The many facets of aerosol science

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J. A"osol Sci.• Vol. 27. 5uppl . I. pp . 51-56, 1996 Copyright 4::> 1996 Elsevier Science Ltd Printed in On:at Bnl&in. All righLJ merved

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The Many Facets of Aerosol Science Othmar Preining, Clean Air Commission, Austrian Academy of Sciences Aerosol Science has its roots in the 19th century. Basic concepts of physics had to be verified and this led e.g. to the study of Brownian motion, of Aitken nuclei, and of the elementary electric charge. In the first half of the 20th century industrial health became the focus of interest and since then environmental problems at all scales like greenhouse issue, acid rain, photochemical smog, stratospheric ozone depletion, and also the industrial use of aerosols have found great interest. Aerosols are at the core of these problems and hence not only important for science but to a considerable portion also for decision making. Nonlinear reactions and inhomogenous large and very large systems must be predicted. New concepts, new insights, new instruments are in demand to permit Aerosol Science to live up to expectations. Many questions will be asked by policy makers and by the public at large, and Aerosol Science will have to provide the answers.

Aerosol is particles suspended in gas, gas is molecules in free motion, but gas is a myth, there are always particles present, only aerosols are real systems. This is not new, it has been in the mind of those who created the term "aerosol" , Schmauss 1920 and Whytlaw-Gray et al. 1923 quoting a suggestion of Donnan, Green and Lane 1964. Number and massof the particles are very small compared to number and mass of the gas-molecules but the particles make the essential properties of the system. Aerosol and man: When we breathe we inhale aerosols not gas. When Prometheus stole fire from heaven and gave it to man, anthropogenic air pollution started. The users of fire inhaled soot; e.g. the corpse of a man who died about 5000 years ago at a high Alpine pass at the Austrian Italian border and remained deep frozen until recently had a soot-blackened lung, Seidler 1996. A historical perspective of air pollution problems was given by Chambers 1962. Aerosols are very important for health aspects in the following fields: industrial hygiene, working place hazards in industry, mining, nuclear industry, general air pollution at all scales from local to global, indoor pollution including passive smoking, a fact recognized at all times, e.g. Sigerson 1870, and Hinds 1982, Williams and Loyalka 1991, Vincent 1995. Historical remarks: Since the age of enlightenment observation of nature in natural philosophy and science has led to the recognition and investigation of phenomena which are aerosol related: Brownian motion, Smoluchowski 1906, Gibbs 1924. Whytlaw-Gray 1932, Chandrasekhar 1943; optical effects like Tydall spectra, Middelton 1952, van de Hulst 1957; thermal diffusion - the dust free zone around hot bodies, Green and Lane 1964; electrification of dusts and droplets, and condensation of water, to mention a few, Fuchs 1964, Hidy and Brock 1970, Friedlander 1977. The study of Brownian motion was of great importance for the development of physics, big names like Smoluchowski 1906, 1915, and Einstein 1905, 1906 are associated with it, a whole new branch of science, statistical physics, developed, Chandrasekhar's monography, Chandrasekhar 1943, initiated the breakthrough. The famous experiments in physics to quantify the elementary electric charge and the Millikan-Ehrenhaft controversy and many follow-up studies led directly to aerosol 51

