Radioactive Aerosols

RADIOACTIVE AEROSOLS

RADIOACTIVITY IN THE ENVIRONMENT A companion series to the Journal of Environmental Radioactivity Series Editor M.S. Baxter Ampfield House Clachan Seil Argyll, Scotland, UK Volume 1: Plutonium in the Environment (A. Kudo, Editor) Volume 2: Interactions of Microorganisms with Radionuclides (F.R. Livens and M. Keith-Roach, Editors) Volume 3: Radioactive Fallout after Nuclear Explosions and Accidents (Yu.A. Izrael, Author) Volume 4: Modelling Radioactivity in the Environment (E.M. Scott, Editor) Volume 5: Sedimentary Processes: Quantification Using Radionuclides (J. Carroll and I. Lerche, Authors) Volume 6: Marine Radioactivity (H.D. Livingston, Editor) Volume 7: The Natural Radiation Environment VII (J.P. McLaughlin, S.E. Simopoulos and F. Steinhäusler, Editors) Volume 8: Radionuclides in the Environment (P.P. Povinec and J.A. Sanchez-Cabeza, Editors) Volume 9: Deep Geological Disposal of Radioactive Waste (R. Alexander and L. McKinley, Editors) Volume 10: Radioactivity in the Terrestrial Environment (G. Shaw, Editor) Volume 11: Analysis of Environmental Radionuclides (P.P. Povinec, Editor) Volume 12: Radioactive Aerosols (C. Papastefanou, Author)

RADIOACTIVE AEROSOLS

Constantin Papastefanou Physics Department, Aristotle University of Thessaloniki Thessaloniki, Greece

AMSTERDAM – BOSTON – HEIDELBERG – LONDON – NEW YORK – PARIS SAN DIEGO – SAN FRANCISCO – SINGAPORE – SYDNEY – TOKYO

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK First edition 2008 Copyright © 2008 Elsevier B.V. All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-08-044075-0 ISSN: 1569-4860

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The wilderness and the parched land shall be glad, and the desert shall rejoice, and blossom as the rose. It shall blossom abundantly, and rejoice even with joy and singing. . . And the parched land shall become a pool, and the thirsty ground springs of water. Isaiah 35: 1,2,7

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Contributions

Andersen, A.A. (1976). ACFM Ambient Particle Sizing Samplers. Operating Manual TR 76900016, pp. 1–21. Andersen, A.A. (1979). Low-Pressure Impactor. Operating Manual TR 76-900016, pp. 1–29. Berner, A., Lurzer, C. (1980). Mass size distributions of traffic aerosols at Vienna. J. Phys. Chem. 84, 2079–2083. Bondietti, E.A., Papastefanou, C. (1989). Large particle nitrate artifacts in the aerodynamic size distributions of ambient aerosols. J. Aerosol Sci. 20 (6), 667–670. Bondietti, E.A., Papastefanou, C. (1993). Estimates of residence times of sulfate aerosols in ambient air. Sci. Total Environ. 136, 25–31. Bondietti, E.A., Papastefanou, C., Rangarajan, C. (1987). Aerodynamic size associations of natural radioactivity with ambient aerosols. In: Hopke, P.K. (Ed.), Radon and Its Decay Products: Occurrence, Properties, and Health Effects, In: ACS Symposium Series, vol. 331, pp. 377–397. Cember, H. (1987). Inhaled radioactivity. In: Introduction to Health Physics, second ed., revised and enlarged. Pergamon Press, New York. Chamberlain, A.C. (1991). Radioactive Aerosols. Cambridge University Press, Cambridge, UK. Cheng, Y.S., Guilmette, R.A., Zhou, Y., Gao, J., LaBone, T., Whicker, J.J., Hoover, M.D. (2004). Characterization of plutonium aerosol collected during an accident. Health Phys. 87 (6), 596–605. Demokritou, P., Lee, S.J., Ferguson, S.T., Koutrakis, P. (2004). A compact multistage (cascade) impactor for the characterization of atmospheric aerosols. J. Aerosol Sci. 35, 281– 299. Grundel, M., Porstedorfer, J. (2003). Characterisation of an electronic radon gas personal dosemeter. Radiat. Prot. Dosim. 107 (4), 287–292. Hinds, W.C. (1999). Aerosol Technology: Properties, and Measurement of Airborne Particles. John Wiley & Sons, New York. Jost, D.T., Gaggeler, H.W., Baltensperger, U. (1986). Chernobyl fallout in size-fractionated aerosol. Nature 324, 22–23. Kauppinen, E.I., Hillamo, R.E., Aaltonen, S.H., Sinkko, K.T.S. (1986). Radioactivity size distributions of ambient aerosols in Helsinki, Finland, during May 1986 after the Chernobyl accident. Environ. Sci. Technol. 20, 1257–1259.

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Contributions

Kesten, J., Butterweck, G., Porstendorfer, J., Reineking, A., Heymel, H.-J. (1993). An online α-impactor for short-lived radon daughters. Aerosol Sci. Technol. 18, 156–164. Kim, D.S., Lee, K.W., Kim, Y.J. (2006). Characterization of a particle trap impactor. J. Aerosol Sci. 37, 1016–1023. Kondo, K., Muramatsu, H., Kanda, Y., Takahara, S. (1984). Particle size distribution of 7 Beaerosols formed in high energy accelerator tunnels. Int. J. Appl. Radiat. Isotopes 35 (10), 939–944. Kulmala, M., Mordas, G., Petaja, T., Gronholm, T., Aalto, P.P., Vehkamaki, H., Hienola, A.I., Herrmann, E., Sipila, M., Riipinen, I., Manninen, H.E., Hameri, K., Stratmann, F., Bilde, M., Winkler, P.M., Birmili, W., Wagner, P.E. (2007). The condensation particle counter battery (CPCB): A new tool to investigate the activation properties of nanoparticles. J. Aerosol Sci. 38 (3), 289–304. Kwon, S.B., Lim, K.S., Jung, J.S., Bae, G.N., Lee, K.W. (2003). Design and calibration of a 5-stage cascade impactor (K-JIST cascade impactor). J. Aerosol Sci. 34, 289–300. Marple, V.A., Willeke, K. (1979). Inertial impactors. In: Lundgren, D.A., Harris Jr., F.S., Marlow, W.H., Lippmann, M., Clark, W.E., Durham, M.D. (Eds.), Aerosol Measurement. University Presses of Florida, Gainesville, FL, pp. 90–107. Marple, V.A., Rubow, K.L., Behm, S.M. (1991). A microorifice uniform deposit impactor (MOUDI): Description, calibration, and use. Aerosol Sci. Technol. 14, 434–446. McMahon, T.A., Denison, P.J. (1979). Empirical atmospheric deposition parameters—A survey. Atmos. Environ. 19, 571–585. National Research Council (1978). Airborne Particles. University Park Press, Baltimore. Papastefanou, C. (2006). Residence time of tropospheric aerosols in association with radioactive nuclides. Appl. Radiat. Isotopes 64, 93–100. Papastefanou, C. (2006). Radioactive nuclides as tracers of environmental processes. J. Radioanal. Nucl. Chem. 267 (2), 315–320. Papastefanou, C., Bondietti, E.A. (1987). Aerodynamic size associations of 212 Pb and 214 Pb in ambient aerosols. Health Phys. 53 (5), 461–472. Papastefanou, C., Bondietti, E.A. (1991). The unattached fraction of radon progeny in ambient aerosols. J. Environ. Radioact. 13 (1), 1–11. Papastefanou, C., Bondietti, E.A. (1991). Mean residence times of atmospheric aerosols in the boundary layer as determined from 210 Bi/210 Pb activity ratios. J. Aerosol Sci. 22 (7), 927–931. Papastefanou, C., Ioannidou, A. (1995). Aerodynamic size association of 7 Be in ambient aerosols. J. Environ. Radioact. 26, 273–282. Papastefanou, C., Ioannidou, A. (1996). Influence of air pollutants in the 7 Be size distribution of atmospheric aerosols. Aerosol Sci. Technol. 21, 102–106. Porstendorfer, J. (1994). Properties and behaviour of radon and thoron and their decay products in the air. J. Aerosol Sci. 25 (2), 219–263. Porstendorfer, J. (2001). Physical parameters and dose factors of the radon and thoron decay products. Radiat. Prot. Dosim. 94 (4), 365–373. Porstendorfer, J., Pagelkopf, P., Grundel, M. (2005). Fraction of the positive 218 Po and 214 Pb clusters in indoor air. Radiat. Prot. Dosim. 113 (3), 342–351. Raabe, O.G. (1979). Design and use of the Mercer-style impactor for characterization of aerosol aerodynamic size distributions. In: Lundgren, D.A., Harris Jr., F.S., Marlow, W.H.,

