AEROSOL CHEMICAL PROCESSES IN THE ENVIRONMENT - CHAPTER 10 docx

Spurny CONTENTS Introduction ...213 Laboratory Methods of Preparing Radioactively Labeled Aerosols...214 Labeling by Means of Decay Products of Radon and Thoron ...214 Ultra-fine Aerosol

Experimental Aerosol Research

Kvetoslav R Spurny

CONTENTS

Introduction 213

Laboratory Methods of Preparing Radioactively Labeled Aerosols 214

Labeling by Means of Decay Products of Radon and Thoron 214

Ultra-fine Aerosols by Radiolysis 215

Labeling by Means of Radioactive Gases 217

Labeling by Means of Radioactively Labeled Elements and Compounds 218

Equipment and Procedures 219

Labeling by Means of Radiolabeled Condensation Nuclei 222

Radioactively Labeled Carbonaceous Aerosols 226

Radioactively Labeled Fibrous Mineral Aerosols 228

Radioactive Labeling of Sampling Filters 231

Radioactive Aerosol Labeling in Animal Inhalation Toxicology and Medical Research 234

Animal Inhalation Toxicology 235

Generation Techniques 235

Radioactively Labeled Model Aerosols in Human Medicine 236

Choice of Particles and Radiolabel 236

Radiolabeled Aerosols for Ventilating Imaging 237

Radioactive Labeling of Atmospheric Aerosols 238

Labeling by Decay Products of Radon and Thoron 238

Labeling by Cosmic Radiation 238

Labeling by Artificial Radioactivity 238

Fission Products 238

Industrial Sources 239

Nuclear Power Plants 239

Chernobyl Aerosol Characterization 239

Radiolabeled Atmospheric Aerosols and Radiation Smog 239

References 243

INTRODUCTION

From an experimental point of view, in studying physical, chemical, and biological behavior and effects of solid and liquid aerodisperse systems under atmospheric and laboratory conditions, the application of radioactive labeling procedures is very useful

Radioactive aerosols and radioactively labeled aerosols have existed in nature probably as long

as our planet has existed In 1995, Renoux published an overview of the history of the natural atmospheric radioactivity The discovery of the rare radioactive gas radon is attributed to Pierre

214 Aerosol Chemical Processes in the Environment

and Marie Curie in 1898 and Dorn in 1900 Thoron (220Rn) was discovered by Rutherford andOwens in 1899–1900, and actinon (219Rn) by Debierne and Geisel about the same time

The first scientist to find radioactive aerosols was Marie Curie in 1905.2 She studied the influence

of gravitational field on the decay products of radon Radon’s radiotoxicity was first studied inFrance in 1904 by Bouchard and Balthazard, and in 1924 it was hypothesized that the great mortalityobserved in uranium mines of Schmeeberg in Germany and Joachimsthal in Czechoslovakia wasdue to radon In 1939, Read and Mottram found that radioactive aerosols are biologically moreeffective than radon itself.2,3 Elster and Geitel were the first to see (in 1901) that radioactivity ispresent in the atmosphere.1

Since World War II, radioactive aerosols have become well known, and the object of increasingstudies and use Their physical properties and effects started to be intensively studied in the 1950s.Wilkening estimated the size distribution of the natural radioactive aerosols in the atmosphere in

1952 and, in 1959, Jacobi found that more than 50% of the natural atmospheric radioactivity isdeposited on aerosol particles smaller than 0.2 µm The first theory of small particle labeling byradioactive ions was developed by Bricard in 1949

The exploitation of radioactive labeling in aerosol research also dates back to the 1950s.Nevertheless, a very fast development started about ten years later.3 Since that time, basic theoreticalinvestigations have led to a complex description of the nuclear methods applied in physical andchemical research.4

What is the difference between a radioactive aerosol and a radioactively labeled aerosol? Theremay be no precisely definable difference From a historical point of view, all radioactively labeledaerosols in the atmosphere and space are called radioactive aerosols But from a radiochemicalpoint of view, for aerosols used in the laboratory conditions, it is best to use the expression

