of the circumpolar trough (Taijaard, 1967). Sources of particles at the surface over the continent are limited and consequently concentrations are small. The source of primary aerosols, generated by mechanical means at sites away from open water, is limited to minerals present in sea-ice and snow and to small areas of exposed rock. Mount Erebus is a volcanic source in the Ross Sea region but atmospheric stability prevents the plume mixing to ground level and affecting local particle measure-

ments. A coarse mode (d > 1.0/zm) in the mass distribution has only been observed at coastal sites during strong onshore winds (Shaw, 1985). Previous studies have identified Aitken (d~0.01~m) and accumulation (d~0.1-1.0/~m) modes in the number size distributions (e.g. Bigg, 1980; Ito, 1989). These and other studies at the South Pole (Maenhaut et al., 1979; Hogan et al., 1984; Bodhaine et al., 1986) confirm the ubiquity of sulphur gas which is a precursor to the Aitken mode of secondary aerosol generated photochemically during the polar day. The accumulation mode, on the other hand, is an aged aerosol mainly consisting of sulphates, but also consisting of sea-salt minerals during intrusions of maritime air into the continent which occur with moderate frequency. There is some debate over sources of sulphur aerosol on the plateau. Cadle et al. (1968) suggested that the source of sulphate aerosol is stratospheric and later evidence from dust sondes shows transport of particles to the Antarctic in the stratosphere to be very effective (Hofmann and Rosen, 1985). However, much evidence points to in situ photochemical production of nuclei. The major source of sulphur for secondary aerosol is likely to be gases of biogenic origin emitted from open water surrounding the continent (Berresheim, 1987). Aircraft

569

570

M.J. HARVEY et al.

studies lead H o g a n (1979) to conclude that most t r a n s p o r t to the plateau occurs in the lower to midtroposphere. This study concentrates on particles in the accumulation m o d e which interact directly with visible light (Shaw, 1987). In the Antarctic, few attempts have been m a d e at in situ measurements of the size distribution of particles in the accumulation m o d e a n d a b o v e using optical particle counters (OPCs). In the Ross Sea region, upper a t m o s p h e r e work with dust sondes has been published by H o f m a n n et al. (1989). R a d k e and Lyons (1982) have m a d e a i r b o r n e m e a s u r e m e n t s near lhe surface. Their results indicate a generally 'clean' atmosphere of well-aged aerosol with excess sulphate in the a c c u m u l a t i o n m o d e a n d a super-micron maritime c o m p o n e n t which falls off rapidly with distance from open water. The potential for local c o n t a m i n a t i o n is greater for studies done from bases. This p a p e r describes a g r o u n d - b a s e d study using an optical particle counter continuously for a period of three weeks during the polar day at a site remote from base. The aim of this study was to characterize the physical a n d chemical properties of the s u m m e r t i m e aerosol a n d investigate meteorological factors t h a t govern these properties at a site on the edge of the c o n t i n e n t where b o t h maritime a n d continental influences might be expected.

2. EXPERIMENTAL 2.1. Collection site and sampling procedure

The New Zealand Antarctic Research programme is centred on Scott Base at Hut Point Peninsula on Ross Island (77°51'S, 166°45'E). Logistic limitations dictated that the experimental site had to be within easy reach by surface travel from Scott Base, yet it is desirable to be away from the concentrated human activity on Ross Island. The site chosen was near Butter Point in the New Harbour Region of the Ross Dependency (77°40'S, 164°12'E) at an elevation of 67 m (Fig. 1). A field party of four established a camp at this site on Bowers Piedmont Glacier (slope 2°) over a smooth snow surface. Measurements were made between 6 and 26 November 1988. Aerosol particles (0.12-3.12/~m diameter) were sized and counted by a 15 channel active scattering aerosol spectrometer probe ASASP-100X (PMS Inc., Boulder, Colorado, U.S.) mounted on a mast at 4.3 m height. Pre-calibration of the ASASP was done with pre-sized polystyrene latex spheres and the instrument was found to be within specification. Condensation nuclei (> 0.0025 #m diameter) were measured by CN monitor (Model Rich I00, Environment One Corp., Schenectady, New York, U.S.), through a sampling tube with an inlet at 4 m height; a correction was applied to allow for diffusional losses in the sampling tube. The CN monitor was run with a 25: 75 antifreeze mixture of ethanol and water in place of the distilled water normally used. Two methods were used to collect particles for subsequent chemical analysis. For bulk sampling, an high-volume pump (575/1, Sccomak Air Products Ltd., Stanmore, Middlesex, U.K.) was used to draw air through 10"x 8" quartz fibre filters (QMA, Whatman Inc., Clifton, New Jersey, U.S.) 2m above the surface at a 'clean' site 100 m distant from the main sampling mast. The nominal flow rate of l m 3 min- 1 was monitored by a very-low differential-pressure transducer