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Lippmann, M., Clark, W.E., Durham, M.D. (Eds.), Aerosol Measurement. University Presses of Florida, Gainesville, FL, pp. 135–140. Ruiperez, L.G., Garcia, B.A., Uria, J.M.J., Iglesias, J.M.P. (1984). A new counter of Aitken particles. Atmos. Environ. 18 (8), 1711–1714. Sinclair, D., Countess, R.J., Liu, B.Y.H., Pui, D.Y.H. (1979). Automatic analysis of submicron aerosols. In: Lundgren, D.A., Harris Jr., F.S., Marlow, W.H., Lippmann, M., Clark, W.E., Durham, M.D. (Eds.), Aerosol Measurement. University Presses of Florida, Gainesville, FL, pp. 544–563. Thomas, J.W. (1953). The Diffusion Battery Method for Aerosol Particle Size Determination. ORNL Report No. 1648, pp. 1–68. Tokonami, S., Takahashi, F., Iimoto, T., Kurosawa, R. (1997). A new device to measure the activity size distribution of radon progeny in a low level environment. Health Phys. 73 (3), 494–497. Wileke, K., Baron, P.A. (1993). Aerosol Measurement: Principles, Techniques, and Applications. Van Nostrand Reinhold, New York. Yu, C.C., Tung, C.S., Hung, I.F., Tseng, C.L. (1993). Analyses of radioactive aerosols to support accurate internal dose assessments at Chin-Shan nuclear power plant. Health Phys. 65 (2), 147–153.

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Contents

Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Chapter 1: Atmospheric aerosol particles . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction and definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. The size distribution of atmospheric aerosol particles . . . . . . . . . . . . . . 3. Aerosols and radiation: Generation of radioactive aerosols . . . . . . . . . . . 4. Aerodynamic size distribution of radionuclide-associated aerosol particles (radioactive aerosols) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Aitken nuclei mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Accumulation mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Coarse particle mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Mechanisms of formation and growth of aerosol particles . . . . . . . . . . . 5.1. Coagulation and condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Gas-to-particle conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 3 4 6

Chapter 2: Radioactive aerosols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Radioactive aerosols associated with the cosmic-ray produced radionuclides 2.1. Beryllium-7 aerosols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Radon and thoron decay product aerosols . . . . . . . . . . . . . . . . . . . . 3.1. Formation of radon decay product aerosols . . . . . . . . . . . . . . . . . . . . . . 3.2. Diffusivity of radon decay products . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Attachment of radon decay products to aerosol particles . . . . . . . . . . . . . . . . 3.4. Activity size distributions of the radon product decay aerosols . . . . . . . . . . . . . 3.4.1. Lead-214 and 212 Pb aerosol size distributions . . . . . . . . . . . . . . . . . 3.4.2. Lead-210 vs 214 Pb aerosol size distributions . . . . . . . . . . . . . . . . . 3.4.3. Lead-212 vs sulfate, SO2− 4 aerosol size distributions . . . . . . . . . . . . .

11 11 11 12 16 16 18 19 21 21 26 26

7 7 7 8 8 8 8 9

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Contents 3.4.4. 3.4.5. 3.4.6. 3.4.7.

214 Pb and 212 Pb aerosol size distriLead-210, 7 Be, 35 S, 32 P and sulfates, SO2− 4 vs butions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . α-Recoil model: An explanation for the 214 Pb shift in the aerosol size distributions Recoil redistribution of 210 Pb following 214 Po α-decay . . . . . . . . . . . .

Radioactive aerosol particle sizes relative to growth mechanisms of sulfate, SO2− 4 aerosols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The unattached fraction of radon decay product aerosols . . . . . . . . . . . .

27 29 32

4. Mine aerosols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Fission product radionuclide aerosols . . . . . . . . . . . . . . . . . . . . . . . 6. Radioactive aerosols associated with the operation of high-energy accelerators 7. Plutonium aerosols due to nuclear weapons testing or nuclear reactor accidents References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

32 33 38 40 49 52 54

Chapter 3: Radioactive nuclides as tracers of environmental processes . . . . . . . 1. Radioactivity in the environment . . . . . . . . . . . . . . . . . . . . . . . . . 2. Atmospheric particle deposition . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Dry deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Wet deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Resuspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Air-to-vegetation transfer of radionuclides associated with submicron aerosols References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 61 61 63 66 67 68

3.4.8.

Chapter 4: Residence time of tropospheric aerosols in association with radioactive nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Residence time of aerosol particles . . . . . . . . . . . . . . . . . . . . . . . . 2. Residence time of tropospheric aerosol particles associated with the cosmic ray produced 7 Be . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Residence time of tropospheric aerosol particles associated with the radon decay product radionuclides 210 Pb, 210 Bi, 210 Po and the fission product radionuclides 89 Sr, 90 Sr, and 140 Ba and their activity ratios . . . . . . . . . . . . . . . 4. Residence time of tropospheric aerosol particles associated with the fission product radionuclides 89 Sr, 90 Sr, 140 Ba and their activity ratios . . . . . . . . 5. Residence times of sulfate aerosols in the atmosphere . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 5: Radioactive particles and human subjects . . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Radioactive dose from inhalation of radon decay product aerosols 3. Deposition of radioactive aerosol particles in the lung . . . . . . . 4. Risk assessment due to inhalation of radon decay product aerosols References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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71 71 73

75 79 80 82

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85 85 85 102 106 110

Chapter 6: Aerosol sampling and measurement techniques . . . . . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Cascade impactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

113 113 116

Contents

3. 4.

