Size distribution of traffic derived aerosols

The Science of the Total Environment, 36 (1984) 299--303 Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands

299

SIZE DISTRIBUTION OF TRAFFIC DERIVED AEROSOLS A. BERNERI , G. REISCHLI and H. PUXBAUM2 I I n s t i t u t e f o r Experimental Physics of the University of Vienna 21nstitute f o r Analytical Chemistry, Technical University of Vienna

ABSTRACT Measurements with low pressure impactors show that automotive t r a f f i c produces larger amounts of nucleation mode material in the size range below 0,I ~m. Most of this material is soot and other organic carbon compounds. After aging, i . e . coagulation and d i l u t i o n in the urban atmosphere, the nucleation mode material contributes to the accumulation mode and an accumulation mode of carbon is found at 0,5 um. The size d i s t r i b u t i o n s of s u l f u r indicate that the s u l f u r compounds measured in the present work are not l i k e l y produced by the t r a f f i c d i r e c t l y . INTRODUCTION Automotive t r a f f i c

is one of the major sources of the urban aerosol. As

demonstrated by WILSON ( r e f .

I) size d i s t r i b u t i o n s measured in a group of cars

d r i v i n g on a highway e x h i b i t three d i s t i n c t modes each of which can be a t t r i buted to certain processes of p a r t i c l e production. Stationary impactor measurements near a s t r e e t in a c i t y have shown that the modal structure is a general c h a r a c t e r i s t i c of the urban aerosol. The modes can be observed d i r e c t l y or can be deduced mathematically. Their concentrations vary. Whereas the nucleation mode and the coarse mode concentrations at d i f f e r e n t hights above s t r e e t levels are correlated to some extent, hereby i n d i c a t i n g a common source or local group of sources, the accumulation mode concentrations are f a i r l y independent from both the nucleation and coarse mode concentrations, at least f o r the measuring s i t e studied so f a r ( r e f . 2). EXPERIMENTAL The impactor studies have been continued by measuring the t r a f f i c aerosol from a moving car. The impactors were mounted on the f r o n t bumper of a passenger car and were operated during d r i v i n g in public streets with heavy automotive traffic.

The impactors are low pressure impactors with a sampling rate of

30 I/min and a measuring range from 15 nm to 16 pm a.e.d. ( a . e . d . : aerodynamic equivalent diameter). The impactors have eleven stages, the c u t o f f points of which gradually increase from 15 nm to 16 pm by a f a c t o r of two.

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The p a r t i c l e s are collected on aluminium f o i l s of 10 Nm thickness in order to obtain a high accuracy of weighing the samples. The errors introduced by manipulating and exposing the f o i l s to d i f f e r e n t laboratory conditions do not exceed I Ng. The aerosol samples have been analysed f o r t o t a l s u l f u r and t o t a l carbon by a thermoanalytical technique ( r e f . 3). Other analysis could not be performed as the sample volumes were too small in these experiments. The size d i s t r i b u t i o n s considered here are averages with respect to the duration of the sampling period (4 hours), to the t r a v e l l i n g distance (100 km to 120 km) and to the t r a f f i c

s i t u a t i o n , which changes continuously. No e f f o r t s

could be made to separate the influence of diesel cars and trucks. THE AEROSOL MASS SIZE DISTRIBUTIONS The mass size d i s t r i b u t i o n s e x h i b i t three d i s t i n c t shapes, a unimodal, a bimodal and a trimodal one (Fig. la and Ib). In the bimodal d i s t r i b u t i o n s the two modes occur at 0,08 Nm a.e.d, and around 5 Nm a . e . d . . The modes can be i d e n t i f i e d as a nucleation mode (0,08 ~m) and a coarse mode (5 Nm). The nucleation mode concentrations of 15 Ng/m3 to 30 ~ig/m3 agree with data in the l i t e r a t u r e ( r e f . 4). The modal diameter of the nucleation mode appears somewhat large, but taking the density of the p a r t i c l e s into account ( ~ :

1,8 g/cm 3)

brings the modal diameter down to 0,05 Nm f o r a volume size d i s t r i b u t i o n

on a

geometric diameter scale. This value corresponds to a modal diameter of 0,02 Nm f o r the respective number size d i s t r i b u t i o n . Both values are in agreement with data reported elsewhere ( r e f . 4). The coarse mode is concentrated at a f a i r l y small diameter. But we expect lower values as i s o k i n e t i c sampling conditions have not been achieved in c o l l e c t i n g the aerosol. Bounce and blow o f f also can be of some influence ( r e f . 2). In the trimodal d i s t r i b u t i o n s there appears a t h i r d mode at sizes around 0,5 ~m a.e.d. (the other modes occur at the sizes reported above). The t h i r d mode represents the accumulation mode ( r e f . 2, 4, 5, 6, 7). F i n a l l y , in the unimodal d i s t r i b u t i o n s the accumulation mode predominates and the nucleation and the coarse modes occur as bumps on the slopes of the accumulation mode d i s t r i b u t i o n s , again at the diameters reported above. We observe that i n - t r a f f i c

aerosols always have f a i r l y

strong nucleation

modes and coarse modes with only s l i g h t l y varying concentrations. The accumulation modes, however, undergo considerable v a r i a t i o n s in concentrations, ranging from a few Ng/m3 to 100 Ng/m3 and more. The rather constant concent r a t i o n s of the nucleation mode, which are higher than those measured near a street ( r e f . 2), indicate that the nucleation mode material is generated by the traffic,

e i t h e r emitted d i r e c t l y from the exhaust pipes of the cars or formed

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in the atmosphere. The t r a f f i c