“radioactively labeled aerosols.” This means that only some aerosol particles are radioactive, andthat only a portion of each particle is in fact radioactive In contrast, the expression “radioactiveaerosol” means that all particles are radioactive, and that each particle consists predominantly ofradioactive species

LABORATORY METHODS OF PREPARING RADIOACTIVELY

L ABELING BY M EANS OF D ECAY P RODUCTS OF R ADON AND T HORON

This method is used very often and is similar to the natural radioactive labeling of fine aerosolparticles in the atmosphere Through a diffusion process, the natural aerosols are labeled by means

of radon and thoron decay products.3 The relative distribution of the activity on particles of differentsizes was first described by Lassen in 1965.5 This distribution function was constructed assumingthe validity of Junges’s distribution of natural aerosols,6 including the condition of coagulation (seeTable 10.1) Wire screen diffusion batteries have been found to be the most suitable method formeasuring the activity size distribution of radon and thoron progeny.7-9

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The short-lived decay products of 222Rn and 220Rn are formed initially in an atomic, positivelycharged state that rapidly combines with submicron (mainly with nano-sized) aerosols The resultingultra-fine aerosols consist of a complex mixture of charged and neutral particles Under normalconditions, the average electrical charge of the 222Rn and 222Rn progeny atmosphere is substantiallyless than one elementary unit The electrical charge distribution is mostly symmetrical.10-13

The decay-product method of labeling is relatively easy to use in the laboratory An artificialinactive aerosol is passed through a cylinder filled with radon or thoron When aerosol particlesremain in this atmosphere for a sufficient length of time, they become alpha-radioactive It should

be noted, however, that if concentrations of thoron greater than about 1 µCi (27 kBq)/liter are used,aerosols may be produced by radiolytic reactions with impurities in the air these may also becomelabeled with ThB and confuse the picture.3

Ultra-fine Aerosols by Radiolysis

It has been reported for many years that condensation nuclei can be produced by ionizing radiation.For example, radiolysis following the decay of 222Rn results in the production of ultra-fine aerosols.Recent studies were able to improve the measurement of activity size distribution of these ultra-fine particles produced by radon and its daughters It has been found that the activity that wasconventionally referred to as the “unattached” fraction is actually an ultra-fine particle aerosol fromwater molecule radiolysis with a size range of 0.5 nm to 3 nm.14 Oxidizable species such as SO2react promptly with hydroxyl radicals and form a condensed phase These molecules coagulate andbecome ultra-fine particles The size distribution of these ultra-fine particles can be shifted upwardwith the increase of SO2 concentrations

Further investigation13 showed that 218Po formed — during radon decay in well-controlledcomposition atmospheres (e.g., N2) — clusters in the size range between 0.7 nm and 2.0 nm Figure10.1 shows the diagram of such a 218Po cluster generation system The size of the produced clusterscould be efficiently measured by means of a SMEC (spectrometre de mobilite electrique circulaire)device

The clusters formed in the radiolysis of radon include progeny particles and nonradioactiveparticles In more recent investigations, the activity size distributions of 212Pb- and 212Bi-bornenanometer particles were produced and measured When thoron gas enters the spherical chamber

charge, they attract polar molecules and form clusters The cluster sizes measured by means of adiffusion battery (DB) were less than 2 nm.9

TABLE 10.1 Some Methods for Preparing Radioactively Labeled Aerosols

neutron source (not very suitable).

activity on particles of different sizes (L Lassen):

A(r) dr = Φ (r) N(r) dr

at low pressure.

aerosols, disperse aerosols, and plasma aerosols).

216 Aerosol Chemical Processes in the Environment

FIGURE 10.1 Schematic diagram showing the system for the generation of 218 Po cluster aerosols (From Mesbah, B., Fitzgerald, B., Hopke, P.K., and Pouprix, M., Aerosol Sci Technol., 27, 381-393, 1997 With permission.)