Inertial impactors . . . . . . . . . . Andersen cascade impactors . . . . 4.1. 1 ACFM ambient cascade impactor . 4.2. Low-pressure cascade impactor . . .

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. . . . 4.3. Four-stage low-pressure cascade impactor . 4.4. High volume cascade impactor . . . . . .

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Activity size distribution of radioactive aerosol particles . . . . . . . . . . . . 5. Online α-impactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. A micro-orifice uniform deposit cascade impactor (MOUDI impactor) 7. Particle trap impactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. The diffusion battery method for aerosol particle size determination . 9. Condensation particle counter battery . . . . . . . . . . . . . . . . . . . 10. The Mercer-style impactor . . . . . . . . . . . . . . . . . . . . . . . . . 11. Berner-type impactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Dichotomous sampler . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. Compact multistage cascade impactor . . . . . . . . . . . . . . . . . . 14. K-JIST 5-stage cascade impactor . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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117 121 123 124 127 128 129 130 135 139 141 146 148 150 151 153 154 157

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

161

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

169

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1

Foreword

The radioactive nuclides, such as radio-iodine and other fission products, that remain airborne for more than a few hours after the explosion of atomic bombs (Hiroshima and Nagasaki, August 1945) or nuclear weapons testing (Nevada, April 1953; Bikini Atolls, 1953; Mururoa and Fangatoufa, 1996) or nuclear reactor accidents (particularly at Chernobyl, April 26, 1986) might become attached to aerosol particles producing radioactive aerosols. Radioactive aerosols can also contain radionuclides of terrestrial origin, such as radon (222 Rn) and thoron (220 Rn) decay products and those of cosmogenic origin, such as 7 Be, which are continuously present in ambient air. The pathways of exposure that must be considered primarily concern the radioactivity which is inhaled through the respiratory tract and the actual cases of exposure to radioactive aerosols considered include the exposure of workers to radon and its decay products in uranium and other mines, such as coal mines. The subject of this particular volume relates to aerosol particle physics including aerosol characterisation, the formation mechanism, the aerodynamic size distribution of the activity and aerosol residence time, instrumentation techniques, aerosol collection and sampling, various kinds of environmental (atmospheric aerosols), particularly radioactive aerosols and the special case of radon decay product aerosols (indoors and outdoors) and the unattached fraction, thoron decay product aerosols, the deposition patterns of aerosol particles in the lung and the subsequent uptake into human subjects and risk assessment. The objective of this special volume is to provide today’s readers of aerosol science and atmospheric physics, in general, with an up-to-date summary of knowledge about radioactive aerosols and, perhaps most importantly, with a vision of future developments in this field of research. I hope that readers will find that this objective has been achieved. I would like to express my gratitude to Murdoch Baxter, the series editor and my friend, for his idea of this book series on radioactivity in the environment and for continual encouragement in writing this book and the contributors for their research on aerosol science which has appeared in the literature over more than three decades. Constantin Papastefanou Aristotle University of Thessaloniki Atomic & Nuclear Physics Laboratory Thessaloniki 54124, Greece E-mail: [email protected]

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3

Chapter 1

Atmospheric aerosol particles

1. Introduction and definitions The aerosol particles in the atmosphere are liquid or solid particles. Their sizes range from a fraction of a micron to several hundreds of microns (µm). Various names, such as Aitken nuclei, smokes, fumes or hazes (for the smaller particles) and dusts, mists, fogs or ash (for larger ones) have been used in the nomenclature of atmospheric aerosols. All aerosol particles are formed by condensation of gases or vapours or by mechanical processes. They may be transformed by coagulation or condensation at the same time as they are transported by air movement and dilution. They might disappear from the atmosphere and settle on some surfaces which act as a sink. The residence times of aerosol particles in the atmosphere vary from some days near the earth’s surface in the troposphere to a year or more in the stratosphere. A particle is a small quantity of liquid or solid. Many particles are unstable. They can change by growing or can even disappear on contact with a surface, such as a raindrop striking a surface and coalescing or an ion losing its charge after contact with a surface or an oppositely charged particle (radioactive aerosols). The atmospheric aerosols are a mixture of gases and particles that exhibits some stability in a gravitational field. The definition of fine and coarse particles as smaller and larger than 1 µm diameter, respectively, originated from the fact that atmospheric aerosol size distributions are bimodal, i.e. one mode occurs below 1 µm diameter, the other above 1 µm diameter (Whitby, 1975) (Figure 1.1). In atmospheric aerosol particle size distributions, fine particles result almost entirely from condensation and coagulation, whereas coarse particles are produced by mechanical processes. Some particles serve as nuclei, cloud condensation nuclei and ice nuclei reflecting their effects. Small and large ions carry electrical charges. Small ions are clusters of atoms or molecules around a charge. Their stability depends on their having a charge. Large ions are merely particles that carry an electric charge. Large particles, for those 0.1 to 1.0 µm in diameter, and giant particles, for those above 1.0 µm diameter, are terms also applied in characterising the atmospheric aerosol particles. As is known, the size of an atom is of the order of 10−10 m or 10−4 µm or 0.1 nm (the Bohr radius r1 = 0.5292 × 10−10 m), while the size of an atomic nucleus is of the order of 10−14 m or 10−8 µm or 10−5 nm. Thus an aerosol particle having a diameter of some nanometres (or 10−2 µm) is a hundred times or more larger than an atom and so includes some number of atoms or clusters of atoms. RADIOACTIVITY IN THE ENVIRONMENT VOLUME 12 ISSN 1569-4860/DOI: 10.1016/S1569-4860(07)12001-5

© 2008 Elsevier B.V. All rights reserved.

4

C. Papastefanou

Fig. 1.1. Bimodal mass distributions measured with a set of special impactors and a cascade impactor. Run 14 contains many more coarse particles than the average because of construction activity upwind. Note the negligible effect of this increased concentration of coarse particles on the fine particle mode. From Whitby (1975, p. III-21) in NRC (1979).

Primary particles, such as road dust, salt (sea-) spray from the oceans and cement dust do not change form after emission, whereas a substantial fraction of mass of the secondary particles, such as photochemically produced sulfates and photochemical smog, is formed by in situ chemical reactions involving gases. Monitoring and particular aerosol characterisation studies have led to a revolution in both our knowledge and our understanding of atmospheric aerosols. Atmospheric sciences, particle technology, industrial hygiene and health effect studies of pollutants are different contributing fields in that context.