)

ROSSSEA|

Fig. I. Location of field site in Antarctica. (1221F1VL Rosemount Inc., Minneapolis, Minnesota, U.S.t. In order to eliminate the possibility of local contamination from petrol-engined generators, power to the pump was switched automatically under the control of the output from a wind direction sensor. For individual particle analysis by electron microscopy (e.m.), a dual impactor sampler designed by Bigg (1980) was used. This automated instrument collects particles at pre-determined intervals in two parallel impaction systems: a low-pressure impactor with a flow rate of 34 cm 3 min- 1, nozzle diameter of 100 tzm and a cut-off at 0.035 vm diameter, an inertial impactor with a flow rate of 4.6 ~ min- ~, 1 mm nozzle and a cut-off at 0.25 ~m diameter. Particles were collected on copper e.m. grids (400 mesh) prepared under clean conditions with a carbon-coated pioloform film. Meteorological variables were measured from the ASASP sampling mast. Wind speed u and direction were monitored at 5 m height by a Climet 011-2B anemometer and a Climet 012-2C wind vane, respectively (Climet, Redlands, California, U.S.). Temperature was measured by two 3 x l0 -3'' copper/ constantan thermocouples (Campbell Scientific Inc., Logan, Utah, U.S.) mounted at 4.1 and 0.5m height, Ttop and Thor, respectively, and the gradient of temperature A T/Az between the two sensors was calculated. Additional instrumentation included a surface 0 3 monitor (Dasibi Environmental Corp., Glendale, California, U.S.), a sonic anemometer (Campbell CA27T) and sensitive propeller anemometers (three-axis Gill) used to calculate the micrometeorological parameters-friction velocity u., Monin-Obukhov stability parameter and heat flux C. Net radiation R, was measured directly (SRI4, Solar Radiation Instruments, Melbourne, Victoria, Australia). Meteorological and analogue data were logged continuously and stored as 15 rain averages by a Campbell 21X microiogger. ASASP data were counted on 15 digital channels of a Taupo logger (Solid State Eqpt. Ltd., Lower Hutt, New Zealand) and stored at 5 min intervals. A.C. power was provided by two portable petrol-engined generators (Honda 1.5 kVA and 2 kVA) run continuously apart from twice daily refuelling stops and weekly routine maintenance. D.C. power was provided by sealed lead-acid batteries. The logged data were down-loaded twice daily onto a portable IBMcompatible computer and stored on floppy diskette with multiple back-ups. 2.2. Data analysis On return to New Zealand, the data were analyzed as 15 min averages. Bad data from faulty sensors were removed

