ABSTRACT Condensation Nucleus (CN) Concentrations Have Been Measured at Mawson (67.6O5, 62.9"8) Since Ml-D 1981

Journal of AtmosphericChemistry 3 (1985), 93-106. 076'7-7 7 64185.15. 93 @ 1985 by D. Reidel Publishing Company.

CoNCENTRATTON AND SrZE VARTATTON OF CONDENSATfON NUCLET AT MAWSON, ANTARCTICA

J.L. Gras and A. Adriaansen CSIRO Division of Atmospherj-c Researcht Private Bag # L, Mordialloc, Victoria, Australia' R. Butler, B. Jarvis, P. Magill and B. Lingen Department of Science, Antarctic Division

ABSTRACT Condensation nucleus (CN) concentrations have been measured at Mawson (67.6o5, 62.9"8) since ml-d 1981. Week1y median have an annual cycle with a maximum of around 300 tg concentqations - 400 cm' in summer and a minimum of a few tens of particles per cm in winter. rn this respect Mawson behaves very much like an Antarctic continental l-ocation. Preliminary measurements of the size distribution of CN particles taken over a nine month period suggest a seasonal change in typical particle radius from around 0.Ol- Um in wj-nter to around 0.04 Um in summer. Diurnal variation in the cN concentration is generally very weak and does not show any systematic relation to the pronounced diurnal variation in wind-speed at Mawson.

ICEYWORDS Antarctic, condensation nucleus. aerosol.

]- INTRODUETION Few data sets covering sufficientiy long periods to satisfactorily determine the seasonal variation of condensation nucleus (cN) concentration are available for the Antarctic continent. Even fewer give information on the size distribution. Such data however hold a promise of helping to understand the compLex process of natural particle production and transPort over the vast area of Antarctica and the surrounding Southern Ocean. An excellent data set is available for the geographic South Pole from the NOAA GMCCprogra:n from t974 to the present. Much of the presi:nt knowi.edge on the Antarctic t continental aerosol has been derived from work associated with this progran for example by Bodhaine (1983). Isolated series I as summarized of measurements have been reported for two coastal locations, for I Mirny 1965 by Voskresenski (l-968) and Syowa 1977-L978 by'Ono et i aL. (1981). Coastal observations are possibly more difficuLt to interpret than those obtained on the high polar plateau' due to a greater intermixing of the effects of the strong continental drainage (sloped inversion and katabatic) flows and the baroclinic disturbances generated over the Southern Ocean areas surrounding the Antarctic continent. Such coastal observations are however of considerable importance as they represent conditions some 30o of latitude 94 J L. GRAS AND A, ADRIAANSEN

equator.wards of the pole and, because they are coastal, may be better indicators of conditions over the vast southern oceanic areas than observations made on the high Polar plateau.

In this paper we present a number of observations from the first of a new series of CN measurements made at Mawson, on the Antarctic coast, conmrencing in 1-981 and comprising approximately three annual cycles in CN concentration and eight months of preliminary data related to size distribution.

EXPERIMENTAL CONDITIONS

CN concentrations have been measured at Mawson (67.605, 62.9oE) since mid l-981 using a replica 1957 photoelectric nucLeus counter (2.5 cm diam. with convergent illumination) described. by Pollak and Metnieks (1960), this is subsequenlly referred to sinrply as a "Pollak counter". The Mawson Pollak counter is an automated version which records extinction ratio once every 90 minutes. fn its present form the Mavrson Pollak counter can detect better than 1 particle per cubic centimeter. Supplenentary meteorological data are recorded by the Australian Bureau of Meteorology.

For this study the Pollak counter was installed within the general station area such that for winds from the most frequent directions the air sampled is free from loca1 contamination. Figure 1 gives the sampling location and the wind frequency as a function of wind direction.