2. The size distribution of atmospheric aerosol particles Very early on, Aitken (1923) showed that most particles in the atmosphere are smaller than 0.1 µm diameter and that their concentrations vary from some hundreds per cm3 over the ocean to millions per cm3 in urban areas. Junge (1955, 1963, 1972) measured the atmospheric aerosol number size distribution and concentration in urban and non-urban areas as functions of altitude and site. He established the standard form for plotting size distribution data: log of dN/dDp versus log Dp , where N = number and Dp = particle diameter. He observed that this plot was a straight line that could be described by the equation: dN/dDp = ADp−k , where A and k were constants. He also noted that in the range from 0.1 to 10.0 µm particle diameter, k was approximately equal to 4.0. This distribution mode was widely known as the Junge distribution or the power law distribution. Friedlander (1961) later showed that by balancing aerosol source and removal rates a portion of the resultant theoretical number distribution steady state could be fitted reasonably well by the Junge distribution. Clark and Whitby (1967), by fitting the Junge distribution to 52 at-

Atmospheric aerosol particles

5

Fig. 1.2. Normalised frequency plots of number, surface, and volume (particle volume times particle density) distributions for the grand average 1969 Pasadena, California smog aerosol. Note the bimodal distribution of mass. Each weighting shows features of the distribution not shown by the other plots. From Whitby (1975, p. II-11) in NRC (1979).

mospheric distributions, found that the constant A was equal to 0.4 multiplied by the aerosol volume concentration (µm3 /cm3 ). This agrees with the value predicted by Friedlander. Whitby et al. (1972) found that the number size distribution established by Junge was not a good model for the surface and mass or volume size distribution which was normally bimodal, with one mode being around about 0.3 µm diameter and the other ranging from 5.0 to 15.0 µm diameter. In Figure 1.2 (NRC, 1979), the normalised frequency plots of number, surface and volume or mass distributions are presented. In this figure, the apparent area under the curves is proportional to the number, surface area and volume or mass in a given size range. Most particles, i.e. the number distribution, are of approximately 0.01 µm diameter. The number of particles decreases sharply with increasing size. Most of the surface area is provided by particles averaging 0.2 µm diameter. The volume or mass distribution is bimodal: one mode is around 0.3 µm diameter, the other about 10.0 µm diameter. The mass of fine particles of size smaller than 2.0 µm is almost equal to the mass of coarse particles of size larger than 2.0 µm. Atmospheric aerosol size distributions consist basically of three separate modes: (i) Aitken nuclei mode for particles smaller than 0.1 µm diameter, i.e. 0.015 µm, (ii) accumulation mode for smaller than 2.0 µm, but larger than 0.1 µm, diameter particles, i.e. 0.3 µm, and (iii) coarse mode for particles larger than 2.0 µm diameter, i.e. 10.0 µm. Depending on their source there may be from one to three distinct maxima in the surface and volume or mass distributions. The activity size distribution of a radionuclide-associated aerosol particle is a surface distribution (Papastefanou and Bondietti, 1987).

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Washout by rain greatly reduces the Aitken nuclei mode and the coarse particle mode but has little effect on the accumulation mode in the trimodal size distribution (Whitby, 1975). The origin of each mode of atmospheric aerosol size distribution can be associated with various aerosol formation mechanisms, such as Brownian motion of the particles smaller than 0.1 µm in diameter, which causes the particles to diffuse and by collisions to coagulate to larger sizes. Coagulation generates multimodal distributions and affects the shape and the chemical composition of the particles.

3. Aerosols and radiation: Generation of radioactive aerosols When a radioactive nuclide decays, electrons are stripped from the parent atom by its recoil and decay products are formed as positive ions. These ions can attract liquid and even solid material, thus forming clusters of atoms or particles in the submicron region ranging from 0.001 to 0.01 µm. Air is permanently ionised by radiation from the natural radioactivity of air and by cosmic radiation which consists mostly of positively charged particles, 85% protons, 10% alpha particles with a smaller percentage of positively charged stripped nuclei of heavier elements, such as Fe, Co and Ni, etc. Production of an ion pair requires 35.6 eV if ionisation is by alpha particles and 32.5 eV if by fast electrons. In the free atmosphere, the rate of production of small ions is in balance with the rate of neutralisation by recombination and the rate of attachment to condensation nuclei. Condensation nuclei are mostly the Aitken nuclei, which are submicrometre particles in the range 0.005 to 0.01 µm. In air containing water vapour, the positive ions are mostly hydrated protons, p+ or H+ (H2 O)n , where n may be any number between 1 and 8. Negative ions are probably mostly hydrated O−− or OH− . The formation of clusters of water molecules round ions is very rapid, but the clusters do not grow beyond about 0.001 µm (1 nm) diameter and remain as small ions until they become attached to condensation nuclei. They then become large ions. Large ions themselves can be classified in two size ranges. The nuclei mode (Aitken nuclei) centred on 0.01 µm (10 nm) is distinct from the accumulation mode, centred on 0.1 µm (100 nm). The number of particles in the nuclei mode is greater than the number in the accumulation mode, but their total surface area is less, and it is surface area which determines the probability of attachment of small ions to particles with diameter of the order of 0.1 µm (100 nm) or less (Papastefanou and Bondietti, 1987). Thus, the large ions are mostly in the accumulation mode. The distinction between small and large ions is well established in the study of atmospheric electricity. The existence of intermediate ions in the size range 0.001 to 0.010 µm (1–10 nm) has been confirmed. If sulfate or nitrate vapours are present in air, molecules of these will dissolve in the water clusters and these will then grow into the intermediate ion range. A possible mechanism for formation of radiolytic nuclei is radiolysis of water, leading to formation of H2 O2 , which then oxidises traces of SO2 to give H2 SO4 . Addition of O3 to the air also increased nuclei production, whereas addition of NO, a well-known radical scavenger, inhibits it. The radionuclide ions in the atmosphere exist in two forms: (1) as “unattached clusters” with a diffusion equivalent diameter size ranging from 0.5 to 5 nm and (2) as “aerosolattached clusters” with particle diameters varying between 5 and 3000 nm. Reported values of the diffusion coefficients in the literature range from 0.01 to 0.1 cm2 s−1 (Raabe, 1968;

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7

Porstendörfer and Mercer, 1979). Most of the previous studies on the diffusion coefficient did not consider the importance of the electrical charge of the radionuclide ions and of the air humidity. Neutralisation of radionuclide ions depends on their concentrations and on the humidity in air. Recently, Dankelman et al. (2001) determined the neutralisation rates of 218 Po ions in air by an electron transfer process in “normal” environmental outdoor and indoor air with natural trace gases and found that they are small and can be neglected under “normal humidity” (RH: 309–95%) and normal radionuclide concentrations.