FIGURE 10.2 Experimental setup for the generation and measurement of nanometer-sized 212 Pb- and 212 borne particles (From Chen, T.R., Tung, C.J., and Cheng, Y.S., Aerosol Sci Technol., 28, 173-181, 1998 With permission.)

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L ABELING BY M EANS OF R ADIOACTIVE G ASES

This method consists of exposing an aerosol or an aerosol sample to a high-frequency discharge atlow pressure in a mixture of radon, krypton, or xenon, etc., and air The atoms of a radioactive gas,ionized and accelerated in the electric discharge, penetrate and are retained on the surface of theaerosol particles The method of labeling has two attractive features First, the position of individualparticles can be determined by autoradiography When radon is used for labeling and radiography iscarried out with nuclear emulsion, individual particles show up in the radiogram as stars consisting

of the tracks of alpha-rays (Figure 10.3) The frequency of the tracks in each star is an indication ofthe particle’s size Second, the action of a suitable gaseous medium (a chemical surface reaction) onthe aerosol particles can release the radioactive gas from the aerosol sample This feature providesthe possibility of chemically identifying individual particles in the aerosol sample

An aerosol can be activated directly in a suspended state, independent of its chemical sition, in a stream of gas By repeated measurements of aerosol activity, the aerosol concentrationcan be measured continuously Labeling with decay products of radon is most suitable for thesepurposes, because the radon is not used in gaseous form and is attached to the surface of the solidsubstances Radon can be firmly fixed, for example, on the inner wall of a glass tube with the aid

compo-of an electric discharge at low pressure.15,16 The radon is retained near the surface and a largeproportion of the RaA atoms originating from the decay are ejected by recoil into the gas insidethe tube Because of their low energy, these atoms traverse a small distance, roughly 0.1 mm; and

if the air is free of aerosols, they quickly diffuse back to the surface of the tube, where they areretained If the air contains aerosol particles, however, some of the RaA atoms are retained by theaerosol; and the retention of RaA atoms increases with increasing concentration of the aerosol

An instrument for continuous measurements of inactive aerosol concentration, based on thisprinciple, was built and described by Jech in 1963.16 The function of the instrument is shownschematically in Figure 10.4 The aerosol sample in air flows at a speed of roughly 0.25 1/minthrough the activating tube (A), which contains 5 to 10 mCi (185 to 370 MBq) radon The activatedaerosol emerges from the tube, is filtered by a Millipore filter (F), and its activity is measureddifferentially The activity of the filter was continuously measured by a Geiger-Müller counter (inits proportional region); and the counts were integrated and registered by the ratemeter (Rm) and

FIGURE 10.3 The tracks of alpha-rays from single aerosol particles.

218 Aerosol Chemical Processes in the Environment

recorder (Rg) The relative amount of RaA atoms retained by individual particles was dependent

on the size of the particles as well as on the numerical concentration of the aerosol Therefore, theinstrument had to be calibrated for an aerosol of given dispersity.17

L ABELING BY M EANS OF R ADIOACTIVELY L ABELED E LEMENTS AND C OMPOUNDS

In this case, there are two principle possibilities: (1) preparation of dispersed aerosols by spraying

or nebulizing solutions or powders; and (2) preparation of condensation aerosols by spontaneousvapor condensation, or vapor condensation in the presence of radioactive condensation nuclei.The first method has some disadvantages: the possibility of contamination is great; the con-sumption of radioactive material is large; and the aerosol particles show little specific radioactivity.Nevertheless, it was used in some cases to great effect in the 1960s and thereafter.18

