Urban aerosols, analysed by secondary ion mass spectroscopy

The Science o f the Total Environment, 31 (1983) 263--275 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

263

URBAN AEROSOLS, ANALYSED BY SECONDARY ION MASS SPECTROSCOPY*

N. KLAUS Institute for Experimental Physics of the University of Vienna, A-1090 Wien (Austria) (Received January 26th, 1983;accepted March 4th, 1983)

ABSTRACT Urban aerosols, collected on membrane filters by an eleven stage low pressure impaetor have been analysed by SIMS (Secondary Ion Mass Spectroscopy). An atomic beam of argon neutrals, 3.5 mm in diameter, with an energy equivalent of 1.4 keV and a flux of 6" 1013 atoms cm -2 sec, generates the emission of positive and negative secondary ions (SI). The intensity of these SI vs. their ratio m/e has been recorded. As a result the distributions of the relative concentrations vs. the particle size of several elements, as well as chemical compounds of these aerosols beginning with hydrogen up to a m/e of 100, are shown.

INTRODUCTION T h e c h e m i c a l analysis o f aerosols, c o l l e c t e d b y i m p a c t o r s o r o t h e r classifiers, b e c o m e s m o r e a n d m o r e i m p o r t a n t f o r t h e u n d e r s t a n d i n g o f a e r o s o l h i s t o r y a n d a e r o s o l e f f e c t s o n m a n a n d n a t u r e . Whereas t h e p h y s i c s o f a e r o s o l s are k n o w n t o a large e x t e n t , t h e c h e m i s t r y is n o t well u n d e r s t o o d in v i e w o f t h e c o m p l e x i t y o f c o m p o u n d s o n a e r o s o l particles a n d t h e c h e m i cal r e a c t i o n s w h i c h o c c u r b e t w e e n t h o s e c o m p o u n d s o r w i t h t h e s u r r o u n d i n g a t m o s p h e r e . This l a c k reflects t o s o m e e x t e n t t h e l a c k o f suitable a n a l y t i c a l t e c h n i q u e s . F o r m i c r o c h e m i c a l analysis, larger a m o u n t s o f m a t e r i a l are r e q u i r e d , w h i c h o f t e n c a n n o t b e c o l l e c t e d in r e a s o n a b l e p e r i o d s o f t i m e . T h e m i n i m u m s a m p l e size is s m a l l e r f o r t e c h n i q u e s like n e u t r o n a c t i v a t i o n analysis ( N A A ) , X - r a y f l u o r e s c e n c e ( X R F ) , o r a t o m i c a b s o r p t i o n spect r o m e t r y ( A A S ) , T h e s e t e c h n i q u e s , h o w e v e r , deliver c o n c e n t r a t i o n s o f e l e m e n t s a n d e l e m e n t mass size d i s t r i b u t i o n s , b u t are n o t a p p l i c a b l e t o the study of chemical compounds.

* This work was supported by the Austrian Fonds zur F~rderung der wissenschaftlichen Forschung under Project No. 4088.

0048-9697/83/$ 03.00

© 1983 Elsevier Science Publishers B.V.

264 The very sensitive physical techniques of Secondary Ion Mass Spectroscopy (SIMS) or Laser Microprobe Mass Analysis (LAMMA) can provide data on chemical compounds. However, t h e y suffer from ambiguity of the results, because the mass to charge ratio (m/e) of the measured ions is not a unique property (problem of interferences). Moreover, the secondary ions reaching the mass filter m a y be fractions of partly u n k n o w n precursor molecules, the disintegration of which depends to some extent on the composition of the aerosol material and the gaseous environment, even under ultra-high vacuum conditions. Nevertheless, the investigation of urban aerosols by SIMS seems to be very promising.

