J. Phys. Chem. 1903, 87, 4130-4134
4130
change is a result of the disordered ice produced by rapid freezing of supercooled water, since the change is proportional to the rate of freezing and it relaxes with times comparable to those for mechanical relaxation. There is considerable evidence for a noncrystalline “liquidlike” layer on the surface of ice with some degree of dipole alignment permitted. It seems plausible that in a highly disordered ice surface with many defects it will be energetically favorable for more dipoles to align thus producing the more negative contact potentials observed. Further work on the surface properties of rimed ice would confirm this suggestion. Armed with a knowledge of the variation of the contact potential of rime with the temperature of formation we find it is now possible to reinterpret many of the recent laboratory on charge transfer between (26) Takahashi, I. J. Atmos. Sci. 1978,35, 1536-48. (27) Gaskell, W.; Illingworth, A. J., Q J . R. Meterol. SOC.1980, 106, 841-54.
colliding ice particles in a more consistent manner. Although some aspects cannot be quantified (for example, the positive charging of frost surfaces and the dependence on impurities), the charge transfer can usually be predicted by considering the difference in contact potentials between ice formed in different ways and the capacitance when the colliding particles separate. If collisions between ice crystals and hailstones are responsible for the electrification of thunderstorms then the assymetry causing the charge transfer could be more negative contact potential of the rimed surface of the hailstone.
Acknowledgment. This work was performed with the help of a scholarship from the CONICET, Buenos Aires, Argentina, and further financial support from Grant AFOSR 81-0189. Registry No. Water, 7732-18-5. ~~
~~
~~
~
(28) Jayaratne, R.; Hallett, J.; Saunders, C. P. R. Soc., accepted for publication.
Q. J . R. Meterol.
Evidence of the Biogenic Nuclei Involvement in Antarctic Coastal Cloudst V. K. Saxena Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27650-5068 (Received: August 23, 1982: In Final Form: April 14, 1983)
Antarctic coastal clouds that form as a result of advection of marine air over the Ross ice shelf during the austral summer were investigated during a field experiment conducted in October-November 1980. Observational data on the spatial and temporal distribution of cloud condensation nuclei (CCN) activity spectra, Aitken nuclei concentrations, and chemical characteristics of the cloud water that was collected through direct aircraft penetrations of clouds were obtained. These measurements were made aboard a C-130 aircraft using a CCN spectrometer, Environment-One Aitken nucleus counter, and bulk-water collecting teflon probes. The collected cloud water was transferred to a scrupulously cleaned germanium internal reflection prism and examined by a Perkin-Elmer infrared spectrophotometer, electron microscope, and X-ray energy dispersive analysis (XEDA). Part of the water sample was allowed to stand at room temperature under diffused light. These analyses demonstrated, first, the growth of an algae, suspected to be Planktonema lauterbornii, in the “clean” Antarctic cloud water, second, the presence of proteinaceous matter as indicated by the infrared absorption spectra, and third, an abundance of ionically bonded potassium chloride salt rather than sodium chloride. The salt KCl is known to be enriched in biological cells. It has already been well documented that precipitation in southern latitudes (e.g., New Zealand) is more nutrient-laden than in the northern hemisphere although the evidence produced here is the fiit to suggest the presence of biogenic material in clouds formed in the southern hemisphere.
Introduction Although processes leading to the transfer of biogenic material from the ocean surface to the atmosphere are well studied1,2 and following Soulage3 the effectiveness of biogenic nuclei in initiating ice phase at elevated freezing temperatures has been well documented,4+ their presence in natural clouds has not been demonstrated so far. Recently, Jayaweera and Flanagan’O have reported the presence of two bacterial strains, namely, Pseudomonas and Erwinia herbicola, and five different fungal spores in air samples collected over the Arctic Ocean. Obviously, one of the principal difficulties in sampling the biogenic nuclei is their extremely low number concentration in the atmosphere. This necessitates processing of unmanageably ‘Paper presented at the VI International Symposium on the Physics and Chemistry of Ice, August 2-6, 1982, Uni\*ersity of Missouri-Rolla.