However, the second possibility — the use of condensation methods — provides highlydispersed aerosols, approximately monodisperse, and the particles show a high specific radioactivity.Through the nucleation process, the particle size and the aerosol concentration can be changed bychanging the supersaturation of the vapor From nucleation theory, it is known that the particleconcentration for a given time is an exponential function of the supersaturation of a vapor Thissupersaturation is controlled in practice by changing the evaporation temperature of the substanceand the flow rate of dilution gas When all conditions are constant, the concentration can becalibrated and the particle size determined as a function of evaporation temperature and gas flow,the particle size being measured with an electron microscope, diffusion battery, etc.19

The chemical elements and compounds for preparing condensation aerosols have to be stable;they should not decompose on heating Evaporation is often accompanied by oxidation, so that theaerosol being prepared becomes oxidized Tables 10.2 and 10.3 describe the elements and inorganicand organic compounds that are suitable for preparing condensation aerosols and which are easy

to label with different radioisotopes Table 10.4 shows more detail concerning some radioactivelylabeled inorganic condensation aerosols that were described and used in laboratory experiments inthe 1960s.19-27

FIGURE 10.4 Schematic diagram of apparatus for continuous recording of aerosol concentration, and an example (inset) of a recording that shows aerosol concentration in unfiltered and filtered air from the laboratory.

A = activating tube; F = Millipore ® filter; GM = Geiger–Müller tube; Rm = ratemeter; Rg = recorder; S = lead shield; D = revolving metal disk; P = pump; CPM = counts per minute, t = time.

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Equipment and Procedures

Different kinds of equipment can be used for spontaneous condensation under constant conditions

Three of them have proven to be very suitable for the generation of highly dispersed radiolabeled

condensation aerosols Such model aerosols make it possible to measure more rapidly and

sensi-tively numerous processes in the mechanics of aerosols (e.g., coagulation, phase transformation,

filtration, deposition, etc.)

TABLE 10.2 Inorganic Material Suitable for Preparing Radioactively Labeled Condensation Aerosols

Element or Compound

Melting Point (°C)

220 Aerosol Chemical Processes in the Environment

Furnace Generators

The apparatus for the generation of condensation aerosols by sublimation of the solid phase or by

evaporation of the liquid phase or inorganic substances consists of an electric furnace in which the

substance under study is heated to an adjustable and controlled temperature The dry gas passing

through the furnace at an adjustable flow rate is enriched with the vapor or aerosol particles of the

same substance used After passing through the furnace, the gas with aerosol particles is led into

a condenser and then into a homogenizer Several types of furnace, each specially designed for an

individual aerosol or aerosol group, proved suitable.19-27

A longitudinal furnace (Figure 10.5) was employed to prepare sodium chloride aerosols.19 A

ceramic tube was heated by two electric coils In the first part of this tube, radiolabeled NaCl [24Na,

10 to 500 mCi (370 MBq to 18.5 GBq)] was heated to the desired temperature in a porcelain boat

The second part of the tube was heated to a temperature about 10% higher than that in the first

part A vertical furnace (Figure 10.6) was employed to prepare silver iodide aerosols The furnace

consisted of two halves that were heated by electric, ceramic heating elements with a power output

of 800 W The gas entered the space over the substance (AgI) in the middle of a sealed silica tube

The vapor and the aerosols of silver iodide were drawn off from the upper part of the furnace The

yellow powder of AgI was added through a wider tube into a platinum crucible placed on the

bottom of the tube The AgI can be radiolabeled by 131I or by 110Ag

For substances with a low melting point or high vapor pressure (e.g., Se, SeO2, H2SO4, H4P2O7),

an apparatus with a double glass orifice was found suitable (Figure 10.7) Here, a vapor was

condensed in a gas stream After going through the double orifice (2), the vapor and cold clean

gas were combined in the mixing reservoir (5) The produced aerosols were radiolabeled by 35S,

75Se, and 32P.19

TABLE 10.3 Organic Compounds Suitable for Preparing Condensation Aerosols (Radioactive Labeling by Means of Radioactive Condensation Nuclei)

Compound Formula

Melting Point (°C)