Summertime aerosol measurements in Antarctica and a summary of the data was produced. The primary data set was produced by further editing which removed aerosol data when there was falling or blowing snow. The following parameters of the aerosol distribution were calculated: number concentration (cm -3) in all channels of the ASASP N(ch >I1) (Appendix 1, where ch is channel number), number concentration (0.195-3.12/an diameter) N(ch>2), number concentration of coarse particles (0.495-3.12 pan diameter) N(ch>5), the slope k of the size distribution plotted as dN/dlogd vs diameter d on log/log axes, and the total mass concentration M calculated assuming spherical particles of a uniform density of 2 g c m -a. Parameters of the aerosol distribution were partitioned by time of day and wind direction to examine the effect of these variables on the distribution. A subset of the primary data was used to plot aerosol distributions typical of a particular regime: the criterion for selection was that wind had to have been in a particular regime continuously for at least 1 h before data were selected. The distributions were compared with past results collected in the 1986-1987 field season and with results obtained in the Pacific free troposphere using the N.Z. Met. Service instrumented aircraft facility (Lechner et al., 1989). Analysis of individual particles collected on e.m. grids was done using a Philips EM400T transmission e.m. fitted with an X-ray analyzer (EDAX) coupled to a multi-channel analyzer for calculation of elemental composition (z > 10). The bulk filter samples were analyzed for soluble ions by aqueous elution using careful clean-lab techniques. Anions were detected by ion chromatography (Dionex QIC, Dionex Corp., Sunnyvale, California, U.S.A.) and cations were measured by flame atomic absorption spectrophotometry using an Hitachi Z6000 analyzer for Na + and a Perkin-Elmer 305 instrument for K +, Ca 2+, and Mg 2+ . 3. ltF.SULTS ANt) DISCUSS,ON 3.1. General description T a b l e 1 presents a s u m m a r y of the p r i m a r y d a t a set including p a r a m e t e r s of the aerosol distribution for all

571

d a t a a n d cases w h e n there was n o precipitation. The n u m b e r c o n c e n t r a t i o n of largest particles N(ch>2) a n d N(ch> 5) a n d hence total mass M increased d u r i n g precipitation. This increase in light winds was caused by detection of ice crystals by the A S A S P whereas in strong winds there was a n increase in blowing snow a n d in mechanical generation of aerosol at the surface. Figure 2 presents 15 rain averages of wind speed a n d direction as a wind-rose frequency plot. T h e m o s t frequent wind directions at the site were southeasterly, southerly a n d southwesterly. F o r these directions, w h e n the wind was < 2.5 m s-1, there was a gentle d r a i n a g e flow d o w n the slope of the Bowers P i e d m o n t Glacier, defined here as a 'light-southerly'. T h e winds >i 2.5 m s - 1 were m o s t c o m m o n l y driven by synoptic scale processes a n d are defined as 'strong-southerly'. Less frequent northerlies came over the sea-ice alt h o u g h all winds t h a t were n o t southeasterly, southerly a n d southwesterly are defined here as 'northerly', except those winds from the direction of the petrolengined generators (37-97 °) defined as 'polluted'. W i t h i n these wind classes, variables a n d p a r a m e t e r s were p a r t i t i o n e d between two times of day: low-sun, defined as between 18:00 a n d 0 6 : 0 0 and, high-sun, defined as between 0 6 : 0 0 a n d 18:00. T a b l e 2 shows the resulting frequency table. T h e d a t a are divided f a i r y evenly between light- a n d strong-southerlies at low- a n d high-sun. T a b l e 3 presents the p a r t i t i o n e d p a r a m e t e r s of the aerosol distribution as averages of the 15 rain values within each d a t a class. T h e m a i n difference between the classes is the greater n u m b e r c o n c e n t r a t i o n s N(ch>2) a n d N(ch>5) a n d mass M for low-sun

Table 1. Data summary for Butter Point, Antarctica: 6-26 November 1988

Units All data

All data

No ppt.

u u, Tb., Ttop

AT/Az

m s- 1 m s- ~ °C °C °C m - 1

C R, 03

W m -2 W m -2 ppbv

CN

N(ch>~1) N(ch > 2) N(ch > 5)

cm- 3 crn -3 cm - 3 era- 3

k M

/~g m - 3

CN

N(ch>>,l) N(ch>2) N(ch>5)

cm -3 cm -3 cm -3

cm -3

k M

?tg m - 3

* Errors at maxima and minima.

Number of observations

Mean

Std. dev.

