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. t_ C o o c g c o 1965 I E c o ,o c o c o MAWSON U I"ASONOJFMAMJ JA SOND 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.