SIMS TECHNIQUE The Secondary Ion Mass Spectroscopy technique [1--4] is a m e t h o d of analysis of the composition of a solid surface. An ion beam bombarding a solid surface generates the emission of secondary particles. These sputtered particles originate from the outermost monolayers of the bombarded solid surface. T h e y are emitted as neutral atoms or molecules, as excited particles, as positive or negative charged atomic or molecular ions, as electrons, or as photons. The extraction of the positive and negative secondary ions and their analysis by a mass spectrometer yields a mass spectrum (SI intensity vs. m/e) of the topmost layer of the solid surface under investigation. The mass range extends from m/e = 1 (thus including hydrogen) to an m/e of about 500, with sufficient resolution to discriminate between single mass numbers and at poorer resolution up to several thousand (very complex molecular ions). The main capabilities of this m e t h o d are: (1) Information depth in the monolayer range. (2) Isotope separation. (3) Detection of chemical compounds. (4) Detection of hydrogen and its compounds. (5) Detection limit about 10 -is g for m a n y elements and compounds, and for some anions down to 10 -18 g. The SIMS analysis has to be performed in ultra-high vacuum (UHV). The time required for the analysis of a specific top layer has to be some orders of magnitude smaller than the time needed to build up a monolayer on a surface by contamination from the residual gas atmosphere. As a general rule, a contamination layer arises within one second at a residual gas pressure of 10 -4 Pa. As a consequence, there are high requirements on the purity of the inlet gas for the primary beam and the residual gas pressure in the vacuum vessel. Bombarding a dielectric layer or an insulator with ions will charge the surface and thus disturb or even prevent the extraction of the secondary ions into the mass filter. The use of an atomic beam instead of a primary ion beam practically eliminates any charging effects, and permits the analysis

265 of dielectric and insulating samples. The neutral beam is generated from the primary ion beam by resonant charge transfer in a reaction chamber filled with the neutral gas (see reference 11). The primary ions become neutrals, but their velocity and direction are maintained. The residual ions are deflected by a plate capacitor and the neutral beam bombards the sample surface and produces the emission of the secondary particles.

INTERPRETATION OF SIMS RESULTS SIMS results represent the secondary ion intensities of the elements and chemical compounds of the top layer of the investigated solid surface; these secondary ion intensities, therefore, provide information about the chemical composition of the sample surface. The identification of elements and compounds is based on their m/e ratio. In most cases, the stable isotopic ratio of the specific element is a good help for the identification. Secondary ions from an element can appear as single or multicharged ions as well as molecular ions i.e., cluster ions. From chemical compounds, secondary ions may be emitted as molecular, as radical, or as atomic ions from the components of the compound. A main problem of identification is, therefore, the existence of interferences, i.e., superpositions of different atomic or molecular species of the same nominal mass number (for example 4°At+ and 4°Ca+; 12C21H~ and 27A1+; or 4°Ca 160+ and S6Fe+). Despite these interferences, most of the peaks of a secondary ion spectrum can be ascribed to certain elements or molecules as shown later in the discussion. For quantitative analysis, variation of secondary ion yield for different species must be considered; the secondary ion yield Sx is defined as the ratio of the number of emitted secondary ions of an element X to the number of bombarding primary ions. Sx is element specific: its value ranges from 10 -s to 10 -2 for pure elements. Additionally, the secondary ion yield for a specific ionic species from a specific element or compound varies by a factor of up to 103, depending on the oxygen content of the sample [5, 6]. This large variation may be overcome by the use of oxygen as primary ions, or by flooding the sample area with oxygen in order to oxidize the sample surface and stabilize secondary ion yields. However, the quantification of SIMS signals raises serious problems. Some theories exist for a quantitative description of the secondary ion emission. Examples are Andersen's model, which is based on thermodynamic principles [7], Schroeer's model, which deals with quantum mechanical principles [8], Sigmund's cascade theory of sputtering [9], or a combined theory such as Ruedenauer's model [10]. These theories, while applicable in specific cases, are not generally successful, especially in multicomponent layers and/or layers of rapidly changing composition. A semiquantitative description, however, can be obtained by the comparison of similar samples (samples of the same or nearly the same matrix)

266

under identical conditions of analysis. With these constraints, the intensity of a specific secondary ion signal, while n o t a measure of the absolute concentration o f this specific element or c o m p o u n d at the sample surface, can be correlated to the distribution of the relative concentration of this species occurring in a n u m b e r of similar samples.