Maximum

1.95 0.001 4.00 3.74 0.587 19.73 6.82 14.22 2

9.37 0.520 - 3.36 - 3.45 2.759 * * * 27

Minimum

1658 1621 1632 1628 1632 1637 1615 1351 1003

3.03 0.054 - 10.94 - 9.46 0.409 2.00 -0.94 7.9 19

0.20 0.00 - 24.78 - 18.97 -0.317 * * * 13

1689 1351 1403 1403 1405 1403

960 27.0 5.0 0.46 - 2.65 1.22

684 12.4 2.5 0.38 0.52 1.14

6008 91.0 19.2 4.0 - 1.35 13.64

39 9.30 1.04 0.05 - 4.05 0.08

1228 1208 1260 1260 1218 1260

997 26.4 4.3 0.32 -2.82 0.84

733 13.4 2.0 0.21 0.47 0.58

6008 91.0 13.1 1.2 - 1.84 3.66

39 9.30 1.04 0.05 -4.05 0.08

572

M.J. HARVEYet al.

Butter 6

-

Point,

Antarctica

26 Nov. 1988

PERCENTAGE FREQUENCY I [ I 0

5

I

10 15

I I 20

25

WIND SPEED 0.2-2A 2.5-4.9 5,0-7.4 7,5-10.0 M/S

Fig. 2. Wind-rose frequency diagram.

Table 2. Observed frequencytable for 15 rain values of wind regime and time of day for non-precipitating cases Wind regime

Time of day High sun Low sun

Northerly Polluted (E) Light-southerly Strong-southerly

129 34 173 183

74 20 265 211

compared to high-sun. Figure 3 shows the diurnal cycle in temperature gradient between the two thermocouples: the surface layers tended to be very stable with a large positive gradient at low-sun. During this very strong surface inversion, aerosol at the sampling height of 4.3 m was in air that was decoupled from the surface. Turbulent deposition was eliminated, allowing the aerosol concentration to build up in the near-surface stable air. Further work is needed to examine the relationship between atmospheric stability and aerosol deposition. 3.2. Distributions Number density distributions are plotted as dN/dlogd vs diameter d along with the volume distribution derived assuming particle sphericity and plotted as d V/d log d vs d, both on log/log axes. Table 4 presents the observed frequency table and mean wind speeds for the data subset used to plot the distributions. The polluted data is omitted. Figure 4 shows the distribution for each of the six data classes. All distributions look similar and lie between the range defined by the low-sun strong-southerly (LO ST

SOUTH) and high-sun strong-southerly (HI ST SOUTH) data subsets. The accumulation mode occurred at sizes smaller than those measured, i.e. <0.12/~m diameter. A coarse mode was present in the volume distributions at around 2 pm. It was surprising that all distributions were similar. The shape of the volume distributions with their coarse mode indicates a strong maritime influence in all wind directions, not just those in the direction of open water to the north. However, at times the surface wind direction was misleading and significant wind shears were observed by tethered balloon flights. On occasions, during strong surface inversions, a light southerly at the surface was accompanied by a northerly aloft. It is important to consider the special synoptic meteorology of the region in order to explain similarities in the distributions. Embayments in the Antarctic coastline and in particular the Ross Sea have a high frequency of inward spiralling cyclonic systems (Taljaard, 1967). Indeed, during our sampling period there was an unusuallyhigh frequency of precipitation at the site and approximately a dozen different depression centres mapped in the vicinity of Butter Point. Figure 5 shows the influence that snow had on the concentration of particles at the larger end of the spectrum The distribution calculated from the entire data-set (including periods of precipitation) for lowsun in strong-southerlies lies above the distributions calculated from the primary data set in non-precipitating conditions, the difference being most noticeable above 0.5 pm diameter, As pointed out by Shaw (1986) McMurdo Sound receives back-flow of air from cyclonic systems that have entered the western Ross Sea to eventually dissipate on the eastern Ross Ice Shelf. Under these conditions, air from the south at Butter Point is maritime. The intrusion of maritime air is aided by general inflow of air due to rising pressure over the continent between October and January (Schwerdtfeger, 1970). Although we had hoped to sample both maritime and continental air, it is unlikely that we sampled air solely from the plateau at any time. When compared to distributions from the 1986-1987 Antarctic data set in Fig. 6 (Lechner et al., 1989) from a number of sites in the region, it is interesting that the distributions are similar in shape and concentration to those found in non-precipitating

