Size distribution of atmospheric aerosols at Ross Island

Size distribution of atmospheric aerosols at Ross Island G.E. SHAW Geophysical of Alaska University of Alaska Fairbanks, Alaska 99775-080()

I report and interpret measurements of the size distribution of microscopic particles (aerosols) suspended in air masses at Ross Island. The measurements indicate that distinct seasonal changes take place in the aerosol. There were also tendencies in the microscopic size distribution that related to the type of air mass examined. Examples of the size distribution of aerosol particles at Ross Island are shown in figure 1; these distributions were selected from data taken when the winds were out of a clean sector and are believed to be representative of regional tropospheric air masses. Notice that the size spectra are bimodal, possessing a "transient mode" (the smaller of the two) and a "permanent mode" centered around the Greenfield Gap at a half-micron diameter (Greenfield 1957). A similar bimodal structure of aerosols has been reported by Ito (1983). The Greenfield Gap region, where the larger aerosol mode exists, is a size where the removal of particles from the atmosphere undergoes a minimum. Particles in the Greenfield Gap have sufficiently small inertia to be carried around obstacles such as hydrometeors, rather than slipping across hydrodynamic streamlines and impacting onto them. These larger particles are also too immobile to be removed from the atmosphere very efficiently by diffusive processes. Thus, the large "permanent" mode of aerosols about a half micron in diameter probably consists of material that has resided in the

atmosphere for considerable lengths of time and which therefore has likely been transported over large distances (e.g., from surrounding continents). Figure 2 shows the mean particle size distributions for two predominant types of air masses: cold, continental antarctic air masses contain particles more finely dispersed than maritime antarctic polar air-mass types. Additionally, we found a positive correlation between tropospheric ozone and the occurrence of fine particles at Ross Island; similar findings were reported at the South Pole by Hogan and Barnard (1978). Continental Antarctic air mass types at Ross Island are associated with anticyclonic circulation and subsidence over the ice cap in conjunction with cyclonic systems (figure 3) which spin 400

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DIA (sm) Figure 2. Mean aerosol particle size distributions for air mass types mP (maritime antarctic Polar) and cA (continental Antarctic). ("m" denotes "micrometer?')

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Figure 1. Inverted aerosol size distributions from Ross Island. ["p.m" denotes "micrometer," "DIA" denotes diameter of the particles, "dn/ d log r" denotes the number concentration (in cubic centimeters) of particles in the radius range r to r + dr, or equivalently, in the diameter range d to d + dd.]

1986 REVIEW

Figure 3. Cyclonic tracks in the Antarctic (after Alt, Astapenko, and Ropar 1959).

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into the Ross Sea and Marie Byrd Land area. This type of air mass is frequently associated with strong air flow over the Ellsworth and Queen Maud mountain ranges. It is suggested on the basis of the correlation between fine mode particles and ozone and continental antarctic air masses, that the fine transient mode of particles may arise from the entrainment of stratospheric or upper tropospheric air, perhaps driven by breaking waves associated with air flowing over the mountain barriers as suggested by Robinson et al. (1983). This work was supported by National Science Foundation grant DPP 82-19625. Thanks to A. Hogan for making ozone data available and to B. McKibben, A. Anger, and AG1 Crayne for help with the experiments.

Atmospheric boundary measurements in eastern Antarctica C. WENDLER

References Alt, S., P. Astapenko, and N.J. Ropar, Jr. 1959. Some aspects of the Antarctic atmospheric circulation in 1958. 1GY Word Data Center, A, iGYGeneral Report, Series No.4. Washington, D.C.: National Academy of Sciences. Greenfield, S.M. 1957. Rain scavenging of radioactive particulate matter from the atmosphere. Journal of Meteorology, 14, 115-125. Hogan, A., and S. Barnard. 1978. Seasonal and frontal variations in Antarctic aerosol concentrations. Journal of Applied Meteorology, 17(10), 1458-1465. Ito, T. 1983. Study on properties and origin of aerosol particles in the Antarctic atmosphere. Papers in Meteorology and Geophysics, 34(3), 151-219. (Meteorological Research Institute of Japan.) Robinson, E., D. Clark, D.R. Crom, and W.L. Bamesberger. 1983. Stratospheric tropospheric ozone exchange in Antarctica caused by breaking waves. Journal of Geophysical Research, 88, 19708-19720.

wind speed was observed at 120 meters, which is the so-called katabatic wind. The picture shown here is rather typical, and little variation was observed from day to day. The wind direction changes with height as well, turning somewhat to the left within boundary layer. This also is rather representative of the katabatic wind in the Southern Hemisphere. Besides these bound-

Geophysical Institute University of Alaska Fairbanks, Alaska 99701 J.C. ANDRE

Centre National tie Recherches Meteorologiques loulouse, Trance

A major field study in Adélie Land, Eastern Antarctica was carried out this year as a joint U.S.-French experiment. The goal was to obtain a better understanding of the boundary layer in Antarctica, with special attention being given to the katabatic wind. Long-term climatological and upper-air data could be obtained from Dumont d'Urville. There are, further, five automatic weather stations, which stretch from close to the coast to Dome C at the end. D-10 is the closest station, some 10 kilometers from Dumont d'Urville, while Dome C is some 1,080 kilometers inland at a height of 3,280 meters. These stations have given us climatological data along the icy slopes of Adélie Land for the last 6 years, on which we reported last year. For our intensive measuring period of about 40 days, three stations were occupied, two by the French and one by us, located some 5, 110, and 210 kilometers from the coastline. Balloons, air foils, and drones were used as carriers for our meteorological packages. The meteorological data were transmitted via radio to ground stations, where they were recorded on magnetic tape. Figure 1 shows the air foild, which is one of those used at the U.S. station. Some difficulties were experienced in very strong winds (above 20 meters per second), which could break the line. A typical morning profile obtained from these measurements is given in figure 2. A strong surface temperature inversion can be observed, which was established in the night, and is now beginning to erode. This is typical for most of Antarctica most of the time. The height of the inversion is about 500 meters. Within this inversion layer, a maximum 242

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Figure 1. An air foil, which will be used as a carrier for the meteorological package, is released at D-47, Adélie Land, Antarctica. ANTARCTIC JOURNAL