Wire Generators

Aerosol generators in which metal wires can be evaporated have also proved very suitable Thistype of condensation aerosol generator produces a constant concentration of aerosol particles andconstant particle sizes; these are reproducible Furnace generators require a few hours before theywork stably On the other hand, wire generators, 10 minutes after they are turned on, produceconstant particle sizes The preparation of radioactively labeled aerosols from platinum wire andnickel-chromium wire were reported in the middle of the 1960s.28,29

The principle of such a generator is shown in Figure 10.8 Clean, dry, and preheated air (G)flows across a platinum wire, which is heated electrically The produced aerosols can be labeled

by 197Pt and 199Au Similarly, other types of metal wires have been found suitable, such as Re (186Re,

188Re), Au (198Au), etc.22

Another apparatus that can be used for preparing radiolabeled aerosols by wire evaporation is

a “plasma” aerosol generator.30 The principle of this method is shown in Figure 10.9 The tungsten

or platinum wire (W) is exploded using energy stored in a bank of condensers (about 30 J) Suchwire explosions are possible in atmospheres of various gases.31

Sintering Metal Generators

Highly dispersed silver aerosols have found useful applications in various physical, chemical, andbiological investigations Generation procedures for this metallic aerosol have been reported inseveral publications since the middle 1970s.32

TABLE 10.4 Radioactively Labeled Inorganic Condensation Aerosols

Compound or Element a

Temperature Range (°C)

Range of Particle Radii ( µµµµm)

Radioactive Isotopes (half-life)

222 Aerosol Chemical Processes in the Environment

Sutugin et al.33-36 have developed a fundamental theoretical basis for the nucleation of metaland metal oxide molecules, and their results were later exploited for practical aerosol preparation.The Ag-aerosol can be easily radiolabeled At the end of the 1970s, Spurny developed anddescribed a generator for highly dispersed (Ag + 110Ag) aerosols.27 In this generator, disks of sinteredsilver particles (produced as “silver membrane filters” by Flotronics, U.S.) were used as the initialmaterial (Figure 10.10) The disks were labeled with 110Ag by neutron activation

A schematic and a photograph of the apparatus for preparation of condensation aerosols ofradiolabeled silver are shown in Figure 10.11 The silver filter disk (Ag) was heated by electriccurrent Nitrogen, helium, or argon was used as the inert gas Approximately monodisperse radio-labeled aerosols of silver were prepared at furnace temperatures between 400 and 1000°C (Agmelting point is 960.8°C.) In the temperature range below the melting point (400 to 950°C), veryfine aerosols could be obtained at concentrations between 10 ng l–1 and 5 µg l–1 Mean particlediameters ranged between about 2 nm and 6 nm At temperatures above the melting point (sinteredsilver was maintained in a porcelain boat), the particle sizes increased rapidly up to over 0.1 µm(Figure 10.12) This aerosol reacts easily with gases and vapors, such as O2, H2S, Cl2, Br2, I2, etc

L ABELING BY M EANS OF R ADIOLABELED C ONDENSATION N UCLEI

The condensation methods described can be used to prepare aerosols of relatively small particlesize; for example, those smaller than 0.5 µm in diameter However, these methods are not convenientfor labeling organic aerosols because oxygen has no usable radioisotopes, and carbon and hydrogenyield soft radiation In such cases, preparation by means of radiolabeled condensation nuclei should

be considered

A combination of two kinds of aerosol generators is useful for the preparation of liquid organicaerosols labeled by radioactive nuclei It is composed of a furnace generator for preparing radio-active condensation nuclei, and a modified Sinclair-LaMer generator (Figure 10.13) An organic

FIGURE 10.5 A longitudinal furnace for preparing inorganic condensation aerosols 1 = boat containing

porous ceramic and an inorganic substance; 2 = metal shield; 3 = ceramic and asbestos shield; 4 = reheater;

5 = adjusting screws; 6 = quartz tube; N2 = nitrogen; T = thermometer; Va = Variac.