Table 3. Variation of aerosol distribution parameters with wind regime and time of day for non-precipitating cases Parameter Time of day Wind regime Northerly Polluted Light-southerly Strong-southerly

N(ch >~1) H

L

30.3 20.6 20.8 23.1

25.2 25.2 24.9 34.1

N(ch > 2) (cm 3) H L 4.2 3.1 3.7 3.6

4.5 3.4 4.0 5.5

N(ch > 5)

k

H

L

H

L

0.33 0.26 0.32 0.26

0.39 0.27 0.34 0.38

- 2.8 - 2.9 -2.7 - 3.0

2.6 2.9 -2.9 - 2.8

M (#g m- 3) H L 0.90 0.60 0.71 0.68

0.95 0.67 0.85 1.07

Summertime aerosol measurements in Antarctica "

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Time/hr

Fig. 3. Variation in temperature gradient between 4.1 and 0.5 m with time of day, plotted as average of 15 min values + 1 std. deviation.

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,

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101

Diameter/Fro

Fig. 5. Comparison of number and volume distributions obtained when there was no precipitation compared to the whole low sun strong southerly data-set including periods of precipitation.

574

M. J. HARVEY et al.

Table 4. Mean wind speeds (m s ~) and number of observations for data set used to plot size distributions Time of day

Wind regime Light-south

Northerly Low sun

Mean

1.86 32 2.76 75

No. obs.

High sun

Mean No. obs.

conditions in the present study. Figure 6 shows low sun strong-southerly and high sun strong-southerly as Butter Pt. (1) and Butter Pt. (2), respectively. When compared with example distributions from earlier work in the free troposphere over New Zealand and the S.W. Pacific in Fig. 7 (Lechner et al., 1989) there is a clear difference. Free troposphere number distributions are found to describe a straight line of gradient - 3.5 when plotted on the axes shown. The gradient in the present study is similar for particles <0.5/~m diameter although the concentration is greater than in the free troposphere. However, there is significantly greater proportion of particles > 0.5 #m in the distributions of the present study, compared with the free

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1.48 156 1.52 80

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4.00 155 4.57 129

troposphere measurements. Measurements in the present study were close to the surface source of mechanically generated particles. Figure 8 shows comparisons with data published by Gras and Ayres (1983) at southern mid-latitudes and other workers in the polar region: Bigg (1980) collected particles at the Pole by impaction; Radke and Lyons (1982) used an airborne OPC in the Ross Sea region and lto (1985) used an OPC at Syowa station. In all studies, the particle concentrations at 0.2/~m are of the same order of magnitude. There is greater variability in the coarse fraction, i.e. at 2.0/~m. Of the background distributions plotted, the largest concentrations were found in marine air at mid-latitudes

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Fig. 6. Comparison of number and volume distributions in the present study with examples from previous Antarctic work (1986-1987 field season).

10 `3 1

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100

101

Diameter/~m

Fig. 7. Comparison of number and volume distributions in the present study with examples from measurements made in the free troposphere of the Southwest Pacific.

Summertime aerosol measurements in Antarctica 10 3

10 3 q~ 101

r.,

RADKE (82A)

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575

NO3. Problems were encountered because of the large but variable blank concentration of Na + and PO~- in the filter, and pre-treatment of these filters will be needed for future work of this type. Amounts of other ions in the blank filters were negligible. It is unlikely that there was any significant amount of PO 3- in the sample and values are not presented. The ionic concentrations detected were of the same order of magnitude as those found in the Pacific free troposphere. The concentrations of sulphate were similar to those reported by Bigg (1980) at the pole (0.14 pg m - 3 in the size range 0.06-2 pm dia.) and Radke and Lyons (1982) 90 m above the ice shelf (0.24 pgm-3). Table 6 shows the enrichment factors for these ions calculated as ( [X]/[C1- ] )filter/( I ' X ] / [ C I - ] ) , , where IX] is the concentration of the ion in question. Chloride was used in place of sodium because of the large uncertainty in the Na + blank values. Enrichment values showed there was a significant amount of non-marine sulphate in both the Antarctic [in agreement with other workers, e.g. Cadle et al., (1968), Wagenbach et al., (1988)] and in Pacific free