0 100m EAST HORSE5HOE BAY HARBOUR

Fig. 1 Location of sampling site (arrowed) and major buildings at tlawson 1981-1983. The wind-rose, frequency of occurrence of wind direction for all wind speeds by 10o steps? is centredl on the sampling site. fhe maximum frequency of 27* is in the 115o-125" sector. Contour units are metre. CONDENSATION NUCLEI AT MAWSON, ANTARCTICA 95

WLnds at Mawson have a remarkable clirectional stability as noted for example by Streten (1968) i fot the three hourly surface wind observations from June 1981 to May 1983, 868 were in the quadrant 75"-L65" and 77* qtere between 1050 anal 1550. The meclian wind spegd was 10 r"-' (19 kts) with only 128 of observations less than 2.5 ns-'. For speeds greater than 2.5 ms-' 848 of observed wind directions fel-I between 105" and 155o(that is, from the direction of the Antarctic Plateau). These latter conditions were sel-ected as data editing criteria for deliving weekly medians with data obtained for speeds * less than 2.5 ms or outside the 1050-1550 sector exclucled.

3 CALIBRATION

In keeping with general practice, the prirnary calibration for our 1957 Potlak counters is based on the calibration of Metnieks and PoLlak (1959). Before shipping the llawson Poll-ak counter to Antarctica (in Novenlcer 1980) it was compared with a "standard" counter, CSIRO 3, maintained in Sydlney soleJ-y for standardisation purposes- ft wAs again compared yrith cslRo 3 in February 1983 by means of a portable counter. Details of these comparisons which indicated a systernatic difference fron CSIRO 3 but only minor shift in calibration over the tntervening period are given in Gras (1984). Concentrations given later have all been converted to equivalent concentrations on CSIRO 3 using the 1980 comParison data.

C o o

c g c o

1965

I E c o ,o c o c o MAWSON U I"ASONOJFMAMJ JA SON D JF AMJJAON JFMAM 1981 1982 1983 1981.

Fig. 2 Lower surve: the variation of condensation nucleus concentration at Mawson 1981-1984. Individual points are weekly medians edited for continental trajectory onJ-y and the continuous Line is a five point nean of these.UpPer curves: the variation of condensation nucleus concentration at l,tirny L965 f'ron Voskresenski (L968) ancl Syowa t977-L97a fron Ono et aI. (1981), both given as nonthly means. 96 J.L. GRAS AND A. ADRIAANSEN

ANNUAL VARIATION IN CN CONCENTRATION

CN concentrations have been measured at Mawson for approxinateJ-y three annual cycles. Weekly median values of measurements made at 90 ninute intervals and a running five point mean are shown in Fignrre 2. A two year series of CN concentrations at Syovra (69.005, 39.608) reported by Ono et al. (1981) at Mirny, (66.5os, 93.OoE) both given as monthJ-y means, are included for comparison. These represent the full extent of earlier CN measurement series at Antarctic coastal locations. In Figure 3 the geonretric mean of the three cycles of Mavrson data is atso plotted in Linear form.

,E o

o o c o cI o o

MAM

Fig. 3 Annual variation in condensation nucleus concentration at l'lawson for the period ;Iune 1981 to May 1984, edited to include only data obtainedrwith wlnd direction 1150-1550 and -. speed greater than 2.5ms Values plotted are goemetric averages of the vreekly medians. CONDENSATIONNUCLEI AT MAWSON,ANTARCTICA 97