D E S C R I P T I O N O F T H E SIMS A P P A R A T U S

The apparatus consists of t w o turbomolecular p u m p e d vessels, connected b y a UHV transport mechanism and a UHV lock. The first vessel acts as a sample preparation chamber with the facilities of vacuum deposition, removal of layers b y ion b o m b a r d m e n t , heat treatment at up to 1000°C, cold treatm e n t b y liquid N2 and a gas inlet system to expose the sample to well defined atmospheres. The second vessel is constructed as a storage and analysis chamber. Its facilities are, again, heat and cold treatment of the sample, a special gas inlet system to flood the sample area with oxygen, and a residual gas analyser to monitor the composition of the residual gas. The primary ion gun can form either an ion beam or a neutral beam of argon or other rare gases. The beam energy ranges from 0.5 to 3.0 keV and the beam density is regulated from 1 0 -11 to 10 -s A c m -2 at a beam diameter of 3.5 mm (see reference 11). The analysing system consists of an asymmetric extraction lens system and a quadrupole mass filter. The ions, passing the mass filter (mass range 1 to 511) are deflected to an off,axis (90 °) secondary electron multiplier (SEM). After amplification, the SEM signal is registered b y a ratemeter and is finally recorded b y an x--y recorder on a linear or logarithmic scale, where the x signal is derived from the H F signal controlling the mass filter. All parts of the sample preparation chamber and the analysis chamber can be baked to 400°C. As soon as the apparatus has been baked and the residual gas pressure in the analysis chamber is in the lower 1 0 - 8 Pa range, the sample can be m o u n t e d on the receptacle and set into the vented preparation chamber. When the total pressure in the preparation chamber is below 10 -s Pa, after a pumping time of 10--15 h, the UHV lock is opened and the samples are transported into the analysis chamber in which the UHV has been maintained. In this way, the samples are brought from atmospheric pressure to the UHV w i t h o u t baking. A description of this apparatus is given in greater detail elsewhere [ 1 2 ] . SAMPLE PREPARATION

The aerosol was sampled by an AERAS low pressure impactor with eleven stages. This impactor has a sampling range from 0 . 0 1 5 # m to 1 6 p m in particle diameters and a flow rate of 25 liters per min. Other data of the impactor, which has been described in more detail elsewhere [13, 1 4 ] , are listed in Table 1. The sampling station was situated on a balcony, 10 meters above ground,

267 TABLE 1 DATA OF THE IMPACTOR LPI 25/0.015 Impactor LPI/O.015 operated with critical orifice: •

Volumetrm flow rate: Measuring range:

.

Q

251/m,n (at 20 C). 0.015pro a.e.d, to 16pm a.e.d. (Aerodynamic Equivalent Diameter).

Number of stages: eleven. Cut off sizes D(0, i) pm a.e.d, of the stages and respective average particle diameter D(i) = (D(O, i)" D(O, i + 1))1/2 pm a.e.d. Vacuum pump capacity: C >1 35 m3/h at 40 mbar. Stage

Stage No.

index i

--1

D(0, i) 0.015 D(i) 0.021

0

1

2

3

4

5

6

7

8

0.030 0.042

0.060 0.087

0.125 0.18

0.25 0.35

0.50 0.71

1.0 1.4

2.0 2.8

4.0 8.0 5.7 11.3

9 16 --

facing into the back yard o f t he institute building. The aerosol was collected f o r 25 h. The i m p a c t o r was equi pped with aluminum foils for d e t e r m i n a t i o n o f the mass vs. particle size distribution, and with m e m b r a n e filter foils f o r th e SIMS analysis. Membrane filters were used for the SIMS because t he b a c k g r o u n d signal o f this material is substantially lower than t h a t of metallic foils. In order to achieve t he simultaneous sampling with one impactor, pieces o f t he filter were layed o u t on t he Al-foils covering one sixth o f t he deposition area. T h e y were held in place by a spacer ring only, w i t h o u t adhesives. Th e normalized mass size distribution, which is det erm i ned directly by gravimetry, and t he normalized surface size distribution are represented in Fig. 1. The diameters o f the modes are D m = 0.52 pm for the mass size distribution and D, = 0.24/zm f o r t he surface distribution.

ANALYSIS OF SAMPLED AEROSOLS BY SIMS The aerosol samples were m o u n t e d on a receptacle and brought into t he analysis cham be r in the m a nne r described above. The sample area in the analysis c ha m ber was f l o o d e d with o x y g e n (2" 1 0 - 4 Pa) in order t o provide a constant secondary ion yield per element or c o m p o u n d over a n u m b e r o f similar samples. T he specimens f r o m stages - - 1 t o 7 collected on membrane filters were analysed b y SIMS. The generation o f t he secondary ions was p e r f o r m e d b y a primary beam o f argon atoms. T he beam parameters were: A const ant flux o f 6" 1013 atoms cm -2 sec, a beam diameter o f 3.5 m m at an incident angle of 50 °, and a beam energy equivalent to 1.4 keV. The p r o c e d u r e o f t he SIMS analysis was, first, the removal of t he t o p 30 m onol ayers (range o f cont am i nat i on) by

268 %

0

mass

•

surface

25

P, 20 u) "0 (D

15,

N

"0 OJ N D

10'