10"2 10 .3 10

1 0 .2

Table 5. Concentration of soluble major ions in atmospheric aerosol determined by high-volume filtration 1 0 "1

10 0

101

Ion

Diameter lpm

Fig. 8. Comparison of number and volume distributions in the present study with previous work referenced by principal author I-Bigg, 1980; Radke and Lyons, 1982; Ito, 1985; Gras and Ayres, 1983].

shown as 'Gras (83)'. The coarse fraction falls off rapidly as sampling location moves away from the coast. 'Radke (82A)' was measured over the Ross Sea when the wind speed in the boundary layer was < 7 m s- 1 and 'Radke (82B)' over the Ross lee Shelf within the boundary layer. There is a good agreement between the distributions measured by Radke and Lyons (1982) and those of the present study. The coarse particle concentrations recorded by 'Ito (85)' at Syowa are lower than at Butter Point: there is a greater frequency of winds from the ice cap at Syowa. The measurements of Bigg (1980) do not extend to large enough sizes to comment on the coarse fraction although concentrations at the South Pole are likely to be below those recorded at other sites.

Na + K+ Mg2÷ Ca 2+ CIBrNO~SO,z-

Concentration (#g m- 3) Antarctic Pacific free surface troposphere Julian day in 1988 311-322 322-330 69-78 0.089 0.012 0.017 0.012 0.258 0.001 0.145 0.290

0.256 0.19 0.037 0.041 0.322 0.002 0.190 0.483

0.20 0.10 --0.14 --0.57

, Large blank concentration on filter. - - Not determined.

Table 6. Enrichment factor of soluble major ions in atmo[X] [X] spheric aerosol determined as ( - ~ /(--~ \ [CI-] / f i l t e r / \ [ ' C I - ] / s e a Ion

3.3. Chemical composition

Na~

The bulk ionic composition of two consecutive high-volume filter samples is shown in Table 5, and compared with the ionic composition of aerosol sampied during an aircraft flight in the Pacific free troposphere in March 1988. The major anion species in the Antarctic samples were, in order, SO~-, CI- and

K÷ Mg2+ Ca 2+ SO2_

Enrichment factor Antarctic Pacific free surface troposphere Julian day in 1988 311-322 322-330 69-78 0.6 2.3

1.4 2.9

2.5 35

1.0

1.7

--

2.2 7.8

5.9 10

28

, Large blank concentration on filter. - - Not determined.

576

M.J. HARVEYet

troposphere samples. The source of this non-sea-salt sulphate was investigated further by individual particle analysis. Figure 9 shows the results of the elemental analysis done by e.m. for three size classes: <0.15#m, 0.15-1.0/tm and >l.0/~m diameter. The smallest class was difficult to analyze and although particles were numerous (Plate 1), they were extremely volatile in the e.m. When successful, the analysis gave S as the only element present indicating a likely composition of H2SO 4 or (NH4)2SO 4. Comparison between the middle and largest size class shows there to be a greater proportion of Na and CI and less S in the larger particles, indicating that the largest particlesare primarily sea salts and predominantly NaCI. Sea salts also extend down in size into the accumulation mode. Many of these larger particles were complex in shape with a combined cube and rod (Plate 2). The cube in Plate 2 contained equal proportions of Na and CI (48%:47%) although the rod contained 71%Mg, I 1% Na and 15% C1. Other particles had Na to S in the ratio of 2:1. The various salt mixtures may be explained by a difference between the composition of