4. Aerodynamic size distribution of radionuclide-associated aerosol particles (radioactive aerosols) The aerodynamic size distribution of radionuclide-associated aerosol particles is, as mentioned in Section 2, a surface distribution, and so it is trimodal, the first mode being the socalled Aitken nuclei mode, the second the accumulation mode and the third the coarse particle mode. Analytically, these modes can be summarised as follows: 4.1. Aitken nuclei mode The nuclei or Aitken nuclei mode accounts for most of the Aitken nuclei count and originates primarily from the condensation and coagulation of highly supersaturated vapours. There is evidence that a prominent nuclei mode in the size distribution indicates the presence of substantial amounts of fresh aerosol. Many particles in the nuclei mode raise the Aitken nuclei. Usually, they do not greatly increase the aerosol mass concentration because the nuclei mode rarely accounts for more than a few percent of the total mass. Whitby et al. (1975) found that the nuclei mode may contain over 25 µg m−3 of aerosol. Whitby et al. (1976) also observed distributions in which the nuclei mode contained more volume than the accumulation mode. Because particles may serve as nuclei for the condensation of water vapour, condensation is an important growth mechanism for submicrometre aerosol particles. Examples are fogs and hazes formed when the humidity exceeds 60%. 4.2. Accumulation mode The twin mechanisms of coagulation and heterogeneous nucleation (condensation of one material to another) tend to accumulate submicrometre aerosol particle mass in this mode (Whitby and Cantrell, 1976; Willeke and Whitby, 1975). Because of the sharp decrease in particles larger than 0.3 µm in diameter, little mass is transferred from the accumulation mode to the coarse particle size range. Sedimentation and impaction tend to increase the relative concentration of the smallest mechanically produced particles, and then accumulate in this mode. Salt from sea spray is typically present as particles in the 1–5 µm size range, outside the normal accumulation mode. The radionuclide-associated aerosols (radioactive aerosols) peak in the accumulation mode as this mode is the main one in terms of surface area distribution (Papastefanou and Bondietti, 1987; NRC, 1979) (Figure 1.2).

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4.3. Coarse particle mode In the coarse particle mode, practically all aerosol particles at relative humidities below 100% originate from mechanical processes. Most of the particles originate from condensation processes occurring in the atmosphere. Coarse particles are produced from natural and/or man-made (anthropogenic) mechanical processes. The origin, behaviour and removal processes of fine particles are almost entirely independent of the coarse particles. The multimodal nature of the size-mass distribution is supported by evidence from the size distributions of chemical elements, radionuclides or compounds in aerosols. These are emitted as smokes or fumes and exist mostly in the fine particle size range. These aerosols eventually coagulate and become mixed throughout the accumulation mode. In the accumulation mode, the geometric mean size is nearly equal to the mass or volume geometric mean size. Elements in the soil, e.g. silicon, are mostly in the coarse particle size range. Elements, such as sulphur, which are produced by condensation processes from anthropogenic sources, are principally in the fine particle size range. The size distribution of elements in soil is similar to the mass distribution of the coarse particle mode. 5. Mechanisms of formation and growth of aerosol particles The aerosol particles are formed either by coagulation and condensation processes or by gasto-particle conversion. Analytically: 5.1. Coagulation and condensation Aerosol particles tend to coalesce when they collide with each other. Since at normal humidities most particles are sheathed with moisture, the sticking probability is close to unity. Collisions between two particles lead to the formation of a new particle of larger size. This process, called coagulation, causes the size distribution to change in favour of large particles. Coagulation must be distinguished from condensation, which describes the deposition of vapour-phase material onto particulate matter. In the absence of pre-existing particles, condensation lead to the formation of new Aitken nuclei, provided that the vapour pressure of the condensing substance is sufficiently high. The last process is termed homogeneous nucleation or gas-to-particle conversion. 5.2. Gas-to-particle conversion Atmospheric gas-phase reactions may lead to the formation of condensible products, which subsequently associate with the atmospheric aerosol. Condensation may either cause the formation of new particles in the Aitken size range (homogeneous nucleation) or deposit material onto pre-existing particles (heterogeneous condensation). The gas-to-particle conversion usually starts with air free from particles. The development of the particle size range goes through three successive stages, dominated by nucleation, coagulation and heterogeneous condensation. In the atmosphere, all three processes take place concurrently. The generation of new particles requires conditions that allow the growth of molecular clusters by condensation in the

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phase of competition from heterogeneous condensation. Molecular clusters are formed due to weakly attractive forces between molecules, the van der Waals’ forces. Except under conditions of low temperature, it is difficult to observe clusters containing more than a few molecules.

References Aitken, J. (1923). In: Knott, C.G. (Ed.), Collected Scientific Papers of John Aitken. Cambridge University Press, London, 591 pp. Clark, W.E., Whitby, K.T. (1967). Concentration and size distribution measurements of atmospheric aerosols and a test of the theory of self-preserving size distributions. J. Atmos. Sci. 24, 677–687. Dankelman, V., Reineking, A., Porstendörfer, J. (2001). Determination of neutralisation rates of 218 Po ions in air. Radiat. Prot. Dosim. 94, 353–357. Friedlander, S.K. (1961). Theoretical considerations for the particle size spectrum of the stratospheric aerosol. Am. Meteor. Soc. J. Meteor. 18, 753–759. Junge, C.E. (1955). The size distribution and ageing of natural aerosols as determined from electric and optical data on the atmosphere. Am. Meteor. Soc. J. Meteor. 12, 13–25. Junge, C.E. (1963). Air Chemistry and Radioactivity. Academic Press, New York, 382 pp. Junge, C.E. (1972). Our knowledge of the physico-chemistry of aerosols in the undisturbed marine environment. J. Geophys. Res. 77, 5183–5200. National Research Council, NRC (1979). Airborne Particles. University Park Press, Baltimore. Papastefanou, C., Bondietti, E.A. (1987). Aerodynamic size associations of 212 Pb and 214 Pb in ambient aerosols. Health Phys. 53, 461–472. Porstendörfer, J., Mercer, T.T. (1979). Influence of electric charge and humidity upon the diffusion coefficient of radon decay products. Health Phys. 37, 191–199. Raabe, O.G. (1968). Measurement of the diffusion coefficient of radium, A. Nature 217, 1143–1145. Whitby, K.T. (1975). Modeling of Atmospheric Aerosol Particle Size Distributions, A Progress Report on EPA Research Grant No. R800971. Sampling and Analysis of Atmospheric Aerosols, Particle Technology Laboratory Report No. 253. Environmental Division, Mechanical Engineering Department, University of Minnesota. Whitby, K.T., Cantrell, B.K. (1976). Atmospheric aerosols—characteristics and measurements. In: International Conference on Environmental Sensing and Assessment, Las Vegas, Nevada, September 14–17, 1975. Institute of Electrical and Electronics Engineers, New York. Whitby, K.T., Husar, R.B., Liu, B.Y.H. (1972). The aerosol size distribution of Los Angeles smog. J. Colloid Interface Sci. 39, 177–204. Whitby, K.T., Clark, W.E., Marple, V.A., Sverdrup, G.M., Sem, G.J., Willeke, K., Liu, B.Y.H., Pui, D.Y.H. (1975). Characterization of California aerosols—1. Size distribution of freeway aerosol. Atmos. Environ. 9, 463–482. Whitby, K.T., Kittelson, D.B., Cantrell, B.K., Barsic, N.J., Dolan, D.F., Tervestad, L.D., Nieken, D.J., Wolf, J.L., Wood, J.R. (1976). Aerosol size distributions and concentrations measured during the General Motor Proving Grounds sulfate study. In: The General Motors/Environmental Protection Agency Sulfate Dispersion Experiment, Rep. EP A-600/3-76-035. Environmental Protection, Research Triangle Park, NC, pp. 29–80. Willeke, K., Whitby, K. (1975). Atmospheric aerosols: size distribution interpretation. J. Air Pollut. Control Assoc. 25, 529–534.