AJ-though the Mawson data have been edited to the extent that essentially only occasions where air arriving at the station that has an immediately Antarctic "continental" trajectory are included, there is virtually no difference in the fo:m of variation of the median when all the data are included. The form of the annual variation at Mawson is clearly dominated by the concentration of particles in the cold boundary layer of air which almost continually flows down from the central plateau region (Mather, L969). Examination of Figures 2 and 3 shows that the concentration from.october to March remains relatively - constant in the range 300-400 cm and then falls very rapid{y during April-May to typical leve1s of sone tens of particles per cm-. There is some indication that concentrations may rise slightly over winter although the most rapid rise, back to summer concentrations, occurs in September with a time constant very similar to the autrunn decline. There are many points in common in the annual variation at Mawson and at South PoIe as discussed in detail by Bigg et aI. (1984). The close similarity of the form of this annual variation in particle concentrations to the annual- variation in solar radiation at a nunber of remote southern hemisphere sites and the aslmmetry of the particle variation around the winter solstice as shown by Bigg et al. clearly indicate that the forrn of the annual variation in condensation nuclei over the Antarctic continent is dominated by the availability of solar radiation during the autumn-winter-spring period and that the source of these nuclei is photochemical. (A conclusion also reached by Ito et al. (1982) from the relationship between particle concentrations and solar zenith angJ-e.) This line of reasoning indicates a mean lifetine of twenty to thirty days for Antarctic aerosol, a value similar to that deduced by Shaw (1982) from the variation of optical depth across the continent. The slight increase in particle concentrations during July-August at Mawson and at Syowa Figures 2 and 3, which is less evident at South PoIe (Parungo et a1., 1981; Hogan et al.,1982), does not appear to be photochenical in origin but is more likely to be associated with the penetration of maritime air fron off-shore cyclonic disturbances. Ono et al. (198L) observed increases in large particles at Syowa in ilune and ,fuly (although the largest j-ncreases were in September-october) .

At South Pole Parungo et al. (1981) observed a maximum fraction of sodirur and chloride containing particles in July and Cunninghan and zoLLer (1981) observed a factor of about eight to ten times more sodiun and chloride in winter than srumer. In winter oqcasional intrusions of rel-atively warm moist air at Mawson and Syowa associated with offshore cyclonic disturbances are accourpanie6 by_a sustained. periods of high particle concentration (to around 500 cur ") Iasting from several hours to over a day. SnaLler increases are also observed at Mawson in winter associated with a slight diurnal variation in particle concentratl-on.

5. VARTATTONrN SrZE DrSTRrBrrrrON, WTNTER-SUI,IUER1982-1983

A nanually operated 8 stage screen diffusion battery siuilar in design 98 J.L. GRAS AND A. ADRIAANSEN

to that described by Sinclair and Hoopes (1975) was used in conjunction with the station Pollak counter to obtain size distribution information over the period June 1982 to February 1983. Penptrations through stages Or2r4 and 6 of the battery in number per - cm are plotted in Figure 4 only for samPles in which the direct readings (penetration through stage 0) are similar to the relevant weekly median concentration.

Medion0 0 f E o 100 c 9 c' L c o u C o O

J J A SON D JF 1982 1983 Dqte

Fig. 4 Diffusion battery transmission values obtained in the period June 1982 to February 1983 for stageq 0 (no screens) , 2,4 and 6 given as concentration per cm-. The weekly nedian values for the same period were used to deter:mine whether conditions at the times of measurenent were representative. The isolated points plotted for 16 .Tuly were obtained during a period of sustained increase in Particle concentration - similar to the 'events' of Ito and fwai (1981).

Data obtained during the period when concentrations are _rlinter-) exceedingly low (10 to 20 q are obviously exceedingly difficult to eLze by this (or any) nethod. However there is an indication that the typical sLze is very small- as shown by the several occasions when virtually alL particles were lost on passing the first or second stage of the diffusion battery (5Og transmission for r=0.075 Un and r=0.019 gm respectively). From late winter in L982 total particle concentrations rose relatively steadily until about the end of spring when a marked decrease occurred, see Figrure 2. This is similar to the sutrmer variation of CN at South Pole but differs from the previous and following Eurmers (1981-1982 and 1983-1984) at Hawson when more constant levels were maintained over suuner. The concentration of particles penetrating up to stage 2 of the diffusion battery (50t transmission at r=0.019 um) were sinilar to the total concentration. For J-arger particles, for exanple those penetrating stage 6 (tnax. CONDENSATIONNUCLEI AT MAWSON,ANTARCTICA 99

transmi.ssi.on = 42* at r=0.30 Um), the variation in concentration bears tittle relati-on to the variation of total particle concentration, exhibiting two broacl increases, one in late winter and the other over surmler. These penetration variations appear to suggest a conplicated seasonal variation in particle size distributions. It is consistent with a large particle increase in late winter followed in spring by an increase in both concentration and relative size. Initial-ly in spring the concentration increase occurs ln smaller size ranges but later in large sizes (penetrating up to stage 6).