compound (e.g., dioctyl-phthalate, dioctyl-sebacate, etc.) is absorbed on the surface of silica gel(S) The organic vapor diffuses through the inner orifice (I) after heating The condensation nucleipass through the diluting space (D2) and reach the outer orifice (O) radioactively labeled Undersuitable conditions, organic vapor condensed on these nuclei The degree of supersaturation in themixer D1 depends on the nature of the liquid, the velocity of both gas streams, the temperature ofthe vaporizer, and the concentration of nuclei.37 Good results were obtained with radiolabeled nuclei

of NaCl, Se, and H4P2O7.20

Another useful generator for preparing radiolabeled organic aerosols was the apparatusdescribed by Prodi in 1970.38 A collision generator disperses very diluted solutions of inorganicsubstances (e.g., NaCl) (see Figure 10.14) After drying (C) fine condensation nuclei are introducedinto a thermostated bubbler (D) containing melted carnauba was The outcoming particles act ascondensation nuclei and, as a result, a solid monodisperse wax aerosol is formed This procedurealso works very well using radiolabeled nuclei Good results were obtained by nuclei of 24NaCland (NH4)699Mo7O24⋅ 4H2O (see Figure 10.15).25

Solid nuclei of 99MoO3 were also successful.26 The ammonium paramolybdate decomposes byheating, and fine molybdenum trioxide is produced (Figure 10.16)

FIGURE 10.6 A vertical furnace for preparing inorganic condensation aerosols 1 = platinum or gold dish

containing an inorganic substance; 2 = quartz tube; 3 = shields; 4 = metal cover; 5 = heating bodies; 6 = metal network; T = thermometer; Va = Variac; A = aerosol; N2 = nitrogen.

224 Aerosol Chemical Processes in the Environment

FIGURE 10.7 Evaporation equipment with double orifice 1 = dish containing an inorganic substance; 2

= double orifice; 3 = path of cold gas stream (G1); 4 = furnace; 5 = mixing reservoir; 6 = glass evaporator; G2 = gas supply; T = thermometer; W = water for cooling; A = aerosol.

FIGURE 10.8 Platinum wire aerosol generator 1 = platinum wire in a ceramic insulator; 2 = cooling

section; 3 = glass sphere for diluting; G = gas; A = aerosol.

FIGURE 10.9 Schematic of a plasma aerosol generator W = wire (e.g., a tungsten wire about 0.05 mm

in diam and 2 mm long); N2 = nitrogen; A = aerosol.

FIGURE 10.10 Scanning electron micrographs of the surface of a silver membrane filter (pore diameter

= 0.2 µm; different magnifications).

226 Aerosol Chemical Processes in the Environment

R ADIOACTIVELY L ABELED C ARBONACEOUS A EROSOLS

Carbonaceous (black carbon or soot) aerosols in polluted atmospheres are totally respirable, withparticle sizes less than 1 µm They carry many, mostly organic, toxic substances (e.g., PAH, nitro-PAH, etc.)

Radiolabeling is a very useful methodology for physico-chemical studies as well as animaltoxicological investigations of the production, behavior, and health effects of this group of aerosols.Combustion processes are the most important source of production of carbonaceous aerosols in theatmospheric environment

FIGURE 10.11 Equipment for the preparation of silver aerosols A = aerosol; Ag = silver filter; N =

nitrogen; HB = reheater; RO = tubular furnace; QR = quartz tube; TR = transformer.