E L. 0 t-"

I

I

I

I

-1

0

1

2

stage

l

l

3

l

4

l

l

5

l

6

7

8

number

Fig. 1. N o r m a l i z e d mass size distribution and n o r m a l i z e d surface d i s t r i b u t i o n o f the aerosol investigated.

sputtering, and then the registration of a positive and a negative secondary ion spectrum of each of the samples. The mass range was 1 ~ m/e ~ 100. The analysis area as given by the diameter and the incidence angle of the primary beam is about 15 mm 2. In comparison, the spot areas, according to light microscope measurements, are substantially smaller, with sizes ranging from 0.3mm 2 at stage --1 to 10.5mm 2 at stage 7. As the SIMS signal depends on the spot area, an attempt was made to correct the SIMS signal to a specific area, i.e. to counting rates per mm 2. This procedure turned out to be inconsistent. In investigating this problem, another part of the aerosol sample was distributed over the membrane filter surface by smearing out the spot material. The SIMS signals of these samples were in general lower than the corresponding signals of the untreated samples by a factor of four to five for the finer fractions (stages --1, 0, 1, 2, 3) and by a factor of two or nearly one for the coarser fractions (stages 4, 5, 6, 7). In addition, a further comparison was performed by X-ray fluorescence. In this experiment the dark spot was separated together with the underlaying filter substrate from the fairly clean complementary piece of the filter. Both parts were analysed for sulfur, which is a marginal impurity of the filter matrix but a main constituent of the aerosol material. The sulfur content of the spot was twice that of the complement piece, in spite of the fact that the spot covers about 95% of the visible deposition area. From both experiments the conclusion

269

106 6

HCH~

105~- H+

C2H ~ Na +

CN-

i,,,I,,c,: ,, ,

10

O-

llili]llt II1!1!11111i1~I111 ,I,Ill

til , 1 'i'i i

li

I~tlI I IIII.!

Mill I!111111ti~i111IIIIIIII , II

~ 'I

L

[t!1!II!ill'Ill 1!1 I'IIIIIIEM,,=,,I,],I,

,,wli,

1,1

l lll,llu :

1C I

10

I

I

3J0

I

5'0

I

7qO

--

9:0 m / e

I

10

I

I

3~0

I

I

50

I

70

',

:

-=--

90 m/e

Fig. 2. Positive and negative SI spectrum of the blank membrane filter.

can be drawn that some of the aerosol material, a liquid phase, diffuses into the filter matrix, thus covering a larger area than the spot, which mainly contains the solid material. As a consequence, the secondary ion intensities have n o t been reduced to the specific areas covered b y the spot of solid materials.

RESULTS

The positive and negative secondary ion spectra of the blank filter substrate are shown in Fig. 2. In the positive secondary ion spectrum, mainly the low molecular weight hydrocarbon ions are visible. The negative spectrum shows the elements and simple c o m p o u n d s of H, C, N, O, but no substantial amounts of higher molecular weight compounds. In order to illustrate the difference, for the secondary ion spectra of an aerosol sample, the spectra obtained from a sample from stage 3 are shown in Fig. 3. The signal of the aerosol is definitely higher than that of the blank filter, b y up to a factor of 10 2 for the higher masses (the ion intensities of the aerosol samples have not been corrected for the blank), and many identifications of specific ions from the aerosol material can be made, as can be seen in Fig. 3.

270

10 ~

H-

O-CN-

CPS4

Na + K + !~

NON OSO-

~T

o5 CaOH +

so~. //----I I II'~ II1~1 ~,~.~ illll.~[llllll!l ]h I.

I, ~ II I~1 Illrll? ~lTILIl~'ll~l!~l,lilll, ll

]I q I I,! / I IJi iltll ~ll~i~JlllillllJll,

10

I II / i, lllr,lSUIl![/~

,

I

, , i,i, ~,~ i , llU,lllllll4,,,llll,lll ,~

l!ll '11' 'If f'l"r'~l"tJrtl~l,t,.i 110

I

3~0

I

J

50

~

:

70

Ii

I

I !ill' # I I I!l IIIllq,I 1.1~ill

.~ ~ vT :

--

90 rn/e

I

10

I

3JO

~'t~', ~'~t~ ~.

I

5to

I

710

I

9~0

!

m/e

Fig. 3. Positive and negative SI spectrum of an aerosol deposited on a membrane filter at stage 3 of the impactor.