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Chapter 2

Radioactive aerosols

1. Introduction Radioactive aerosols can be classified in the following categories: (a) Radioactive aerosols associated with radioactive nuclides of cosmogenic origin, such as 7 Be, 22 Na, 32 P and 35 S, (b) radon and thoron decay product aerosols associated with 218 Po 214 Pb, 212 Pb, 210 Pb, 210 Bi and 210 Po, (c) fission product radionuclide aerosols associated with 89 Sr, 90 Sr, 137 Cs, 103 Ru, 131 I, 132 Te and 140 Ba, (d) radioactive aerosols associated with the operation of high-energy accelerators, such as 7 Be, 22 Na, 24 Na and 52 Mn, (e) plutonium aerosols due to nuclear weapons testing or nuclear reactor accidents, and (f) mine aerosols.

2. Radioactive aerosols associated with the cosmic-ray produced radionuclides Relatively short-lived radionuclides with half-life of about a few days, of cosmogenic origin, such as 7 Be (T1/2 = 53.3 d), 32 P (14.3 d), 33 P (25.3 d), and 35 S (87.4 d) occur permanently in the atmosphere. There is another category of long-lived radionuclides with half-life larger than a year, such as 3 H (T1/2 = 12.5 y), 10 Be (1.5 × 106 y), 14 C (5730 y), 22 Na (2.6 y), 26 Al (7.3×105 y), 32 Si (280 y), 36 Cl (3.01×105 y), 39 Ar (269 y) and 81 Kr (2.1×105 y), which also occur in the atmosphere. All the above-mentioned radionuclides are formed continuously by the interaction of cosmic-ray particles with matter (atmosphere). Most of them are formed by spallation processes of light atmospheric nuclei, such as nitrogen (Z = 7), oxygen (Z = 8) and even carbon (Z = 6), or heavier atmospheric nuclei, such as sodium (Z = 11), phosphorus (Z = 15), sulphur (Z = 16), potassium (Z = 19) and calcium (Z = 20) (NCRP, 1987), when they absorb protons and even neutrons of cosmic origin. Exceptions are the production of 24 Na and 38 Cl by neutron activation of the stable isotopes 23 Na and 37 Cl, respectively. Global average production rates and concentrations of cosmogenic radionuclides in the atmosphere are summarised in Table 2.1 (Lal and Suess, 1968; UNSCEAR, 2000). Of the above radionuclides, 7 Be has a high isotope production rate in the atmosphere (8.1 × 10−2 atoms cm−2 s−1 ) and as it is a gamma-emitter (477.6 keV gammas, 11% yield), it can easily be detected and measured in the atmospheric air, in precipitation and in vegetation as well. Its average concentration in the tropospheric air is about 12.5 mBq m−3 (UNSCEAR, 2000) and 700 Bq m−3 in rainwater (UNSCEAR, 1982). Sodium-22 is also a RADIOACTIVITY IN THE ENVIRONMENT VOLUME 12 ISSN 1569-4860/DOI: 10.1016/S1569-4860(07)12002-7

© 2008 Elsevier B.V. All rights reserved.

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Table 2.1 Production rates and concentrations of cosmogenic radionuclides in the atmosphere (UNSCEAR, 2000) Radionuclide

3H 7 Be 10 Be 14 C 22 Na 26 Al 32 Si 32 P 33 P 35 S 36 Cl 37 Ar 39 Ar 81 Kr

Production rate Per unit area (atoms m−2 s−1 )

Annual amount (PBq a−1 )

2500 810 450 25,000 0.86 1.4 1.6 8.1 6.8 14 11 8.3 56 0.01

72 1960 0.000064 1.54 0.12 0.000001 0.00087 73 35 21 0.000013 31 0.074 1.7 × 10−8

Global inventory (PBq)

Fractional amount in troposphere

Concentration in troposphere (mBq m3 )

1275 413 230 12,750 0.44 0.71 0.82 4.1 3.5 7.1 5.6 4.2 28.6 0.005

0.004 0.11 0.0023 0.016 0.017 7.7 × 10−8 0.00011 0.24 0.16 0.08 6 × 10−8 0.37 0.83 0.82

1.4 12.5 0.15 56.3 0.0021 1.5 × 10−8 0.000025 0.27 0.15 0.16 9.3 × 10−8 0.43 6.5 0.0012

positron- and gamma-emitter (511 keV gammas, annihilation peak), but its production rate in the atmosphere is too small (8.6 × 10−5 atoms cm−2 s−1 ) and its concentration in the tropospheric air is 0.0021 mBq m−3 (UNSCEAR, 2000). All the other radionuclides referred to above are mostly beta-emitters with low production rates in the atmosphere and very low concentrations in the troposphere. 2.1. Beryllium-7 aerosols Beryllium-7 is a relatively short-lived (T1/2 = 53.3 d, τ = 1/λ = 77 d) naturally occurring radionuclide of cosmogenic origin formed by spallation processes of light atmospheric nuclei, such as carbon (Z = 6), nitrogen (Z = 7) and oxygen (Z = 8), when they absorb protons and even neutrons of the primary component of cosmic radiation (Bruninx, 1961, 1964; Rindi and Charalambous, 1967; Silberberg and Tsao, 1973), according to the following reactions: 12 C + p

→ 7 Be + 6 Li, 14 N + p → 7 Be + 24 He, 16 O + p → 7 Be + 10 B, 16 O + p → 7 Be + 7 Li + 3 He, 7 Be

12 C + n

→ 7 Be + 6 He, 14 N + n → 7 Be + 8 Li, 16 O + n → 7 Be + 10 Be, 16 O + n → 7 Be + 6 He + 4 He.