Therefore, it is also reasonable to use the combustion of well defined fuels for generating modelcarbonaceous aerosols under laboratory conditions and to apply them in physical, chemical, andanimal toxicological studies For radioactive labeling, 14C is the most suitable radioisotope.Very fine dispersed carbonaceous aerosols can be produced in the laboratory by an incompletecombustion of acetylene or acetylene + benzene.39,40 A mixture of acetylene, benzene, and oxygen,introduced into a small special burner, is additionally and continuously labeled by a radioactivegas or vapor (e.g., by 14C-acetylene, 14C-benzene, etc.) The system for very sensitive dosage ofthe radiolabeled benzene vapor is schematically shown in Figure 10.17 The radiolabeled benzene

is available in glass vials (Ra-S) After opening, the vial is heated (heater D, regulation transformer

RT, and thermometer T) so that the rate of diffusion of the radiolabeled benzene vapor throughthe glass fritt (F) into a chamber (M) can be well controlled The original gas mixture (B) islabeled by the 14C-benzene (Ra-B) by this procedure, and is then introduced into the burner Thefine carbonaceous aerosol thus produced is labeled by 14C (Figure 10.18) and has specific activities

in the range 1 to 10 µCi (37 to 370 kBq)/mg Such a model carbonaceous aerosol can be loadedwith different PAH (non-active or radioactive) and used in physico-chemical as well as toxico-logical studies.39 By studying the behavior of a benzo(a)pyrene (BaP) aerosol itself and incombination with soot particles, different resublimation characteristic curves can be observed(Figure 10.19)

Radiolabeled BaP-aerosol and a mixture of “soot” + BaP were sampled on silver membrane

or glass fiber filters Then, the filter sample was gradually heated and the released radioactivitywas measured The BaP alone could be completely resublimated very quickly The BaP “coated”

on soot particles sublimated much more slowly, at higher temperatures, and incompletely Afterbeing heated at temperatures over 200°C, about 10% of the BaP was still bound on the sootsample

Similar useful labeling procedures can be applied directly in a diesel engine Radiolabeleddiesel exhaust is then produced and can be used for physical, chemical, and mainly for inhalationtoxicological studies on animals.41 A single-cylinder diesel engine was used to burn diesel fuelcontaining trace amounts of 14C-labeled hexadecane, dotriacontane, benzene, phenanthrene, orbenzo(a)pyrene Greater than 98% of the 14C in all additives was converted to volatile materialsupon combustion It has been found that aromatic additives labeled carbon particles more efficientlythan aliphatic additives

FIGURE 10.12 Scanning electron micrograph and size distribution curves of silver aerosol particles

(par-ticle diameter D and par(par-ticle number frequency F) prepared at furnace temperature of 980°C.

228 Aerosol Chemical Processes in the Environment

R ADIOACTIVELY L ABELED F IBROUS M INERAL A EROSOLS

Fibrous mineral aerosols belong in the group of aerosols consisting of nonspherical particles Theparticle shape, size, and chemical composition are parameters characterizing the physical, chemical,

as well as toxic effects of any fibrous aerosol.42-51

The procedure of radioactive labeling is therefore of basic importance in physico-chemical andtoxicological studies in this field The fibrous mineral aerosols are produced by dispersing natural(mainly asbestos) and man-made (glass, ceramic, etc.) mineral fibers These are mostly silicatefibers containing several elements, which are often characteristic for different types of fibers, andcan be radiolabeled, for example, by irradiation with thermal neutrons in a reactor (by fluxes ofabout 2.1014/cm2 s)

Particle size — fiber diameter and fiber length — is involved in the dynamic behavior and inthe toxic health effects Before labeling, the fibrous powders to be used should be well classifiedwith respect to fiber diameter and fiber length.46 Such quasi-monodispersed powders (Figure 10.20)can be irradiated for a period of 7 days.42-45

FIGURE 10.13 Schematic of a condensation aerosol generator: aerosol particle size multiplier (APSM).

B = boat containing nucleus material; N2 = nitrogen; S = silica gel; DO = double glass orifice; I = inner orifice; O = outer orifice; D1, D2 = glass spheres for diluting; H = heaters; T = thermometer; C = cooler; W

= water supply for cooler.

FIGURE 10.14 Schematic diagram of the “Prodi”-generator.

FIGURE 10.15 Activity size distribution spectra with ammonium paramolybdate nuclei obtained at three

different temperatures.