A rigorous analysis of the diffusion battery data using a non-linear iterative procedure developed by Twomey Q'975) and further discussed by Gras (1-983) to j-nvert the observed penetrations produced somewhat disappointing results. Residuals (fractional differences between observed and computed transmission values) were generally quite large (typicalLy 20+ to 3ot) indicating noisy transmission data and reflecting the low total particle concentrations, although several consistent and important features \itere observed in the derived size distributions.

Distributions with a single mode were approxiurateJ-y log-normal and were observed slightly more often than bimodal distributions. Typical examples of the inverted distributions are given in Figure 5. Geometric mean radii determined by fitting log-norrnal distributions Lo the inverted distributions are plotted in Figure 5. Bimodal distributions are distinguished \,vith a cross for the larger size node. In the cases urarkecl "C' the distributions were obtained either cluring or just foLlowing periods of calm or light wind with a northerly direction. In these conditions it is difficult to be certain of the origin of the air sampled. Bimotlal size distributions obtained on .Tuly 16 and 18, marked "En in Figure 6 (the distribution for .Tuly 18 is also shown in Figure 5b) correspond with a period of enhanced particle concentration sinilar in nature to the 'eventsn reported by ito and Iwai (1981). Following the approach of Hogan and Barnard (1-978) these authors tentatively classified events as due to either subsidence of dry air associated with upper frontal passage during sunner or advection of warm noist air from lower latitudes with Low leve1 frontal activity. Examination of the meteorological data for the period around the particle enhancement showed a concomitant increase in moisture (although little change in temperature) suggesting the advection of oceanic air. Thus it ls probable that in these cases the J.arger slze mode is sea-sa1t. rn the case depicted in Figure 5b there is a strong siurilarity to the salt mod.e derived using inpaction techniques in Southern Ocean air and reported in Gras and Ayers (1983) (Figure 5). rt should be noted however that when the air arrivedl at ltla\rson its trajectory was south-east or wcontinentaln. Sinilar particle concentration enhancements have been observed on several occasiong during each winter at Mawson accompanied by increases in either relative hunidity or temPerature (or both). Interpretation of trends in particle sLze fron Figure 5 is difficult due to the snall number of distributions reuaining after editing and 100 J,L. GRAS AND A, ADRIAANSEN

o o) -9 E z E

Logrodrus (,um)

Fig. 5 Four representative size distributions obtained by inversion of the diffusion battery transnission values. Tsto are winter distributions a) 3 June 1982 and b) 18 July 1982. Distribution b was obtained during a period with moist; particLe rich (maritime ?) air, similar to 'events" described by Ito andl Iwai (198L). Distributions c) and d) were obtained on 10 November, 1982 and L6 February, 1983 in late spring and summer respectively.

? 3 o f o o c o o E ro- .o

q) E o o (D

s 1982 r983 Dotc Fig. 6 Geometric mean radii deternined by fitting log-no:mal- distributions to size distributions derived from the diffusion battery transmission values. For bimodal distributions the larger node is indicatecl with an x. Distributions marked with an E were made during or just following enhanced particle concentration events and those marked C were obtained during or just following calm or light wind conditions fron the N or NNw. I CONDENSATIONNUCLEI AT MAWSON,ANTARCTICA 101