large volumes of air in an environment usually dominated by inorganic and/or anthropogenic sources. Absence of anthropogenic and some prolific natural sources of biogenic nuclei in polar regions makes them potentially useful for investigating their presence and studying their role in natural cloud forming processes. Sampling of cloud water provides an unusual and unique opportunity since clouds do process large volumes of air naturally during their horizontal and vertical development. In this paper, evi(1) D. C. Blanchard and D. Syzdek, J. Geol. Res., 77, 5087 (1972). (2) D. C. Blanchard and D. Syzdek, J . Rech. Atmos., 8 , 529 (1974). (3) G. Soulage, Ann. Geophys., 13, 103 (1957). (4) R. C. Schnell and G. Vali, Nature (London),236, 163 (1972). (5) R. C. Schnell and G. Vali, Nature (London),246, 212 (1973). (6) R. C. Schnell, Geophys. Res. Lett., 2, 500 (1975). (7) R. C. Schnell and G. Vali, Tellus, 27, 321 (1975). (8) R. C. Schnell and G. Vali, J . Atmos. Sci., 33, 1554 (1976). (9) G. Vali, M. Christensen, R. W. Fresh, E. L. Gaylan, L. R. Maki, and R. C. Schnell, J . Atmos. Sci., 33, 1565 (1976). (10)K. Jayaweera and P. Flanagan, Geophys. Res. Lett., 9,94 (1982).
0022-3654/83/2087-4130$01.50/0 ’ 1983 American Chemical Society
The Journal of phvsical Chemistry, Vol. 87, No. 21, 1983 4131
Biogenic Nuclei in Antarctic Coastal Clouds ATLANTIC OCEAN
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Figure 1. Sampling site in Antarctica where coastal clouds were penetrated for the collection of cloud water samples.
dence is presented indicating that biogenic nuclei actively participate in the formation of Antarctic coastal clouds. The evidence is derived from the analysis of cloud water samples collected over the Ross Ice Shelf in Antarctica on November 3,1980.
,
Experimental Section During the 1980-81 austral summer, measurements of microphysical parameters were made along the Antarctic coast. We set out to accomplish the following objectives: (1)the determination of cloud nucleation characteristics of those a e r m l particles which participate in the formation of Antarctic coastal clouds, (2) simultaneous measurements of Aitken nuclei concentrations, and (3) the determination of the primary chemical constituents of cloud water collected directly through aircraft penetrations. So that the above objectives could be accomplished, a cloud condensation nucleus (CCN) spectrometer"J2 capable of measuring spatial and temporal distributions of the CCN activity spectrum in real time was installed in the instrumented C-130 aircraft which was also equipped with sensors for pressure, humidity, and aircraft position. The Aitken nuclei concentration was measured with an Environment/One automatic counter which was calibrated against a similar counter a t the National Center for Atmospheric Research (NCAR), Boulder, CO. The calibration of the NCAR counter could be traced to the original Pollak counter. In spite of all these precautions in measurements, when Aitken nuclei Concentrations dropped below 300 ~ m - large ~ , errors were encountered. The absolute values could easily be off by a factor13 of two. The sampling site is shown in Figure 1. Measurements were made on November 3,1980. The cloud water was collected by using teflon probes which were exposed to supercooled cloud droplets through a hole in the fuselage. The probes consisted of teflon cylinders as shown in Figure 2. Each cylinder was 5.0 cm in diameter and 122 cm long. On the surface of the cylinder were provided little cups which were 0.63 cm in diameter at the rim. Following the procedure of Hegg and Hobbs,14 the probe was triply washed with deionized water before deployment. Being thermodynamically unstable, the supercooled droplets (11) N. Fukuta and V. K. Saxena, J. Rech. Atmos., 18,1352 (1979).
(12) N. Fukuta and V. K. Saxena, J. Appl. Meteorol., 13,169 (1979). (13) V. K. Saxena, D. J. Alofs, A. 0. Tabelak, and P. A. Nee,J. Rech. Atmos., 6, 11 (1972). (14) D. A. Hegg and P. V. Hobbs,Atmos. Enuiron., 15,1597 (1981).