al,

sea-ice and sea-water. On progressive freezing of the sea, different salts crystallize at different rates: calcium carbonate and sodium sulphate are the salts which crystallize first. Wellman and Wilson (1963) recorded this phenomenon with observations of extensive salt deposits on the sea-ice in McMurdo Sound. The bulk composition of particles entrained from the sea-ice was altered by fractionation on freezing. Sea-ice and open water were to the north and northeast of the sampling site, but the predominant surface wind was southeasterly. It is likely that the sea-salt minerals collected at Butter Point arrived at the site in southerly flows that had previously crossed McMurdo Sound. Marine particles in these samples had been brought in to the Ross Sea in cyclonic systems travelling in a southeasterly direction. It is likely also that high-level northerly flows that were occasionally observed at the site by tethered balloon flights brought marine air inland. Further work is needed to establish the predominant air-mass backtrajectories from the site. In addition to sea-salt minerals, a few gypsum particles, alone or attached to sea-salt (Plate 3) were collected. A likely source is the

Individual particle analysis (TEM/EDAX)

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(126)

SIZE

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Fig. 9. Elemental composition of particles (mole %) for elements z > 10 for three size classes (<0.15 tzm diameter, 0.15-1.0/~m diameter and > 1.0 #m diameter). Number of particles in each class is given at bottom of figure.

Summertime aerosol measurements in Antarctica

577

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G Plate 1. Sub-micron sized sulphur particles collected by low-pressure impaction (scale = 1 #m).

gypsum deposits to the south of Butter Point recorded by Lyon (1978).

4. CONCLUSIONS

Data collected during this field study provided a valuable insight into the physical and chemical properties of Antarctic aerosol. Two seasons of measurement at a number of sites in the region showed the

aerosol to be maritime influenced and remarkably invariant. Over the experimental period, the most important factor found to influence the size distribution (0.12-3.12 #m diameter) was the occurrence of precipitation. The concentration of aerosol was found to be influenced by atmospheric stability: concentrations were greater in low-sun when the surface air was extremely stable. There is good evidence that the atmospheric aerosol below about 0.5 #m diameter in different remote regions has similar properties, which corresponds to the

578

M.J. HARVEYet al.

!i i!~ii ~/~i~;iil i!/~ ¸ +i

Plate 2. Complex sea-salt particle (scale-= 1 ~tm).

suggestion by Miiller (1986) that there are also invariant properties in the atmospheric aerosol in polluted regions. Although concentrations at the smaller end of the spectrum were up to an order of magnitude greater in the present study compared with measurements in the temperate and tropical free troposphere of the Southwest Pacific, the slope of the distribution for particles <0.5/~m diameter was similar and

around -3.5. In both environments, this fine aerosol consists mainly of sulphur species, likely to have come from the common source of photochemical transformation of reduced sulphur gases emitted from marine plankton (Charlson et al., 1987). For sizes greater than about 0.5 #m diameter in this study at the edge of the Antarctic continent, the aerosol was found to consist mainly of sea salts.

Summertime aerosol measurements in Antarctica

579

Plate 3. Combined sea-salt and gypsum particle (scale = 1/~m).

Acknowledoements--This work was done as part of the N.Z. Antarctic Research Programme and the Climate Change Programme of the N.Z. Meteorological Service. It was made possible by the high level of logistic and field support provided by Antarctic Division, Dept. of Scientific and Industrial Research. We thank Graeme Lyon of D.S.I.R., colleagues at N.Z. Met. Service and the anonymous referees for their helpful comments during the preparation of this manuscript. REFERENCES Berresheim H. (1987) Biogenic sulfur emissions from the Subantarctic and Antarctic Oceans. J. geophys. Res. 92, D I I , 13,245-13,262.