(2.1)

Once is formed in the troposphere, it rapidly associates primarily with submicronsized aerosol particles (Bondietti et al., 1987). Beryllium-7 in these fine aerosols may subsequently enter the marine as well as the terrestrial environment and vegetation via wet or dry depositional events. Following deposition, 7 Be will tend to associate with particulate material (a particle-reactive element). Beryllium-7 has come to be recognised as a potential tool in studying the description of environmental processes such as aerosol transit and residence times in the troposphere (Martell,

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1970), aerosol deposition velocities (Young and Silker, 1980) and aerosol trapping by aboveground vegetation (Bondietti et al., 1984). Beryllium-7, as well as other natural radionuclides like 212+210 Pb, 22 Na, 35 S and 32+33 P, participates in the formation and growth of the accumulation mode aerosols (0.07 to 2 µm diameter), which are a major reservoir of pollutants in the atmosphere. Following its production by gas-phase nuclear transformation, this isotope condenses on the aerosol population, growing by condensation of non-radioactive species (e.g., H2 SO4 or organics) and the fate of 7 Be will become the fate of the carrier aerosols (Bondietti et al., 1988). On the behaviour of 7 Be with atmospheric aerosols it was concluded from early aerosol studies that considerable coagulation occurred during migration from the stratosphere and upper troposphere to ground level (Lockhart et al., 1965b; Gaziev et al., 1966). A histogram of the activity size distribution of 7 Be versus aerodynamic diameter (Dp ) is presented in Figure 2.1. This distribution was selected by Papastefanou and Ioannidou (1995) from 11 atmospheric aerosol sampling measurements made over an almost 2-year period at Thessaloniki, Greece (40◦ 38 N, 22◦ 58 E) by using Andersen 1 ACFM cascade impactors at a flow rate of 1.7 m3 h−1 (28.3 l min−1 or 1 ft3 min−1 ). Atmospheric aerosol size appeared to follow a trimodal distribution expected for condensation-derived aerosols. This trimodal distribution of atmospheric aerosols showed the following mode ranges: the Aitken nuclei mode ranges from 0.003 to 0.07 µm (average 0.015 µm); the accumulation mode ranges from 0.07 to 2 µm (average 0.3 µm); and the coarse mode ranges from 2 to 36 µm (average larger that 10 µm) (NRC, 1979). Young et al. (1975) reported that 7 Be is attached primarily to submicron-sized particles in the atmosphere. About 88% of 7 Be was found to be present as particles smaller than 1.1 µm in diameter, and less than 1% was on particles larger than 7 µm in diameter. This means that 7 Be aerosols are accumulation mode aerosols. This is also evident from the plot of Figure 2.1a. From the 11 measurements carried out over a 2-year period including all seasons, Papastefanou and Ioannidou (1995) reported that the activity median aerodynamic diameter (AMAD) varied from 0.76 to 1.18 µm (average 0.90 µm) and the geometric standard deviation (σg ) varied from 1.86 to 2.77 (average 2.24). The AMAD and (σg ) calculations were made by plotting the cumulative distributions on log-normal probability paper. They also showed that 60% of the 7 Be activity was associated with particles with diameter smaller than 1.1 µm. Beryllium-7 aerosol measurements carried out by Papastefanou and Ioannidou (1995) at sea level in a coastal area, in a hilly area, at 250 m height and on the top of a mountain, at 1000 m altitude, showed that the 7 Be activity size distribution dominated a smaller size range of aerosol particles with an AMAD of 0.68 µm (σg = 2.18) at a height of 250 m and an AMAD of 0.68 µm (σg = 2.24) at a height of 1000 m, showing a dependency on altitude. In marine environments at sea-level, the 7 Be activity size distribution dominated a higher size range of aerosol particles with an AMAD of 0.82 µm (σg = 1.88). Bondietti et al. (1987) in 13 measurements in an almost one-year period (June 1985–March 1986) at Oak Ridge, Tennessee at temperate latitude (35◦ 58 N, 84◦ 30 W) and with a wet climate showed that the activity median aerodynamic diameter, AMAD, varied from 0.29 to 0.50 µm (average 0.35 µm) and that the fraction of 7 Be-associated aerosols above 1.4 µm was usually between 5 and 10%, i.e. analytically 4.5% was found in the 1.4 to 2.1 µm size range, 1.1% in the 2.1 to 4.2 µm size range, and only 0.2% in sizes greater than 4.2 µm.

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(a)

(b) Fig. 2.1. (a) Aerodynamic size distribution of 7 Be ambient aerosols. (b) Relative activity size distribution of 7 Be in outdoor air.

They also concluded that cosmogenic radionuclides such as 7 Be and 35 S were associated with smaller aerosols than the longer-lived radionuclides such as 210 Pb, a decay product of 222 Rn of terrestrial origin, which were associated with larger aerosols. Röbig et al. (1980) reported that the distribution of the long-lived radionuclide 7 Be was shifted to large particle sizes due to long residence times of 7 Be in the atmosphere. An equivalent aerodynamic diameter of about 0.65 µm for 7 Be might have resulted from the plot of the activity size distribution of the ambient air obtained by a high volume cascade impactor

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(flow rate 68 m3 h−1 ) at Göttingen, Germany (51◦ 32 N, 9◦ 55 E). Shifts to large particle sizes were also observed when the relative humidity increased during rainfall. Much later, Grundel and Porstendörfer (2004) observed that the results of the 7 Be measurements for a period of 4 weeks, carried out in outdoor air in a suburb of the town of Göttingen, showed no activity fraction in the nucleation (Aitken nuclei) mode, but a small amount of the activity (5%) in the coarse mode size range (Figure 2.1b). The accumulation mode of 7 Be-aerosols with an activity fraction of 95% has an activity median aerodynamic diameter AMAD value of 702 nm. The activity size distribution of 7 Be-aerosols depends probably on their location of formation. Most of the 7 Be atoms and the 7 Be-aerosols are generated in the upper region of the atmosphere, where different aerosol conditions exist than those in the lower atmosphere. In the activity size distribution of ambient aerosols, 7 Be is shifted to large particle sizes in the presence of pollutants. Removal of small aerosol particles in the submicron size range of the activity size distribution either by scavenging or by deposition of particles on any surface may result in a depletion of small particles in the activity size distribution. Subsequent 7 Be condensation on all aerosols effectively enriches large particles in the activity size distribution. The radioactive aerosol is only generated by attachment and there is no nucleation, in contrast to the sulfate aerosol (nucleation + attachment) (Hopke, 1991). Freshly produced 7 Be should attach to an existing aerosol or coagulate with other nuclei during its lifetime, as mean attachment half-lives are of the order of a minute or less (Porstendörfer and Mercer, 1980). Papastefanou and Ioannidou (1996) reported that sites where the AMAD of 7 Be aerosol varied between 0.62 and 0.74 µm were located on lines of direction of wind blow (local wind). These streamlines of air masses transfer pollutants from an industrial zone to urban air and the marine environment over the sea surface. In the area influenced by the industrial zone, the AMAD of the 7 Be aerosol varied between 0.82 and 1.00 µm which is higher than that observed outside the industrial zone. Winkler et al. (1998) in 46 measurements in a period of 1 1/3 years (December 1994– March 1996) at Munich-Neuherberg, Germany (48◦ 13 N, 11◦ 36 E) showed that the activity median aerodynamic diameter, AMAD, of 7 Be-aerosols ranged from 0.44 to 0.74 µm (average 0.57 µm) and that because of seasonal effects during the period of high 7 Be air concentrations, i.e. in the summer period, relatively low values of the AMAD (0.45–0.52 µm) occur. They also concluded that the activity median aerodynamic diameters, AMADs, ranged between the mass median aerodynamic diameters, MMAD, and the surface median aerodynamic diameters, SMAD, of the ambient aerosols, indicating that this radionuclide is involved in the transformation process of the tropospheric aerosols after their formation in the stratosphere and upper troposphere. Yu and Lee (2002) in 14 measurements in Hong Kong (22◦ 18 N, 114◦ 10 E) for a 3 1/2 month period (26 November 2001–8 March 2002) demonstrated that the activity median aerodynamic diameter, AMAD, of 7 Be-aerosols varied from 0.33 to 1.15 µm (average 0.67 µm). They concluded that the AMADs of 7 Be-aerosols are anticorrelated with 7 Be concentrations in air, are correlated with relative humidity, RH and mean cloud cover, while temperature does not affect the AMADs of the 7 Be-aerosols. Apart from these, Lujaniene et al. (2001) reported larger 7 Be AMAD values varying from 1.12 to 2.06 µm (average 1.45 µm) at a northern latitude in Vilnius, Lithuania (54◦ 41 N,