Flgue 2. Location of the cloud water probe and Fukuta-SaxCCN spectrometer11g12 aboard the LC-130 aircraft used for akbome sampling in Antarctica. The dimensions are not drawn to the scale.
froze upon impinging on the probe, which was projected upward from the fuselage while the aircraft (see Figure 2) was making tracks within the Antarctic coastal clouds. The cups on the surface of the probe provided preferential sites for ice accumulation. The iced cylinder was then removed and stored in a clean plastic bag. Collected cloud water was latter retrieved from the bag and stored in scrupulously cleaned plastic vials. A plastic vial containing the test sample was allowed to stand at ambient room temperatures (about 20 "C) under diffused sunlight for 4 weeks and the growth of an algae was noticed in the sample. No such growth occurred in a control vial containing ordinary distilled water and exposed to identical environmental conditions. An electron micrograph of the algae was monitored. For comparison, Jayaweera and Flanagan'O had analyzed their Arctic aerosol samples after the lapse of 3 and 9 months from the collection time.
Results and Discussion In Figure 3, CCN concentration (active at 1% supersaturation), dew point, and air temperature are plotted as a function of altitude at a location of about (76"06'S, 168'35'E) for the flight of November 3,1980. The CCN concentration varied from 50 to 325 cm-l, the minimum occurring at 640 m MSL and the maximum at 830 m MSL. The CCN activation spectrum measurements in the subcloud layer are usually correlated15with the cloud droplet size spectrum. The low CCN concentrations shown in Figure 3 are characteristic of maritime air masses. The CCN concentration increases with height below the surface inversion layer (-260 m MSL), then decreases sharply by a factor of three within a thin layer of 50 m. It stays constant at -100 C M - ~ up to a height of 700 m MSL and then starts to increase with height. On the other hand, the Aitken nuclei concentration (cf. Figure 4) shows a steady decrease up to 300 m MSL and then increases monotonically, the maximum being recorded at 2500 m MSL. Aerosol concentrations measured at any given time represent a net result of production and depletion mechanisms.16 Elevated concentrations close to the surface in Figures 3 and 4 are suggestive of exposed oceanic surface as a wide area source of aerosol while a monotonic increase with height points out the presence of an atmospheric (15) M. J. Manton and J. Warner, Q. J. R. Meteorol. SOC.,108,917 (1982). (16) H. R. Pruppacher and J. D. Klett, 'Microphysics of Clouds and Precipitation", Reidel, Boston, 1980.
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The Journal of Physical Chemistry, Voi. 87,
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Flgure 4. Number concentration of Aitken nuclei as a function of altitude at a location of about (75'50's. 169'24'E).
Flgure 3. Number concentration of cloud condensation nuclei (CCN) active at 1% supersaturation as a function of altitude at a location of about (76OO6'S, 186'35'E). Dew point and air temperatures are also shown plotted.
source which may be the result of stratospheric-tropospheric exchange17 or in situ production of aerosol particles as a result of gas-to-particle conversion mechanisms.l8 Analysis of collected cloud water samples was done with a Perkin-Elmer infrared spectrophotometer, an electron microscope, and an X-ray energy dispersion spectrometer. In Figure 5 is shown an infrared absorption spectrum recorded by the Perkin-Elmer infrared spectrophotometer for the cloud water collected on November 3,1980 around (75O53'S, 172'20'E) and analyzed after about 8 weeks from the time of collection. In the figure, the minimum in transmittance at 3400 cm-' represents OH and NH groups and three consecutive minima in the frequency range of 1600-1400 cm-' represent the presence of proteinaceous matter. For the desiccated water specimen, scanning electron micrographs and energy dispersive X-ray scans were made. These are shown in Figures 6 and 7, respectively. In Figure 6 are shown the rings formed by the residue submicron particles contained in the cloud water. Within each ring, there are several submicron particles and although elemental composition of each of these particles may be slightly different, an average representation may be worked out and is displayed in Figure 7. Various peaks in the figure indicate the presence of sodium, silicon, phosphorus, sulfur, chlorine, potassium, and calcium. The silicon seems to derive from silicates which are indicated by the infrared scans shown in Figure 5 at 1040 cm-' and (17) R. D. Cadle, W. H. Fischer, E. R. Frank, and J. P. Lodge, Jr., J. A h " . Sei., 25, 100 (1968). (18) S. K. Friedlander, "Smoke, Dust and Haze", Wiley, New York, 1977.