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mechanics, Fuchs 1964. Radioactivity and later nuclear and elementary particle physics profited from Wilson's cloud chamber which in turn is associated with Aitken nuclei. Global Warming: Aitken Nuclei Counters have been used to investigate outdoor and indoor atmospheres since the 80ies of the 19th century, Aitken 1890, and more extensively after World War II, e.g. Rich 1957, Pollak 1959. Follow-up investigations led to subjects like cloud condensation nuclei and ice nuclei and in this way entered the global climate debate about 10 years ago, Twomey 1977, Preining 1993. Another approach to the climate issue was aerosol optics; light scattering, the color of the sky, the rainbow, vision through the atmosphere, Middelton 1952, van de Hulst 1957, then radiative transfer where Chandrasekhar 1950 made again a great contribution by comprehending the available information in a monography. In the beginning, aerosols were not adequately treated in climate models, they are now included but they are still the sources of great uncertainties. Some new reviews image recent developments: Lenoble 1993, Pueschel 1995, Wuebbles 1995, Schwartz 1996, Horvath 1997. Acid Rain: The chemical composition of atmospheric aerosols is essentially influenced by anthropogenic emissions of oxides of sulfur and nitrogen mainly from fossil fuel combustion, Rodhe et al. 1995. The aerosol acidity as well as the acidity of cloud droplets, rain, and dew has increased, especially around industrialized regions during the past century, Galloway 1995. Acid deposition causes damage to ecosystems, corrodes building materials and pieces of art, and most important acidic aerosols have adverse effects on human health, see e.g. Marijnissen and Gradon 1996. Photochemical Smog: Volatile organic substances are oxidized photochemically by solar radiation in the presence of oxides of nitrogen, Haagen-Smit 1962. Some "final" reaction products form new particles or attach to preexisting aerosols. This is firstly a natural process leading to the "Blue Haze" but secondly in case of air pollution a process leading to the aggressive "L.A.-type-smog" which damages materials, has negative effects on plants, reduces visibility considerably, and even impairs human health, features known at least since the 50ies, Went 1955, Marijnissen and Gradon 1996. Ozon Hole: The observed ozon depletion in the lower stratosphere, especially over Antarctica during spring, can be understood only by using a heterogenous chemistry model combining gas phase and aerosol chemistry, Shen et al. 1995. The issue raised great public concern and as a consequence led to political actions, the ban of CFC's. The future demand for air transportation may cause a transfer from the troposphere to the lower stratosphere; such plans cause some concern, Stolarski (NASA) 1995. Technical Use: There are some attempts to use aerosol technologies to create new materials, especially in the nanometer size range, e.g. Seta 1995, another example is the preparation of titaniadioxide, Powell et at. 1995.

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The future of Aerosol Science: To understand the complex chemical and physical reactions of aerosol systems, particularly in the atmosphere, new experimental techniques became available but these have so far hardly been applied in classical aerosol studies. It is the study of clusters which is opening new ways, e.g. Bernstein 1995, Preining 1995. It is important to understand when and why particles are resuspended from solid or even liquid surfaces, to this end the wetting-drying transitions of particle-surface systems have to be investigated; some methodologically interesting work is going on in the surface science community and the aerosol field could profit from it, Aveyard 1996. A special important subfield is Ultrafine Aerosols, Marijnissen and Pratsinis 1993. Aerosols and the public at large: Aerosols are essential components of most of the local, regional, and global pollution problems and they have been with us throughout history. The general results of Cowling and Nilson 1995 from acidification research can be readily converted and applied to aerosol research. Scientific lesson: Aerosols have a much greater impact than generally anticipated; Social dynamics lesson: Scientists spend more time and energy with accurate detailed measurements, especially in networks, than with analysis and interpretation of existing data. There is a great deficiency in informing properly the public as well as decision makers. The reasons are partly personal/psychological and social/psychological as well as budgetary/logistical. There are differences of attitudes in European, North American, and especially Developing Countries. We have to see the Aerosol Issues together with the other Global Problems within the frame of Alternative Futures for Democratic Societies and we have to see the ethical context. Aerosol and environmental scientists have been among the first to see the problem, Bolin 1994, but they also experience the vastly different facets seen by other scientists, e.g. economists, Nordhaus 1994. The concept of critical loads, Bull 1995, Klockow 1996, will be very important for public understanding. These "Problems" are only partly scientific, the scientists have to accept the fact that public and political understanding and decision making is not only influenced by sience but also by anti-science, Holton 1993, Preining 1995.

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Hinds, W.A., Aerosol Technology, John Wiley &. Sons, N.Y. 1982 Holton, G., Science and Anti-Science, Harvard University Press, Cambridge, Mass. 1993 Horvath, H. Influence of atmospheric aerosol upon the global radiation balance, in: Environmental Particles, Vol. IV, Atmospheric Particles, Harrison, R.M. and R. van Griecken, ed's, John Wiley &. Sons, N.Y. 1997, in press K1ockow, D., Critical Atmospheric Parameters: Research Needs and Challenges for Analytical Chemistry, presented: 26th International Symposium onEnvironmental Analytical Chemistry, Vienna, Austria, April 9·11, 1996 Lenoble, J., Atmospheric radiation transfer, A. Deepak, Publ., Hampton, VA, 1993

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