In Figs. 4a--e the secondary ion counting rates for specific ions are plotted on the ordinate in a logarithmic scale against the particle size, represented here by the stage number (see Table 1) on the abscissa. The secondary ion intensities have been taken from the recorded spectra, measured under standard conditions for each aerosol fraction (i.e. stage number). If the secondary ion signal is proportional to the specific surface density of the ion's parent material, then the signal intensity can be correlated on a relative basis with the specific surface density for that particular element or compound. We may then define a relative specific surface density with respect to an internal reference, i.e., a certain sample or sample average from the set obtained by the impactor. We do not assume, however, that secondary ion signal ratios of different ions correspond to equal ratios of the respective materials, since secondary ion yields vary by large factors from one ion species to another. Many of the surface density distributions can be grouped together according to the similarity of variation with stage number. One group, which is represented in Fig. 4a, comprises the ions H-, C-, COOH-, Na ÷ and oxides of nitrogen. Another group with the ions O -, O H , CNO-, CaOH + and Mg+ is very similar in shape, but definitely differs at stages 6 and 7 (see Fig. 4b),

271

e~ (a)

(b)

O -

lOS

H-

°H-

Na+

10

C-

=

CNO-

COOH104

CaOH+

NO~

10 4 -

.

~

_~

NO-

10 ~

NO;

10 3

• ~ error bar i

J

-1 0

i

,

=

=

i

i

i

1

2 3

4

5

6

7

t ' ' r r ° r b ar. . . . . . I

.

-1

.

.

0

.

1

.

.

2

.

.

3

.

4

5

6

7

stage number stage number

(d)

(c) 10 5

10' K+

o

10 4

10 4

o

-~

~

,e

¢~

~error bar

=; 1 0 3 ~

- ~

H20+

= o

° 10

SO~

?,-,, i , , i

1o, -1

~ .

.....

-1

0

1

80-

2

3

0

1

2

3 4

5

6

7

stage number 4

5

6

7

stage number

(e) ~=

o

10 ~ - -j -

~

a r

~

/

F-

CI-

10 4 SiO;

o, =

~= 103 o 102 -1

0

I

2

3

4

5

8

7

stage number

Fig. 4. SI intensity distributions of (a) H-, Na +, C-, C O O H - and NO~; (b) O-, OH-, CNO-, CaOH + and Mg*; ( c ) H 2 0 +, SO~ and SO~; ( d ) K +, SO-, SO~ and Li+; ( e ) F - , C1 - and SiO ~.

272

~, C a O H + 0 0 4

• Mg+

• OH-

t

O3

N

~ 0.6 L-

~ 0.4

0.2 -10

1

2

stage

3

4

5

6

7

number

Fig. 5. Normalized intensity distributions

ct/~ of CaOH +, Mg*, O-, O H - a n d CNO-

in which there is an increase in relative surface density. A group with a predominant modal structure contains the ions SO ~, SO ~ and H 2 0 + with a m a x i m u m at stage 3 (see Fig. 4c). In comparison, the ions S O - a n d SO~ exhibit a different distribution with a maximum at stage 3, b u t almost constant values for larger particles (see Fig. 4d). Interestingly, the K ÷ curve seems to possess some relation to the sulfur curves. A further group, F-, C1- and SiO ~, is shown in Fig. 4e. These ions are represented mainly in the coarse particles, indicating a correlation to soil materials, b u t they can o b v i o u s l y be f o u n d in the smallest particles as well.

DISCUSSION

To compare the results in more detail, the distributions have been normalized by dividing the surface densities ct b y average densities ~ = Y, c i / N , where ~ is the total sample average. An example of the normalized distributions with the results of Fig. 4b are shown in Fig. 5. The consequence of the normalization has been to bring the group of measured curves into close coincidence, showing unambiguously the correlation of these ion signals and the species t h e y represent in the aerosols. The sulfur-containing ions all have a maximum at stage 3, with a modal diameter of 0.37 pm which corresponds well with ~ the modes of the mass and surface size distributions (see Fig. 1.). The m o d e falls in the "accumulation m o d e " , a term which has been introduced b y Whitby et al. [15] in

273 TABLE 2 C O R R E L A T I O N C O E F F I C I E N T S O F Li +, K +, Na +, Mg + A N D CaOH + D I S T R I B U T I O N S