does not seem to be a very s i g n i f i c a n t d i r e c t

source of accumulation mode p a r t i c l e s , else the strongly varying concentrations and e s p e c i a l l y the low values in t r a f f i c

can not be explained. The accumulation

mode concentrations, however, depend on the weather conditions. The accumulation aerosol is an "aged" aerosol. When streaming i n t o the area of the c i t y i t carries p a r t i c l e s with l i f e times of hours and days, the concentrations of which depend on the o r i g i n of the a i r masses and on t h e i r path to the measuring s i t e . Over the c i t y the accumulation mode p a r t i c l e s c o l l e c t nucleation mode material by coaqulation, and even new accumulation mode p a r t i c l e s can be formed. Soot material is indicated by the o p ti c a l appearence of the aerosol deposits. Whereas the deposits of the coarse p a r t i c l e s are greyish, those of the accumul a t i o n mode e x h i b i t a black c o l o r , which even deepens in the nucleation mode size range. Soot therefore is a main component of the nucleation mode and accumulation mode m a te r i a l . (Remark: The color of aerosol deposits is in general not an unambiguous i n d i c a t o r of the aerosol m a t e r i a l . In our case however we feel j u s t i f i e d to exclude other dark materials such as metal oxides or metals themselves.) TOTAL SULFUR AND TOTAL CARBON SIZE DISTRIBUTIONS The size d i s t r i b u t i o n s f o r t o t a l carbon (C d i s t r i b u t i o n s ) and t o t a l s u l f a t e (S d i s t r i b u t i o n s ) y i e l d more information on the r e l a t i o n between t r a f f i c the urban aerosol. In f i g . bution of an i n - t r a f f i c

and

2a and 2b a bimodal and a trimodal mass size d i s t r i -

aerosol is shown, together with the carbon and s u l f a t e

d i s t r i b u t i o n s . Carbon in both cases has a very pronounced peak at 0,08 Nm a . e . d . , in complete correspondence with the nucleation mode discussed above. In the bimodal case the carbon concentrations are f a i r l y low at sizes around 0,5 ~m a . e . d . , but rise again at larger p a r t i c l e s . In the trimodal case the carbon concentrations at 0,5 ~m a.e.d, are higher than in the bimodal case, but as f a r as the present measurements are concerned they do not exceed the concentrations of the nucleation mode. Evidently carbon is a main constituent of the nucleation mode produced by the t r a f f i c . This r e s u l t is supported by comparative measurements, an example of which is represented in f i g . 3a and 3b. Here the i n - t r a f f i c

aerosol has been

measured along aroute around the i n s t i t u t e b u i l d i n g , where another impactor was operated in a back yard. The C d i s t r i b u t i o n in t r a f f i c e x h i b i t s again a peak at 0,08 Nm a.e.d, and a bump at the position of the accumulation mode. At the remote s i t e , the nucleation mode disappears and an accumulation mode remains in the C d i s t r i b u t i o n . This r e s u l t which is found in a l l other comparat i v e measurements is again evidence that nucleation mode material is d i r e c t l y produced by the t r a f f i c

and moreover demonstrates the rapid decay of the

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nucleation mode by coagulation and d i l u t i o n . We suppose that the accumulation mode of the C d i s t r i b u t i o n is mainly a coagulation product of the nucleation mode material. The S d i s t r i b u t i o n s show a d i f f e r e n t behaviour. In a l l measurements a single mode appears at sizes around 0,5 Nm a . e . d . , but not at 0,08 Nm a.e.d. or in the coarse p a r t i c l e range. In a d d i t i o n , there is no substantial difference in concentration and shape of the t r a f f i c conclude that the t r a f f i c ,

and back yard d i s t r i b u t i o n s . We

under the conditions at Vienna, is not d i r e c t l y

c o n t r i b u t i n g to the aerosol s u l f u r on a s i g n i f i c a n t l e v e l . ACKNOWLEDGEMENT The project was supported by the "Magistrat der Gemeinde Wien, MA 22 Umweltschutz" under contract number MA 22-850/79 and contract number MA 22-943/79. REFERENCES I

W.E. R.K. K.T. Ass.

Wilson, L.L. S p i l l e r , T.G. Ellestad, P.J. Lamothe, T.G. Dzubay, Stevens, E.S. Macias, R.A. Fletcher, J.K. Husar, R.B. Husar, Whitby, D.B. K i t t e l s o n and B.K. C a n t r e l l , J. A i r P o l l u t . Control 27 (1977) 46-51

2

A. Berner and Ch. LUrzer, J. Phys. Chem. 84 (1980) 2079-2083

3

H. Puxbaum and J. Rendl, Mikrochim. Acta (Wien), 1983 I , 263-272

4

K.T. Whitby, Atmos. Environ. 12 (1978) 135-159

5

A. Berner and G. Reischl, J. Aerosol Sci. 14 (1983) 324-325

6

A. Berner, Ch. L~rzer, F. Pohl, O. Preining and P. Wagner Sci. Total Environ., 13 (1979) 245-261

7

R. Hitzenberger and R.B. Husar, Atmos. Environ. (1983) in Press

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i:=o,o21~m; 2:=o,o42~m; 3:=o,o87~m; 4:=o, 18~m; 5:=o,~5~m; 6:=0, 71~m; 7:=1,4~m; 8:=2,8~m; 9:~5,7~m; Io:=11,3~m. of mass,

M: mass distributions,

elemental C: carbon

carbon or sulfate distributions,

, ~ g / m 3 per stage

S: sulfate

distributlons