the mix of birnodal and single mode distributions. The bimodal distributions generalJ.y indicate a fine-particle mode with a mode radius typically less than 0.01 pm and with no obvious seasonal trend (the geometric mean for the distributions plotted in Figure 6 is 0.06 Um). Considering the remaining single-mode distributions and the observation that in winter on several occasions inversions were not possible (due to the capture of all particles in the first stages of the diffusion battery) there aPpears to be evidence for a seasonal change frora quite small typical size rS0.01 gm in vtinter to a value of around 0.03 to 0.04 urn in summer in Antarctic continental air. It is clear however that more observations are necessary both to verify this conclusion and to firmly establish the nature of the air-masses giving rise to the different distribution types. within the limitation of the sizing techniques the typical val-ue of the mode radius at Mawson in summer, 0.03 to 0.04 Um, agrees quite well with the typical size of about 0.025 pm observed for the fine particle sulfate mode in maritime air in southern rnid-latitudes by Gras and Ayers (L983). A very smal-l typical size of the winter aerosol and its persistence over the sunless part of the winter can be understood in view of the low concentrations and the consequent weak thermal coagnrlation. Bigg et al. (1984) have argued that the tinre constant for the decay of aerosol concentrations over the Antarctic continent is 20 to 30 days which implies a mid-tropospheric source or reservoir. Assuming for exantple that the qource of particles is at 500 mb and the number concentration is 50 cm-" (numerically similar to the Mawson (sea leveI) winter value but significantly greater than South Pole winter concentrations) then for particles tl4gicalJ.y 0.01 pm radius, coagulation only accounts for a number decrease of around 10t in two weeks. Growth to nuch larger sizes even with a 20 to 30 day lifetiure will clearly be inhibited while concentrations are this tow. An increase in typical size from winter to sumner is consistent with a photochemical source of the fine particle fraction and a shift from mal-nly new particle nucleation in spring to heterogeneous growth during sunmer as suggested by Bigg et aI. (1984). This is possibly most clearly indicated in the penetration d.ata (Figure 4) where the initial springtime increase in particle concentrations occurs for particles removed before stage 4 of the diffusion battery (50t transmission at 0.07 Um) and the increase in larger particles as shown by the nurnber penetrating stage 6 of the battery only occurs once the total concentration has reachecl its tlpical srumer value and is levelling off .

6 I,IETEOROLOGICALINFLUENCES ON PARTICLE CONCENTRATION

Bigg et al-. (1984) demonstrated the importance of photochemical particle production at several remote southern locations by examining the annual concentration cycles. llhese were obtained by suPressing all short-tenn variations in the particle record. Possible local influences such as inversion depth, wind. speed or local meteorology were not investigated. some of these influences, of relevance at Mawson, will now be eonsidered. t02 J.L, GRAS AND A, ADRIAANSEN

The tropospheric transport of vast quantities of lower latitude wa::ur moist air above about 600 mb into the central regions of Antarctica by a mean meridional flow augrmented by transient eddies is summarised by Schwerdtfeger (l-970). This advected air undergoes heat loss by radiational cooling and through subsiclence balances the cold surface drainage f1ow. Hogan et aI. (1982) have shown that aerosol particles observed at South Pole are transported similarly to both heat and moisture with the most particle rich layer coincident with the warm moist layer just above the surface inversion.

The surface itself is esse.ntially source-free thus the concentration of particles reaching the coast in the outflowing surface air should depend on the transport tirne from the area where particles are mixed into the surface layer and subsequent loss processes. Possible infl-uences on concentration include inversion depth, wind speed, diffusion to drifting snow or the surface, and entrainment of air from the overlying region. Wind speed is an important factor or indicator in virtually aJ.I of these processes.

The surface inversion, sloped with the underlying terrain rising up to the polar plateau is one of the main driving forces behind the characteristic winds of the Mawson area. Various theoretical relations between the equilibrium flow strength and inversion strength have been proposed, although as shown by Gosink (1982) equilibriun may not necessarily be attained. Observations of wind strength at Mawson by Streten (L962) showed that over the period October to March (in the absence of active coastal depressions) the strength of the surface wind shows a marked diurnal variation. The extent of the effects of wind related processes on particle concentration therefore night be gauged by the extent that particle concentrations undergo siniLar diurnal variation.

Voskresenski (1968) repoltFd a diurnal variation in CN concentration - at Mirny of up to 200 cm present only during summer. At Mawson we have found that diurnal variations are generalJ.y weak although there are some extended periods where such variations are identifiable in the daily CN data and these periods appear to be more typical in or near winter.