Li+ K+ Na+ Mg+ CaOH +

Li +

K +

Na +

Mg +

1 +0.29 --0.77 + 0.I 7 + 0.15

1 --0.17 --0.73 --0.74

1 -- 0.07 --0.10

1 + 1.00

CaOH

+

order to describe the structure of atmospheric aerosols. The mode indicates that sulfurous c o m p o u n d s and H 2 0 are major constituents of the accumulation m o d e particles. In following Whitby's model of atmospheric aerosols, we expect that accumulation mode particles are primarily built up b y coagulation of finer particles. They grow further b y agglomeration, condensation of vapors, and b y conversion of gases. In case of a pure coagulation, the surface densities of the ions should n o t depend on particle size over a wider range. The occurrence of a maximum thus indicates either a high conversion rate of SO2 on the accumulation mode particles, or a local source of some strength that emits sulfurous aerosols with sizes around 0.3 ~m. A strong correlation exists between the sulfurous ions with K + and Li+, b u t n o t with Na +, Mg + and CaOH + or the other groups. Interestingly enough, the ions H 2 0 +, S O ~ and SO~ follow K ÷, with correlation coefficients of 0.95, 0.88 and 0.90 respectively, whereas the ions SO~ and S O - f o l l o w lithium with correlation coefficients of 0.82 and 0.79. The other correlation coefficients of this group fall between 0.29 for Li+/K ÷ and 0.62 for S O ~ / S O ~ , with an average coefficient of 0.47 (1 + 0.24). Consequently, the sulfurous ion distributions decompose into a Li group and a K group. We want to add here that the distributions of t w o other mass numbers, i.e. m / e = 50 and m / e = 83, which could be attributed to SNH~ and SONH a NH~, correlate well with the Li group, but have been omitted because of possible interferences with other ions. The ions Na +, Mg ÷ and CaOH + are also correlated with their specific groups, with correlation coefficients of 0.88, 0.97 and 0.92 respectively. For the ions Li +, Na +, K +, Mg + and CaOH ÷ we obtain correlation coefficients listed in Table 2. Obviously the elements Li, Na, K and Mg (or Ca) are n o t correlated and can be regarded as characteristic elements for certain ion groups, at least in this aerosol sample. It seems to us that these results require a more detailed discussion of the identification of the elements. The assignment of mass number 7 to 7Li +, mass n u m b e r 23 to 23Na + and mass number 39 to 39K+ is quite definite. Li + and K + are verified b y comparing the stable isotope ratios 7Li+/6Li + = 12.5 and agK+/41K+ = 13.5 with the corresponding SI signal intensities. For

274

Na + there could be an interference only with 7Lil60 +, b u t as there is no peak for 6Li ' 6 0 + we can exclude this possibility. At mass number 24, which has been attributed to Mg, interferences could occur with either 12C ~, Nail ÷ or LiOH +. C~ cannot be a main component, as there is no large corresponding C2H + peak, whereas the CH ÷ peak is even larger than the C + peak (reference is made here to the SI spectra in Fig. 3). Additionally, LiOH + can be excluded as Li ÷ has a surface density distribution that does n o t correlate with the distribution of mass number 24. A similar argument holds for Nail +. Of course, we assume that the SIMS signal for ion pairs such as Li÷and LiOH ÷, or Na ÷ and Nail +, would occur in constant proportions, irrespective of surface densities. The minimum, which occurs at stage 3 in the Mg and CaOH group and the Na group, merely reflects the diminished surface densities resulting from the additional material. Assuming a dilution factor of roughly three, we think that the total a m o u n t of material added to stage 3 and its neighbours b y gas conversion, or b y particles of a local source, is fairly high.

CONCLUSIONS

The analysis of aerosol samples b y SIMS has been performed with a newly developed primary gun producing an atomic beam of argon. Elements, as well as chemical c o m p o u n d s with mass numbers from 1 (hydrogen) up to complex molecules, can be identified. The secondary ion intensities vary by two to three orders of magnitude for some aerosol components. The minimum a m o u n t of aerosol for SIMS analysis is a b o u t 0.1 ~g. This m e t h o d had been applied to aerosol samples collected on membrane filters b y a low pressure cascade impactor. As indicated b y the size distributions, accumulation mode particles seem to possess the potential to convert SO2 into particulate matter, and certain ion groups are associated with the elements Li, K, Na and Mg. At present it is n o t clear whether these correlations are specific to the one sample investigated, or would occur systematically in the urban aerosol at Vienna.

ACKNOWLEDGMENTS

The author acknowledges the help of Dr. A. Berner in providing the aerosol samples, and for his contributions to the discussions. Further thanks are due to Dr. V. Berner for the X-ray fluorescence crosscheck measurements of some o f the samples.

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