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sulfur from sulfates that are indicated at 1100 cm-'. As mentioned earlier, when cloud water was allowed to stand at 20 "C under diffused sun light for 4 weeks, growth of an algae was noticed. An electron micrograph of this algae is shown in Figure 8. The X-ray energy dispersive analysis of the specimen is shown in Figure 9. For the analysis, the water specimen was allowed to dry directly onto the face of a scrupulously cleaned germanium internal reflection prism. The right-hand peak in Figure 9 is characteristic of the germanium substrate. Elemental composition consisted of aluminum, silicon, sulfur, chlorine, and potassium. Relative abundance of silicon and potassium is about the same but sulfur seems to be the most abundant. Mount Erebus being an active volcano is the only natural source1gof sulfur on the Antarctic (19) L. F. Radke, Nature (London),299, 710 (1982).
Bbgenk Nuclei in Antarctic Coastal Clouds
The Jownal of phvsrcer Chem&by, Vd. 87, No. 21, 1983 4133
1
Flgura 6. Electroll micrograph of the desiccated cloud water sample on the face of a germanium internal reflection prism. Submicron particles are clustered around rings.
Flgwe 8. Electron micrograph of the algae that grew in ttk Antarctic cloud water WMCh was collected through direct aircraft penetratkm on November 3, 1980 around 75O53'S, 172O20'E.
Ftgwo 7. Typical X-ray energy dispersive spectrum of a submicron particle shown in Figure 6. Presence of sodium, silicon, phosphorus, SUM., chlorine, potassium, and calcium is indicated by peaks showing the relathre abundance.
Figure 9. X-ray energy dispersive spectrum of the sample shown in Figure 8. Aluminum, silicon, sulfur, chlorine, and potassium are r e p resented by the indicated peaks. The end peak is characteristic of the germanium substrate.
continent. Previous fiidings17 have led to the hypothesis that the origin of sulfate particles is in the Junge sulfate layer and stratospheric-tropospheric exchange brings them into the lower troposphere where they may get involved, in the cloud formation processes. Antarctic sulfur exists mostly in the form of sulfates and consequently such particles are very effective CCN. Although Na and Mg are present in the sample (6. Figure 9), they seem to be much less abundant in comparison to K. From the foregoing discussion of the sample analyses, some tentative conclusions may be drawn. Algae represented in Rigure 8 as the fibrous and sheetlike matter may be identifiedmas PZanktonepza luuterbornii. The growth
of an algae in the "clean" Antarctic cloud water, which was sampled through direct aircraft penetration with utmost care to avoid any contamination, indeed warrants further investigation. It indicates the presence of biogenic material in the Antarctic clouds. As reported earlier,21infrared absorption spectra of the cloud water specimen have consistently indicated the presence of the proteinaceous matter. (901 1. \-"I
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Algae", Sparkes Press, Raleigh, 1973. (21) V. K. Saxena and R. E. Baier, "Programe and Abstracts of the InternationalConference on Condensationand Ice Nuclei",Hamburg, FFt Germany, Aug 2628,1981, p 56.