Bigg E. K. (1980) Comparison of aerosol at four baseline monitoring stations. J. appl. Met. 19, 521-533. Bodhaine B. A., Deluisi J. J., Harris J. M., Houmere P. and Bauman S. (1986) Aerosol measurements at the South Pole. Tellus 38B, 223-235. Cadle R. D., Fisher W. H., Frank E. R. and Lodge J. P., Jr. (1968) Particles in the Antarctic atmosphere. J. atmos. Sci. 25, 100-103. Charlson R. J., Lovelock J. E., Andreae M. O. and Warren S. G. (1987) Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, Lond. 326, 655-661. Gras J. L. and Ayres G. P. (1983) Marine aerosol at southern mid-latitudes. J. oeophys. Res. 88, 10,661-10,666. Hofmann D. J. and Rosen J. M. (1985) Antarctic observations of stratospheric aerosol and high altitude condensation

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nuclei following the El Chichon eruption. Geophys. Res. Lett. 12, 13-16. Hofmann D. J., Rosen J. M., Harder J. W. and Hereford J. V. (1989) Balloon-borne measurements of aerosol, condensation nuclei, and cloud particles in the stratosphere at McMurdo Station, Antarctica, during the spring of 1987. J. geophys. Res. 94, Dg, 11,253-11,269. Hogan A. W. (1979) Meteorological transport of particulate material to the South Polar plateau. J. appl. Met. 18, 741-749. Hogan A., Samson J., Kebschull K., Townsend R., Barnard S., Murphey B. and Hare T. (1984) On the interaction of aerosol with meteorology. J. Rech. atmos. 1, 41-67. lto T. (1985) Study of background aerosols in the Antarctic troposphere. J. atmos. Chem. 3, 69-91. lto T. (1989) Antarctic submicron aerosols and long-range transport of pollutants. Ambio 18, 34-41. Lechner I. S., Fisher G. W., Larsen H. R., Harvey M. J. and Knobben R. A. (1989) Aerosol size distributions in the Southwest Pacific. J. geophys. Res. 94, D2, 14,893-14,903. Lyon G. L. (1978) The stable isotope geochemistry of gypsum, Miers Valley, Antarctica. In Stable Isotopes in the Earth Sciences (edited by Robinson B. W.). DSIR Bulletin 220, 97-103. Maenhaut W., Zoller W. H., Duce R. A. and Hoffman G. L. (1979) Concentration and size distribution of particulate trace elements in the South Polar atmosphere. J. geophys. Res. 84, C ~ 2421£2431. Miiller J. (1986)lnvariant properties of the atmospheric aerosol. J. aerosol Sci. 17, 277-282. Radke L. F. and Lyons J. H. (1982) Airborne measurements of particles in Antarctica, In 2nd Symposium on Composition of the Non-urban Troposphere, pp: 159-163. Am. Met. Soc., Massachusetts, U.S.A. Schwerdtfeger W. (1970) The climate of the Antarctic. In Climates of the Polar Regions, ch. 4 (edited by Orvig S.). Elsevier, Amsterdam. Shaw G. E. (1979) Considerations on the origin and properties of the Antarctic aerosol. Rev. geophys, space Phys. 17, 1983-1998. Shaw G. E. (1985) The nature and size of microscopic airborne particles at McMurdo Station. Ant. J. U.S.-1985 Review 19, 204.

Shaw G. E. (1986) On physical properties of aerosol at Ross Island, Antarctica. J. aerosol ScL 17, 937-945. Shaw G. E. (1987) Aerosols as climate regulators: a climate-biosphere linkage? Atmospheric Environment 21, 985-986. Taljaard J. J. (1967) Development, distribution and movement of cyclones and anticyclones in the southern hemisphere during the IGY. J. appl. Met. 6, 973-987. Wagenbach D., G6rlach U., Moser K. and Mfinnich K. O. (1988) Coastal Antarctic aerosol: the seasonal pattern of its chemical composition and radionuclide content. Tellus 40B, 426-436. Weiiman H. W. and Wilson A. T. (1963) Salts on the sea ice in McMurdo Sound, Antarctica, Nature, Lond. 200, 462-463.

APPENDIX

ASASP channel divisions (particle sizing between 0.12 and 3.12 ~m diameter). Channel no.

Upper limit (#m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0.145 0.195 0.270 0.370 0.495 0.645 0.820 1.020 1.245 1.495 1.770 2.070 2.395 2.745 3.120