To examine systematic diurnal variations the data was divided into four week bLocks. tledian concentrations were derived for each of the sixteen sample times (per day) and an overall mean concentration of the four week bl-ock calculated. The percentage variation of the sixteen medians from the overaLl mean for the four week block were calculated. lhese were then smoothed by averaging each point with its neighbouring points (in tine and date) and the smoothed dlata used to derive a contour plot (Figure 7), A sinilar procedure using three-hourly observations yras followed for wind-strength giving Figure 8. CONDENSATIONNUCLEI AT MAWSON,ANTARCTICA 103

z 1'5 7.5 l0'3 13.5 15'5 19'5 n$ 15 /.'5tocol Timelhours)

FLg. 7 Contours of condensation nucleus concentration percentage variation from the four weekly median at each sampling time. contour intervals are 5t anil positive variation shacled. Little systematic variation is evident although there is possibty some evidence of a tendancy to a diurnal variation in the winter months.

when applied to the wind data this procedure clearLy shows that a systeuratic diurnal wind speed variation, with early morni-ng maximum and late afternoon minimum, reported by Streten (1962) for the suruner months is present over the whole year apart from the sunless winter months. During the winter however there is apparentl-y still a iteak diurnaL variation somewhat shifted in phase. In comparison with the wind-strength variation the CN concentrations show little consistent variation in relation to time of day or season and hence wind strength. There is some possibility that more pronounced and organised diurnal variations occur during or near winter aLthough clearly the "noise level" dictates a much longer data period to estabLish thls trend with certainty. It is therefore evident that the influence of the strength of the katabatic wind on particle concentrations via mechanisms such as particle losses to blowing Snow is very weak (in an average sense). Inclirectly it is also aPparent that the diurnal variation in inversLon strength and depth has very littLe significant effect on particle concentrations. On the other 104 J.L. GRAS AND A. ADRIAANSEN

hand a source for a winter diurnal variation in particle concentrations is not obvious in view of the lack of diurnal heating but may be related to the observation of Streten (1962) that the winter wind patterns are apparently much more strongly controlj_ed by the general synoptic pattern. Another still unanswered question reLates to the mode and geographic location where the transport of particles into the surface layer occurs.

z 4'5tocol Time(hoursl

Fig. I Contours of wind-strength percentage variation from the four weekly mean (at three hourly recording times) at Mawson from mid 1981 to mid L984. The contour interval is 2C and positive variation is shaded. There is a clear diurnat variation during the sunlit portion of the year mod.ulated by an annual variation centred close to the sunmer solstice. fhe diurnal maximum occurs around 0430 local time and the minimun around 1630 Local. During the sunless part of the year there is a weak (less than 4t) diurnat variation tending to maximum around Local niclday.

7 CONCI,USIONS

Although Mawsgn is Located on the Antarctic coast the high persistence and directional stability of the outflow of surface air from the interior plateau regions means that for the urajority of the time in CONDENSATIONNUCLEIATMAWSON.ANTARCTICA 105

terms of airborne particles (CN in particular) it has many of the properties of an Antarctic continental location. There a iq- pronounced annual variation in CN concentration, from arond 30 crn in - winter to around 400 crn in sununer. Particle sizes in the outflowing continental air also appear to vary seasonally with typically very small modal- radii in winter, r-<0.01 ym, and larger values in surnrner, around 0.03 to 0.04 Um. The3'e l-atter values are similar to those observed at southern mid-latitudes. During winter, conditions indicating the advection of lower latitude, relatively warm, moist air accompanied by increased CN concentrations and in the cases studied indicated the presence of an additional large particle mode tentatively identified as sea-sal-t. Diurnal variation in CN concentration is generally weak (less than +L0E of the mean) and not wel-l organised-with respect to tinre of day or season, although some periods with persistent diurnal variation are observed. There is little evidence that these variations are linked to either wind-strength or diurnal heating or indeed that wind-strength or dj-urnal heating affect the CN concentration to any significant extent.

REFERENCES

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