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The Journal of Physical Chemistry, Vol. 87,No. 21, 1983
Evidence presented in Figures 7 and 9 points out that potassium and chlorine are relatively more abundant than sodium, thus the major ionically bonded salt present in the cloud water is potassium chloride rather than sodium chloride. It is well understood that the former is enriched inside biological cells. The above three pieces of evidence seem to confirm that submicroscopic aerosol particles of biogenic origin, such as those derived from plankton in the Antarctic ocean, actively participate in cloud processes over the Ross Ice Shelf. Frequently, these clouds produce snow as precipitation. The biogenic nuclei may be initiating the condensation-freezing mechanism which helps these clouds to precipitate. Previous s t ~ d i e s ~ have - ~ indicated that biogenic nuclei exhibit good ice nucleating activity. Condensation followed by freezing is one of the prevalent mechanisms16for initiating ice phase in clouds. It is in order here to note that precipitation in the southern hemisphere is known to be more nutrient-laden than in the northern hemisphere. MacIntyre22has discussed the recipe of rainwater for a small town on the New Zealand coast. The biological material in the precipitation samples represented one thousand times the amount of organics normally present in an equal volume of seawater. The processes1*2J0 by which this material finds its way into the atmosphere are still least understood and heretofore in the cloud water no evidence of the presence of biogenic material which is an integral part of the rainwater content in New Zealand has been reported. Our measurements have indicated that the biogenic material, which may consist of phytoplankton aerosols, is present in the cloud water. This finding provides a n insight into t h e relation between macroscopic meteorology and microlayer oceanography. In the Antarctic region, biogenic nuclei might be treated as a tracer for air masses that advect from the plankton-rich ocean to the continent.
Concluding Remarks Antarctic continent provides an unique and unusual opportunity to investigate trends in the global pollutants. Its remoteness from anthropogenic pollution sources makes it ideally suitable for studying the long-range transport"% of pollutants and the strength of natural sources. Because of its hostile environment, only limited time is usually (22) F. MacIntyre, Sci. Am., 230 (No. 5), 62 (1974). (23) G. E. Shaw, Rev. Geophys. Space Phys., 17, 1983 (1979). (24) A. W. Hogan and S. Barnard, J.Appl. Meteorol., 17,1458 (1978). (25) E. K. Bigg, J. Appl. Meteorol., 19, 521 (1980). (26) A. W. Hogan, J. Appl. Meteorol., 14, 550 (1975).
Saxena
available to procure aerosol samples and costs for doing so are indeed prohibitively high. A methodology is presented above for in situ, airborne measurements of CCN and Aitken nuclei, and for collecting samples of cloud water. As a case study, measurements made on November 3,1980 over the Ross Ice Shelf are presented and analyzed. The following remarks seem to hold good. Concentrations of CCN (active at 1% supersaturation) within the surface boundary layer (SBL) over the Ross Ice Shelf suggest contributions from a wide area, surface source of aerosol such as exposed ocean water. Close to the top of the planetary boundary layer (PBL), CCN concentrations show a steady increase thus pointing out the presence of an atmospheric source. Aitken nuclei concentrations within the SBL are found similar to those measured earlier26 and show the same general features as the CCN concentrations. Four possible sources may be identified for the aerosol found over the Ross Ice Shelf, namely, wave action at the exposed oceanic surface,lg2tropospheric-stratospheric exchange processes,l' in situ formation of aerosol particles as a result of photochemical reactions, and wind erosion over the dry valleys.23 Biogenic material identified in the colIected cloud water through direct cloud penetrations suggest that surrounding ocean may be one of the important sources for producing cloud-active aerosols. Future field experiments are needed to estimate the source strength of oceanic surfaces, to make our understanding quantitative about the extent of and mechanisms for the involvement of biogenic nuclei in Antarctic coastal clouds, and to understand interactions between microlayer oceanography and macroscopic meteorology over the Antarctic continent. Evidence presented above and recently provided by Jayaweera and FlanaganlO for the presence of biogenic material over the Antarctic and Arctic oceans, respectively, warrants further similar investigation in areas devoid of excessive anthropogenic pollution.
Acknowledgment. This work was supported by the National Science Foundation, Washington, DC under Grant No. DPP79-22058. The help of Dr. Robert E. Baier of Arvin Calspan Advanced Technology Center is gratefully acknowledged in analyzing the cloud water samples. Mr. R. S. Rathore helped in plotting the data. Registry No. Sodium chloride, 7647-14-5; potassium chloride, 1447-40-7.