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25◦ 19 E). The data rather indicate that the activity median aerodynamic diameter, AMAD, of 7 Be-aerosols increases with increasing latitude (latitudinal effect). As cosmic radiation increases with latitude, the numbers of 7 Be atoms and ions formed also increase with latitude and so there are more 7 Be atoms and ions available either to form small aerosol particles in the nucleation (Aitken nuclei) mode and then growing or to be attached directly to the existing large particles in the accumulation and in the coarse particle modes thereby increasing the AMAD of the 7 Be-aerosols.

3. Radon and thoron decay product aerosols The gases radon (222 Rn) and thoron (220 Rn) are formed as decay products of uranium and thorium in the uranium and thorium series in soil and rocks. They are emitted from the ground into the atmosphere, where they decay and form decay products, radioisotopes of polonium (Z = 84), bismuth (Z = 83) and lead (Z = 82), which either remain airborne till they decay or are deposited in rain and by diffusion to the ground. The decay of 222 Rn (radon) and 220 Rn (thoron) in the atmosphere produces low vapour pressure decay products which coagulate with other freshly produced nuclei or condense on existing accumulation-mode aerosols. These radioisotopes include 218 Po (T1/2 = 3.05 min), 214 Pb (26.8 min), and 212 Pb (10.64 h). A longlived radioisotope in the 222 Rn decay chain, 210 Pb (22.3 y), is produced about an hour after attachment of 218 Po. There is considerable information about radon decay product aerosols in ambient air (outdoors) and even in residences (indoors) (Mercer and Stowe, 1971; Kruger and Andrews, 1976; Porstendörfer and Mercer, 1978; Kruger and Nöthling, 1979; Porstendörfer and Mercer, 1979; Porstendörfer et al., 1979; George and Breslin, 1980; Porstendörfer and Mercer, 1980; Becker et al., 1984; George et al., 1984; Bondietti et al., 1987; Papastefanou and Bondietti, 1987; Porstendörfer, 1994; Grundel and Porstendörfer, 2004). The concentration of radon (222 Rn) in ambient air is about 10 and 40 Bq m−3 in indoor air, while the concentration of thoron (220 Rn) in outdoor air is about 10 Bq m−3 and approximately the same indoors (UNSCEAR, 1993, 2000). The concentrations of the radon (222 Rn) decay products in outdoor air are 19.05 Bq m−3 for 218 Po, 3.88 Bq m−3 for 214 Pb, 5.26 Bq m−3 for 214 Bi, 0.5 mBq m−3 for 210 Pb and 0.050 mBq m−3 for 210 Po, while for thoron (220 Rn) decay products in outdoor air, they are 0.055 Bq m−3 for 212 Pb and 0.575 Bq m−3 for 212 Bi (UNSCEAR, 2000). 3.1. Formation of radon decay product aerosols Alpha decay carries away positive charge and electrons are stripped from the parent atom by its recoil. Therefore, the decay products are formed as positive ions. Air is ionised by radiation from the naturally occurring radionuclides in the air and on the ground and by cosmic rays. Production of one ion pair requires 32.5 eV if ionisation is caused by fast electrons, 35.6 eV if by alpha particles. The total energy dissipated in air per decay of 222 Rn depends on the equilibrium ratio of the radon decay products. In the free atmosphere, the rate of production of small ions is in balance with the rate of neutralisation by recombination and the rate of attachment to condensation nuclei. Condensation nuclei, otherwise called Aitken nuclei, are submicrometre particles mainly pro-

Radioactive aerosols

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duced by combustion processes. In air containing water vapour, positive ions are mostly hydrated protons, H+ (H2 O)n , where n may be any number between 1 and 8. Negative ions are mostly hydrated O−− or OH− . The formation of clusters of water molecules round ions is very rapid, but in unpolluted air the clusters do not grow beyond 1 nm diameter and remain as small ions until they become attached to condensation nuclei. They then become large ions. Large ions themselves can be classified in two size ranges, the nuclei mode centred on 0.01 µm which is distinct from the accumulation mode, centred on 0.1 µm (Whitby, 1978). In urban air, the number of particles in the nuclei mode is greater than the number in surface area which determines the probability of attachment of small ions to particles with diameters of order 0.1 µm or less. Thus the large ions are mostly in the accumulation mode. The distinction between small and large ions is well established in the science of atmospheric electricity. Intermediate ions exist in the size range 0.001–0.01 µm. If sulfates and nitrates are formed photochemically in air, molecules of acid will dissolve in the water clusters and these will then grow into the intermediate ion range (Raes, 1985). Reaction products can be created in air by radiolytic as well as photolytic processes. The radiolytic nuclei are initially uncharged, indicating that ions are not required for their formation. A possible mechanism for formation of radiolytic nuclei is radiolysis of water vapour, leading to formation of H2 O2 , which then oxidises traces of SO2 to give H2 SO4 . Increased formation of radiolytic nuclei occurs in air when SO2 is added. Addition of O3 to the air also increases nucleus production, whereas addition of NO, a well-known radical scavenger, inhibits it. The radon decay product aerosols in the atmosphere are generated in two steps. After formation from the radon isotope (222 Rn), the freshly generated decay product radionuclides react very fast (